Regular Article
Bioinspired Total Synthesis of (+)-Kopsiyunnanine B
2024 年 72 巻 1 号 p. 68-74
詳細
2024 年 72 巻 1 号 p. 68-74
The first enantioselective total synthesis of kopsiyunnanine B, which has a unique folded and complex pentacyclic structure containing six contiguous chiral centers, has been achieved along our originally proposed biosynthetic pathway. The key reaction of this synthesis includes a bioinspired cascade that builds three ring structures and three chiral centers in one step and features the stereoselective reduction of a β-acrylate and oxidation to an oxindole.
The monoterpenoid indole alkaloids, of which more than 3000 have been discovered, are a group of alkaloids found in higher plants, such as the Apocynaceae and Rubiaceae families.1–10) Some of them, such as vinblastine, quinine, and ajmaline, have already been used as pharmaceuticals. Kopsiyunnanine B (1) was isolated as a monoterpenoid oxindole alkaloid from Yunnan Kopsia arborea by Takayama and colleagues in 200811) (Chart 1). It has a unique folded and complex pentacyclic structure containing six contiguous chiral centers. In addition, the C–C bond at the indole C2 and C21 positions is extremely rare in this class of alkaloids.12) Based on its proposed biosynthetic pathway by Takayama, compound 1 was classified as a corynanthe-type indole alkaloid. The biosynthetic hypothesis they proposed is shown in Chart 1. Thus, tetrahydroalstonine (3), derived from tryptamine and secologanin (2), is cleaved along the C2–C3 bond under acidic conditions, followed by isomerization of the iminium ion across the N4-nitrogen to intermediate 6, in which C21 is converted to an electrophilic carbon (3→6). The intermediate 6 recyclizes in a Pictet–Spengler reaction and yields indole 7. Subsequent reduction of the β-acrylate residue and oxidation to oxindole leads to 1.
We have recently achieved a collective total synthesis of monoterpenoid indole alkaloids along a biosynthetic tree diagram.13–21) In this study, we conceived an alternative biosynthetic hypothesis for 1 (Chart 2). Our proposed biosynthesis begins with a connection by the reductive amination of tryptamine and secologanin (2). This results in the formation of secondary amine 8. This molecule is not found in nature but was previously semi-synthesized as a potent inhibitor of the enzyme strictosidine synthase.22) After the sugar chain is removed by β-glycosidase, the ring-opening reaction of the resulting hemiacetal 9 affords aldehyde 10. The resulting C21 aldehyde of compound 10 and the nitrogen at the N4 position form a Schiff base. The activated iminium ion in compound 11 promotes isomerization of the C18–19 terminal double bond, converting it to compound 12 with an electronically stabilized E olefin. A subsequent stereoselective oxa-Michael reaction leads to intermediate 6 proposed by Takayama et al. We have previously developed a similar stereoselective oxa-Michael reaction in our bioinspired total synthesis of tetrahydroalstonine (3).15)
Herein, we achieved the total synthesis of (+)-kopsiyunnanine B (1) following our originally proposed biosynthetic pathway using synthetic optically active secologanin aglycone.
To execute a strategy consistent with our proposed biosynthetic hypothesis, secologanin aglycone silyl ether 14 was synthesized (Chart 3). Compound 14 is a key intermediate in our previous collective total synthesis of monoterpenoid indole alkaloids, which was successfully synthesized from a commercial propargylic aldehyde in six steps, in a total yield of 31%, on a gram scale.15) Aldehyde 14 was then subjected to reductive amination with tryptamine as in the proposal biosynthesis, affording the desired secondary amine 15 in 21% yield. However, the major product was lactam 16 (55%), generated by the condensation of the resulting amine and the methoxycarbonyl group at C22. Because we believed that the lactamization reaction had progressed due to the Lewis acidity of the boron reductant, we subsequently switched to a condensation using Mitsunobu reaction protocols. Thus, aglycone 14 was treated with NaBH4 to provide alcohol 17 in quantitative yield. Then, a Mitsunobu reaction was carried out using diisopropyl azodicarboxylate and triphenylphosphine in the presence of nosylated tryptamine 18 (83%). After the removal of the nosyl group of the coupled product 19, key precursor 15 of our proposed bioinspired reaction was obtained in excellent yield (92%). Gratifyingly, the basic conditions for the removal of the nosyl group, benzenethiol and potassium carbonate, did not lead to undesired lactamization. In addition, compound 15 could be purified by silica gel and stored at low temperatures for extended periods, although careful handling was required.
With a sufficient amount of secondary amine 15 in hand, the key bioinspired reaction was examined. To obtain the desired tetracyclic compound 7 from secondary amine 15, a six-step cascade of reactions was required (Charts 1, 2, and 4): 1) removal of the silyl group, 2) ring opening of the in situ-generated hemiacetal, 3) cyclic imine formation between positions N4 and C21, 4) isomerization of the terminal double bond (C18–19), 5) dihydropyran ring formation by a stereoselective oxa-Michael reaction, 6) C ring formation by a stereoselective Pictet–Spengler cyclization. Therefore, key intermediate 15 was treated with aqueous hydrochloric acid as a type of condition that could occur in biosynthesis (room temperature (r.t.), 3 d). To our great delight, the desired pentacyclic product 7 was obtained in 39% yield. Compound 20, the isomer of compound 7 at C19 and 20, was also obtained in 11% yield. The relative stereochemistry of 7 and 20 was confirmed by nuclear Overhauser effect spectroscopy (NOESY) analysis (see details in supplementary materials). Unexpectedly, the byproduct 21, in which the β-acrylate residue was hydrated, was obtained in 35% yield as a mixture of several diastereomers. When the diastereomeric mixture was treated with BF3·Et2O to promote the dehydration reaction, they converged to single isomer 20 (75%). Thus, we speculated that compound 21 was mainly a diastereomeric mixture of the C17 and 16 positions. We wondered about the fixed stereochemistry of the C19 and 21 positions of compound 21 and tried to hydrate 20 under the same conditions as the key bioinspired reaction. However, the hydration reaction did not proceed, and the starting material 20 was completely recovered. This fact suggests that the hydration reaction at C17 proceeds during the reaction to produce 21. At present, it is not clear at which stage in the reaction the hydration occurs and stereoselectivity is generated. On the other hand, the stereoselectivity in the Pictet–Spengler reaction was perfectly controlled to C21S. After further investigation, we found that this cascade reaction could proceed under weakly acidic and anhydrous conditions as long as the silyl group was removed. Thus, secondary amine 15 was treated with tetrabutylammonium fluoride (TBAF, 2.0 equivalent (equiv.)) at low temperatures to remove the silyl group. After the workup, the crude hemiacetal mixture was stirred at room temperature with an excess of acetic acid under anhydrous conditions to provide pentacyclic product 7 in 45%. Interestingly, 7 was the only product that could be isolated in this two-step protocol (compounds 20 and 21 were not yielded).
The two remaining bioinspired reactions for the total synthesis of kopsiyunnanine B (1) were then examined. First, stereoselective reduction of the β-acrylate residue was accomplished with L-selectride (3 equiv., −40 °C, 3 h). In the 1,4-reduction, kinetic protonation proceeded at C16 position, and compound 22 with the desired R configuration was selectively obtained. Finally, an oxidation/rearrangement reaction from indole to oxindole was performed. Thus, compound 22 was treated with N-chlorosuccinimide (NCS), and a clean single spot was observed on TLC. In NMR experiments, compound 23, in which a chlorine atom was at the C7 position, was observed as a single isomer (the stereochemistry of C7 was not determined). Since intermediate 23 was a single isomer, it was expected that the rearrangement reaction to the oxindole could proceed stereoselectively if it occurred in a concerted manner. However, the subsequent rearrangement reaction with heating in acetic acid/aqueous methanol provided a 5 : 4 diastereomeric mixture at the C7 spiro center, which implied either initial ionization of the chloride prior to rearrangement or subsequent isomerization. Two separable diastereomers were isolated, and the total synthesis of kopsiyunnanine B (1) was achieved (49% isolated yield of 1).
All analytical data of the synthetic 1 agreed with the reported data of the natural product, except for the optical rotation. The sign of the rotation of the natural and synthetic products were opposite, leading us to question the absolute stereochemistry (synthetic 1; [α]23D +19.2 (c 0.3, CHCl3), reported 1; [α]18D −15.9 (c 0.1, CHCl3). On the other hand, the electronic circular dichroism (ECD) spectra were in perfect agreement. Then, we remeasured the rotation of the originally isolated natural product 1, and found it to be in perfect agreement with the synthetic product ([α]23D +21.0 (c 0.1, CHCl3).23) Therefore, the rotation of the natural product was found to be opposite to what was reported.
Regarding the lack of selectivity of the rearrangement reaction to the oxindole, we became aware of the isomerization of 7-epi-kopsiyunnanine B (24) to 1. That is, when 7-epi-kopsiyunnanine B (24) was stored in an NMR tube in CDCl3, it gradually isomerized to natural product 1 over a long period of time. Among the monoterpenoid indole alkaloids, oxindoles are known to isomerize via a retro-Mannich reaction under acidic conditions24) (CDCl3 decomposes over time to release DCl). Thus, the isolated 24 was heated again under acetic acid/aqueous MeOH conditions, and, as expected, the isomerization reaction proceeded to give a mixture of 1 and 24. The results suggest that this isomerization is thermodynamically controlled, and that there is almost no energy difference between the products (see also Chart S1 in supplementary materials). The isolated yield of 1 in the isomerization was 51%.
In conclusion, we accomplished the enantioselective total synthesis of kopsiyunnanine B (1) via 14 steps in 4% overall yield from commercially available propargyl aldehyde 13. The stereochemistry of 1 was set by a bioinspired cascade cyclization, stereoselective reduction of a β-acrylate, and a spirocycle formation via a semi-pinacol rearrangement. This synthesis supported our proposed biosynthetic hypothesis. Now, to support our biosynthetic hypothesis, we will try to isolate compound 7, which could exist as a biosynthetic intermediate in the original plants.
All reactions were monitored by TLC using Merck 60 F254 precoated silica gel plates (0.25 mm thick) and Fuji Silysia Chemical (Aichi, Japan) precoated amino-silica gel plates (0.25 mm thick). UV spectra were recorded in MeOH on a JASCO V-560 instrument. Specific optical rotations were measured using a JASCO P-2200 polarimeter. Electronic circular dichroism spectra were recorded on a JASCO J-1100 spectrometer. Fourier transform (FT) IR spectra were recorded on a JASCO FT/IR-4700. 1H- and 13C-NMR spectra were recorded on a JEOL ECZ 400 (400 MHz for 1H-NMR, 100 MHz for 13C-NMR) and ECZ 600 (600 MHz for 1H-NMR, 150 MHz for 13C-NMR) FT-NMR instrument. Data for 1H-NMR are reported as chemical shifts (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, dd = double doublet, ddd = doublet of doublet of doublet, dt = doublet of triplet, q = quartet, m = multiplet, br = broad), coupling constant (Hz), integration, and assignment. Data for 13C-NMR are reported as chemical shifts. Chemical shifts were reported using residual CDCl3 [δH 7.26 and δc 77.16 ppm or δc 77.0 ppm (for 1)] as internal standard. The high-resolution mass spectra (HR-MS) were recorded on a JEOL AccuTOF LC-plus JMS-T100LP. Preparative thin layer chromatography (PTLC) was performed using Merck 60 F254 precoated silica gel plates (0.25 mm thickness) and Fuji Silysia Chemical precoated amino-silica gel plates (0.25 mm thick). The crude materials were purified by flash chromatography was performed using Kanto Chemical (Tokyo, Japan) silica gel 60N and Fuji Silysia Chemical amino-silica gel (SiO2–NH, NH-DM2035).
Reductive Amination of Compound 14To a solution of secologanin aglycone silyl ether 1415) (18.7 mg, 0.05 mmol) in tetrahydrofuran (THF) (550 µL) were added acetic acid (AcOH) (3.1 µL, 0.05 mmol), NaBH3CN (11.6 mg, 0.05 mmol), and tryptamine (29.4 mg, 0.17 mmol) at −20 °C under Ar atmosphere. The reaction mixture was stirred for 1 h at −20 °C under Ar atmosphere. The mixture was quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by flash chromatography repeated four time (SiO2–NH, 10% MeOH/CHCl3, then SiO2, 3–20% MeOH/CHCl3 gradient, then SiO2–NH, 10% MeOH/CHCl3, then SiO2, 3–20% MeOH/CHCl3 gradient) provided a secondary amine 15 (5.6 mg, 21%) and lactam 16 (13.7 mg, 55%).
Compound 15: colorless oil; [α]23D −78.0 (c 1.2, CHCl3); IR (attenuated total reflectance (ATR)) νmax cm−1 2932, 2853, 1698, 1634, 1460, 1442, 1250, 1163, 1078, 836, 727; HR-MS (electrospray ionization (ESI)) [M + H]+ Calcd for [C27H41N2O4Si]+: 485.2830. Found: 485.2831; UV (MeOH) λmax 222, 281, 290 nm; 1H-NMR (600 MHz, CDCl3) δ: 8.10 (br s, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.41 (s, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.19 (ddd, J = 8.0, 7.0, 1.1 Hz, 1H), 7.11 (ddd, J = 8.0, 7.0, 1.1 Hz, 1H), 7.05 (d, J = 2.3 Hz, 1H), 5.67 (m, 1H), 5.26–5.09 (3H, overlapped), 3.65 (s, 3H), 3.03–2.89 (4H, overlapped), 2.75–2.56 (3H, overlapped), 2.41 (m, 1H), 1.93 (br s, 1H), 1.75 (m, 1H), 1.63 (m, 1H), 0.88 (s, 9H), 0.11 (s, 6H); 13C-NMR (150 MHz) δ: 168.1, 152.9, 136.6, 134.8, 127.6, 122.2, 122.1, 119.4, 119.1, 119.0, 113.9, 111.3, 109.8, 95. 6, 51.4, 49.9, 48.0, 47.4, 31.9, 31.2, 25.74, 25.72, 18.1, −4.1, −5.1.
Compound 16: colorless oil; [α]23D −119.5 (c 0.4, CHCl3); IR (ATR) νmax cm−1 3251, 2928, 2857, 1652, 1589, 1483, 1450, 1333, 1258, 1171, 1100, 1013, 833, 739; HR-MS (ESI) [M + Na]+ Calcd for [C26H36N2NaO3Si]+: 475.2387. Found: 475.2394; UV (MeOH) λmax 204, 221 nm; 1H-NMR (600 MHz, CDCl3) δ: 8.09 (br s, 1H), 7.68 (dd, J = 7.9, 1.1 Hz, 1H), 7.49 (d, J = 2.5 Hz, 1H), 7.35 (d, J = 7.9 Hz, 1H), 7.18 (ddd, J = 7.9, 7.0, 1.1 Hz, 1H), 7.12 (ddd, J = 7.9, 7.0, 1.1 Hz, 1H), 7.04 (d, J = 2.3 Hz, 1H), 5.53 (m, 1H), 5.21–5.18 (2H, overlapped), 5.16 (m, 1H), 3.74 (ddd, J = 13.2, 8.8, 6.0 Hz, 1H), 3.67 (ddd, J = 13.2, 8.7, 6.6 Hz, 1H), 3.37 (ddd, J = 12.2, 12.2, 3.4 Hz, 1H), 3.19 (ddd, J = 12.1, 4.6, 2.4 Hz, 1H), 3.12–2.99 (2H, overlapped), 2.82 (m, 1H), 2.38 (ddd, J = 10.0, 5.3, 1.9 Hz, 1H), 1.60–1.45 (2H, overlapped), 0.89 (s, 9H), 0.15 (s, 3H), 0.12 (s, 3H); 13C-NMR (150 MHz, CDCl3) δ: 164.4, 147.4, 136.4, 133.9, 127.7, 122.1 (2C), 119.5, 119.4, 119.1, 113.7, 111.2, 107.1, 95.4, 48.9, 47.7, 46.2, 28.0, 25.8, 25.3, 23.5, 18.1, −4.3, −5.2.
Preparation of Compound 17To a solution of secologanin aglycone silyl ether 14 (358.6 mg, 1.05 mmol) in MeOH (11 mL) was added NaBH4 (39.8 mg, 1.05 mmol) at −40 °C under Ar atmosphere. The reaction mixture was stirred for 1 h at −40 °C under Ar atmosphere. The mixture was quenched with saturated aqueous NH4Cl and the resulting mixture was extracted three times with CHCl3. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by flash chromatography (SiO2, 20% AcOEt/n-hexane) provided a primary alcohol 17 (370.2 mg, quant.).
Compound 17: colorless oil; [α]23D −99.4 (c 1.9, CHCl3); IR (ATR) νmax cm−1 3450, 2938, 2867, 1713, 1622, 1258, 1163, 1100, 844, 777; HR-MS (ESI) [M + H]+ Calcd for [C17H31O5Si]+: 343.1935. Found: 343.1940; 1H-NMR (600 MHz, CDCl3) δ: 7.52 (s, 1H), 5.70 (ddd, J = 17.0, 10.6, 8.6 Hz, 1H), 5.29 (d, J = 8.3 Hz, 1H), 5.23–5.18 (2H, overlapped), 3.73 (s, 3H), 3.61 (m, 1H), 3.53 (ddd, J = 11.6, 9.3, 4.1 Hz, 1H), 3.17 (br s, 1H), 2.88 (ddd, J = 9.9, 5.0, 5.0 Hz, 1H), 2.47 (m, 1H), 1.96 (dddd, J = 14.0, 9.2, 4.5, 4.5 Hz, 1H), 1.40 (dddd, J = 14.0, 9.2, 4.5, 4.5 Hz, 1H), 0.88 (s, 9H), 0.13 (s, 3H), 0.12 (s, 3H); 13C-NMR (150 MHz, CDCl3) δ: 169.4, 154.3, 134.6, 119.1, 109.1, 95.6, 60.1, 51.8, 47.5, 34.4, 31.1, 25.7, 18.1, −4.1, −5.1.
Preparation of Compound 19To a solution of primary alcohol 17 (374.3 mg, 1.09 mmol), PPh3 (429.9 mg, 1.64 mmol) and N-nosyl tryptamine 18 (452.9 mg, 1.31 mmol) in dry THF (5.5 mL) was added diisopropyl azodicarboxylate (40% in toluene, 863 µL, 1.64 mmol) at 0 °C under Ar atmosphere. The reaction mixture was stirred for 5 h at r.t. under Ar atmosphere. The mixture was concentrated under reduced pressure. The crude materials were purified by flash chromatography, repeated twice (SiO2, 20–50% AcOEt/n-hexane gradient then SiO2, 3% AcOEt/benzene) provided a N-nosyl amine 19 (607.6 mg, 83%).
Compound 19: yellow amorphous powder; [α]23D −91.9 (c 1.0, CHCl3); IR (ATR) νmax cm−1 3413, 2951, 2928, 2887, 2857, 2299, 1698, 1626, 1539, 1464, 1430, 1341, 1292, 1246, 1153, 1122, 1078, 833, 736; HR-MS (ESI) [M + Na]+ Calcd for [C33H43N3NaO8SSi]+: 692.2432. Found: 692.2458; UV (MeOH) λmax 216, 284 nm; 1H-NMR (600 MHz, CDCl3) δ: 7.97 (s, 1H), 7.89 (dd, J = 7.0, 1.2 Hz, 1H), 7.63–7.54 (3H, overlapped), 7.50 (m, 1H), 7.43 (s, 1H), 7.31 (d, J = 8.0 Hz, 1H), 7.17 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 7.10 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 7.04 (d, J = 2.2 Hz, 1H), 5.62 (ddd, J = 17.1, 9.8, 9.8 Hz, 1H), 5.27–5.20 (2H, overlapped), 5.16 (d, J = 6.2 Hz, 1H), 3.70 (s, 3H), 3.59 (2H, overlapped), 3.52–3.39 (2H, overlapped), 3.06–3.01 (2H, overlapped), 2.69 (ddd, J = 5.5, 5.5, 5.5 Hz, 1H), 2.51 (ddd, J = 9.0, 5.8, 5.8 Hz, 1H), 2.00 (m, 1H), 1.71 (m, 1H), 0.89 (s, 9H), 0.14 (s, 3H), 0.12 (s, 3H); 13C-NMR (150 MHz, CDCl3) δ: 167.8, 152.9, 148.1, 136.3, 134.4, 133.8, 133.2, 131.6, 130.7, 127.3, 124.1, 122.5, 122.2, 119.7, 119.6, 118.7, 112.4, 111.3, 109.3, 95.6, 51.4, 48.2, 46.8, 46.7, 30.7, 29.3, 25.7, 24.7, 18.1, −4.2, −5.1.
Preparation of Compound 15To a solution of N-nosyl amine 19 (20.0 mg, 0.03 mmol) in dry N,N-dimethylformamide (DMF) (300 µL) were added benzenethiol (3.1 µL, 0.03 mmol) and K2CO3 (8.3 mg, 0.06 mmol) at r.t. under Ar atmosphere. The reaction mixture was stirred for 1 h at r.t. under Ar atmosphere. The mixture was concentrated under reduced pressure. The crude materials were purified by PTLC (SiO2, methanol : AcOEt : benzene = 2 : 9 : 9) provided a secondary amine 15 (13.3 mg, 92%).
Preparation of Compound 7<Acidic protocol >
To a solution of secondary amine 15 (14.0 mg, 0.029 mmol) in THF (290 µL), 1M HCl (290 µL) was added at r.t. under Ar atmosphere. The reaction mixture was stirred for 3 d at rt under Ar atmosphere. The mixture was then quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with 10% MeOH/CHCl3. The combined organic layer was dried over Na2SO4 and then concentrated under reduced pressure. The crude materials were purified by PTLC (SiO2, 10% MeOH/CHCl3) provided a pentacyclic product 7 (4.0 mg, 39%) and diastereomer 20 (1.1 mg, 11%) and hydrated compound 21 as a diastereomer mixture (3.7 mg, 35%).
<Basic protocol >
To a solution of secondary amine 15 (69.9 mg, 0.144 mmol) in THF (1.4 mL) was added tetrabutylammonium fluoride (1.0 M in THF solution, 288 µL, 0.288 mmol) at −20 °C under Ar atmosphere. The reaction mixture was stirred for 30 min at −20 °C under Ar atmosphere. The mixture was then quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was washed three times with brine, dried over Na2SO4 and then concentrated under reduced pressure. Obtained crude materials were dissolved in CH2Cl2 (7.2 mL), then AcOH (82.5 µL, 1.44 mmol) added at 0 °C under Ar atmosphere. The reaction mixture was stirred for 2 h at r.t. under Ar atmosphere. The mixture was then quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by flash chromatography (SiO2–NH, 15% AcOEt/n-hexane) provided a pentacyclic product 7 (22.8 mg, 45%).
Compound 7: white amorphous powder; [α]23D −71.4 (c 1.1, CHCl3); IR (ATR) νmax cm−1 3364, 2924, 1671, 1618, 1446, 1258, 1194, 1111, 1078, 735; HR-MS (ESI) [M + H]+ Calcd for [C21H25N2O3]+: 353.1860. Found: 353.1858; UV (MeOH) λmax 202, 219, 269, 283, 291 nm; 1H-NMR (600 MHz, CDCl3) δ: 7.98 (br s, 1H), 7.56 (s, 1H), 7.47 (d, J = 8.0 Hz, 1H), 7.32 (dd, J = 8.0, 1.2 Hz, 1H), 7.16 (ddd, J = 8.0, 7.1, 1.2 Hz, 2H), 7.11 (ddd, J = 8.0, 7.1, 1.2 Hz, 1H), 4.70 (dq, J = 9.9, 6.2 Hz, 1H), 4.52 (s, 1H), 3.64 (s, 3H), 3.26 (dd, J = 13.8, 6.4 Hz, 1H), 3.17 (ddd, J = 13.2, 12.5, 5.5 Hz, 1H), 2.99 (m, 1H), 2.88 (dd, J = 12.4, 12.4 Hz, 1H), 2.58 (ddd, J = 10.8, 3.0, 3.0 Hz 1H), 2.55 (dd, J = 16.1, 5.1 Hz, 1H), 2.39 (m, 1H), 2.02 (dd, J = 10.2, 4.4 Hz, 1H), 1.93 (m, 1H), 1.61 (m, 1H), 1.51 (d, J = 6.2 Hz, 3H); 13C-NMR (150 MHz, CDCl3) δ: 168.0, 155.3, 135.7, 131. 5, 127.8, 121.7, 119.7, 118.2, 111.1, 110.2, 108.4, 71.8, 54.7, 51.7, 51.3, 44.7, 40.1, 31.7, 29.6, 27.3, 19.3, 16.9.
Compound 20: white amorphous powder; [α]23D +128.0 (c 0.3, CHCl3); IR (ATR) νmax cm−1 3390, 2920, 2853, 1694, 1615, 1438, 1292, 1190, 1092, 739; HR-MS (ESI) [M + H]+ Calcd. for [C21H25N2O3]+: 353.1860. Found: 353.1881; UV (MeOH) λmax 290, 281, 276, 225 nm; 1H-NMR (600 MHz, CDCl3) δ: 7.58 (s, 1H), 7.49 (d, J = 8.0 Hz, 1H), 7.44 (br s, 1H), 7.32 (dd, J = 8.0, 1.0 Hz, 1H), 7.18 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 7.12 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 4.60 (s, 1H), 4.30 (dq, J = 10.4, 6.5 Hz, 1H), 3.65 (s, 3H), 3.36 (dd, J = 13.6, 6.3 Hz, 1H), 3.29 (ddd, J = 13.6, 12.8, 5.5 Hz, 1H), 3.07 (m, 1H), 2.92 (ddd, J = 12.1, 11.9, 3.2 Hz, 1H), 2.72 (ddd, J = 11.9, 4.3, 2.5 Hz, 1H), 2.58 (ddd, J = 16.2, 4.8, 1.8 Hz, 1H), 2.42 (ddd, J = 12.6, 3.0, 3.0 Hz, 1H), 2.21 (m, 1H), 2.06 (m, 1H), 1.61 (d, J = 6.5 Hz, 3H), 1.27 (m, 1H); 13C-NMR (150 MHz, CDCl3) δ: 167.4, 155.3, 135.8, 130.6, 127.3, 122.3, 120.0, 118.3, 111.1, 110.3, 110.2, 74.7, 55.5, 51.5, 51.1, 49.0, 45.0, 30.9, 30.9, 18.2, 16.9.
Compound 21 (two diastereomer mixture): white amorphous powder; IR (ATR) νmax cm−1 2924, 2326, 1727, 1438, 1371, 1351, 1239, 1036, 742; HR-MS (ESI) [M + H]+ Calcd for [C21H27N2O4]+: 371.1965. Found: 371.1954; UV (MeOH) λmax 205, 219, 281, 291 nm; 1H-NMR (600 MHz, CDCl3) δ: 7.99 (s), 7.77 (s), 7.48 (d, J = 7.7 Hz), 7.39 (m), 7.18 (m), 7.12 (m), 5.46 (d, J = 3.4 Hz), 5.01 (d, J = 8.9 Hz), 4.65 (m), 4.51 (m), 4.46 (m), 4.12 (m), 3.68 (s), 3.67 (s), 3.30 (m), 3.24 (m), 3.03 (m), 2.94 (m), 2.64 (m), 2.59 (m), 2.53 (dd, J = 11.3, 3.3 Hz), 2.26 (m), 2.10 (m), 1.85 (m), 1.76 (m), 1.57 (m), 1.51 (d, J = 6.2 Hz), 1.42 (d, J = 6.2 Hz,), 1.31 (m).; 13C-NMR (150 MHz, CDCl3) δ: 172.8, 172.6, 135.9, 135.8, 131.1, 130.7, 127.2, 122.3, 122.1, 120.0, 119.8, 118.2, 118.1, 111.4, 111.3, 110.0, 109.7, 96.3, 91.1, 73.3, 65.0, 56.4, 55.6, 52.2, 52.1, 51.88, 51.85, 51.8, 49.8, 49.7, 44.5, 44.3, 35.2, 31.7, 31.1, 29.1, 19.12, 19.06, 17.2, 17.1.
Dehydration of Compound 21To a solution of 21 (18 mg, 0.049 mmol) in CH2Cl2 (490 µL) was added BF3⋅OEt2 (61 µL, 0.060 mmol) at r.t. under Ar atmosphere. The reaction mixture was stirred for 3 h at r.t. under Ar atmosphere. The mixture was quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by PTLC (SiO2, 10% MeOH/CHCl3) provided 20 (12.9 mg, 75%).
Preparation of Compound 22To a solution of pentacyclic product 7 (7.0 mg, 0.020 mmol) in THF (275 µL) was added L-Selectride (1M THF solution, 60 µL, 0.060 mmol) at −40 °C under Ar atmosphere.
The reaction mixture was stirred for 3 h at −40 °C under Ar atmosphere. The mixture was then quenched with saturated aqueous NH4Cl and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and then concentrated under reduced pressure. The crude materials were purified by PTLC (SiO2, 10% MeOH/CHCl3) provided a 1,4-reduced compound 22 (3.7 mg, 53%) and stating material 7 (1.7 mg, 25%).
Compound 22: white amorphous powder; [α]23D +50.0 (c 1.2, CHCl3); IR (ATR) νmax cm−1 3405, 2932, 1721, 1460, 1235, 1190, 1126, 1104, 1024, 840, 746; HR-MS (ESI) [M + H]+ Calcd. for [C21H27N2O3]+: 355.2016. Found: 355.2020; UV (MeOH) λmax 201, 225 nm; 1H-NMR (600 MHz, CDCl3) δ: 8.20 (br s, 1H), 7.46 (d, J = 8.0 Hz, 1H), 7.43 (d, J = 8.0 Hz, 1H), 7.16 (ddd, J = 8.0, 7.0, 1.2 Hz, 1H), 7.10 (ddd, J = 8.0, 7.0, 1.2 Hz, 1H), 4.39 (s, 1H), 4.24 (d, J = 12.2 Hz, 1H), 4.17 (dq, J = 10.1, 6.0 Hz, 1H), 3.95 (dd, J = 12.2, 3.4 Hz, 1H), 3.77 (s, 3H), 3.26 (dd, J = 13.7, 6.6 Hz, 1H), 3.20 (ddd, J = 13.7, 13.7, 5.2 Hz, 1H), 3.00 (m, 1H), 2.84 (ddd, J = 11.0, 11.0, 3.2 Hz, 1H), 2.69 (br d, J = 11.0 Hz, 1H), 2.57 (dd, J = 15.6, 5.2 Hz, 1H), 2.30 (m, 1H), 2.25 (m, 1H), 2.22–2.13 (2H, overlapped), 1.31 (d, J = 6.0 Hz, 1H), 1.29 (m, 1H); 13C-NMR (150 MHz, CDCl3) δ: 174.3, 135.9, 132.1, 128.0, 121.6, 119.5, 118.0, 111.5, 108.0, 71.1, 64.7, 55.5, 52.2, 51.4, 45.4, 45.0, 40.5, 29.2, 25.5, 20.1, 17.0.
Preparation of Compounds 1 and 24To a solution of 1,4-reduced compound 22 (5.8 mg, 0.016 mmol) in CH2Cl2 (300 µL) was added N-chlorosuccinimide (2.3 mg, 0.017 mmol) at 0 °C under Ar atmosphere. The reaction mixture was stirred for 3 h at 0 °C under Ar atmosphere. The mixture was then concentrated under reduced pressure. Obtained crude materials were dissolved in methanol (100 µL) and H2O (100 µL), then AcOH (10 µL, 0.175 mmol) was added at r.t. under Ar atmosphere. The resulting mixture was stirred at 100 °C for 10 min. The mixture was quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by PTLC (SiO2, 10% MeOH/CHCl3) provided kopsiyunnanine B (1) (2.9 mg, 49%) and 7-epi-kopsiyunnanine B (24) (2.3 mg, 39%).
Isomerization of 24 to 1To a solution of 7-epi-kopsiyunnanine B (24) (3.5 mg, 9.5 µmol) in methanol (100 µL) and H2O (100 µL) was added AcOH (10 µL, 0.175 mmol) at r.t. under Ar atmosphere. The reaction mixture was stirred for 10 min at 100 °C under Ar atmosphere. The mixture was quenched with saturated aqueous NaHCO3 and the resulting mixture was extracted three times with AcOEt. The combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude materials were purified by PTLC (NH–SiO2, 70% AcOEt/n-hexane) provided kopsiyunnanine B (1) (1.8 mg, 51%) and 7-epi-kopsiyunnanine B (24) (1.4 mg, 40%).
Kopsiyunnanine B (1): white amorphous powder; [α]23D +19.2 (c 0.3, CHCl3); IR (ATR) νmax cm−1 2932, 2803, 1713, 1618, 1464, 1314, 1197, 1171, 1149, 1051, 750, 678; HR-MS (ESI) [M + H]+ Calcd for [C21H27N2O4]+: 371.1965. Found: 371.1965; UV (MeOH) λmax 210, 252 nm; CD (0.3 mM, MeOH, 23 °C) λ nm (Δε): 213 (−2.31), 217 (−3.49), 226 (0), 238 (6.26), 251 (0), 261 (−3.31), 275 (−1.58), 290 (−1.80), 312 (0); 1H-NMR (600 MHz, CDCl3) δ: 7.56 (br s, 1H), 7.40 (d, J = 7.4 Hz, 1H), 7.20 (ddd, J = 7.8, 7.4, 1.2 Hz, 1H), 7.03 (ddd, J = 7.8, 7.4, 1.2 Hz, 1H), 6.87 (d, J = 7.4 Hz, 1H), 3.71 (dd, J = 11.5, 5.1 Hz, 1H), 3.66 (s, 3H), 3.58 (dd, J = 11.5, 11.5 Hz, 1H), 3.23 (m, 1H), 3.22 (m, 1H), 3.07 (ddd, J = 11.5, 11.5, 5.1 Hz, 1H), 2.98 (m, 1H), 2.96 (d, J = 11.0 Hz, 1H), 2.59 (ddd, J = 9.0, 9.0, 9.0 Hz, 1H), 2.39 (m, 1H), 2.38 (m, 1H), 2.34 (ddd, J = 13.0, 9.0, 1.8 Hz, 1H), 1.99 (ddd, J = 13.0, 9.5, 8.3 Hz, 1H), 1.78 (ddd, J = 13.3, 13.3, 4.7 Hz, 1H), 1.58 (m, 1H), 1.48 (dd, J = 11.0, 4.7 Hz, 1H), 0.75 (d, J = 6.8 Hz, 3H); 13C-NMR (150 MHz, CDCl3) δ: 181.8, 173.8, 139.9, 133.2, 127.8, 125.3, 122.5, 109.4, 69.0, 68.0, 61.1, 56.7, 54.0, 51.7, 47.7, 40.9, 40.2, 37.3, 29.7, 28.3, 16.6.
7-epi-Kopsiyunnanine B (24): white amorphous powder; [α]23D +26.5 (c 0.4, CHCl3); IR (ATR) νmax cm−1 2942, 2800, 1725, 1690, 1472, 1329, 1179, 1133, 1036, 739; HR-MS (ESI) [M + H]+ Calcd for [C21H27N2O4]+: 371.1965. Found: 371.1959; UV (MeOH) λmax 203, 253 nm; CD (0.3 mM, MeOH, 23 °C) λ nm (Δε): 213 (−15.88), 227 (0), 238 (+6.84), 250 (0), 261 (−4.52), 275 (0), 290 (+1.71), 308 (0); 1H-NMR (600 MHz, CDCl3) δ: 8.02 (s, 1H), 7.29 (d, J = 7.0 Hz, 1H), 7.20 (ddd, J = 7.7, 7.0, 1.2 Hz, 1H), 7.08 (ddd, J = 7.7, 7.2, 1.2 Hz, 1H), 6.87 (d, J = 7.2 Hz, 1H), 3.66 (s, 3H), 3.64–3.53 (2H, overlapped), 3.33 (t, J = 8.3 Hz, 1H), 3.23 (qd, J = 6.8, 1.8 Hz 1H), 3.07 (br d, J = 11.4 Hz, 1H), 3.00 (m, 1H), 2.93 (d, J = 10.6 Hz, 1H), 2.57 (m, 1H), 2.49 (m, 1H), 2.44 (m, 1H), 2.37 (ddd, J = 11.4, 11.4, 2.4 Hz 1H), 2.01 (dd, J = 10.6, 4.7 Hz, 1H), 1.94 (m, 1H), 1.93 (m, 1H), 1.55 (br d, J = 14.2 Hz, 1H), 1.11 (d, J = 6.8 Hz, 3H); 13C-NMR (150 MHz, CDCl3) δ: 181.3, 173.7, 139.5, 135.4, 128.3, 123.3, 109.9, 69.7, 68.7, 61.0, 55.4, 54.7, 51.9, 47.7, 41.4, 40.1, 37.6, 29.4, 28.1, 17.0.
We gratefully acknowledge financial support through a Grant-in-Aid for Scientific Research (B) (21H02608 to H. I. and 20H03395 to M. K.) from JSPS, and a JST SPRING, Grant Number JPMJSP2109 to S. I. and Grant Number JPMJSP2109 to Y. N.
The authors declare no conflict of interest.
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