First Total Synthesis of the Pavine Alkaloid (±)-Neocaryachine and Its Optical Resolution

Abstract

The first total synthesis of (±)-neocaryachine (1) was achieved using a radical cyclization to produce the dibenzo-9-azabicyclo[3.3.1]nonane pavine skeleton, following a Bischler–Napieralski reaction to construct an intermediate benzylisoquinoline. The resulting racemic mixture was separated by chiral column chromatography to provide pure (+)- and (−)-1.

Introduction

Pavine alkaloids are structurally based on dibenzo-9-azabicyclo[3.3.1]nonane, which contains two aromatic rings fused on opposite sides of an eight-membered ring bridged by a nitrogen atom. Various functional groups are found on the aromatic rings and the nitrogen atom. Pavine alkaloids have been reported in limited genera of only five plant families, including three genera Argemone, Eschscholzia and Roemeria in the Papaveraceae, two genera Beilschmiedia and Cryptocarya in the Lauraceae, two genera Berberis and Leontice in the Berberidaceae, one genus Thalictrum in the Ranunculaceae, and one genus Hernandia in the Hernandiaceae, and are biosynthesized through tetrahydrobenzylisoquinoline.^1–3) Several total syntheses have been accomplished^4–9) due to the interesting structures and biological activities of this compound class.^10–13) However, to date, all synthesized pavine alkaloids have no functional group at the C-7 position. Thus, neocaryachine, which has hydroxy at C-7, methoxy at C-8, methyl on nitrogen, and methylenedioxy at C-2/C-3, has not yet been synthesized. We describe here the first total synthesis of (±)-neocaryachine (1, Fig. 1) and its optical resolution by chiral column chromatography.

Fig. 1. Structure of the Pavine Alkaloid (−)-Neocaryachine (1)

Results and Discussion

The retrosynthetic analysis began with the disconnection of the C-6 and C-6a bond, which could be formed by radical cyclization between the enamine and bromobenzene of 2 (Fig. 2). Benzylisoquinoline 3, the precursor of 2, could be constructed by a Bischler–Napieralski reaction of 4, obtained by amidation of 5 with 6. The two latter compounds would be synthesized from aldehydes 7 and 8, respectively.

Fig. 2. Retrosynthetic Analysis of (±)-Neocaryachine (1)

According to a literature method,¹⁴⁾ benzaldehyde 7 was converted to amine 5¹⁵⁾ in three steps, conversion to a nitroalkene, followed by methoxylation and reduction, in 52.3% overall yield (Chart 1). The substituted phenylacetic acid 6¹⁶⁾ was obtained from commercially available 2-bromo-3-hydroxy-4-methoxybenzaldehyde 8 in five steps, including benzyl protection of the phenol, reduction of the aldehyde, and homologation through cyanidation, in 83.8% overall yield. The resulting amine 5 and carboxylic acid 6 were condensed using carbonyldiimidazole to provide amide 10 in 91% yield. A Bischler–Napieralski reaction of 10 with POCl₃ in CH₃CN formed benzylisoquinoline 11 together with the de-benzylated product 12 in 60 and 18% yield, respectively. In this reaction, a small amount of intermediate 13 (1–11%) remained even at reflux temperature or with a longer reaction time. The treatment of 11 with methyl iodide (MeI) provided quaternary methyl amine 14 in 77% yield. The reduction of 14 with NaBH₄ at room temperature (r.t.) generated the enamine 16 along with the unwanted amine 15 as the major product. To increase the proportion of 16, various reduction conditions were examined as shown in Table 1. The optimal conditions were reaction in MeOH at −78°C for 5 min to provide 16 in about 83% yield (16/15 ratio = 10 : 1). Purification was difficult due to the instability of enamine 16; consequently, the cyclization was performed directly on the crude product. The treatments of enamine 16 with Bu₃SnH in the presence of 2,2′-azobis(isobutyronitrile) (AIBN)⁶⁾ provided the target pavine alkaloid 17 (Table 2). All other conditions, including the use of (TMS)₃SiH and the reductive Heck reaction, were not effective. After several optimizations, such as the use of microwave (entries 1–4), the dropwise addition of a solution of AIBN and Bu₃SnH in toluene to a refluxing solution of 16 produced 17 in 20% yield (entry 5). The low yield was probably caused by the decomposition of enamine 16 before cyclization. Finally, removal of the benzyl group by catalytic reduction produced (±)-neocaryachine (1) in 72% yield. Alternatively, benzylisoquinoline 11 was converted quantitatively to carbamate 18, which is relatively stable compared with enamine 16. The radical cyclization of 18 under the above conditions produced pavine alkaloid 19 in a better 59% yield. However, the reduction of the carbamate to the methylamine of 17 using LiAlH₄ failed due to decomposition. The obtained (±)-1 was further separated with chiral column chromatography to obtain both enantiomers. The retentions time were 7.36 and 19.42 min for (−)-1 with [α]²⁷_D value of −251° (c 0.07, MeOH) and (+)-1 with [α]²⁷_D value of +263° (c 0.08, MeOH), respectively.

Chart 1. Total Synthesis of (±)-Neocaryachine (1)

Reagents and Conditions: a) MeNO₂, NH₄OAc, 100°C, 5 h, 76%; b) MeONa, MeOH, THF, room temperature (r.t.), 15 min, 86%; c) H₂, Pd/C, EtOH, r.t., 24 h, 80%; d) BnBr, K₂CO₃, DMF, 85°C, 3 h, 100%; e) NaBH₄, MeOH, r.t., 2 h, 100%; f) SOCl₂, DMF, CH₂Cl₂, 0°C, 3.5 h, 99%; g) KCN, DMF, H₂O, 100°C, 1 h, 91%; h) NaOH, 1,4-dioxane, MeOH, H₂O, 100°C, 23 h, 93%; i) CDI, CH₂Cl₂, r.t., 2 h, 91%; j) POCl₃, MeCN, 50°C, 18 h, 60%; k) MeI, 80°C, 1.5 h, 77%; l) NaBH₄, MeOH, −78°C, 5 min; m) Bu₃SnH, AIBN, toluene, reflux, 16.5 h, 20%; n) H₂, Pd/C, EtOH, r.t., 17 h, 72%; o) ClCO₂Me, Bu₃SnH, MeCN, 0°C to r.t., 14 h, 95%; p) Bu₃SnH, AIBN, toluene, reflux, 17 h, 59%.

Table 1. Reduction Conditions for Conversion of 14 to 15 and 16

Entry	NaBH4	Solvent	Temp.	Time*	Result (16 : 15)
1	1.0 eq	MeOH	r.t.	10 min	1 : 2
2	0.2 eq × 3	MeOH	0°C	15 min × 3	2 : 1
3	0.2 eq × 3	MeOH/DMF (10 : 1)	0°C	10 min × 1 and 15 min × 2	1 : 2
4	0.2 eq × 3	CH3CN	0°C	15 min × 3	0 : 1
5	0.2 eq × 4	MeOH	−20°C	10 min × 2 and 15 min × 2	1 : 1
6	1.1 eq	MeOH	−20°C	5 min	2 : 1
7	1.1 eq	MeOH	−78°C	5 min	10 : 1

*The reaction was monitored by TLC until starting material had almost disappeared. eq: equivalent.

Table 2. Optimizations for Radical Cyclization of 16^a)

Entry	AIBN	Bu3SnH	Temp.	Yields of 17 (%)
1b)	0.2 eq	1.0 eq	100°C	0
2b)	1.1 eq	2.3 eq	100°C	10
3b)	1.0 eq	2.1 eq	120°C	12
4b)	1.0 eq	1.2 eq	120°C	13
5c)	1.0 eq	2.1 eq	Reflux	20
6c,d)	1.0 eq	1.9 eq	Reflux	0

a) The reactions were performed in toluene. b) The reactions were carried out using microwave. c) A solution of AIBN and Bu₃SnH in toluene was added dropwise to a refluxing solution of 16. d) The reaction was performed in toluene/DMF (1 : 1, v/v) solution.

Neither enantiomer exhibited significant antiproliferative activity against several human tumor cell lines, including A549, MDA-MB-231, MCF-7, KB, and KB-VIN. While a prior report stated that (−)-1 showed potent cytotoxic activity,¹⁷⁾ careful examinations including ¹H-NMR finally revealed that the natural material isolated from Cryptocarya laevigata was contaminated with a trace amount of (−)-13aα-antofine, which exhibited an IC₅₀ of single digit nM level.

Experimental

General Experimental Procedures

All solvents were used as purchased. All NMR spectra were recorded on a JNM-ECS400 or JNM-ECA600 spectrometer with tetramethylsilane (TMS) as the internal standard. All chemical shifts are reported in ppm, apparent scalar coupling constants J in Hz. High-resolution (HR) mass spectroscopic data were obtained on a JMS-SX102A (FAB) or JMS-T100TD (DART) mass spectrometer. IR spectra were measured with a SHIMADZU FTIR-8700 instrument for samples in CHCl₃. Optical rotations were recorded on a JASCO P-2200 digital polarimeter. Analytical TLC was carried out on Merck precoated glass silica gel sheets (TLC silica gel 60 RP-18 F-254S).

N-[2-(Benzo[1,3]dioxol-5-yl)-2-methoxyethyl]-2-(3-benzyloxy-2-bromo-4-hydroxyphenyl)acetamide (10)

1,1′-Carbonyldiimidazole (CDI, 50.5 mg, 0.31 mmol) was added to a solution of 6 (109 mg, 0.310 mmol) in 3.0 mL of CH₂Cl₂, and the mixture was stirred at room temperature for 2 h. Then, compound 5 was added and the mixture was stirred for additional 12 h. The resulting solution was washed with saturated aqueous Na₂CO₃, dried over Na₂SO₄, and concentrated. The residue was purified by silica gel column chromatography (hexane/EtOAc = 1 : 1) to give 10 as a colorless oil (149.0 mg, 91%).

¹H-NMR (400 MHz, CDCl₃) δ 7.57–7.35 (m, 5H), 7.07 (d, J = 8.8 Hz, 1H), 6.89 (d, J = 8.8 Hz, 1H), 6.75–6.68 (m, 3H), 5.94 (m, 2H), 5.80 (br s, 1H), 5.03 (s, 2H), 4.11 (m, 1H), 3.89 (s, 3H), 3.68 (d, J = 4.4 Hz, 2H), 3.61 (m, 1H), 3.17 (s, 3H), 3.14 (m, 1H); ¹³C-NMR (400 MHz, CDCl₃) δ 169.9, 153.0, 147.9, 147.4, 145.7, 137.1, 132.9, 128.4, 128.3, 128.1, 127.7, 126.6, 121.1, 120.4, 111.6, 108.2, 106.6, 101.0, 81.9, 74.6, 56.7, 56.1, 45.6, 43.7; HRMS-FAB (m/z): [M + H]⁺ calcd for C₂₆H₂₇BrNO₆, 528.1022, 530.1001; found, 528.0954, 530.0984.

1-(3-Benzyloxy-2-bromo-4-methoxy)benzyl-6,7-methylenedioxyisoquinoline (11)

To a solution of 10 (54.7 mg, 0.10 mmol) in dry CH₃CN (2.0 mL) was added POCl₃ (0.10 mL, 1.07 mmol). The mixture was heated at 50°C under N₂ for 18 h. After cooling to room temperature, the mixture was placed in an ice bath and its pH was adjusted to 8 by addition of saturated aqueous NaHCO₃. After extraction with CH₂Cl₂ (× 3), the organic layer was dried over anhydrous Na₂SO₄ and concentrated. The residue was purified by silica gel column chromatography (hexane/EtOAc/Et₃N = 67 : 33 : 0.3) to give 11 as a pale orange solid (60%).

¹H-NMR (400 MHz, CDCl₃) δ 8.36 (d, J = 6.0 Hz, 1H), 7.57–7.60 (m, 2H), 7.43–7.34 (m, 5H), 7.08 (s, 1H), 6.69 (d, J = 8.8 Hz, 1H), 6.57 (d, J = 8.8 Hz, 1H), 6.06 (s, 2H), 5.06 (s, 2H), 4.60 (s, 2H), 3.80 (s, 3H); ¹³C-NMR (600 MHz, CDCl₃) δ 157.6, 152.1, 150.5, 148.4, 145.3, 141.5, 137.3, 134.9, 132.0, 128.5, 128.3, 128.0, 125.0, 124.5, 120.6, 119.5, 111.3, 103.1, 101.9, 101.6, 74.5, 56.1, 41.7; HRMS-FAB (m/z): [M + H]⁺ calcd for C₂₅H₂₁BrNO₄, 478.0654, 480.0633; found, 478.0627, 480.0618.

1-(3-Benzyloxy-2-bromo-4-methoxy)benzyl-6,7-methylenedioxyisoquinoline Methiodide (14)

A solution of 11 (58.4 mg, 0.12 mmol) in methyl iodide (1.5 mL) was refluxed at 80°C for 3.5 h. After cooling to room temperature, the solid was collected by filtration and washed with Et₂O to give 14 as a yellow solid (58.8 mg, 78%).

¹H-NMR (400 MHz, CDCl₃) δ 9.10 (d, J = 6.8 Hz, 1H), 8.13 (d, J = 6.8 Hz, 1H), 7.54–7.35 (m, 7H), 6.74 (d, J = 8.4 Hz, 1H), 6.29 (s, 2H), 6.22 (d, J = 8.0 Hz, 1H), 5.09 (s, 2H), 4.78 (s, 2H), 4.46 (s, 3H), 3.83 (s, 3H); (600 MHz, dimethyl sulfoxide (DMSO)-d₆) δ 8.62 (d, J = 7.2 Hz, 1H), 8.27 (d, J = 7.2 Hz, 1H), 7.90 (s, 1H), 7.75 (s, 1H), 7.54–7.35 (m, 5H), 6.91 (d, J = 8.4 Hz, 1H), 6.42 (s, 2H), 6.20 (d, J = 8.4 Hz, 1H), 5.03 (s, 2H), 4.88 (s, 2H), 4.16 (s, 3H), 3.80 (s, 3H); ¹³C-NMR (600 MHz, DMSO-d₆) δ 156.0, 155.1, 153.1, 152.6, 145.6, 138.5, 137.4, 137.3, 128.9, 128.8, 128.7, 127.2, 126.5, 124.2, 123.6, 120.2, 112.9, 104.7, 103.9, 103.3, 74.5, 56.7, 46.2, 35.3; HRMS-FAB (m/z): [M]⁺ calcd for C₂₆H₂₃BrNO₄, 492.0810, 494.0790; found, 492.0802, 494.0795.

1-(3-Benzyloxy-2-bromo-4-methoxy)benzyl-2-methyl-6,7-methylenedioxy-1,2-dihydroxyisoquinoline (16)

To a solution of 14 (40.1 mg, 0.06 mmol) in dry MeOH (10.0 mL) was added NaBH₄ (2.73 mg, 0.07 mmol). After the mixture was stirred at −78°C under N₂ for 5 min, it was diluted with water and extracted with Et₂O. The organic layer was dried over Na₂SO₄ and concentrated to give 16 as an air-sensitive oil, which was used in the next step without further purification.

¹H-NMR (400 MHz, CDCl₃) δ 7.57–7.33 (m, 5H), 6.70 (d, J = 8.8 Hz, 1H), 6.54 (d, J = 8.8 Hz, 1H), 6.46 (s, 1H), 6.04 (dd, J = 7.2, 1.6 Hz, 1H), 5.95 (s, 1H), 5.82 (dd, J = 1.6, 8.0 Hz, 2H), 5.28 (d, J = 7.2 Hz, 1H), 5.04 (s, 2H), 4.45 (t, J = 6.8 Hz, 1H), 3.85 (s, 3H), 3.11 (dd, J = 12.8, 6.8 Hz, 1H), 2.85 (dd, J = 12.8, 6.8 Hz, 1H), 2.79 (s, 3H).

(±)-7-Benzyloxyneocaryachine (17)

A solution of 16 (35.0 mg, 0.07 mmol) in dry toluene (50.0 mL) was refluxed in an Ar atmosphere at 130°C in an oil bath. A solution of AIBN (11.6 mg, 0.07 mmol) and Bu₃SnH (40.0 µL, 0.149 mmol) in dry toluene (5.0 mL) was added dropwise to the mixture, which was refluxed for 16.5 h. After the solvent was evaporated, the residue was purified by silica gel column chromatography (hexane/EtOAc/Et₃N = 25 : 75 : 0.3) to give 17 as a colorless solid (approx. 5.0 mg, approx. 20%).

¹H-NMR (400 MHz, CDCl₃) δ 7.52–7.33 (m, 5H), 6.76 (d, J = 8.8 Hz, 1H), 6.71 (d, J = 8.8 Hz, 1H), 6.55 (s, 1H), 6.38 (s, 1H), 5.84 (s, 1H), 5.80 (s, 1H), 5.22 (d, J = 11.2 Hz, 1H), 5.01 (d, J = 11.2 Hz, 1H), 4.16 (d, J = 5.6 Hz, 1H), 3.95 (d, J = 5.6 Hz, 1H), 3.84 (s, 3H), 3.38–3.21 (m, 2H), 2.67 (d, J = 16.4 Hz, 1H), 2.57 (d, J = 16.4 Hz, 1H), 2.39 (s, 3H); ¹³C-NMR (600 MHz, CDCl₃) δ 150.5, 146.2, 145.8, 144.3, 138.1, 132.0, 130.8, 128.5, 128.0, 127.9, 125.9, 125.0, 124.3, 111.2, 108.7, 106.9, 100.5, 74.3, 56.5, 55.8, 52.1, 40.7, 32.8, 32.6, 29.8; HRMS-FAB (m/z): [M + H]⁺ calcd for C₂₆H₂₆NO₄, 416.1862; found, 416.1818.

(±)-Neocaryachine (1)

To a solution of 17 (8.90 mg, 0.02 mmol) in EtOH (2.0 mL) was added 5% Pd/C (13.8 mg). After being stirred at room temperature for 8 h with H₂, the reaction mixture was heated at 50°C and stirred for 33 h. The resulting mixture was filtered through Celite and concentrated. The residue was purified by silica gel column chromatography (CH₂Cl₂/MeOH/Et₃N = 100 : 2 : 0.3) to give (±)-1 as a colorless solid (5.0 mg, 72%).

¹H-NMR (400 MHz, CDCl₃) δ 6.66 (d, J = 8.2 Hz, 1H), 6.58 (s, 1H), 6.50 (d, J = 8.2 Hz, 1H), 6.43 (s, 1H), 5.85 (d, J = 1.4 Hz, 1H), 5.80 (d, J = 1.4 Hz, 1H), 5.69 (s, 1H), 4.34 (d, J = 6.0 Hz, 1H), 3.98 (d, J = 6.0 Hz, 1H), 3.83 (s, 3H), 3.37 (dd, J = 6.0, 16.5 Hz, 1H), 3.32 (dd, J = 6.0, 16.5 Hz, 1H), 2.72 (d, J = 16.5 Hz, 1H), 2.59 (d, J = 16.0 Hz, 1H), 2.53 (s, 3H).

Acknowledgments

We appreciate valuable comments, suggestions, and editing on the manuscript by Dr. Susan L. Morris-Natschke (UNC-CH). This study was supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT KAKENHI, Japan) awarded to K.N.G. (Grant Number 25293024).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

The online version of this article contains supplementary materials. NMR and IR spectra of compound 1, and NMR data of compounds 2–11 are available as supplementary materials.

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