2013 Volume 61 Issue 8 Pages 853-869
Tricyclic lactam model compounds of the left half (ABC ring) of renieramycin and saframycin anticancer natural products were prepared from common intermediate 6a. Readily available alcohol 6a was converted into enamide 8, and this was followed by transformation into 6b through a hydrobromination reaction in a stereoselective manner. Some diastereomers at C-6 to C-11a of the tricyclic lactam model compounds having several functional groups at C-6 were prepared from 6a or 6b in good yields. We presented also an unexpected reductive acetylation of the p-quinone to produce the corresponding 3,4-dehydro derivative.
Tetrahydroisoquinoline natural products, including renieramycins, saframycins, ecteinascidins, naphthyridinomycins, and quinocarcins, have persistently attracted considerable interest for more than 30 years due to their novel structures and meager availability in nature, as well as their potent antitumor activity1,2) (Fig. 1). Ecteinascidin 743, in particular, has received considerable attention for its strong in vivo activity.3) It is currently marketed in over fifty countries for the treatment of soft-tissue sarcoma and phase II/III clinical trials for the treatment of other cancers are ongoing.4,5)
In the course of our research of new metabolites from marine animals, we isolated renieramycin M in gram scale from the Thai blue sponge Xestospongia sp. by pretreatment with potassium cyanide.6,7) Structure–activity relationship (SAR) investigations of a variety of derivatives of renieramycin M8–10) yielded several pieces of evidence pointing to the fact that virtually all biologically active members of this family of natural products, such as 1, possess a carbinolamine or cyano function at C-21, which permits the formation of potent electrophilic iminium ion species 2, a compound that has been implicated in the formation of covalent bonds with DNA and possibly other bio macromolecules (Fig. 2). We are also very interested in the biological activities of renieramycin G11) and cribrostatin 4 (=renieramycin H12–14)) because both compounds show antiproliferative activity. Renieramycin G and cribrostatin 4 are tetrahydroisoquinolines possessing a C-21 lactam carbonyl group; however, they exhibit low cytotoxicity and are less active than their respective C-21 cyano or carbinolamine containing relatives. To date, five total syntheses of renieramycin G15–21) and four total syntheses of cribrostatin 422–25) have been published. Several studies on the SAR of renieramycins, including renieramycin G and cribrostatin 4, were conducted,8,26–30) but the cytotoxicity profiles of these compounds were not addressed.
Among a handful of studies of simple models containing a lactam ring, such as 3, Ong and coworkers reported that pentacyclic scaffold 4 showed low cytotoxicity.31,32) Avendaño and coworkers discovered that pyrazino[1.2-b]isoquinoline-4-one derivative 5 induced apoptosis from the G2/M phase of the cell cycle without causing DNA damage.33) These findings have stirred much excitement and inspired the search for other simplified tetrahydroisoquinolines having antitumor activities that are similar to or more potent than those displayed by parent marine natural products. We report here the preparation of tricyclic lactam model compounds and their diastereomers. A series of C-6 to C-11a cis stereoisomers, such as 14b, 16b, 23b, and 27b, were prepared from 6b, a compound obtained from trans alcohol 6a in four steps. The trans isomers, such as 14a, 16a, 23a, and 27a, were also prepared from 6a by using the same process. An unexpected reductive acetylation of the p-quinone to produce the corresponding 3,4-dehydro derivative was also presented.
We have reported the preparation of alcohol 6a having trans configuration at C-6 to C-11a, which involved the stereoselective Pictet–Spengler type cyclization of N-methyl-3-arylmethylpiperazine-2,5-dione34) with ethyl diethoxyacetate, followed by chemoselective transformation.35) We first studied the conversion of trans isomer 6a into cis-alcohol 6b (Chart 1).
Treatment of 6a with [(PhO)3P+CH3]I− (methyl triphenoxy phosphonium iodide) in N,N-dimethylformamide (DMF) gave alkyl iodide 7a in 87.6% yield. Subjecting 7a to an elimination reaction with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in ethyl acetate under reflux afforded enamide 8 in 88.2% yield. Transformation of 8 with pyridinium bromide perbromide (PBP) in tetrahydrofuran (THF), followed by NaBH3CN and trifluoroacetic acid (TFA) produced bromide 9b in 65.5% overall yield along with ketobromide 10 in 9.2% yield. The reduction of iminium ion 11B via bromonium ion intermediate 11A might occur from the α-face to generate 9b stereoselectively (Fig. 3). On the other hand, the production of 10 could be explained by the attack of 11B by water, followed by B ring opening. According to the procedure of Dibble and coworkers,36) we heated 9b with CaCO3 in aqueous 1,4-dioxane at 80°C for 27 h and obtained 6b in 53.5% yield. X-Ray crystallographic analysis of 6b showed that the stereochemistry was cis between C-6 and C-11a protons (Fig. 4).
Next, we turned our attention to the conversion of alcohol 6b into the tricyclic lactam model compound of renieramycin G 16b (Chart 2). The conversion of the trimethoxyarene into a p-quinone system was initiated by oxidative demethylation. All attempts at the direct oxidative demethylation of 6b employing conventional agents, such as ceric ammonium nitrate (CAN),37) argentic oxide (AgO),38) silver(II) dipicolinate,39) and nitric-acid-impregnated manganese dioxide,40) gave only the starting material. Furthermore, the oxidative demethylation of 6b with nitric acid41) or [bis(trifluoroacetoxy)iodo]benzene (BTI)42) gave only a polar polymeric material. This problem was solved by using the partial demethylation and oxidative demethylation sequence. Thus, the partial demethylation of 6b with 4.5 equiv of BBr3 in CH2Cl2 at −78°C for 10 min and then at −20°C for 24 h gave 12b and its regio isomer 13b in 51% and 11% yields, respectively, along with recovered 6b (21%). The orientation of the phenolic hydroxyl group in 12b and 13b was determined on the basis of heteronuclear multiple bond connectivity (HMBC) correlations. The chemical shifts of C-7 (δ 142.6 ppm), C-8 (δ 149.7 ppm), and C-10 (δ 148.5 ppm) in 12b indicated oxygen substitution and two of the three carbons were found to have long-range 1H–13C correlations to methoxy protons (δ 3.77, 3.53 ppm). The remaining carbon at δ 149.7 ppm showed long-range 1H–13C correlations to C-11 methylene protons. The long-range 1H–13C correlations of 9-CH3 protons (δ 2.45 ppm) to C-8 and C-10 carbons also revealed the positions of those two carbons. In contrast, in 13b, the long-range 1H–13C correlations of 9-CH3 protons (δ 2.43 ppm) to C-8 (δ 147.9 ppm) and C-10 (δ 151.7 ppm) carbons were observed. No long-range 1H–13C correlations of the C-8 carbon (δ 147.9 ppm) to any methoxy protons (δ 3.77, 3.55 ppm) were detected, whereas C-10 carbon (δ 151.7 ppm) showed long-range 1H–13C correlations to methoxy protons (δ 3.55 ppm) as well as C-11 methylene protons.
The oxidative demethylation of 12b with CAN in aqueous acetonitrile at −17°C for 30 min gave p-quinones 14b and 15b in 60% and 19% yields, respectively. According to the method of Williams,23) esterification of the primary alcohol group in 14b with angeloyl chloride gave tricyclic lactam model compound 16b in 78% yield.
With our first target model compound 16b in hand, we next investigated the conversion of the hydroxymethyl group in 6b into a pyruvoyl amide group by utilizing the Mitsunobu procedure43) (Chart 3). Treatment of 6b with diethyl azodicarboxylate (DEAD), triphenylphosphine, and phthalimide in THF at 25°C for 3 h gave imide 17b in 70% yield along with 8 (15%). Heating 17b with hydrazine hydrate in ethanol at 65°C for 1 h gave amine 18b in 82% yield,44) which was acylated with pyruvoyl chloride to give amide 19b in 84% yield. After an extensive investigation of the conversion of 19b into 20b, the following procedure was considered the best in terms of product yield and reproducibility. Treatment of 19b with BBr3 (5 eq) in dichloromethane at −20°C gave 20b, 21b, and 22b in 44.6%, 9.0%, and 9.0% yields, respectively. The oxidative demethylation of 20b with CAN provided the model of saframycin A 23b in 62.2% yield. We were able to obtain our second target model 23b, but the overall yield of the three-step process from 17b into 20b was disappointingly low (30.6%). This result prompted us to examine another approach that involved the conversion of 17b into 20b via 24b and 25b. The reaction of 17b with BBr3 (4.5 eq) at −20°C for 45 h gave 24b in 83.0% yield. Treatment of 24b with hydrazine hydrate in ethanol at 25°C for 6 h gave amine 25b in 88.0% yield, which was acylated with pyruvoyl chloride to give amide 20b in 99% yield (72.3% overall yield from 17b).
Another target model was N-acetyl derivative 27b (Chart 4). Acetylation of phenol 25b with acetyl chloride and triethylamine (TEA) in dichloromethane at 0°C for 30 min gave 26b in a quantitative yield. Oxidative demethylation of 26b with CAN afforded 27b in 86.0% yield. In contrast, the oxidative demethylation of 24b with CAN gave 28b and cleavage of the phthaloyl group with hydrazine monohydrate gave primary amine 29b. However, attempts to purify 29b by silica gel column chromatography were unsuccessful because 29b was easily converted into 30b. Thus, crude 29b was used in the next acylation step without further purification to give 27b in 85.0% overall yield.
Encouraged by the result of the above C-6 to C-11a cis configuration model conversion from 6b, we carried out a similar transformation in a C-6 to C-11a trans series model from 6a. Partial demethylation of 6a with BBr3 gave 12a and 13a in 81.0% and 10.0% yields, respectively (Chart 2). Oxidative demethylation of 12a with CAN afforded 14a and 15a in 78.6% and 7.2% yields, respectively. Acylation of 14a with angelic chloride gave 16a in 72.1% yield. Reaction of 6a with DEAD, triphenylphosphine, and phthalimide in THF at 25°C for 3 h produced imide 17a (100%), which was subsequently treated with hydrazine monohydrate to afford amine 18a (Chart 3). Acylation of 18a with pyruvoyl chloride gave pyruvamide 19a in 88.7% yield. Partial demethylation of 19a with 5.0 equiv of BBr3 gave phenols 20a and 22a in 80.0% and 2.5% yields, respectively. Oxidative demethylation of 20a with CAN provided 23a in 88.0% yield. In addition, partial demethylation of 17a with BBr3 gave 24a in 91.3% yield (Chart 5). Oxidative demethylation of 24a with CAN afforded 28a in 92.5% yield. Cleavage of the phthaloyl group of 28a with hydrazine monohydrate furnished primary amine 29a, which was used in the next acylation step without further purification to give 27a in 64.5% overall yield.
During the course of our research, we were surprised to find that when 29a and acetic anhydride in pyridine were stirred at 25°C for a long time, triacetate 31 was generated in 80.0% yield. The 1H-NMR spectrum of 31 displayed no signals corresponding to the methylene proton at C-11; however, a characteristic singlet at δ 7.21 ppm was observed together with three acetyl methyl proton signals at δ 2.74, 2.52, and 2.26 ppm. The 13C-NMR spectrum of 31 showed one doublet at δ 107.4 ppm, which was assigned to the olefinic carbon, but the carbon signals of p-quinone (δ 180–186 ppm) and C-11 methylene carbon were not observed. Triacetate 31 was also obtained from 27a in the same manner in 66.8% yield. The results obtained by using other substrates are shown in Table 1. The results indicated that the transformation rate was relative low and the addition of 4-dimethylaminopyridine (DMAP) accelerated the reaction. The yields of this transformation of the C-6 to C-11a trans series of p-quinones were much higher than those of the C-6 to C-11a cis series of compounds, because the stabilities of the cis series of p-quinones were much lower than those of the corresponding trans series. A probable mechanistic pathway for the formation of 31 (X=NHCOCH3) is shown in Chart 6. Reductive acetylation of 29a and 14a with zinc powder and acetic anhydride produced 36 and 37 in 87% and 93% yields, respectively.
| Entry | Substrate | Time (h) | Product | Yield (%) | 11-H singlet δ | C11-signal δ |
|---|---|---|---|---|---|---|
| 1 | 29a | 87 | 31 | 80.0 | 7.21 s | 107.4 d |
| 2 | 27a | 48 | 31 | 66.8 | ||
| 3 | 27b | 48 | 31 | 29.0 | ||
| 4 | 16a | 20 | 32 | 87.0 | 6.95 s | 108.8 d |
| 5a) | 16a | 2 | 32 | 70.0 | ||
| 6 | 16b | 96 | 32 | 22.0 | ||
| 7 | 23a | 51 | 33 | 66.0 | 6.90 s | 108.7 d |
| 8 | 23b | 24 | 33 | 7.0 | ||
| 9 | 28a | 21 | 34 | 93.0 | 7.04 s | 108.8 d |
| 10a) | 28a | 2 | 34 | 70.0 | ||
| 11 | 28b | 72 | 34 | 71.0 | ||
| 12 | 14a | 20 | 35 | 83.0 | 6.95 s | 108.8 d |
| 13a) | 14a | 2 | 35 | 70.0 | ||
| 14 | 14b | 96 | 35 | 46.0 |
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In conclusion, we prepared some diastereomers at C-6 to C-11a as simple tricyclic lactam model compounds of renieramycin and saframycin anticancer natural products from common intermediate 6b. We discovered that the transformation of p-quinone (14, 16, 23, 27, 29) into diacetylhydroquinone took place by using unprecedented C-11 to C-11a unsaturated analogues (32–35) via enolate formation. Efforts geared toward the preparation of other analogues and the examination of their biological activities to evaluate the mechanism of action are under way in our laboratory.
All melting points were determined with a Yanagimoto micromelting point apparatus and are uncorrected. IR spectra were obtained with a Shimadzu IRAffinity-1 Fourier Transform Infrared Spectrometer. 1H-NMR spectra were recorded at 300 MHz on a JEOL AL-300 spectrometer and at 400 MHz on a JEOL AL-400 spectrometer. 13C-NMR spectra were recorded at 100 MHz [multiplicities were determined from distortionless enhancement by polarization transfer (DEPT) spectra]. NMR spectra were measured in CDCl3, pyridine-d5 or DMSO-d6 and the chemical shifts were recorded in δH values relative to (CH3)4Si as the internal standard. All proton and carbon signals were assigned by extensive NMR measurements using correlation spectroscopy (COSY), HMBC, and heteronuclear multiple quantum coherence (HMQC) techniques. Mass spectra were recorded on a JMS-700 instrument with a direct inlet system operating at 70 eV. Elemental analyses were conducted on a YANACO MT-6 CHN CORDER elemental analyzer.
(6S*,11aS*)-6-(Iodomethyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (7a)Methyl triphenoxy phosphonium iodide (6.330 g, 14 mmol) was added to a solution of 6a (2.548 g, 7 mmol) in dry DMF (100 mL) at 0°C, and the mixture was stirred for 1 h at 0°C. The reaction mixture was diluted with water (500 mL) and extracted with Et2O (500 mL×3). The combined extracts were washed with brine (500 mL), dried, and concentrated in vacuo to give a residue, which was purified by column chromatography with AcOEt to give 7a (2.908 g, 87.6%) as a colorless amorphous powder. 1H-NMR δ: 5.99 (1H, dd, J=10.9, 3.8 Hz, 6-H), 4.30 (1H, dd, J=12.4, 4.7 Hz, 11a-H), 4.15 (1H, d, J=17.8 Hz, 3-H), 4.06 (1H, d, J=17.8 Hz, 3-H), 3.92 (3H, s, 7-OCH3), 3.80 (1H, dd, J=10.9, 3.8 Hz, 6-CH), 3.79 (3H, s, 8-OCH3), 3.66 (3H, s, 10-OCH3), 3.41 (1H, dd, J=17.0, 4.7 Hz, 11-Hα), 3.29 (1H, t, J=10.9 Hz, 6-CH), 3.07 (3H, s, N-CH3), 2.64 (1H, dd, J=17.0, 12.4 Hz, 11-Hβ), 2.18 (3H, s, 9-CH3). 13C-NMR δ: 164.9 (s, C-1), 161.5 (s, C-4), 152.2 (s, C-10), 150.2 (s, C-8), 145.9 (s, C-7), 125.6 (s, C-9), 124.9 (s, C-6a), 120.8 (s, C-10a), 60.6 (q, 7-OCH3), 60.1 (q, 8-OCH3), 60.0 (q, 10-OCH3), 51.4 (t, C-3), 51.0 (d, C-11a), 49.0 (d, C-6), 33.6 (q, N-CH3), 28.6 (t, C-11), 9.6 (q, 9-CH3), 5.8 (t, 6-CH2). IR (KBr) cm−1: 3453, 2934, 1664, 1464, 1070. Electron ionization-mass spectra (EI-MS) m/z (%): 474 (M+, 26), 347 (78), 333 (100), 319 (15), 305 (15), 234 (13). High-resolution (HR)-EI-MS Calcd for C18H23IN2O5: 474.0652. Found 474.0651.
7,8,10-Trimethoxy-2,9-dimethyl-6-methylene-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (8)DBU (1.17 mL, 7.8 mmol) was added to a solution of 7a (1.85 g, 3.9 mmol) in AcOEt (115 mL) and the mixture was heated under reflux for 1.5 h. The reaction mixture was poured into 1 N aqueous HCl solution (600 mL) and extracted with CHCl3 (600 mL×3). The combined extracts were washed with brine (600 mL), dried, and concentrated in vacuo. The residue was recrystallized from AcOEt to afford 8 (1.19 g, 88.2%) as colorless needles, mp 150–151°C. 1H-NMR δ: 6.34 (1H, s, 6-CH), 5.68 (1H, s, 6-CH), 4.19 (1H, dd, J=12.2, 4.2 Hz, 11a-H), 4.13 (1H, d, J=17.4 Hz, 3-H), 4.04 (1H, d, J=17.4 Hz, 3-H), 3.84 (3H, s, 8-OCH3), 3.82 (3H, s, 7-OCH3), 3.69 (3H, s, 10-OCH3), 3.53 (1H, dd, J=16.2, 4.2 Hz, 11-H), 3.06 (3H, s, N-CH3), 2.83 (1H, dd, J=16.2, 12.2 Hz, 11-H), 2.21 (3H, s, 9-CH3). 13C-NMR δ: 165.5 (s, C-1), 162.5 (s, C-4), 151.6 (s, C-10), 151.4 (s, C-8), 147.4 (s, C-7), 132.9 (s, C-6), 126.0 (s, C-9), 123.7 (s, C-6a), 122.1 (s, C-10a), 114.6 (t, 6-CH2), 60.5 (q×2, 8-, 10-OCH3), 60.1 (q, 7-OCH3), 56.8 (d, C-11a), 52.5 (t, C-3), 33.6 (q, N-CH3), 27.2 (t, C-11), 9.8 (q, 9-CH3). IR (KBr) cm−1: 2943, 1667, 1641, 1441, 1408. EI-MS m/z (%): 346 (M+, 60), 331 (100), 303 (6), 232 (21), 219 (25), 205 (9). HR-EI-MS Calcd for C18H22N2O5: 346.1529. Found: 346.1531. Anal. Calcd for C18H22N2O5: C, 62.42; H, 6.40; N, 8.09. Found: C, 62.21; H, 6.48; N, 8.13.
Bromination of Compound 8PBP (1.95 g, 6.12 mmol) was added to a stirred solution of 8 (1.06 g, 3.06 mmol) in THF (80 mL) and the mixture was stirred at −20°C for 2 h. The reaction mixture was warmed to 0°C and then TFA (2.27 mL, 30.6 mmol) and 1 N THF solution of NaBH3CN (30.5 mL, 30.5 mmol) were added. Stirring was continued at 0°C for 30 h. The reaction mixture was diluted with saturated NaHCO3 solution (450 mL) and extracted with CHCl3 (450 mL×3). The combined extracts were washed with brine (450 mL), dried, and concentrated in vacuo. The residue was subjected to column chromatography with hexane–AcOEt (1 : 2) to afford 9b (854 mg, 65.5%) as a colorless amorphous powder. Further elution with hexane–AcOEt (1 : 4) gave 10 (124 mg, 9.2%) as a pale yellow amorphous powder.
(6R*,11aS*)-6-(Bromomethyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (9b)1H-NMR δ: 5.89 (1H, t, J=3.9 Hz, 6-H), 4.12 (1H, d, J=17.1 Hz, 3-H), 4.01 (1H, dd, J=10.5, 3.9 Hz, 6-CH), 3.99 (1H, d, J=17.1 Hz, 3-H), 3.90 (3H, s, 7-OCH3), 3.89 (1H, dd, J=12.7, 3.9 Hz, 11a-H), 3.80 (3H, s, 8-OCH3), 3.71 (3H, s, 10-OCH3), 3.70 (1H, dd, J=16.4, 3.9 Hz, 11-Hα), 3.61 (1H, dd, J=10.5, 3.9 Hz, 6-CH), 3.09 (1H, dd, J=16.4, 12.7 Hz, 11-Hβ), 3.08 (3H, s, N-CH3), 2.22 (3H, s, 9-CH3). 13C-NMR δ: 166.3 (s, C-4), 166.3 (s, C-1), 151.4 (s, C-10), 150.1 (s, C-8), 145.7 (s, C-7), 125.7 (s, C-9), 124.4 (s, C-6a), 123.2 (s, C-10a), 60.8 (q, 10-OCH3), 60.5 (q, 7-OCH3), 59.9 (q, 8-OCH3), 54.5 (d, C-11a), 52.2 (t, C-3), 51.2 (d, C-6), 37.1 (t, 6-CH2), 33.6 (q, N-CH3), 24.5 (t, C-11), 9.3 (q, 9-CH3). IR (KBr) cm−1: 2940, 2361, 1670, 1410, 1074. EI-MS m/z (%): 428 (M+ + 2, 9), 426 (M+, 9), 333 (100), 305 (9), 234 (8). HR-EI-MS Calcd for C18H23O5N2Br: 426.0790. Found: 426.0794.
3-(2-(2-Bromoacetyl)-3,4,6-trimethoxy-5-methylbenzyl)-1-methylpiperazine-2,5-dione (10)1H-NMR δ: 6.58 (1H, br s, NH), 4.41 (1H, d, J=12.9 Hz, ArCOCH), 4.36 (1H, d, J=12.9 Hz, ArCOCH), 4.30 (1H, dd, J=8.5, 4.6 Hz, 3-H), 3.90 (1H, d, J=17.3 Hz, 6-H), 3.85 (3H, s, ArOCH3), 3.85 (3H, s, ArOCH3), 3.79 (1H, d, J=17.3 Hz, 6-H), 3.73 (3H, s, ArOCH3), 3.28 (1H, dd, J=14.2, 4.6 Hz, ArCH), 2.99 (3H, s, N-CH3), 2.82 (1H, dd, J=14.2, 8.5 Hz, ArCH), 2.23 (3H, s, ArCH3). 13C-NMR δ: 197.2 (s), 166.3 (s), 165.7 (s), 154.0 (s), 151.0 (s), 146.9 (s), 131.4 (s), 129.5 (s), 123.6 (s), 61.5 (q, ArOCH3), 60.5 (q, ArOCH3), 60.3 (q, ArOCH3), 55.1 (d, C-3), 51.9 (t, C-6), 36.6 (t, COCH2Br), 34.1 (q, N-CH3), 30.7 (t, C-3a), 10.0 (q, ArCH3). IR (KBr) cm−1: 2941, 1684, 1464, 1402, 1331, 1065. EI-MS m/z (%): 444 (M+ + 2, 4), 442 (M+, 4), 345 (100), 317 (22), 315 (36), 237 (18). HR-EI-MS Calcd for C18H23O6N2Br: 442.0740. Found: 442.0734.
(6R*,11aS*)-6-(Hydroxymethyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (6b)CaCO3 (1.84 g, 18.4 mmol) in water (40 mL) was added to a stirred solution of 9b (1.565 g, 3.7 mmol) in 1,4-dioxane (40 mL) and the reaction mixture was heated to 80°C and stirred for 27 h. After the reaction mixture was cooled to 25°C, water (500 mL) was added and the resulting mixture was extracted with CHCl3 (500 mL×3). The combined extracts were washed with brine (500 mL), dried, and concentrated in vacuo to give a residue, which was purified by silica gel chromatography with hexane–AcOEt (4 : 1) to afford 9b (561 mg, 30.5%). Further elution with CHCl3–MeOH (20 : 1) afforded 6b (720.3 mg, 53.5%) as a solid, which was recrystallized from AcOEt–MeOH to give colorless prisms, mp 194–195°C. 1H-NMR δ: 5.82 (1H, dd, J=7.0, 4.8 Hz, 6-H), 4.14 (1H, d, J=16.9 Hz, 3-H), 3.97 (1H, d, J=16.9 Hz, 3-H), 3.90 (3H, s, 7-OCH3), 3.88 (1H, dd, J=12.6, 3.2 Hz, 11a-H), 3.80 (3H, s, 8-OCH3), 3.78 (1H, dt, J=11.1, 4.8 Hz, 6-CH), 3.76 (1H, dd, J=15.8, 3.2 Hz, 11-Hα), 3.71 (3H, s, 10-OCH3), 3.60 (1H, ddd, J=11.1, 7.0, 6.3 Hz, 6-CH), 3.07 (3H, s, N-CH3), 2.79 (1H, dd, J=15.8, 12.6 Hz, 11-Hβ), 2.59 (1H, dd, J=6.3, 4.8 Hz, OH), 2.21 (3H, s, 9-CH3). 13C-NMR δ: 167.1 (s, C-4), 166.2 (s, C-1), 151.4 (s, C-10), 150.2 (s, C-8), 145.8 (s, C-7), 125.4 (s, C-9), 124.0 (s, C-6a), 122.8 (s, C-10a), 67.2 (t, 6-CH2), 61.1 (q, 10-OCH3), 60.7 (q, 7-OCH3), 60.0 (q, 8-OCH3), 55.3 (d, C-11a), 52.9 (d, C-6), 52.5 (t, C-3), 33.9 (q, N-CH3), 24.2 (t, C-11), 9.6 (q, 9-CH3). IR (KBr) cm−1: 3433, 2941, 1664. EI-MS m/z (%): 364 (M+, 3), 347 (17), 333 (100), 305 (14), 234 (14), 204 (11). HR-EI-MS Calcd for C18H24N2O6: 364.1634. Found: 364.1629. Anal. Calcd for C18H24N2O6: C, 59.33; H, 6.64; N, 7.69. Found: C, 59.18; H, 6.62; N, 7.66.
X-Ray Structure Determination of Compound 6bCrystals of 6b (C18H24N2O6) belong to monoclinic space group P21/c (#14) with a=6.0410(1) Å, b=24.6043(5) Å, c=13.2970(9) Å, β=115.777(7)°, V=1779.7(2) Å3, Z=4, and Dcalcd=1.360 g/cm3. X-Ray intensities were measured with a Rigaku R-AXIS RAPID diffractometer in the graphite-monochromated CuKα radiation mode (λ=1.54187 Å). The final cycle of the full-matrix least-squares refinement was based on 3241 unique reflections (2θ<136.4°) and 241 variable parameters and converged with unweighted and weighted agreement factors of R=0.0452, Rw=0.1019, and R1=0.0384 for I>2.0σ(I) data. The drawing of the molecule was made by ORTEP. CCDC-No. 889511 contains supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Partial Demethylation of Compound 6bA solution of 6b (36.4 mg, 0.10 mmol) in CH2Cl2 (6.4 mL) was cooled at −78°C and a CH2Cl2 solution of BBr3 (1.0 M, 0.45 mL, 0.45 mmol) was added. The temperature was maintained at −78°C for 10 min. Then, the reaction mixture was warmed to −20°C over 2.5 h and stirring was continued at −20°C for 24 h. The reaction was quenched with water (40 mL) and extraction was carried out with CHCl3 (40 mL×3). The combined extracts were washed with brine (40 mL), dried, and concentrated in vacuo. The residue was purified by silica gel chromatography with hexane–EtOH (17 : 3) to give 12b (17.8 mg, 51.0%) as a colorless amorphous powder and 13b (4.0 mg, 11.0%) as a colorless amorphous powder. Further elution with CHCl3–MeOH (40 : 1) gave 6b (7.6 mg, 21.0% recovery).
(6R*,11aS*)-10-Hydroxy-6-(hydroxymethyl)-7,8-dimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (12b)1H-NMR (pyridine-d5) δ: 10.59 (1H, br s, 10-OH), 6.28 (1H, t, J=4.2 Hz, 6-H), 4.53 (1H, dd, J=11.3, 4.2 Hz, 6-CH), 4.45 (1H, dd, J=15.6, 3.9 Hz, 11-Hα), 4.20 (1H, d, J=16.6 Hz, 3-H), 4.17 (1H, dd, J=11.3, 4.2 Hz, 6-CH), 4.15 (1H, dd, J=12.5, 3.9 Hz, 11a-H), 3.98 (1H, d, J=16.6 Hz, 3-H), 3.85 (3H, s, 7-OCH3), 3.74 (3H, s, 8-OCH3), 3.56 (1H, dd, J=15.6, 12.5 Hz, 11-Hβ), 2.78 (3H, s, N-CH3), 2.45 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 166.6 (s, C-1), 165.6 (s, C-4), 149.7 (s, C-8), 148.5 (s, C-10), 142.6 (s, C-7), 125.5 (s, C-6a), 118.6 (s, C-9), 118.4 (s, C-10a), 65.2 (t, 6-CH2), 60.0 (q, 7-OCH3), 59.3 (q, 8-OCH3), 54.9 (d, C-11a), 53.1 (d, C-6), 52.1 (t, C-3), 32.6 (q, N-CH3), 25.5 (t, C-11), 9.7 (q, 9-CH3). IR (KBr) cm−1: 3509, 3306, 2961, 2924, 2853, 1672, 1636. EI-MS m/z (%): 350 (M+, 5), 319 (100), 291 (16), 220 (13). HR-EI-MS Calcd for C17H22N2O6: 350.1478. Found: 350.1474.
(6R*,11aS*)-8-Hydroxy-6-(hydroxymethyl)-7,10-dimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (13b)1H-NMR (pyridine-d5) δ: 11.12 (1H, br s, 8-OH), 6.28 (1H, t, J=4.2 Hz, 6-H), 4.51 (1H, dd, J=11.2, 4.2 Hz, 6-CH), 4.23 (1H, d, J=16.6 Hz, 3-H), 4.20 (1H, dd, J=11.2, 4.2 Hz, 6-CH), 4.13 (1H, dd, J=12.5, 3.7 Hz, 11a-H), 4.06 (1H, dd, J=15.3, 3.7 Hz, 11-Hα), 4.00 (1H, d, J=16.6 Hz, 3-H), 3.77 (3H, s, 7-OCH3), 3.55 (3H, s, 10-OCH3), 3.52 (1H, dd, J=15.3, 12.5 Hz, 11-Hβ), 2.80 (3H, s, N-CH3), 2.43 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 166.5 (s, C-1), 165.8 (s, C-4), 151.7 (s, C-10), 147.9 (s, C-8), 141.4 (s, C-7), 125.5 (s, C-6a), 118.9 (s, C-10a), 118.6 (s, C-9), 65.2 (t, 6-CH2), 60.0 (q, 10-OCH3), 59.8 (q, 7-OCH3), 55.0 (d, C-11a), 53.1 (d, C-6), 52.1 (t, C-3), 32.6 (q, N-CH3), 24.7 (t, C-11), 9.5 (q, 9-CH3). IR (KBr) cm−1: 3397, 2926, 2852, 1663, 1063. EI-MS m/z (%): 350 (M+, 6), 319 (100), 291 (16), 220 (10), 190 (15). HR-EI-MS Calcd for C17H22N2O6: 350.1478. Found: 350.1475.
Oxidative Demethylation of 12b with Ceric Ammonium Nitrate (CAN)A stirred solution of 12b (21.5 mg, 0.06 mmol) in CH3CN (9.0 mL) was cooled with ice-water and an aqueous solution (0.7 mL) containing CAN (84.0 mg, 0.15 mmol) was added dropwise over 10 min at −17°C. Stirring was continued for 30 min at −17°C. The reaction mixture was diluted with 5% aqueous NaHCO3 solution (60 mL) and extracted with CH2Cl2 (60 mL×3). The combined extracts were washed with brine (30 mL), dried, and evaporated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (50 : 1) to afford 14b (12.0 mg, 60.0%) as a yellow amorphous powder and 15b (4.0 mg, 19.0%) as a colorless amorphous powder.
(6R*,11aS*)-6-Hydroxymethyl-8-methoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4,7,10(6H)-tetraone (14b)1H-NMR (pyridine-d5) δ: 5.47 (1H, td, J=4.0, 1.7 Hz, 6-H), 4.14 (1H, d, J=17.3 Hz, 3-H), 4.02 (3H, s, 8-OCH3), 3.98 (1H, ddd, J=11.3, 6.0, 4.0 Hz, 6-CH), 3.96 (1H, d, J=17.3 Hz, 3-H), 3.87 (1H, dd, J=11.7, 3.4 Hz, 11a-H), 3.67 (1H, ddd, J=11.3, 6.0, 1.7 Hz, 6-CH), 3.66 (1H, dd, J=17.5, 3.4 Hz, 11-Hα), 3.06 (3H, s, N-CH3), 2.61 (1H, ddd, J=17.5, 11.7, 1.7 Hz, 11-Hβ), 2.09 (1H, t, J=6.0 Hz, OH), 1.99 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 184.9 (s, C-10), 180.3 (s, C-7), 166.4 (s, C-4), 165.3 (s, C-1), 155.2 (s, C-8), 140.8 (s, C-10a), 136.3 (s, C-6a), 129.0 (s, C-9), 64.6 (t, 6-CH2), 61.1 (q, 8-OCH3), 53.8 (d, C-11a), 52.4 (d, C-6), 52.3 (t, C-3), 34.0 (q, N-CH3), 23.6 (t, C-11), 9.1 (q, 9-CH3). Fourier transform (FT)-IR (KBr) cm−1: 3422, 2941, 1663, 1431, 1229. HR-FAB-MS (Magic bullet) m/z 335.1248 [M++H] (Calcd for C16H19N2O6: 335.1243).
(2aR*,6aS*,11aR*)-2a,3-Dimethoxy-4,8-dimethyl-1,6,6a,8,9,11a-hexahydrofuro[2,3,4-ij]pyrazino[1,2-b]isoquinoline-5,7,10(2aH)-trione (15b)1H-NMR δ: 5.03 (1H, dd, J=9.0, 8.0 Hz, 1-H), 4.68 (1H, tdd, J=8.0, 3.3, 3.1 Hz, 11a-H), 4.15 (1H, dd, J=11.2, 4.2 Hz, 6a-H), 4.14 (3H, s, 3-OCH3), 4.09 (1H, d, J=17.9 Hz, 9-H), 3.99 (1H, d, J=17.9 Hz, 9-H), 3.98 (1H, dd, J=9.0, 8.0 Hz, 1-H), 3.45 (1H, ddd, J=18.8, 4.2, 3.1 Hz, 6-Hα), 3.16 (3H, s, 2a-OCH3), 3.02 (3H, s, N-CH3), 2.44 (1H, ddd, J=18.8, 11.2, 3.3 Hz, 6-Hβ), 1.82 (3H, s, 4-CH3). 13C-NMR δ: 184.2 (s, C-5), 164.5 (s, C-10), 163.7 (s, C-7), 160.6 (s, C-3), 142.2 (s, C-11b), 128.7 (s, C-5a), 117.3 (s, C-4), 96.9 (s, C-2a), 73.8 (t, C-1), 59.1 (q, 3-OCH3), 55.8 (d, C-6a), 53.1 (d, C-11a), 51.5 (q, 2a-OCH3), 51.3 (t, C-9), 33.7 (q, N-CH3), 28.1 (t, C-6), 7.9 (q, 4-CH3). IR (KBr) cm−1: 2924, 1653, 1238. EI-MS m/z (%): 348 (M+, 7), 333 (28), 317 (100), 219 (15), 204 (10), 191 (12). HR-EI-MS Calcd for C17H20N2O6: 348.1321. Found: 348.1316.
(Z)-((6R*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-2,3,4,6,7,10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl 2-methylbut-2-enoate (16b)A solution of angelic acid (74.4 mg, 0.74 mmol) in ether (2.7 mL) was cooled with ice-water and a DMF (5.3 µL, 69 µmol) solution of oxalyl chloride (63 µL, 0.74 mmol) and toluene (0.25 mL) was added dropwise over 10 min. The mixture was stirred at 25°C for 2 h. A CH2Cl2 (1.5 mL) solution of 14b (13.2 mg, 0.036 mmol) was added to the mixture and stirring was continued at 25°C for 3.5 h. The reaction mixture was directly purified by silica gel chromatography with AcOEt–CH2Cl2 (1 : 4) to afford 16b (11.8 mg, 78.0%) as a dark yellow amorphous powder. 1H-NMR δ: 6.03 (1H, qq, J=7.2, 1.5 Hz, =CHCH3), 5.59 (1H, br td, J=2.9, 1.9 Hz, 6-H), 4.56 (1H, dd, J=11.8, 2.9 Hz, 6-CH), 4.51 (1H, dd, J=11.8, 2.9 Hz, 6-CH), 4.14 (1H, d, J=17.4 Hz, 3-H), 4.06 (3H, s, 8-OCH3), 3.99 (1H, d, J=17.4, 3-H), 3.86 (1H, dd, J=11.7, 3.9 Hz, 11a-H), 3.61 (1H, dd, J=18.0, 3.9 Hz, 11-Hα), 3.05 (3H, s, N-CH3), 2.43 (1H, ddd, J=18.0, 11.7, 1.9 Hz, 11-Hβ), 1.96 (3H, s, 9-CH3), 1.82 (3H, dq, J=7.2, 1.5 Hz, =CHCH3), 1.74 (3H, quint, J=1.5 Hz, =CCH3). 13C-NMR δ: 185.0 (s, C-10), 180.0 (s, C-7), 167.3 (s, C-14), 166.1 (s, C-4), 165.4 (s, C-1), 155.9 (s, C-8), 140.1 (s, C-10a), 140.0 (d, =CHCH3), 135.9 (s, C-6a), 128.4 (s, C-9), 126.7 (s, =CCH3), 63.6 (t, 6-CH2), 61.0 (q, 8-OCH3), 53.3 (d, C-11a), 52.2 (t, C-3), 50.5 (d, C-6), 33.8 (q, N-CH3), 23.5 (t, C-11), 20.5 (q, =CCH3), 15.6 (q, =CHCH3), 8.8 (q, 9-CH3). IR (KBr) cm−1: 2955, 1717, 1668, 1230, 1146. EI-MS m/z (%): 416 (M+, 4), 387 (23), 386 (100), 305 (19), 304 (22), 303 (30), 204 (14). HR-EI-MS Calcd for C21H24N2O7: 416.1584. Found: 416.1586.
(6R*,11aS*)-6-((1,3-Dioxoisoiodolin-2-yl)methyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (17b)A solution of diethyl azodicarboxylate in toluene (40%, 2.28 mL, 5.0 mmol) was added dropwise to a solution of 6b (364.0 mg, 1.0 mmol), phthalimide (736.0 mg, 5.0 mmol), and triphenylphosphine (1.31 g, 5.0 mmol) in THF (25 mL) at 25°C. After the resulting solution was stirred at 25°C for 3 h, the solvent was concentrated in vacuo. The residue was diluted with water (300 mL) and extracted with CHCl3 (300 mL×3). The combined extracts were washed with brine (300 mL), dried, and concentrated in vacuo to give a residue, which was chromatographed on a silica gel column (40 g) with hexane–EtOH (4 : 1) to give 17b (347.5 mg, 70.0%) as a pale yellow amorphous powder. Further purification by silica gel chromatography with AcOEt–EtOH (8 : 1) afforded 8 (50.5 mg, 15.0%) as colorless needles.
17b: 1H-NMR δ: 7.78 (2H, dd, J=4.9, 2.9 Hz, 3′-H2), 7.67 (2H, dd, J=4.9, 2.9 Hz, 4′-H2), 6.04 (1H, dd, J=7.1, 5.4 Hz, 6-H), 4.04 (1H, d, J=16.8, 1.2 Hz, 3-H), 3.96 (1H, dd, J=13.7, 5.4 Hz, 6-CH), 3.91 (3H, s, 7-OCH3), 3.88 (1H, dd, J=13.7, 7.1 Hz, 6-CH), 3.86 (1H, dd, J=12.4, 4.1 Hz, 11a-H), 3.81 (1H, d, J=16.8 Hz, 3-H), 3.74 (1H, dd, J=16.1, 4.1 Hz, 11-Hα), 3.63 (3H, s, 8-OCH3), 3.62 (3H, s, 10-OCH3), 3.06 (3H, s, N-CH3), 2.82 (1H, dd, J=16.1, 12.4 Hz, 11-Hβ), 2.19 (3H, s, 9-CH3). 13C-NMR δ: 167.6 (s, C-2′), 166.2 (s, C-1), 165.8 (s, C-4), 151.6 (s, C-10), 150.2 (s, C-8), 146.0 (s, C-7), 133.6 (d, C-4′), 131.9 (s, C-2a′), 125.5 (s, C-9), 124.0 (s, C-6a), 122.9 (d, C-3′), 122.5 (s, C-10a), 61.0 (10-OCH3), 60.7 (q, 7-OCH3), 59.8 (q, 8-OCH3), 54.9 (d, C-11a), 52.4 (t, C-3), 48.8 (d, C-6), 41.6 (t, 6-CH2), 33.9 (q, N-CH3), 23.9 (t, C-11), 9.5 (q, 9-CH3). IR (KBr) cm−1: 2940, 1717, 1670, 1418, 1395. EI-MS m/z (%): 493 (M+, 5), 333 (100), 305 (6), 234 (5), 204 (7). HR-EI-MS Calcd for C26H27N3O7: 493.1849. Found: 493.1854.
(6R*,11aS*)-6-(Aminomethyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (18b)Hydrazine monohydrate (1.4 mL) was added to a stirred solution of 17b (347.5 mg, 0.7 mmol) in EtOH (14 mL) and the mixture was heated at 65°C for 1 h. The reaction mixture was concentrated in vacuo to afford a residue, which was diluted with benzene (150 mL) and extracted with 1 N HCl (150 mL×3). The combined acidic aqueous layer was made alkaline with 5% aqueous NH4OH and extracted with CHCl3 (150 mL×3). The combined extracts were washed with brine (150 mL), dried, and concentrated in vacuo to give a residue, which was purified by silica gel chromatography with CHCl3–MeOH (4 : 1) to give 18b (208.9 mg, 82.0%) as a pale yellow amorphous powder. 1H-NMR δ: 5.66 (1H, t, J=6.3 Hz, 6-H), 4.12 (1H, d, J=16.6 Hz, 3-H), 3.95 (1H, d, J=16.6 Hz, 3-H), 3.87 (3H, s, 7-OCH3), 3.85 (1H, dd, J=12.5, 3.9 Hz, 11a-H), 3.81 (3H, s, 8-OCH3), 3.74 (1H, dd, J=15.8, 3.9 Hz, 11-Hα), 3.72 (3H, s, 10-OCH3), 3.07 (3H, s, N-CH3), 2.88 (1H, dd, J=13.1, 6.3 Hz, 6-CH), 2.80 (1H, dd, J=13.1, 6.3 Hz, 6-CH), 2.76 (1H, dd, J=15.8, 12.5 Hz, 11-Hβ), 2.22 (3H, s, 9-CH3). 13C-NMR δ: 166.9 (s, C-1), 166.4 (s, C-4), 151.7 (s, C-10), 150.5 (s, C-8), 146.2 (s, C-7), 125.6 (s, C-6a), 125.2 (s, C-9), 122.6 (s, C-10a), 61.1 (q, 10-OCH3), 60.6 (q, 7-OCH3), 60.0 (q, 8-OCH3), 55.1 (d, C-11a), 53.0 (d, C-6), 52.6 (t, C-3), 48.0 (t, 6-CH2), 33.8 (q, N-CH3), 23.9 (t, C-11), 9.4 (q, 9-CH3). IR (KBr) cm−1: 2938, 1665, 1458, 1404, 1072. EI-MS m/z (%): 363 (M+, 3), 345 (40), 334 (100), 333 (99), 305 (11), 234 (18), 204 (20). HR-EI-MS Calcd for C18H25N3O5: 363.1794. Found: 363.1791.
N-(((6R*,11aS*)-7,8,10-Trimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (19b)A solution of 18b (198.6 mg, 0.55 mmol), TEA (152 µL, 1.09 mmol), and 4-(dimethylamino) pyridine (134 mg, 1.09 mmol) in CH2Cl2 (0.9 mL) was cooled at 0°C, and a carbon tetrachloride solution of pyruvoyl chloride (1.0 M, 2.2 mL, 2.2 mmol) was added dropwise over 10 min. Stirring was continued at 0°C for 1 h. Then, the reaction mixture was diluted with water (100 mL) and extracted with CHCl3 (100 mL×3). The combined extracts were washed with brine (100 mL), dried, and concentrated in vacuo. The residue was subjected to column chromatography with AcOEt to afford 19b (199.1 mg, 84.0%) as a pale yellow amorphous powder. 1H-NMR δ: 7.52 (1H, dd, J=6.6, 5.8 Hz, NH), 5.84 (1H, t, J=6.6 Hz, 6-H), 4.11 (1H, d, J=17.0 Hz, 3-H), 3.95 (1H, d, J=17.0 Hz, 3-H), 3.90 (3H, s, 7-OCH3), 3.87 (1H, br dd, J=12.4, 4.1 Hz, 11a-H), 3.78 (1H, dd, J=16.0, 4.1 Hz, 11-Hα), 3.78 (3H, s, 8-OCH3), 3.71 (3H, s, 10-OCH3), 3.50 (1H, dt, J=13.6, 6.6 Hz, 6-CH), 3.44 (1H, ddd, J=13.6, 6.6, 5.8 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.72 (1H, dd, J=16.0, 12.4 Hz, 11-Hβ), 2.39 (3H, s, COCH3), 2.21 (3H, s, 9-CH3). 13C-NMR δ: 196.3 (s, COCH3), 166.4 (s, C-4), 166.2 (s, C-1), 160.2 (s, NHCO), 151.7 (s, C-10), 150.2 (s, C-8), 145.5 (s, C-7), 125.8 (s, C-9), 124.1 (s, C-6a), 122.3 (s, C-10a), 61.2 (q, 10-OCH3), 60.8 (q, 7-OCH3), 60.1 (q, 8-OCH3), 55.1 (d, C-11a), 52.4 (t, C-3), 49.2 (d, C-6), 45.1 (t, 6-CH2), 33.9 (q, N-CH3), 24.5 (q, COCH3), 24.0 (t, C-11), 9.6 (q, 9-CH3). IR (KBr) cm−1: 3339, 2926, 1717, 1674, 1412. EI-MS m/z (%): 433 (M+, 1), 333 (100), 305 (7), 234 (6), 204 (6). HR-EI-MS Calcd for C21H27N3O7: 433.1849. Found: 433.1853.
Partial Demethylation of Compound 19bA solution of 19b (43.3 mg, 0.10 mmol) in CH2Cl2 (6.4 mL) was cooled at −78°C and a CH2Cl2 solution of BBr3 (1.0 M, 0.50 mL, 0.50 mmol) was added. The reaction was kept at the same temperature for 10 min, warmed to −20°C over 2 h, and then continuously stirred at −20°C for 6.5 h. The reaction was quenched with water (40 mL) and the mixture was extracted with CHCl3 (40 mL×3). The combined extracts were washed with brine (40 mL), dried, and concentrated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (50 : 3) to give 20b (18.7 mg, 44.6%) as a colorless amorphous powder. Further elution with CH2Cl2–MeOH (20 : 1) gave 21b (3.6 mg, 9.0%) and 22b (1.8 mg, 9.0%).
N-(((6R*,11aS*)-10-Hydroxy-7,8-dimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexa-hydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (20b)1H-NMR δ: 7.57 (1H, dd, J=6.3, 5.4 Hz, NH), 6.12 (1H, br s, 10-OH), 5.84 (1H, t, J=6.3 Hz, 6-H), 4.14 (1H, d, J=17.1 Hz, 3-H), 3.97 (1H, d, J=17.1 Hz, 3-H), 3.92 (1H, dd, J=12.2, 4.4 Hz, 11a-H), 3.87 (3H, s, 7-OCH3), 3.85 (1H, dd, J=16.1, 4.4 Hz, 11-Hα), 3.78 (3H, s, 8-OCH3), 3.48 (1H, ddd, J=14.7, 6.3, 5.4 Hz, 6-CH), 3.44 (1H, dt, J=14.7, 6.3 Hz, 6-CH), 3.07 (3H, s, N-CH3), 2.69 (1H, dd, J=16.1, 12.2 Hz, 11-Hβ), 2.37 (3H, s, COCH3), 2.19 (3H, s, 9-CH3). 13C-NMR δ: 196.6 (s, COCH3), 166.9 (s, C-1), 166.4 (s, C-4), 160.5 (s, NHCO), 150.2 (s, C-8), 147.6 (s, C-10), 143.0 (s, C-7), 123.9 (s, C-6a), 119.1 (s, C-9), 115.5 (s, C-10a), 60.9 (q, 7-OCH3), 60.2 (q, 8-OCH3), 55.1 (d, C-11a), 52.4 (t, C-3), 49.2 (d, C-6), 45.0 (t, 6-CH2), 33.9 (q, N-CH3), 24.4 (q, COCH3), 23.6 (t, C-11), 9.1 (q, 9-CH3). IR (KBr) cm−1: 3339, 2936, 1722, 1663, 1464, 1422, 1070. EI-MS m/z (%): 419 (M+, 2), 320 (18), 319 (100), 291 (8), 220 (5). HR-EI-MS Calcd for C20H25N3O7: 419.1693. Found: 419.1688.
N-(((6R*,11aS*)-8-Hydroxy-7,10-dimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexa-hydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (21b)1H-NMR δ: 7.41 (1H, dd, J=6.6, 5.4 Hz, NH), 5.79 (1H, t, J=6.6 Hz, 6-H), 5.79 (1H, br s, 8-OH), 4.12 (1H, d, J=17.1 Hz, 3-H), 3.96 (1H, d, J=17.1 H, 3-H), 3.87 (1H, dd, J=12.7, 4.4 Hz, 11a-H), 3.82 (3H, s, 7-OCH3), 3.76 (1H, dd, J=16.1, 4.4 Hz, 11-Hα), 3.71 (3H, s, 10-OCH3), 3.53 (1H, dt, J=13.7, 6.6 Hz, 6-CH), 3.43 (1H, ddd, J=13.7, 6.6, 5.4 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.72 (1H, dd, J=16.1, 12.7 Hz, 11-Hβ), 2.40 (3H, s, COCH3), 2.21 (3H, s, 9-CH3). 13C-NMR δ: 196.7 (s, COCH3), 166.7 (s, C-4), 166.4 (s, C-1), 160.5 (s, NHCO), 152.8 (s, C-10), 146.9 (s, C-8), 139.8 (s, C-7), 123.6 (s, C-6a), 119.1 (s, C-9), 118.4 (s, C-10a), 61.8 (q, 7-OCH3), 61.2 (q, 10-OCH3), 55.1 (d, C-11a), 52.3 (t, C-3), 49.2 (d, C-6), 45.1 (t, 6-CH2), 33.8 (q, N-CH3), 24.4 (q, COCH3), 23.7 (t, C-11), 9.2 (q, 9-CH3). IR (KBr) cm−1: 3341, 2938, 1722, 1668, 1472, 1425, 1061. EI-MS m/z (%): 419 (M+, 2), 320 (18), 319 (100), 291 (7), 220 (4), 190 (7). HR-EI-MS Calcd for C20H25N3O7: 419.1693. Found: 419.1689
N-(((6R*,11aS*)-8,10-Dihydroxy-7-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexa-hydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (22b)1H-NMR (pyridine-d5) δ: 11.04 (1H, br s, 10- or 8-OH), 10.50 (1H, br s, 10- or 8-OH), 9.40 (1H, br t, J=5.9 Hz, NH), 6.39 (1H, t, J=5.9 Hz, 6-H), 4.44 (1H, dd, J=15.9, 3.9 Hz, 11-Hα), 4.20 (1H, d, J=16.6 Hz, 3-H), 4.16 (1H, dd, J=12.6, 3.9 Hz, 11a-H), 4.07 (1H, dt, J=12.8, 5.9 Hz, 6-CH), 4.04 (1H, d, J=16.6 Hz, 3-H), 3.82 (1H, dt, J=12.8, 5.9 Hz, 6-CH), 3.78 (3H, s, 7-OCH3), 3.27 (1H, dd, J=15.9, 12.6 Hz, 11-Hβ), 2.84 (3H, s, N-CH3), 2.65 (3H, s, 9-CH3), 2.38 (3H, s, COCH3). 13C-NMR (pyridine-d5) δ: 196.6 (s, COCH3), 166.5 (s, C-1), 166.4 (s, C-4), 161.3 (s, NHCO), 149.5–148.4 (s, overlapped with solvent signal, C-10), 147.8 (s, C-8), 138.1 (s, C-7), 123.9 (s, C-6a), 113.7 (s, C-9), 112.2 (s, C-10a), 60.1 (q, 7-OCH3), 54.9 (d, C-11a), 51.8 (t, C-3), 49.2 (d, C-6), 44.7 (t, 6-CH2), 32.3 (q, N-CH3), 24.1 (q, C-11), 23.9 (q, COCH3), 9.6 (q, 9-CH3). IR (KBr) cm−1: 3362, 2924, 2853, 1722, 1663, 1458, 1435, 1261, 1059. EI-MS m/z (%): 405 (M+, 3), 319 (22), 306 (23), 305 (100), 277 (9), 191 (10). HR-EI-MS Calcd for C19H23N3O7: 405.1536. Found: 405.1532.
N-(((6R*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-2,3,4,6,7,10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (23b)A stirred solution of 20b (18.9 mg, 0.045 mmol) in CH3CN (6.2 mL) was cooled with ice-water and an aqueous solution (1.1 mL) of CAN (62.0 mg, 0.11 mmol) was added dropwise over 10 min at −17°C. Stirring was continued for 30 min at −17°C. The reaction mixture was diluted with 5% aqueous NaHCO3 solution (60 mL) and extracted with CHCl3 (30 mL×3). The combined extracts were washed with brine (30 mL), dried, and evaporated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (80 : 1) to afford 23b (11.3 mg, 62%) as a yellow amorphous powder. 1H-NMR δ: 7.20 (1H, dd, J=6.8, 6.2 Hz, NH), 5.47 (1H, td, J=4.2, 1.6 Hz, 6-H), 4.13 (1H, d, J=17.5 Hz, 3-H), 4.06 (3H, s, 8-OCH3), 3.99 (1H, d, J=17.5 Hz, 3-H), 3.84 (1H, dd, J=11.4, 4.2 Hz, 11a-H), 3.83 (1H, ddd, J=13.7, 6.2, 4.2 Hz, 6-CH), 3.63 (1H, dd, J=18.1, 4.2 Hz, 11-Hα), 3.57 (1H, ddd, J=13.7, 6.8, 4.2 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.34 (3H, s, COCH3), 2.33 (1H, ddd, J=18.1, 11.4, 1.6 Hz, 11-Hβ), 1.98 (3H, s, 9-CH3). 13C-NMR δ: 196.3 (s, COCH3), 185.0 (s, C-10), 180.3 (s, C-7), 166.7 (s, C-4), 165.2 (s, C-1), 160.9 (s, NHCO), 155.9 (s, C-8), 139.6 (s, C-10a), 136.5 (s, C-6a), 128.5 (s, C-9), 61.1 (q, 8-OCH3), 53.3 (d, C-11a), 52.1 (t, C-3), 50.9 (d, C-6), 42.3 (t, 6-CH2), 33.8 (q, N-CH3), 24.3 (q, COCH3), 23.5 (t, C-11), 8.8 (q, 9-CH3). IR (KBr) cm−1: 3339, 2951, 1722, 1670, 1522, 1425, 1229. HR-FAB-MS (Magic bullet) m/z 404.1461 [M++H] (Calcd for C19H22N3O7: 404.1458).
Improved Synthesis of Compound 20b through 24b and 25bA: A solution of 17b (148.0 mg, 0.30 mmol) in CH2Cl2 (18 mL) was cooled at −78°C and a CH2Cl2 solution of BBr3 (1.0 M, 1.35 mL, 1.35 mmol) was added. The solution was kept at the same temperature for 10 min, warmed to −20°C over 2 h, and then stirred continuously at −20°C for 45 h. The reaction was quenched with water (6 mL) and the mixture was extracted with CHCl3 (60 mL×3). The combined extracts were washed with brine (60 mL), dried, and concentrated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (50 : 3) to give 24b (118.5 mg, 83.0%) as a colorless amorphous powder.
(6R*,11aS*)-10-Hydroxy-7,8-dimethoxy-2,9-dimethyl-6-phthalimidomethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (24b)1H-NMR (pyridine-d5) δ: 10.75 (1H, br s, 10-OH), 7.78 (2H, dd, J=5.3, 2.9 Hz, 3′-H2), 7.51 (2H, dd, J=5.3, 2.9 Hz, 4′-H2), 6.60 (1H, t, J=6.8 Hz, 6-H), 4.50 (1H, dd, J=16.0, 3.9 Hz, 11-Hα), 4.17 (1H, dd, J=12.7, 3.9 Hz, 11a-H), 4.13 (2H, d, J=16.8 Hz, 6-CH2), 4.12 (1H, d, J=16.6 Hz, 3-H), 3.92 (3H, s, 7-OCH3), 3.87 (1H, d, J=16.6 Hz, 3-H), 3.67 (3H, s, 8-OCH3), 3.40 (1H, dd, J=16.0, 12.7 Hz, 11-Hβ), 2.96 (3H, s, N-CH3), 2.44 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 167.4 (s, C-2′), 166.5 (s, C-1), 166.2 (s, C-4), 149.3 (s, C-10), 148.7 (s, C-8), 142.6 (s, C-7), 133.1 (d, C-4′), 131.9 (s, C-2a′), 123.9 (s, C-6a), 122.2 (d, C-3′), 119.6 (s, C-9), 117.1 (s, C-10a), 60.2 (q, 7-OCH3), 59.1 (q, 8-OCH3), 54.7 (d, C-11a), 51.8 (t, C-3), 49.7 (d, C-6), 41.6 (t, 6-CH2), 32.5 (q, N-CH3), 23.9 (t, C-11), 9.4 (q, 9-CH3). IR (KBr) cm−1: 3310, 2926, 1771, 1713, 1661, 1393. EI-MS m/z (%): 479 (M+, 7), 319 (100), 291 (7), 220 (5). HR-EI-MS Calcd for C25H25N3O7: 479.1693. Found: 479.1695.
B: Hydrazine monohydrate (0.4 mL) was added to a stirred solution of 24b (89.0 mg, 0.186 mmol) in EtOH (4.0 mL) and the reaction mixture was heated at 25°C for 6 h. The reaction mixture was concentrated in vacuo to afford a residue, which was diluted with benzene (20 mL) and extracted with 1 N HCl (20 mL×3). The combined acidic aqueous layer was made alkaline with 5% aqueous NH4OH and extracted with CHCl3 (20 mL×3). The combined extracts were washed with brine (20 mL), dried, and concentrated in vacuo to give a residue, which was purified by silica gel chromatography with CHCl3–MeOH (10 : 1) to give 25b (57.0 mg, 88.0%) as a pale yellow amorphous powder.
(6R*,11aS*)-6-(Aminomethyl)-10-hydroxy-7,8-dimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (25b)1H-NMR (pyridine-d5) δ: 6.16 (1H, t, J=5.8 Hz, 6-H), 4.45 (1H, dd, J=15.8, 4.0 Hz, 11-Hα), 4.21 (1H, d, J=16.6 Hz, 3-H), 4.15 (1H, dd, J=12.6, 4.0 Hz, 11a-H), 4.05 (1H, d, J=16.6 Hz, 3-H), 3.80 (3H, s, 7-OCH3), 3.78 (3H, s, 8-OCH3), 3.32 (1H, dd, J=13.2, 5.8 Hz, 6-CH), 3.27 (1H, dd, J=15.8, 12.6 Hz, 11-Hβ), 3.21 (1H, dd, J=13.2, 5.8 Hz, 6-CH), 2.86 (3H, s, N-CH3), 2.47 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 166.6 (s, C-1), 166.3 (s, C-4), 149.9 (s, C-8), 149.0 (s, C-10), 142.4 (s, C-7), 125.3 (s, C-9), 118.7 (s, C-6a), 117.3 (s, C-10a), 59.8 (q, 7-OCH3), 59.2 (q, 8-OCH3), 54.8 (d, C-11a), 52.5 (d, C-6), 51.9 (t, C-3), 47.9 (t, 6-CH2), 32.3 (q, N-CH3), 24.6 (t, C-11), 9.4 (q, 9-CH3). IR (KBr) cm−1: 3366, 3327, 3287, 2928, 1651, 1585, 1418, 1067. EI-MS m/z (%): 349 (M+, 3), 331 (100), 330 (53), 320 (42), 220 (16). HR-EI-MS Calcd for C17H23N3O5: 349.1628. Found: 349.634.
C: A solution of 25b (53.0 mg, 0.15 mmol), TEA (42 µL, 0.30 mmol), and 4-(dimethylamino)pyridine (36.9 mg, 0.30 mmol) in CH2Cl2 (0.5 mL) was cooled at 0°C, and a carbon tetrachloride solution of pyruvoyl chloride (1.0 M, 0.6 mL, 0.60 mmol) was added dropwise over 10 min. Stirring was continued at 0°C for 1 h. Then, the reaction mixture was diluted with water (50 mL) and extracted with CHCl3 (50 mL×3). The combined extracts were washed with brine (50 mL), dried, and concentrated in vacuo. The residue was subjected to column chromatography with AcOEt to afford 20b (64.0 mg, 98.6%) as a pale yellow amorphous powder, whose spectra were identical with those of the authentic sample described above.
N-(((6R*,11aS*)-10-Hydroxy-7,8-dimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)acetamide (26b)A solution of 25b (53.0 mg, 0.15 mol) and TEA (21.0 µL, 0.15 mmol) in CH2Cl2 (1.5 mL) was cooled with ice-water and acetyl chloride (12.8 µL, 0.18 mmol) was added dropwise over 5 min. Stirring was continued at 0°C for 30 min. The reaction mixture was diluted with water (30 mL) and then extracted with CHCl3 (30 mL×3). The combined extracts were washed with brine (30 mL), dried, and concentrated in vacuo. The residue was subjected to column chromatography with CHCl3–MeOH (10 : 1) to afford 26b (58.6 mg, 100%) as a colorless amorphous powder. 1H-NMR δ: 10.60 (1H, br s, OH), 8.60 (1H, t, J=5.9 Hz, N-H), 6.26 (1H, t, J=5.9 Hz, 6-H), 4.45 (1H, dd, J=15.6, 3.9 Hz, 11-Hα), 4.19 (1H, td, J=13.2, 5.9 Hz, 6-CH), 4.19 (1H, d, J=16.6 Hz, 3-H), 4.14 (1H, dd, J=12.7, 3.9 Hz, 11a-H), 4.05 (1H, d, J=16.6 Hz, 3-H), 3.85 (3H, s, 7-OCH3), 3.75 (3H, s, 8-OCH3), 3.75 (1H, dt, J=13.2, 5.9 Hz, 6-CH), 3.31 (1H, dd, J=15.6, 12.7 Hz, 11-Hβ), 2.82 (3H, s, N-CH3), 2.41 (3H, s, 9-CH3), 1.93 (3H, s, COCH3). 13C-NMR δ: 169.3 (s, COCH3), 166.5 (s, C-1), 166.1 (s, C-4), 149.8 (s, C-8), 149.0 (s, C-10), 142.5 (s, C-7), 125.0 (s, C-9), 118.9 (s, C-6a), 117.3 (s, C-10a), 59.9 (q, 7-OCH3), 59.2 (q, 8-OCH3), 54.6 (d, C-11a), 51.8 (t, C-3), 50.1 (d, C-6), 43.9 (t, 6-CH2), 32.4 (q, N-CH3), 24.5 (t, C-11), 22.2 (q, COCH3), 9.4 (q, 9-CH3). IR (KBr) cm−1: 3310, 2926, 1668, 1655, 1549, 1460. EI-MS m/z (%): 391 (M+, 4), 319 (100), 291 (11), 220 (6). HR-EI-MS Calcd for C19H25N3O6: 391.1743. Found: 391.1737.
N-(((6R*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-2,3,4,6,7,10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)acetamide (27b)A stirred solution of 26b (41.1 mg, 0.105 mmol) in CH3CN (17.5 mL) was cooled with ice-water and an aqueous solution (3.5 mL) of CAN (144.0 mg, 0.263 mmol) was added dropwise over 10 min at −17°C. Stirring was continued for 15 min at −17°C. The reaction mixture was diluted with 5% aqueous NaHCO3 solution (40 mL) and extracted with CHCl3 (40 mL×3). The combined extracts were washed with brine (40 mL), dried, and evaporated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (20 : 1) to afford 27b (33.9 mg, 86.0%) as a dark yellow amorphous powder. 1H-NMR δ: 6.19 (1H, t, J=6.5 Hz, NH), 5.38 (1H, td, J=4.2, 1.5 Hz, 6-H), 4.15 (1H, d, J=17.1 Hz, 3-H), 4.06 (3H, s, 8-OCH3), 3.96 (1H, d, J=17.1 Hz, 3-H), 3.86 (1H, dd, J=11.7, 4.4 Hz, 11a-H), 3.65 (1H, dd, J=17.4, 4.4 Hz, 11-Hα), 3.60 (2H, dd, J=6.5, 4.2 Hz, 6-CH2), 3.05 (3H, s, N-CH3), 2.45 (1H, ddd, J=17.4, 11.7, 1.5 Hz, 11-Hβ), 1.97 (3H, s, 9-CH3), 1.86 (3H, s, COCH3). 13C-NMR δ: 185.2 (s, C-10), 180.3 (s, C-7), 170.9 (s, COCH3), 166.7 (s, C-4), 165.5 (s, C-1), 156.2 (s, C-8), 138.6 (s, C-10a), 137.1 (s, C-6a), 128.2 (s, C-9), 61.0 (q, 8-OCH3), 53.4 (d, C-11a), 52.2 (t, C-3), 51.3 (d, C-6), 42.8 (t, 6-CH2), 33.8 (q, N-CH3), 23.3 (t, C-11), 22.9 (q, COCH3), 8.8 (q, 9-CH3). IR (KBr) cm−1: 3308, 2945, 1663, 1285, 1229. HR-FAB-MS (Magic bullet) m/z 376.1505 [M++H] (Calcd for C18H22N3O6: 376.1509).
Oxidative Demethylation of 24b with CANA stirred solution of 24b (121.8 mg, 0.254 mmol) in CH3CN (38 mL) was cooled with ice-water and an aqueous solution (5.7 mL) of CAN (348.0 mg, 0.653 mmol) was added dropwise over 10 min at −17°C. Stirring was continued for 30 min at −17°C. The reaction mixture was diluted with 5% aqueous NaHCO3 solution (100 mL) and extracted with CHCl3 (100 mL×3). The combined extracts were washed with brine (100 mL), dried, and evaporated in vacuo. The residue was purified by silica gel chromatography with CH2Cl2–MeOH (80 : 1) to afford 28b (92.0 mg, 78.2%) as a dark yellow amorphous powder.
(6R*,11aS*)-6-((1,3-Dioxoisoindolin)-2-yl)methyl)-8-methoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4,7,10(6H)-tetraone (28b)1H-NMR δ: 7.76 (2H, dd, J=5.8, 2.9 Hz, 3′-H2), 7.70 (2H, dd, J=5.8, 2.9 Hz, 4′-H2), 5.55 (1H, ddd, J=3.4, 2.9, 2.2 Hz, 6-H), 4.60 (1H, dd, J=14.6, 3.4 Hz, 6-CH), 4.13 (3H, s, 8-OCH3), 4.12 (2H, s, 3-H2), 3.85 (1H, dd, J=14.6, 2.9 Hz, 6-CH), 3.84 (1H, dd, J=11.4, 4.7 Hz, 11a-H), 3.48 (1H, dd, J=18.5, 4.7 Hz, 11-Hα), 3.07 (3H, s, N-CH3), 1.98 (3H, s, 9-CH3), 1.91 (1H, ddd, J=18.5, 11.4, 2.2 Hz, 11-Hβ). 13C-NMR δ: 185.1 (s, C-10), 180.3 (s, C-7), 168.3 (s, C-2′), 166.2 (s, C-4), 164.8 (s, C-1), 156.2 (s, C-8), 139.1 (s, C-10a), 136.1 (s, C-6a), 134.4 (d, C-4′), 131.3 (s, C-2a′), 128.4 (s, C-9), 123.6 (d, C-3′), 61.1 (q, 8-OCH3), 52.8 (d, C-11a), 52.0 (t, C-3), 50.9 (d, C-6), 39.4 (t, 6-CH2), 33.8 (q, N-CH3), 23.7 (t, C-11), 8.8 (q, 9-CH3). IR (KBr) cm−1: 2926, 1771, 1713, 1647, 1626, 1389. EI-MS m/z (%): 463 (M+, 55), 305 (24), 304 (11), 303 (60), 204 (17), 160 (100). HR-EI-MS Calcd for C24H21N3O7: 463.1380. Found: 463.1377.
Treatment of 28b with Hydrazine MonohydrateHydrazine monohydrate (20 µL) was added to a stirred solution of 28b (46.0 mg, 0.1 mmol) in EtOH (2.0 mL) and the mixture was stirred at 25°C for 4 h. The reaction mixture was concentrated in vacuo to afford a residue, which was diluted with benzene (20 mL) and extracted with 1 N HCl (20 mL×3). The combined acidic aqueous layer was made alkaline with 5% aqueous NH4OH and extracted with CHCl3 (20 mL×3). The combined extracts were washed with brine (20 mL), dried, and concentrated in vacuo to give a residue (33.0 mg, quantitative yield), which was used for the next step without further purification. The 1H-NMR spectral data of crude amine 29b [δ: 5.40 (1H, br t, J=4.2 Hz, 6-H), 4.13 (1H, d, J=17.1 Hz, 3-H), 4.02 (3H, s, 8-OCH3), 3.95 (1H, d, J=17.1 Hz, 3-H), 3.86 (1H, dd, J=11.7, 3.9 Hz, 11a-H), 3.65 (1H, dd, J=17.6, 4.2 Hz, 11-Hα), 3.13 (1H, dd, J=13.2, 4.2 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.89 (1H, dd, J=13.2, 4.2 Hz, 6-CH), 2.64 (1H, ddd, J=17.6, 11.7, 1.7 Hz, 11-Hβ), 2.00 (3H, s, 9-CH3), 1.92 (2H, br s, NH2)] indicated that 29b was almost pure, but this compound was easily converted into 30b45) after purification by flash column chromatography on a silica gel column with CHCl3 : MeOH (100 : 1).
A solution of crude 29b (33.0 mg) in acetic acid (0.2 mL) and acetic anhydride (0.2 mL) was stirred at 25°C for 1.5 h. The reaction mixture was poured into water (10 mL) and extracted with AcOEt (20 mL×3). The combined extracts were washed with saturated NaHCO3 solution, dried, and concentrated in vacuo. The residue was subjected to chromatography to give 27b (31.9 mg, 85.0%: 2 steps) as a dark yellow amorphous powder, whose spectra were identical with those of the authentic sample described above.
(6S*,11aS*)-10-Hydroxy-6-hydroxymethyl-7,8-dimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (12a)The reaction conditions for the preparation of this compound from 6a (182.0 mg, 0.5 mmol) were similar to those used in the conversion of 6b into 12b as described above. The isolation yield of 12a was 81.0% (142.0 mg) and 10.0% of 6a (16.0 mg) was recovered. Colorless needles, mp 262–263°C (EtOH). 1H-NMR (pyridine-d5) δ: 10.30 (1H, br s, 10-OH), 6.55 (1H, dd, J=8.0, 3.7 Hz, 6-H), 5.24 (1H, dd, J=12.2, 4.9 Hz, 11a-H), 4.49 (1H, dd, J=11.9, 3.7 Hz, 6-CH), 4.36 (1H, dd, J=11.9, 8.0 Hz, 6-CH), 4.21 (1H, d, J=17.6 Hz, 3-H), 4.01 (1H, d, J=17.6 Hz, 3-H), 3.99 (1H, dd, J=16.8, 4.9 Hz, 11-Hα), 3.78 (3H, s, 7-OCH3), 3.71 (3H, s, 8-OCH3), 3.05 (1H, dd, J=16.8, 12.2 Hz, 11-Hβ), 2.79 (3H, s, N-CH3), 2.41 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 165.0 (s, C-1), 161.8 (s, C-4), 149.5 (s, C-10), 149.5 (s, C-8), 142.4 (s, C-7), 123.9 (s, C-6a), 117.7 (s, C-9), 116.7 (s, C-10a), 62.8 (t, 6-CH2), 59.8 (q, 7-OCH3), 59.4 (q, 8-OCH3), 51.8 (d, C11a), 51.2 (d, C-6), 51.1 (t, C-3), 32.2 (q, N-CH3), 29.2 (t, C-11), 9.5 (q, 9-CH3). IR (KBr) cm−1: 3420, 2926, 1647. EI-MS m/z (%): 350 (M+, 8), 333 (7), 320 (18), 319 (100), 291 (14), 220 (10). HR-EI-MS Calcd for C17H22N2O6: 350.1478. Found: 350.1474. Anal. Calcd for C17H22N2O6·1/2C2H5OH: C, 57.90; H, 6.75; N, 7.50. Found: C, 58.14; H, 6.82; N, 7.12.
Oxidative Demethylation of 12a with CANThe oxidative demethylation of 12a (42.0 mg, 0.12 mmol) was conducted under the same conditions as those described above for the conversion of 12b into 14b. The yields of products 14a and 15a were 78.6% (31.6 mg) and 7.2% (3.0 mg), respectively.
(6S*,11aS*)-6-Hydroxymethyl-8-methoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4,7,10(6H)-tetraone (14a)1H-NMR δ: 5.64 (1H, dt, J=5.7, 2.4 Hz, 6-H), 4.65 (1H, dd, J=11.7, 4.4 Hz, 11a-H), 4.08 (1H, d, J=18.1 Hz, 3-H), 4.05 (1H, dd, J=10.7, 2.4 Hz, 6-CH), 4.00 (3H, s, 8-OCH3), 3.92 (1H, dt, J=10.7, 5.7 Hz, 6-CH), 3.72 (1H, d, J=18.1 Hz, 3-H), 3.34 (1H, dd, J=19.5, 4.4 Hz, 11-Hα), 3.29 (1H, br s, OH), 3.01 (3H, s, N-CH3), 2.44 (1H, ddd, J=19.5, 11.7, 2.4 Hz, 11-Hβ), 1.99 (3H, s, 9-CH3). 13C-NMR δ: 185.6 (s, C-10), 180.8 (s, C-7), 164.3 (s, C-1), 161.8 (s, C-4), 155.3 (s, C-8), 140.5 (s, C-10a), 135.5 (s, C-6a), 128.8 (s, C-9), 63.7 (t, 6-CH2), 61.1 (q, 8-OCH3), 52.2 (d, C-11a), 51.1 (t, C-3), 50.4 (d, C-6), 33.5 (q, N-CH3), 28.4 (t, C-11), 8.9 (q, 9-CH3). IR (KBr) cm−1: 3393, 2941, 1661, 1287. HR-FAB-MS (Magic bullet) m/z 335.1237 [M+H+] (Calcd for C16H19N2O6: 335.1243).
(2aS*,6aS*,11aS*)-2a,3-Dimethoxy-4,8-dimethyl-1,6,6a,8,9,11a-hexahydrofuro[2,3,4-ij]pyrazino[1,2-b]isoquinoline-5,7,10(2aH)-trione (15a)1H-NMR δ: 5.29 (1H, ddd, J=8.7, 6.7, 2.7 Hz, 11a-H), 4.78 (1H, t, J=8.7 Hz, 1-H), 4.07 (3H, s, 3-OCH3), 4.05 (1H, d, J=17.9 Hz, 9-H), 3.98 (1H, d, J=17.9 Hz, 9-H), 3.75 (1H, br dd, J=11.5, 3.4 Hz, 6a-H), 3.66 (1H, dd, J=8.7, 6.7 Hz, 1-H), 3.50 (1H, dd, J=16.4, 3.4 Hz, 6-Hα), 3.08 (3H, s, 2a-OCH3), 2.96 (3H, s, N-CH3), 2.23 (1H, ddd, J=16.4, 11.5, 2.7 Hz, 6-Hβ), 1.77 (3H, s, 4-CH3). 13C-NMR (CDCl3) δ: 183.9 (s, C-5), 163.4 (s, C-7), 163.3 (s, C-10), 160.0 (s, C-3), 144.0 (s, C-11b), 128.7 (s, C-5a), 117.8 (s, C-4), 97.4 (s, C-2a), 72.7 (t, C-1), 59.1 (q, 3-OCH3), 53.2 (d, C-6a), 51.6 (t, C-9), 51.5 (q, 2a-OCH3), 50.6 (d, C-11a), 33.8 (q, N-CH3), 25.4 (t, C-6), 8.1 (q, 4-CH3). IR (KBr) cm−1: 2926, 1663, 1638, 1283. EI-MS m/z (%): 348 (M+, 12), 333 (9), 318 (47), 317 (100), 275 (13), 219 (40), 191 (34). HR-EI-MS Calcd for C17H20N2O6: 348.1321. Found: 348.1318.
Z-((6S*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-1,3,4,6,7,10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl 2-Methylbut-2-enone (16a)The reaction conditions for the preparation of this compound from 14a (16.7 mg, 49.9 µmol) were similar to those for the conversion of 14b into 16b as described above. 16a was isolated in 72.1% (15.0 mg) yield as a yellow amorphous powder. 1H-NMR δ: 6.08 (1H, qq, J=7.3, 1.5 Hz, =CHCH3), 5.90 (1H, ddd, J=5.4, 2.7, 1.0 Hz, 6-H), 4.79 (1H, dd, J=12.0, 5.4 Hz, 6-CH), 4.50 (1H, dd, J=11.7, 4.4 Hz, 11a-H), 4.33 (1H, dd, J=12.0, 2.7 Hz, 6-CH), 4.06 (3H, s, 8-OCH3), 4.06 (1H, d, J=14.9 Hz, 3-H), 4.02 (1H, d, J=14.9 Hz, 3-H), 3.37 (1H, ddd, J=19.6, 4.4, 1.0, Hz, 11-Hα), 3.02 (3H, s, N-CH3), 2.45 (1H, ddd, J=19.6, 11.7, 2.7 Hz, 11-Hβ), 1.95 (3H, s, 9-CH3), 1.89 (3H, dq, J=7.3, 1.5 Hz, =CHCH3), 1.81 (3H, quint, J=1.5 Hz, =CCH3). 13C-NMR δ: 185.4 (s, C-10), 180.2 (s, C-7), 166.9 (s, C-14), 163.7 (s, C-1), 161.3 (s, C-4), 155.8 (s, C-8), 139.9 (d, =CHCH3), 139.8 (s, C-10a), 135.0 (s, C-6a), 128.2 (s, C-9), 126.6 (s, =CCH3), 64.5 (t, 6-CH2), 61.0 (q, 8-OCH3), 52.4 (d, C-11a), 51.2 (t, C-3), 48.1 (d, C-6), 33.6 (q, N-CH3), 28.2 (t, C-11), 20.7 (q, =CCH3), 15.8 (q, =CHCH3), 8.8 (q, 9-CH3). IR (KBr) cm−1: 2957, 1719, 1663, 1233, 1152. EI-MS m/z (%): 416 (M+, 7), 386 (52), 303 (73), 275 (10), 273 (12), 204 (15), 83 (100). HR-EI-MS Calcd for C21H24N2O7: 416.1584. Found: 416.1583.
(6S*,11aS*)-7,8,10-Trimethoxy-2,9-dimethyl-6-phthalimidomethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (17a)The reaction conditions for the preparation of this compound from 6a (21.9 mg, 60.0 µmol) were similar to those for the conversion of 6b into 17b as described above. 17a (29.6 mg) was obtained in a quantitative yield as colorless prisms, mp 201–202.5°C (AcOEt–hexane). 1H-NMR δ: 7.85 (2H, dd, J=5.5, 3.1 Hz, 3′-H2), 7.71 (2H, dd, J=5.5, 3.1 Hz, 4′-H2), 6.11 (1H, dd, J=10.7, 3.3 Hz, 6-H), 4.69 (1H, dd, J=12.0, 4.5 Hz, 11a-H), 4.32 (1H, dd, J=14.0, 3.3 Hz, 6-CH), 4.05 (3H, s, 8-OCH3), 3.92 (1H, dd, J=14.0, 10.7 Hz, 6-CH), 3.86 (1H, d, J=17.0 Hz, 3-H), 3.84 (3H, s, 9-OCH3), 3.69 (3H, s, 10-OCH3), 3.69 (1H, d, J=17.0 Hz, 3-H), 3.57 (1H, dd, J=17.0, 4.5 Hz, 11-Hα), 3.00 (3H, s, N-CH3), 2.78 (1H, dd, J=17.0, 12.0 Hz, 11-Hβ), 2.21 (3H, s, 9-CH3). 13C-NMR δ: 168.1 (s, C-2′), 164.9 (s, C-1), 162.0 (s, C-4), 152.1 (s, C10), 150.3 (s, C-8), 146.1 (s, C-7), 133.8 (d, C-4′), 131.9 (s, C-2a′), 125.3 (s, C-9), 123.1 (d, C-3′), 123.1 (d, C-6a), 121.8 (s, C-10a), 60.6 (q, 7-OCH3), 60.1 (q, 8-OCH3), 60.0 (q, 10-OCH3), 51.4 (d, C-11a), 51.2 (t, C-3), 48.7 (d, C-6), 38.9 (t, 6-CH2), 33.5 (q, N-CH3), 28.4 (t, C-11), 9.5 (q, 9-CH3). IR (KBr) cm−1: 2943, 1771, 1715, 1665, 1466, 1395. EI-MS m/z (%): 493 (M+, 5), 333 (100), 305 (6), 234 (5), 204 (6). HR-EI-MS Calcd for C26H27N3O7: 493.1849. Found: 493.1853. Anal. Calcd for C26H27N3O7: C, 63.28; H, 5.51; N, 8.51. Found: C, 63.19; H, 5.67; N, 8.55.
(6S*,11aS*)-6-(Aminomethyl)-7,8,10-trimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (18a)The reaction conditions for the preparation of this compound from 17a (99.0 mg, 0.2 mmol) were similar to those for the conversion of 17b into 18b as described above. 17a (72.0 mg) was obtained in 99.0% yield as a pale yellow amorphous powder. 1H-NMR (CDCl3) δ: 5.79 (1H, dd, J=10.0, 3.4 Hz, 6-H), 4.42 (1H, dd, J=12.2, 4.4 Hz, 11a-H), 4.12 (1H, d, J=18.8 Hz, 3-H), 4.07 (1H, d, J=18.8 Hz, 3-H), 3.90 (3H, s, 7-OCH3), 3.79 (3H, s, 8-OCH3), 3.67 (3H, s, 10-OCH3), 3.45 (1H, dd, J=17.0, 4.4 Hz, 11-Hα), 3.25 (1H, dd, J=13.6, 3.4 Hz, 6-CH), 3.05 (3H, s, N-CH3), 2.86 (1H, dd, J=13.6, 10.0 Hz, 6-CH), 2.74 (1H, dd, J=17.0, 12.2 Hz, 11-Hβ), 2.18 (3H, s, 9-CH3). 13C-NMR δ: 165.5 (s, C-1), 162.8 (s, C-4), 152.3 (s, C-10), 150.3 (s, C-8), 146.0 (s, C-7), 125.0 (s, C-6a), 124.8 (s, C-9), 121.2 (s, C-10a), 60.4 (q, 7-OCH3), 60.0 (q, 10-OCH3), 60.0 (q, 8-OCH3), 51.8 (d, C-6), 51.5 (d, C-11a), 51.5 (t, C-3), 44.4 (t, 6-CH2), 33.4 (q, N-CH3), 28.4 (t, C-11), 9.4 (q, 9-CH3). IR (KBr) cm−1: 3360, 2941, 1658, 1637, 1458, 1337, 1076, 1001. EI-MS m/z (%): 363 (M+, 3), 334 (92), 333 (100), 305 (11), 234 (14), 204 (16). HR-EI-MS Calcd for C18H25N3O5: 363.1794. Found: 363.1797.
2-Oxo-N-(((6S*,11aS*)-7,8,10-trimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)propanamide (19a)The reaction conditions for the preparation of this compound from 18a (65.4 mg, 0.18 mmol) were similar to those for the conversion of 18b into 19b as described above. 19a (69.1 mg) was obtained in 88.7% yield as colorless prisms. mp 188–189°C (AcOEt–hexane). 1H-NMR δ: 7.48 (1H, br t, J=5.6 Hz, NH), 5.96 (1H, dd, J=10.2, 3.9 Hz, 6-H), 4.47 (1H, dd, J=12.0, 4.5 Hz, 11a-H), 4.03 (1H, d, J=17.7 Hz, 3-H), 3.97 (1H, d, J=17.7 Hz, 3-H), 3.92 (3H, s, 7-OCH3), 3.79 (3H, s, 8-OCH3), 3.75 (1H, ddd, J=14.0, 5.6, 3.9 Hz, 6-CH), 3.68 (3H, s, 10-OCH3), 3.63 (1H, ddd, J=14.0, 10.2, 5.6 Hz, 6-CH), 3.50 (1H, dd, J=17.0, 4.5 Hz, 11-Hα), 3.03 (3H, s, N-CH3), 2.77 (1H, dd, J=17.0, 12.0 Hz, 11-Hβ), 2.45 (3H, s, COCH3), 2.19 (3H, s 9-CH3). 13C-NMR δ: 196.2 (s, COCH3), 164.9 (s, C-1), 162.4 (s, C-4), 160.5 (s, NHCO), 152.2 (s, C-10), 150.2 (s, C-8), 145.7 (s, C-7), 125.3 (s, C-9), 123.3 (s, C-6a), 121.5 (s, C-10a), 60.5 (q, 7-OCH3), 60.1 (q, 8-OCH3), 60.1 (q, 10-OCH3), 51.6 (d, C-11a), 51.4 (t, C-3), 48.8 (d, C-6), 41.7 (t, 6-CH2), 33.6 (q, N-CH3), 28.3 (t, C-11), 24.6 (q, COCH3), 9.5 (q, 9-CH3). IR (KBr) cm−1: 3356, 2941, 1722, 1680, 1661, 1464. EI-MS m/z (%): 433 (M+, 1), 333 (100), 305 (7), 234 (6), 204 (6). HR-EI-MS Calcd for C21H27N3O7: 433.1849. Found: 433.1847. Anal. Calcd for C21H27N3O9: C, 58.19; H, 6.28; N, 9.69. Found: C, 57.99; H, 6.29; N, 9.64.
Partial Demethylation of Compound 19a with 5 Equivalents of Boron TribromideThe partial demethylation of 19a (43.3 mg, 0.10 mmol) was conducted under the same conditions as those described above for the conversion of 19b into 20b. The yields of products 20a and 22a were 80.0% (33.5 mg) and 2.5% (1.4 mg), respectively. Starting material 19a (6.9 mg, 15.9%) was recovered.
N-(((6S*,11aS*)-10-Hydroxy-7,8-dimethoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (20a)1H-NMR δ: 7.47 (1H, dd, J=7.3, 5.3 Hz, NH), 5.97 (1H, dd, J=9.2, 5.3 Hz, 6-H), 5.88 (1H, s, 10-OH), 4.52 (1H, dd, J=12.2, 4.4 Hz, 11a-H), 4.04 (1H, d, J=17.6 Hz, 3-H), 3.96 (1H, d, J=17.6, 3-H), 3.89 (3H, s, 7-OCH3), 3.79 (3H, s, 8-OCH3), 3.72 (1H, dt, J=14.1, 5.3 Hz, 6-CH), 3.67 (1H, ddd, J=14.1, 9.2, 7.3 Hz, 6-CH), 3.56 (1H, dd, J=16.6, 4.4 Hz, 11-Hα), 3.03 (3H, s, N-CH3), 2.67 (1H, dd, J=16.6, 12.2 Hz, 11-Hβ), 2.44 (3H, s, COCH3), 2.19 (3H, s, 9-CH3). 13C-NMR δ: 196.6 (s, COCH3), 165.7 (s, C-1), 162.4 (s, C-4), 160.9 (s, NHCO), 150.1 (s, C-8), 148.4 (s, C-10), 143.0 (s, C-7), 122.9 (s, C-6a), 118.3 (s, C-9), 115.2 (s, C-10a), 60.6 (q, 7-OCH3), 60.2 (q, 8-OCH3), 51.4 (d, C-11a), 51.2 (t, C-3), 48.9 (d, C-6), 41.6 (t, 6-CH2), 33.5 (q, N-CH3), 28.2 (t, C-11), 24.5 (q, COCH3), 8.9 (q, 9-CH3). IR (KBr) cm−1: 3404, 2931, 1719, 1680, 1647, 1460, 1331, 1331, 1074. EI-MS m/z (%): 419 (M+, 2), 320 (17), 319 (100), 291 (10), 220 (6). HR-EI-MS Calcd for C20H25N3O7: 419.1693. Found: 419.1689.
N-(((6S*,11aS*)-8,10-Dihydroxy-7-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (22a)1H-NMR (pyridine-d5) δ: 10.95 (1H, br s, 8- or 10-OH), 10.21 (1H, br s, 8- or 10-OH), 10.00 (1H, dd, J=7.3, 5.4 Hz, NH), 6.60 (1H, dd, J=10.6, 4.0 Hz, 6-H), 5.12 (1H, dd, J=12.5, 4.7 Hz, 11a-H), 4.23 (1H, ddd, J=13.7, 5.4, 4.0 Hz, 6-CH), 4.20 (1H, d, J=17.6 Hz, 3-H), 4.10 (1H, d, J=17.6 Hz, 3-H), 4.07 (1H, ddd, J=13.7, 10.6, 7.3 Hz, 6-CH), 3.99 (1H, dd, J=16.4, 4.7 Hz, 11-Hα), 3.82 (3H, s, 7-OCH3), 3.05 (1H, dd, J=16.4, 12.5 Hz, 11-Hβ), 2.81 (3H, s, N-CH3), 2.66 (3H, s, 9-CH3), 2.43 (3H, s, COCH3). 13C-NMR (pyridine-d5) δ: 196.6 (s, COCH3), 165.0 (s, C-1), 162.2 (s, C-4), 161.9 (s, NHCO), 150.1 (s, C-10), 147.6 (s, C-8), 138.0 (s, C-7), 123.1–122.1 (s, overlapped with solvent signal, C-10a), 112.9 (s, C-9), 111.3 (s, C-6a), 59.8 (q, 7-OCH3), 51.2 (d, C-11a), 50.7 (t, C-3), 48.6 (d, C-6), 40.7 (t, 6-CH2), 32.0 (q, N-CH3), 28.6 (t, C-11), 24.1 (q, COCH3), 9.5 (9-CH3). IR (KBr) cm−1: 3325, 2924, 2853, 1719, 1653, 1466, 1260. EI-MS m/z (%): 405 (M+, 4), 319 (10), 306 (16), 305 (100), 277 (12), 191 (10). HR-EI-MS Calcd for C19H23N3O7: 405.1536. Found: 405.1534.
N-(((6S*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-2,3,4,6,7,10,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)-2-oxopropanamide (23a)The reaction conditions for the preparation of this compound from 20a (37.7 mg, 0.09 mmol) were similar to those for the conversion of 20b into 23b as described above. 23a (32.0 mg) was obtained in 88.0% yield as a dark yellow amorphous powder. 1H-NMR δ: 7.52 (1H, dd, J=7.8, 5.2 Hz, NH), 5.77 (1H, ddd, J=7.8, 3.4, 2.4 Hz, 6-H), 4.43 (1H, dd, J=11.7, 4.4 Hz, 11a-H), 4.05 (3H, s, 8-OCH3), 4.04 (1H, d, J=18.0 Hz, 3-H), 3.96 (1H, d, J=18.0 Hz, 3-H), 3.77 (1H, dt, J=14.0, 7.8 Hz, 6-CH), 3.62 (1H, ddd, J=14.0, 5.2, 3.4 Hz, 6-CH), 3.38 (1H, dd, J=19.8, 4.4 Hz, 11-Hα), 3.01 (3H, s, N-CH3), 2.45 (1H, ddd, J=19.8, 11.7, 2.4 Hz, 11-Hβ), 2.42 (3H, s, COCH3), 1.96 (3H, s, 9-CH3). 13C-NMR δ: 196.2 (s, COCH3), 185.7 (s, C-10), 180.9 (s, C-7), 163.9 (s, C-1), 162.3 (s, C-4), 161.1 (s, NHCO), 155.7 (s, C-8), 140.3 (s, C-10a), 135.7 (s, C-6a), 128.8 (s, C-9), 61.0 (q, 8-OCH3), 51.3 (d, C-11a), 51.1 (t, C-3), 48.5 (d, C-6), 41.2 (t, 6-CH2), 33.5 (q, N-CH3), 28.3 (t, C-11), 24.4 (q, COCH3), 8.7 (q, 9-CH3). IR (KBr) cm−1: 3327, 2951, 1726, 1670, 1612, 1470, 1287, 1219, 1150. HR-FAB-MS (Magic bullet) m/z 404.1460 [M+H]+ (Calcd for C19H22N3O7, 404.1458).
(6S*,11aS*)-6-((1,3-Dioxoisoindolin-2-yl)methyl)-10-hydroxy-7,8-dimethoxy-2,9-dimethyl-2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione (24a)The reaction conditions for the preparation of this compound from 17a (224.4 mg, 0.51 mmol) were similar to those for the conversion of 17b into 24b as described above. 24a (222.3 mg) was obtained in 91.3% yield as a pale yellow amorphous powder. 1H-NMR (DMSO-d6) δ: 8.40 (1H, br s, 10-OH), 7.89 (2H, dd, J=5.3, 2.9 Hz, 3′-H2), 7.84 (2H, dd, J=5.3, 2.9 Hz, 4′-H2), 5.88 (1H, dd, J=10.7, 3.9 Hz, 6-H), 4.57 (1H, dd, J=12.5, 4.4 Hz, 11a-H), 4.05 (1H, dd, J=14.1, 3.9 Hz, 6-CH), 3.96 (1H, dd, J=14.1, 10.7 Hz, 6-CH), 3.96 (1H, d, J=17.6 Hz, 3-H), 3.85 (3H, s, 7-OCH3), 3.70 (3H, s, 8-OCH3), 3.59 (1H, d, J=17.6 Hz, 3-H), 3.23 (1H, dd, J=16.6, 4.4 Hz, 11-Hα), 2.82 (3H, s, N-CH3), 2.63 (1H, dd, J=16.6, 12.5 Hz, 11-Hβ), 2.07 (3H, s, 9-CH3). 13C-NMR (DMSO-d6) δ: 167.8 (s, C-2′), 164.5 (s, C-1), 162.4 (s, C-4), 149.4 (s, C-8), 148.8 (s, C-10), 142.5 (s, C-7), 134.5 (d, C-4′), 131.5 (s, C-2a′), 123.1 (d, C-3′), 122.7 (s, C-6a), 118.5 (s, C-9), 116.8 (s, C-10a), 60.3 (q, 7-OCH3), 59.9 (q, 8-OCH3), 50.6 (d, C-11a), 50.3 (t, C-3), 47.4 (d, C-6), 38.6 (t, 6-CH2), 32.6 (q, N-CH3), 28.1 (t, C-11), 9.5 (q, 9-CH3). IR (KBr) cm−1: 3421, 2943, 1773, 1717, 1663, 1395. EI-MS m/z (%): 479 (M+, 7), 319 (100), 291 (7), 220 (4). HR-EI-MS Calcd for C25H25N3O7: 479.1693. Found: 479.1692.
(6S*,11aS*)-6-(1,3-Dioxoisoindolin-2-yl)methyl)-8-methoxy-2,9-dimethylyl-2,3,4,6,7,10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinoline-1,4,7,10(6H)-tetraone (28a)The reaction conditions for the preparation of this compound from 24a (28.7 mg, 0.06 mmol) were similar to those for the conversion of 24b into 28b as described above. 28a (25.7 mg) was obtained in 92.5% yield as a pale yellow amorphous powder. 1H-NMR δ: 7.84 (2H, dd, J=5.3, 2.9 Hz, 3′-H2), 7.73 (2H, dd, J=5.3, 2.9 Hz, 4′-H2), 5.84 (1H, ddd, J=6.6, 3.7, 2.7 Hz, 6-H), 4.38 (1H, dd, J=14.5, 3.7 Hz, 6-CH), 4.21 (1H, dd, J=11.6, 4.4 Hz, 11a-H), 4.15 (3H, s, 8-OCH3), 4.00 (1H, dd, J=14.5, 6.6 Hz, 6-CH), 3.97 (1H, d, J=17.9, 3-H), 3.91 (1H, d, J=17.9 Hz, 3-H), 3.38 (1H, ddd, J=19.6, 4.4, 1.1 Hz, 11-Hα). 3.00 (3H, s, N-CH3), 2.41 (1H, ddd, J=19.6, 11.6, 2.7 Hz, 11-Hβ), 1.97 (3H, s, 9-CH3). 13C-NMR δ: 185.7 (s, C-10), 180.7 (s, C-7), 168.4 (s, C-2′), 163.7 (s, C-1), 162.2 (s, C-4), 156.1 (s, C-8), 140.0 (s, C-10a), 135.3 (s, C-6a), 134.4 (d, C-4′), 131.7 (s, C-2a′), 128.2 (s, C-9), 123.7 (d, C-3′), 61.0 (q, 8-OCH3), 51.7 (d, C-11a), 51.0 (t, C-3), 48.5 (d, C-6), 38.9 (t, 6-CH2), 33.5 (q, N-CH3), 28.1 (t, C-11), 8.7 (q, 9-CH3). IR (KBr) cm−1: 2920, 2851, 1771, 1709, 1668, 1634, 1466, 1400. EI-MS m/z (%): 463 (M+, 44), 303 (100), 301 (31), 273 (51), 202 (25). HR-EI-MS Calcd for C24H23N3O7: 463.1380. Found: 463.1381.
N-(((6S*,11aS*)-8-Methoxy-2,9-dimethyl-1,4,7,10-tetraoxo-2,3,4,6,7.10,11,11a-octahydro-1H-pyrazino[1,2-b]isoquinolin-6-yl)methyl)acetamide (27a) from 28a via 29aThe reaction conditions for the preparation of this compound from 28a (23.2 mg, 0.05 mmol) were similar to those for the conversion of 28b into 27b via 29b as described above. 27a (12.1 mg) was obtained in 64.5% overall yield as a dark yellow amorphous powder. 1H-NMR δ: 6.14 (1H, dd, J=8.9, 3.9 Hz, NH), 5.96 (1H, ddd, J=8.9, 4.2, 2.4 Hz, 6-H), 4.54 (1H, dd, J=11.7, 4.4 Hz, 11a-H), 4.02 (3H, s, 8-OCH3), 3.99 (2H, s, 3-H2), 3.87 (1H, dd, J=14.7, 8.9 Hz, 6-CH), 3.40 (1H, dd, J=20.0, 4.4 Hz, 11-Hα), 3.36 (1H, dd, J=14.7, 3.9 Hz, 6-CH), 3.01 (3H, s, N-CH3), 2.47 (1H, ddd, J=20.0, 11.7, 2.4 Hz, 11-Hβ), 1.94 (3H, s, 9-CH3), 1.86 (3H, s, COCH3). 13C-NMR δ: 185.8 (s, C-10), 181.0 (s, C-7), 171.4 (s, COCH3), 163.9 (s, C-1), 162.4 (s, C-4), 155.7 (s, C-8), 140.1 (s, C-10a), 135.6 (s, C-6a), 129.0 (s, C-9), 61.0 (q, 8-OCH3), 51.1 (d, C-11a), 51.1 (t, C-3), 49.0 (d, C-6), 41.3 (t, 6-CH2), 33.4 (q, N-CH3), 28.4 (t, C-11), 23.1 (q, COCH3), 8.8 (q, 9-CH3). IR (KBr) cm−1: 3306, 2945, 1761, 1661, 1466, 1285. EI-MS m/z (%): 375 (M+, 10), 345 (58), 303 (100), 275 (27), 232 (19), 204 (26). HR-MS m/z Calcd for C18H21N3O6: 375.1430. Found: 375.1424.
6-(Acetamidomethyl)-8-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (31). From 29aAcetyl anhydride (0.5 mL) was added to a solution of amine 29a (16.7 mg, 0.05 mmol) in pyridine (2.0 mL) and stirring was continued at 25°C for 87 h. After being diluted with water (10 mL), the mixture was extracted with CHCl3 (30 mL×3). The combined extracts were washed with brine (20 mL), dried, and concentrated in vacuo to give a residue, which was subjected to chromatography on a silica gel column with AcOEt–MeOH=10 : 1 to afford 31 (18.4 mg, 80.0%) as a colorless amorphous powder.
From 27aA solution of 27a (12.1 mg, 0.032 mmol) and acetic anhydride (0.5 mL) in pyridine (2.0 mL) was stirred at 25°C for 48 h and general work-up gave 31 (9.9 mg, 66.8%).
From 27bA solution of 27b (11.3 mg, 0.030 mmol) and acetic anhydride (0.5 mL) in pyridine (2.0 mL) was stirred at 25°C for 48 h and general work-up gave 31 (4.0 mg, 29.0%).
1H-NMR (pyridine-d5) δ: 9.13 (1H, dd, J=7.8, 5.2 Hz, NH), 7.21 (1H, s, 11-H), 6.64 (1H, dd, J=8.5, 3.3 Hz, 6-H), 4.13 (2H, s, 3-H2), 3.86 (1H, ddd, J=13.5, 6.2, 3.3 Hz, 6-CH), 3.70 (3H, s, 8-OCH3), 3.39 (1H, ddd, J=13.5, 8.5, 6.2 Hz, 6-CH), 2.74 (3H, s, N-CH3), 2.52 (3H, s, COCH3), 2.26 (3H, s, COCH3), 2.08 (3H, s, COCH3), 2.04 (3H, s, 9-CH3). 13C-NMR (pyridine-d5) δ: 171.1 (s, NCOCH3), 169.4 (s, OCOCH3), 168.9 (s, OCOCH3), 163.2 (s), 158.3 (s), 152.1 (s), 144.2 (s), 139.3 (s), 128.5 (s), 126.1 (s), 123.5 (s), 119.9 (s), 107.4 (d, C-11), 61.0 (q, 8-OCH3), 52.1 (t, C-3), 48.8 (d, C-6), 42.0 (t, 6-CH2), 33.1 (q, N-CH3), 23.0 (q, COCH3), 20.5 (q, OCOCH3), 20.0 (q, OCOCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 3399, 2936, 1767, 1686, 1624, 1387, 1186. EI-MS m/z (%): 459 (M+, 5), 387 (100), 345 (51), 317 (25), 274 (10), 204 (16). HR-EI-MS Calcd for C22H25N3O8: 459.1642. Found: 459.1643.
(Z)-8-Methoxy-2,9-dimethyl-6-(((2-methylbut-2-enoyl)oxy)methyl)-1,4-dioxo-2,3,4,6-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (32)Yield: 87% (from 16a) and 22% (from 16b). 1H-NMR δ: 6.95 (1H, s, 11-H), 6.29 (1H, dd, J=8.8, 2.9 Hz, 6-H), 6.07 (1H, quint, J=7.3 Hz, =CHCH3), 4.29 (1H, dd, J=11.7, 2.9 Hz, 6-CH), 4.14 (1H, d, J=17.8 Hz, 3-H), 4.02 (1H, d, J=17.8 Hz, 3-H), 3.94 (1H, dd, J=11.7, 8.8 Hz, 6-CH), 3.78 (3H, s, 8-OCH3), 3.06 (3H, s, N-CH3), 2.50 (3H, s, COCH3), 2.40 (3H, s, COCH3), 2.10 (3H, s, COCH3), 1.80 (3H, s, 9-CH3). 13C-NMR δ: 169.0 (s, OCOCH3), 168.5 (s, OCOCH3), 167.1 (s, OCO), 162.1 (s, C-4), 158.7 (s, C-1), 151.9 (s), 144.0 (s), 139.4 (d, =CHCH3), 138.9 (s), 127.1 (s), 126.7 (s), 126.5 (s), 120.9 (s), 119.3 (s), 108.8 (d, C-11), 62.6 (t, 6-CH2), 61.1 (q, 8-OCH3), 51.9 (t, C-3), 47.7 (d, C-6), 33.8 (q, N-CH3), 20.6 (q, =CCH3), 20.5 (q, OCOCH3), 20.5 (q, OCOCH3), 15.7 (q, =CHCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 2953, 1769, 1693, 1626, 1384, 1184. EI-MS m/z (%): 500 (M+, 9), 387 (100), 345 (48), 317 (16), 204 (11). HR-EI-MS Calcd for C25H28N2O9: 500.1795. Found: 500.1973.
8-Methoxy-2,9-dimethyl-1,4-dioxo-6-(2-oxopropanamido)methyl-2,3,4,6-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (33)Yield: 66% (from 23a) and 7% (from 23b). 1H-NMR δ: 7.13 (1H, br t, J=6.3 Hz, NH), 6.90 (1H, s, 11-H), 6.18 (1H, dd, J=8.0, 3.4 Hz, 6-H), 4.12 (1H, d, J=17.7 Hz, 3-H), 4.04 (1H, d, J=17.7 Hz, 3-H), 3.78 (3H, s, 8-OCH3), 3.56 (1H, ddd, J=13.6, 6.3, 3.4 Hz, 6-CH), 3.20 (1H, ddd, J=13.6, 8.0, 6.3 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.50 (3H, s, OCOCH3), 2.44 (3H, s, COCH3), 2.39 (3H, s, OCOCH3), 2.10 (3H, s, 9-CH3). 13C-NMR δ: 196.5 (s, COCH3), 169.0 (s, OCOCH3), 168.5 (s, OCOCH3), 162.4 (s), 160.9 (s, NCOCH3), 158.5 (s), 151.9 (s), 144.1 (s), 138.7 (s), 126.7 (s), 126.6 (s), 121.6 (s), 119.0 (s), 108.7 (d, C-11), 61.1 (q, 8-OCH3), 51.8 (t, C-3), 47.7 (d, C-6), 41.7 (t, 6-CH2), 33.8 (q, N-CH3), 24.3 (q, COCH3), 20.6 (q, OCOCH3), 20.5 (q, OCOCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 1767, 1722, 1690, 1626, 1383, 1184. EI-MS m/z (%): 487 (M+, 2), 387 (100), 345 (49), 317 (18), 303 (8), 274 (8), 204 (14). HR-EI-MS Calcd for C23H25N3O9: 487.1591. Found: 487.1589.
8-Methoxy-2,9-dimethyl-1,4-dioxo-6-phthalimidomethyl-2,3,4,6-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (34)Yield: 93% (from 28a) and 70% (from 28b). 1H-NMR δ: 7.83 (2H, dd, J=5.3, 2.9 Hz, 3′-H2), 7.71 (2H, dd, J=5.3, 2.9 Hz, 4′-H2), 7.04 (1H, s, 11-H), 6.28 (1H, dd, J=9.8, 2.4 Hz, 6-H), 4.04 (1H, dd, J=12.1, 9.8 Hz, 6-CH), 3.99 (1H, d, J=17.8 Hz, 3-H), 3.79 (3H, s, 8-OCH3), 3.76 (1H, d, J=17.8 Hz, 3-H), 3.54 (1H, dd, J=12.1, 9.8 Hz, 6-CH), 3.06 (3H, s, N-CH3), 2.61 (3H, s, OCOCH3), 2.39 (3H, s, COCH3), 2.10 (3H, s, 9-CH3). 13C-NMR δ: 169.3 (s, OCOCH3), 168.5 (s, OCOCH3), 168.1 (s, C-2′), 162.6 (s, C-4), 158.2 (s, C-1), 151.9 (s, C-8), 144.1 (s, C-10), 138.7 (s, C-7), 134.0 (d, C-4′), 131.9 (s, C-2a′), 126.6 (s), 126.6 (s), 123.3 (s, C-3′), 121.5 (s, C-6a), 119.0 (s), 108.7 (d, C-11), 61.1 (q, 8-OCH3), 51.7 (t, C-3), 47.8 (d, C-6), 39.1 (t, 6-CH2), 33.8 (q, N-CH3), 20.7 (q, OCOCH3), 20.5 (q, OCOCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 2932, 1773, 1717, 1688, 1626, 1400, 1184. EI-MS m/z (%): 547 (M+, 2), 387 (100), 345 (37), 317 (12), 274 (8), 204 (12). HR-EI-MS Calcd for C28H25N3O9: 4547.1591. Found: 547.1593.
6-(Acetoxymethyl)-8-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (35)Yield: 83% (from 14a) and 46% (from 14b). 1H-NMR δ: 6.95 (1H, s, 11-H), 6.26 (1H, dd, J=9.8, 3.4 Hz, 6-H), 4.25 (1H, dd, J=11.7, 3.4 Hz, 6-CH), 4.15 (1H, d, J=17.6 Hz, 3-H), 4.07 (1H, d, J=17.6 Hz, 3-H), 3.80 (1H, dd, J=11.7, 9.8 Hz, 6-CH), 3.78 (3H, s, 8-OCH3), 3.08 (3H, s, N-CH3), 2.49 (3H, s, COCH3), 2.40 (3H, s, COCH3), 2.09 (3H, s, 9-CH3), 2.00 (3H, s, COCH3). 13C-NMR δ: 170.8 (s, NCOCH3), 169.1 (s, OCOCH3), 168.7 (s, OCOCH3), 162.2 (s), 158.6 (s), 152.0 (s), 144.0 (s), 138.9 (s), 126.6 (s), 126.6 (s), 120.6 (s), 119.2 (s), 108.8 (d, C-11), 62.8 (t, 6-CH2), 61.1 (q, 8-OCH3), 51.9 (t, C-3), 47.6 (d, C-6), 33.8 (q, N-CH3), 20.7 (q, COCH3), 20.6 (q, OCOCH3), 20.5 (q, OCOCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 2938, 1765, 1694, 1379, 1184, 1057. EI-MS m/z (%): 460 (M+, 13), 387 (100), 345 (61), 317 (24), 204 (14). HR-EI-MS Calcd for C22H24N2O9: 460.1481. Found: 460.1481.
Conversion of Compound 16a into 32 in Pyridine and Acetic Anhydride with DMAP (Typical Procedure)A solution of 16a (10.0 mg, 0.030 mmol), acetic anhydride (0.5 mL), and DMAP (1.8 mg, 0.015 mmol) in pyridine (1.5 mL) was stirred at 25°C for 2 h and usual work-up gave 32 (9.6 mg, 69.6%).
(6S*,11aS*)-6-(Acetamidomethyl)-8-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (36)Zinc powder (15.0 mg) was added to a solution of 29a (16.7 mg, 0.05 mmol) in acetic anhydride (1.5 mL) and the reaction mixture was heated at 60°C for 1 h. The reaction mixture was filtered and the combined filtrates were diluted with saturated NaHCO3 solution (30 mL) and then extracted with CHCl3 (30 mL×3). The combined extracts were washed with brine, dried, and concentrated in vacuo to give a residue, which was subjected to chromatography with CHCl3–MeOH (9 : 1) to give 36 (20.1 mg, 87.2%) as a colorless amorphous powder. 1H-NMR δ (pyridine-d5): 9.22 (1H, br t, J=4.9 Hz, NH), 6.41 (1H, dd, J=10.5, 3.0 Hz, 6-H), 4.93 (1H, dd, J=12.2, 4.6 Hz, 11a-H), 4.19 (1H, ddd, J=14.2, 6.6, 3.0 Hz, 6-CH), 4.09 (1H, d, J=17.4 Hz, 3-H), 3.98 (1H, d, J=17.4 Hz, 3-H), 3.67 (3H, s, 8-OCH3), 3.63 (1H, dd, J=14.2, 10.5 Hz, 6-CH), 3.49 (1H, br d, J=13.2 Hz, 11-Hα), 2.85 (1H, dd, J=16.8, 12.5 Hz, 11-Hβ), 2.70 (3H, s, N-CH3), 2.52 (3H, s, COCH3), 2.16 (3H, s, COCH3), 2.08 (3H, s, 9-CH3), 2.03 (3H, s, COCH3). 13C-NMR δ (pyridine-d5): 169.9 (s, NCOCH3), 168.5 (s, OCOCH3), 167.6 (s, OCOCH3), 164.0 (s), 161.8 (s), 149.3 (s), 145.3 (s), 139.0 (s), 125.1 (s), 124.3 (s), 121.9 (s), 60.0 (q, 8-OCH3), 50.6 (t, C-3), 50.1 (d, C-11a), 48.6 (d, C-6), 39.8 (t, 6-CH2), 31.9 (q, N-CH3), 27.8 (t, C-11), 22.1 (q, OCOCH3), 19.8 (q, OCOCH3), 19.1 (q, COCH3), 9.1 (q, 9-CH3). IR (KBr) cm−1: 3397, 2932, 1761, 1668, 1184. EI-MS m/z (%): 461 (M+, 8), 419 (3), 389 (51), 359 (10), 347 (100), 317 (12), 305 (11). HR-EI-MS Calcd for C22H27N3O8: 461.1798. Found: 461.1801.
(6S*,11aS*)-6-(Acetoxymethyl)-8-methoxy-2,9-dimethyl-1,4-dioxo-2,3,4,6,11,11a-hexahydro-1H-pyrazino[1,2-b]isoquinoline-7,10-diyl Diacetate (37)Zinc powder (7.5 mg) was added to a solution of 14a (10.0 mg, 0.03 mmol) in acetic anhydride (0.75 mL) and the reaction mixture was heated at 60°C for 1 h. The reaction mixture was filtered and the combined filtrates were diluted with saturated NaHCO3 solution (20 mL) and then extracted with CHCl3 (20 mL×3). The combined extracts were washed with brine, dried, and concentrated in vacuo to give a residue, which was subjected to chromatography with CH2Cl2–MeOH (50 : 1) to give 37 (12.9 mg, 93.1%) as a colorless amorphous powder. 1H-NMR δ: 5.98 (1H, d, J=9.3 Hz, 6-H), 4.55 (1H, d, J=10.7 Hz, 6-CH), 4.45 (1H, dd, J=12.2, 4.6 Hz, 11a-H), 4.06 (1H, d, J=10.7 Hz, 6-CH), 4.04 (1H, d, J=17.8 Hz, 3-H), 3.99 (1H, d, J=17.8 Hz, 3-H), 3.74 (3H, s, 8-OCH3), 3.26 (1H, br d, J=13.7 Hz, 11-Hα), 3.02 (3H, s, N-CH3), 2.65 (1H, br d, J=13.9 Hz, 11-Hβ), 2.48 (3H, s, COCH3), 2.33 (3H, s, COCH3), 2.08 (3H, s, 9-CH3), 2.02 (3H, s, COCH3). 13C-NMR δ: 170.3 (s, NCOCH3), 168.6 (s, OCOCH3), 168.0 (s, OCOCH3), 164.4 (s), 161.3 (s), 149.8 (s), 145.5 (s), 139.2 (s), 125.5 (s), 122.2 (s), 121.8 (s), 62.8 (t, 6-CH2), 60.9 (q, 8-OCH3), 51.2 (t, C-3), 51.0 (d, C-11a), 47.9 (d, C-6), 33.5 (q, N-CH3), 28.3 (t, C-11), 20.8 (q, OCOCH3), 20.8 (q, OCOCH3), 20.5 (q, COCH3), 10.1 (q, 9-CH3). IR (KBr) cm−1: 3468, 2938, 1761, 1748, 1668, 1472, 1184. EI-MS m/z (%): 462 (M+, 5), 420 (22), 389 (23), 378 (20), 347 (100), 318 (19), 305 (73). HR-EI-MS Calcd for C22H26N2O9: 462.1638. Found: 462.1638.
This work was supported by a Grant-in-Aid (No. 23590019) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also partially supported by a Grant from the High-Tech Research Center Project from MEXT, Japan (No. S080104). We would like to thank Dr. Kazuhiko Takatori (Meiji Pharmaceutical University) for the X-ray crystallographic analysis of 6b.
