First total synthesis of (−)- and (+)-6-O-desmethylantofine

Abstract

The first total synthesis of (−)-6-O-desmethylantofine (A) and its unnatural enantiomer (+)-6-O-desmethylantofine (B) is described. The synthetic route is efficient and practical with easily available glutamic acid dimethyl ester hydrochloride as the chiral material under mild conditions.

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Introduction

In 1965, Haznagy isolated a compound from C. vincetoxicum and named it C-1.¹ In 1969, Wiegrebe also isolated C-1 from C. vincetoxicum and confirmed its configuration as (−)-6-O-desmethylantofine (A)² (Fig. 1).

Fig. 1 Chemical structures of compounds A, B and C.

(−)-Antofine (C) (Fig. 1) is known for its extremely potent inhibition of cancer cell growth. Antofine has IC₅₀ values in the low nanomolar range, comparable to that of clinically employed cytotoxic drugs.³ As an analogue of (−)-antofine (C), (−)-6-O-desmethylantofine (A) has attracted more and more attention in recent years. There were many reports about in vitro cytotoxic activity of (−)-6-O-desmethylantofine (A) against the KB cancer cell lines,^3a,4 with IC₅₀ values lower than that of (−)-antofine (C). Cytotoxicity significantly increased for A relative to C, indicating that the hydroxy group at C6 is of importance.

We have previously reported that (−)-antofine (C), isolated from Cynanchum komarovii, possessed excellent antiviral activity against tobacco mosaic virus (TMV).⁵ (−)-6-O-Desmethylantofine (A) was also isolated from the same plant for the first time and was found to exhibit better anti-TMV activity than (−)-antofine (C).⁶

Up to now, (−)-6-O-desmethylantofine (A) can only be obtained by isolation. However, its natural abundance is very low (about 0.003% of the extraction of the plant mentioned above),⁶ which restricts our research on the antiviral agent. Thus, an efficient and general approach to prepare 6-O-desmethylantofine would be very desirable. Due to the hydroxyl group at C6, the synthesis of 6-O-desmethylantofine is difficult. Only a few syntheses of (±)-6-O-desmethylantofine have been reported.^4b,7 However, optically pure (−)-6-O-desmethylantofine (A) or its unnatural enantiomer (+)-6-O-desmethylantofine (B) has not been reported. Herein, we report the first total synthesis of (−)-6-O-desmethylantofine (A) and its enantiomer B.

Results and discussion

Our synthetic route is shown in Scheme 1. Perkin condensation of 3,4-dimethoxybenzaldehyde (1) and 4-hydroxyphenylacetic acid (2) could easily afford (E)-2-(4-acetoxyphenyl)-3-(3,4-dimethoxyphenyl)acrylic acid (3), which was converted to its corresponding ester 4 with methanol in the presence of concentrated sulfuric acid. Remarkably, acetyl deprotection and esterification of 3 was accomplished in one pot to afford 4.

Scheme 1 Synthesis of (−)-6-O-desmethylantofine (A) and (+)-6-O-desmethylantofine (B). Reagents and conditions: (a) Ac₂O/Et₃N; (b) CH₃OH/concd. H₂SO₄; (c) FeCl₃/CH₂Cl₂; (d) BnBr/K₂CO₃/CH₃COCH₃; (e) LiAlH₄/THF; (f) PBr₃/CH₂Cl₂; (g) Ra or Rb/K₂CO₃/DMF; (h) AcOH/MeOH; (i) KOH/MeOH/dioxane; (j) (COCl)₂/CH₂Cl₂, SnCl₄/CH₂Cl₂ (k) NaBH₄/EtOH, Et₃SiH/CF₃COOH/CH₂Cl₂ (l) LiAlH₄/THF.

To the best of our knowledge, intramolecular oxidative coupling using oxidative coupling reagents such as thallium(III) trifluoroacetate (TTFA),⁸ lead(IV) tetraacetate (Pb(OAc)₄),⁹ vanadium oxytrifluoride (VOF₃),¹⁰ iron(III) chloride (FeCl₃)¹¹ and manganese(IV) dioxides (MnO₂)¹² is the most convenient way to construct phenanthrene ring system. To test the coupling of ester 4, we tried many oxidative coupling reagents such as VOF₃, FeCl₃ and MnO₂. The reaction using FeCl₃ gave the best yield (58%), whereas no oxidative coupling product 5 was found when using VOF₃ or MnO₂. The coupling of ester 4 is much more complicated than that of polymethoxy substituted 2,3-diphenylacrylate.^11b,12 Then we protected the hydroxyl using a series of protecting groups such as acetyl, benzyl and methoxymethyl before coupling, but only to make the coupling reaction more complex.

Ester 5 was converted to its corresponding alcohol using lithium aluminium hydride, but the solubility of the corresponding alcohol was so poor that it was difficult to be purified, so it was impossible to be used in the following route. To improve the solubility in the following route, it is necessary to protect the hydroxy group. We introduced a benzyl group to compound 5 using benzyl bromide to afford 6.

Ester 6 was easily converted to corresponding alcohol 7 by lithium aluminium hydride. 7 was treated with phosphorus tribromide and then reacted with D-glutamic acid dimethyl ester hydrochloride followed by cyclization in warm methanol and acetic acid to afford lactam ester 8a successfully. The yield of the combined three steps from 7 to 8a is high (60%). Ester 8a was easily converted to the corresponding acid 9a with potassium hydroxide.

The benzyl derivative was stable in strong base (KOH) and strong acid (HCl) from 8a to 9a. But interestingly, using SnCl₄ as Lewis acid, deprotection of benzyl group and intramolecular Friedel–Crafts acylation occurred in one pot to afford 10a from 9a.

10a was treated with sodium borohydride and then immediately converted to methylene using triethylsilane and trifluoroacetic acid to give 11a. At last, 11a was reduced with lithium aluminium hydride to afford (−)-6-O-desmethylantofine (A). The overall yield is 9.68%. By the same procedure, we got (+)-6-O-desmethylantofine (B) using L-glutamic acid dimethyl ester hydrochloride in an overall yield of 9.32%. The enantiomeric excesses of A and B were determined. The ee values were 90% and 91% respectively.

Conclusion

Optically pure (−)-6-O-desmethylantofine (A) and its enantiomer (+)-6-O-desmethylantofine (B) were first synthesized efficiently from cheap materials under mild conditions and their ee values is up to 90%. Intramolecular Friedel–Crafts acylation and deprotection of benzyl group of compounds 9a and 9b was successfully achieved in one pot using SnCl₄. The chemistry described here provided a practical synthetic method of optically pure phenanthroindolizidine alkaloids for biochemical and pharmaceutical studies.

Experimental

The melting points were determined with an X-4 binocular microscope melting-point apparatus (Beijing Tech Instruments Co., Beijing, China) and were uncorrected. ¹H NMR spectra were obtained by using Bruker AV 400 spectrometer. Chemical shifts (δ) were given in parts per million (ppm) and were measured downfield from internal tetramethylsilane. ¹³C NMR spectra were recorded by using Bruker AV 400 (100 MHz) and CDCl₃ or DMSO-d₆ as a solvent. Chemical shifts (δ) are reported in parts per million using the solvent peak. High-resolution mass spectra were obtained with an FT-ICR MS spectrometer (Ionspec, 7.0 T). The enantiomeric excesses of A and B were determined by HPLC with a Chiralcel AD-H column using Agilent 1100 instrument. Optical rotations were recorded with a Perkin–Elmer 341 MC polarimeter. All anhydrous solvents were dried and purified by standard techniques just before use.

(E)-2-(4-Acetoxyphenyl)-3-(3,4-dimethoxyphenyl)acrylic acid (3). A mixture of 4-hydroxyphenylacetic acid 2 (66.8 g, 0.44 mol), 3,4-dimethoxybenzaldehyde 1 (83.0 g, 0.44 mol), acetic anhydride (200 mL), and triethylamine (66.7 g, 0.66 mol) was heated at reflux for 9 h. The solution was cooled to room temperature, water (250 mL) was added, and the mixture was heated at reflux for 1 h. After being cooled to room temperature, the mixture was stirred in cold water for another 1 h. The precipitate was filtrated, washed with anhydrous ethanol to give (E)-2-(4-Acetoxyphenyl)-3-(3,4-dimethoxyphenyl)acrylic acid 3 117.4 g (78%). mp 244–247 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 12.59 (br, 1 H, COOH), 7.72 (s, 1 H, C=CH), 7.23 (d, J = 7.6 Hz, 2 H, Ar–H), 7.17 (d, J = 7.6 Hz, 2 H, Ar–H), 6.87 (m, 2 H, Ar–H), 6.41 (s, 1 H, Ar–H), 3.72 (s, 3 H, OMe), 3.30 (s, 3 H, OMe), 2.22 (s, 3 H, COCH₃); ¹³C NMR (100 MHz, DMSO) δ 169.2, 168.3, 149.8, 149.7, 147.8, 139.5, 134.3, 130.7, 129.6, 126.6, 125.2, 122.1, 111.9, 111.1, 55.3, 54.5, 20.8. HRMS (ESI): calcd. for C₁₉H₁₈O₆ [M − H]⁺ 341.1031; found 341.1031.

(E)-3-(3,4-Dimethoxyphenyl)-2-(4-hydroxyphenyl)acrylic acid methyl ester (4). Compound 3 (40.4 g, 118.1 mmol) was added to dry methanol (600 mL). Concentrated sulfuric acid (24.1 g) was added, and the solution was stirred at reflux for 10 h. Most of the methanol was removed by rotary evaporation, and the residue was redissolved in dichloromethane and washed with saturated sodium hydrogen carbonate. The organic phase was dried over sodium sulfate, filtered, and evaporated to give 36.0 g (97%) of 4. mp 127–128 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.76 (s, 1 H, C=CH), 7.10 (d, J = 8.4 Hz, 2 H, Ar–H), 6.83–6.85 (m, 3 H, Ar–H), 6.73 (d, J = 8.4 Hz, 1 H, Ar–H), 6.48 (d, J = 1.6 Hz, 1 H, Ar–H), 3.84 (s, 3 H, OMe), 3.79 (s, 3 H, OMe), 3.46 (s, 3 H, OMe); ¹³C NMR (100 MHz, CDCl₃) δ 169.2, 155.6, 149.9, 148.0, 140.6, 131.2, 129.5, 128.0, 127.5, 125.5, 115.9, 112.4, 110.4, 55.7, 55.1, 52.4. HRMS (ESI): calcd. for C₁₈H₁₈O₅ [M + Na]⁺ 337.1046; found 337.1041

6-Hydroxy-2,3-dimethoxyphenanthrene-9-carboxylic acid methyl ester (5). To a solution of 4 (10.10 g, 32.17 mmol) in dichloromethane (250 mL) was added anhydrous Iron(III) chloride (13.07 g, 80.41 mmol). The reaction solution was stirred at room temperature for 4.5 h, and then quenched with methanol (50 mL). Then H₂O (200 mL) was added, and the mixture was stirred for an additional 0.5 h. The water phase was extracted with dichloromethane, the combined organic phase was dried over sodium sulfate and filtered. The solvent was removed by rotary evaporation, and the crude product was purified by column chromatography (petroleum ether/EtOAc, 3 : 1, v/v) to give 5.82 g (58%) of 5 as a white solid. mp 188–190 °C; ¹H NMR (400 MHz, DMSO-d6): δ 9.91 (s, 1 H, Ar–OH), 8.66 (d, J = 9.2 Hz, 1 H, Ar–H), 8.26 (s, 1 H, Ar–H), 8.03 (s, 1 H, Ar–H), 7.94 (s, 1 H, Ar–H), 7.59 (s, 1 H, Ar–H), 7.18 (d, J = 8.8 Hz, 1 H, Ar–H), 4.04 (s, 3 H, OMe), 3.93 (s, 3 H, OMe), 3.92 (s, 3 H, OMe); ¹³C NMR (100 MHz, CDCl₃) δ 168.3, 154.1, 150.9, 149.7, 131.9, 129.6, 128.6, 126.5, 125.5, 123.7, 123.2, 116.6, 109.1, 106.4, 103.1, 55.9, 52.1. HRMS (ESI): calcd. for C₁₈H₁₆O₅ [M + Na]⁺ 335.0890; found 335.0889

6-Benzyloxy-2,3-dimethoxyphenanthrene-9-carboxylic acid methyl ester (6). To a solution of 5 (15.24 g, 48.85 mmol) in acetone (300 mL) was added benzyl bromide (10.02 g, 58.62 mmol), and potassium carbonate (10.11 g, 73.28 mmol). The reaction mixture was stirred for 10 h at reflux, then filtered. The filtrate was concentrated. The residue was purified by column chromatography (petroleum ether/EtOAc/CH₂Cl₂, 60 : 10 : 1, v/v/v) to give 17.28 g (88%) of 6 as a white solid. mp 156–157 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.94 (d, J = 9.2 Hz, 1 H, Ar–H), 8.31 (s, 1H, Ar–H), 7.97 (d, J = 2.8 Hz, 1 H, Ar–H), 7.78 (s, 1 H, Ar–H), 7.54 (d, J = 7.2 Hz, 2 H, Ar–H), 740–7.44 (m, 2 H, Ar–H), 7.34–7.37 (m, 2 H, Ar–H), 7.26 (s, 1 H, Ar–H), 5.30 (s, 2 H, benzyl-CH₂), 4.10 (s, 3 H, OMe), 4.03 (s, 3 H, OMe), 4.01 (s, 3 H, OMe); ¹³C NMR (100 MHz, CDCl₃) δ 168.1, 157.2, 150.9, 149.6, 136.9, 131.6, 129.6, 128.7, 128.4, 128.1, 127.6, 126.7, 125.5, 123.7, 123.3, 116.2, 109.2, 106.0, 103.1, 70.4, 56.0, 55.9, 52.1. HRMS (ESI): calcd. for C₂₅H₂₂O₅ [M + Na]⁺ 425.1359; found 425.1352

(6-Benzyloxy-2,3-dimethoxyphenanthren-9-yl)methanol (7). To a mixture of lithium aluminium hydride (2.84 g, 74.78 mmol) in tetrahydrofuran (150 mL) at 0 °C was added dropwise a solution of 6 (10.02 g, 24.93 mmol) in tetrahydrofuran (150 mL). The solution was stirred at room temperature for 20 min, then heated at reflux for another 1.5 h. Then the solution was brought back to 0 °C, dichloromethane (200 mL) was added, followed by 2 N hydrochloric acid (70 mL). The solution was separated, the water phase was extracted with dichloromethane, the combined organic phase was dried over sodium sulfate and filtered. The solvent was removed by evaporation to give 9.12 g (97.9%) of 7 as a white solid. mp 184–186 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.10 (d, J = 8.8 Hz, 1 H, Ar–H), 7.98 (d, J = 2.8 Hz, 1 H, Ar–H), 7.78 (s, 1 H, Ar–H), 7.53–7.55 (m, 3 H, Ar–H), 7.40–7.44 (m, 2 H, Ar–H), 7.30–7.37 (m, 2 H, Ar–H), 7.19 (s, 1 H, Ar–H), 5.29 (s, 2 H, benzyl-CH₂), 5.13 (d, J = 6.0 Hz, 2 H, CH₂), 4.08 (s, 3 H, OMe), 4.023 (s, 3 H, OMe), 1.75 (t, J = 6.0 Hz, 1 H, OH); ¹³C NMR (100 MHz, DMSO-d₆) δ 156.6, 149.3, 148.7, 137.2, 133.8, 131.0, 128.4, 127.9, 127.8, 126.7, 125.8, 123.8, 123.4, 121.8, 115.7, 108.5, 106.0, 104.0, 69.5, 61.6, 55.8, 55.4. HRMS (ESI): calcd. for C₂₄H₂₂O₄ [M + Na]⁺ 397.1410; found 397.1402

(R)-1-((6-Benzyloxy-2,3-dimethoxyphenanthren-9-yl)methyl)-5-oxo-pyrrolidine-2-carboxylic acid methyl ester (8a). Compound 7 (9.30 g, 24.87 mmol) was dissolved in dry dichloromethane (500 mL) and cooled to 0 °C. A solution of phosphorus tribromide (3.5 mL, 37 mmol) in dichloromethane (20 mL) was added dropwise. The solution was then stirred at room temperature overnight, then poured over ice water, the water phase was extracted with dichloromethane (40 mL × 2). The combined organic phase was dried over sodium sulfate, filtered and evaporated to give a white solid. The solid was then dissolved in N,N-dimethylformamide (300 mL). D-Glutamic acid dimethyl ester hydrochloride (7.89 g, 37.31 mmol) was added and allowed to stir for 20 min. Potassium carbonate (5.15 g, 37.31 mmol) was added, and the mixture was allowed to stir at room temperature for 12 h. The solution was then distilled under reduced pressure to remove N,N-dimethylformamide. To the residue was added dichloromethane and water. The water phase was extracted with dichloromethane. The combined organic phase was dried over sodium sulfate, filtered and rotary evaporated. The crude product was dissolved in methanol (200 mL) and acetic acid glacial (120 mL) and stirred for 8 h at 45–50 °C. The solution was rotary evaporated, and the crude product was purified by column chromatography (petroleum ether/EtOAc, 2 : 1 then 1 : 1, v/v) to give 7.49 g (60%) of 8a as a white solid. mp 182–183 °C; [α]²⁰_D −56.0 (c 1.0, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 8.03 (d, J = 8.8 Hz, 1 H, Ar–H), 7.97 (d, J = 2.4 Hz, 1 H, Ar–H), 7.78 (s, 1 H, Ar–H), 7.52–7.54 (m, 2 H, Ar–H), 7.40–7.44 (m, 2 H,Ar–H), 7.34–7.37 (m, 2 H, Ar–H), 7.31 (dd, J = 2.4, 8.8 Hz, 1H, Ar–H), 7.16 (s, 1 H, Ar–H), 5.50 (d, J = 14.4 Hz, 1 H, N–CH₂), 5.28 (s, 2 H, benzyl-CH₂), 4.40 (d, J = 14.4 Hz, 1 H, N–CH₂), 4.09 (s, 3 H, OMe), 4.03 (s, 3 H, OMe), 3.84 (dd, J = 2.8, 9.2 Hz, 1 H, N–CH), 3.58 (s, 3 H, OMe), 2.55–2.64 (m, 1 H, N–CH–CH₂), 2.04–2.48 (m, 1 H, NCO–CH₂), 2.08–2.18 (m, 1 H, N–CH–CH₂), 1.95–2.02 (m, 1 H, NCO–CH₂) ¹³C NMR (100 MHz, CDCl₃) δ 174.6, 172.3, 157.4, 149.6, 149.4, 137.0, 131.7, 128.7, 128.1, 127.7, 127.6, 126.6, 126.2, 126.2, 124.6, 124.4, 115.8, 108.2, 106.4, 103.3, 70.4, 58.6, 56.0, 55.9, 52.2, 44.4, 29.8, 22.8. HRMS (ESI): calcd. for C₃₀H₂₉NO₆ [M + Na]⁺ 522.1887; found 522.1883

(R)-1-((6-Benzyloxy-2,3-dimethoxyphenanthren-9-yl)methyl)-5-oxo-pyrrolidine-2-carboxylic acid (9a). Compound 8a (2.18 g, 4.37 mmol) was stirred in a solution of dioxane (50 mL), methanol (40 mL) and 2 mol L⁻¹ aqueous potassium hydroxide (30 mL) for 2 h. The mixture was rotary evaporated to remove dioxane and methanol, and the resulting mixture was cooled to 0 °C and acidified with 2 mol L⁻¹ hydrochloric acid until pH = 1–2, then washed with water and dried to give 2.08 g (98%) of 9a as a white solid. mp 276–278 °C; [α]²⁰_D −67.6 (c 0.5, DMF); ¹H NMR (400 MHz, DMSO-d₆): δ 8.21 (d, J = 2.4 Hz, 1 H, Ar–H), 8.02 (s, 1 H, Ar–H), 7.96 (d, J = 9.2 Hz, 1 H, Ar–H), 7.57–7.59 (m, 2 H, Ar–H), 7.40–7.45 (m, 4 H, Ar–H), 7.33–7.37 (m, 1 H, Ar–H), 7.30 (dd, J = 2.4, 9.2 Hz, 1H, Ar–H), 5.38 (d, J = 14.8 Hz, 1 H, N–CH₂), 5.37 (s, 2 H, benzyl-CH₂), 4.23 (d, J = 15.2 Hz, 1 H, N–CH₂), 4.02 (s, 3 H, OMe), 3.91 (s, 3 H,OMe), 3.75 (dd, J = 2.8, 9.2 Hz, 1 H, N–CH), 2.33–2.43 (m, 2 H, N–CH–CH₂, NCO–CH₂), 2.13–2.23 (m, 1 H, NCO–CH₂), 1.90–1.96 (m, 1 H, N–CH–CH₂); ¹³C NMR (100 MHz, DMSO-d₆) δ 174.1, 173.1, 156.9, 149.4, 149.2, 137.1, 131.3, 128.4, 128.0, 127.9, 127.5, 126.3, 125.5, 124.8, 123.9, 123.7, 116.0, 108.5, 106.2, 104.0, 69.5, 58.0, 55.8, 55.4, 43.3, 29.2, 22.4. HRMS (ESI): calcd. for C₂₉H₂₇NO₆ [M − H]⁺ 484.1766; found 484.1765

(R)-6-Hydroxy-2,3-dimethoxyphenanthro[9,10-b]-11,14-indolizidinedione(10a). To a solution of 9a (1.44 g, 2.97 mmol) in dry dichloromethane (100 mL) was added oxalyl chloride (0.49 g, 3.86 mmol) and N,N-dimethylformamide (0.1 ml). The mixture was stirred for 4.5 h at room temperature. Tin(IV) chloride (0.69 mL, 5.94 mmol) was added slowly under nitrogen at 0 °C, and then the mixture was stirred overnight at 35 °C under nitrogen. The solution was cooled to room temperature, and 2 mol L⁻¹ hydrochloric acid (70 mL) was added. The organic phase was rotary evaporated, and the crude product was purified by column chromatography (CH₂Cl₂–EtOAc, 3 : 1, v/v) to give 0.62 g (55%) of 10a as a light-yellow solid. mp 260–261 °C; [α]²⁰_D −94.4 (c 0.5, DMF); ¹H NMR (400 MHz, DMSO-d₆): δ 10.41(s, 1 H, Ar–OH), 8.99 (s, 1 H, Ar–H), 8.14 (d, J = 8.8 Hz, 1 H, Ar–H), 8.02 (d, J = 2.0 Hz, 1 H, Ar–H), 7.95 (s, 1 H, Ar–H), 7.23 (dd, J = 2.0, 8.8 Hz, 1 H, Ar–H), 5.55 (d, J = 18.4 Hz, 1 H, 9-H), 4.78 (d, J = 18.0 Hz, 1 H, 9-H), 4.54–4.57 (m, 1 H,13a-H), 4.02 (s, 3 H, OMe), 3.91 (s, 3 H, OMe), 2.30–2.44 (m, 4 H, 12-H, 13-H); ¹³C NMR (100 MHz, DMSO-d₆) δ 196.1, 172.8, 159.2, 149.6, 148.4, 139.6, 133.9, 127.3, 123.7, 123.3, 120.3, 119.8, 117.5, 107.4, 106.7, 103.5, 60.5, 55.3, 55.1, 39.9, 29.4, 20.4. HRMS (ESI): calcd. for C₂₂H₁₉NO₅ [M + Na]⁺ 400.1155; found 400.1152

(R)-6-Hydroxy-2,3-dimethoxyphenanthro[9,10-b]-11-quinolizidinone(11a). A mixture of 10a (0.41 g, 1.09 mmol), sodium borohydride (0.12 g, 3.26 mmol) and ethanol (50 mL) was stirred for 40 min at room temperature. The mixture was cooled to 0 °C, and then 2 mol L⁻¹ hydrochloric acid (10 mL) was added slowly. Dichloromethane (100 mL) and water (50 mL) was added to the solution, then the water phase was extracted with dichloromethane (30 mL). The combined organic phase was dried over sodium sulfate, filtered, and rotary evaporated, and the product was dissolved in dry dichloromethane (50 mL), and then triethylsilane (0.63 g, 5.44 mmol) and trifluoroacetic acid (1.24 g, 20.88 mmol) was added. The mixture was stirred for 3.5 h at 33–36 °C. The solution was cooled to room temperature, and dichloromethane (50 mL) and water (50 mL) was added. The water phase was extracted with dichloromethane (20 mL × 3). The combined organic phase was rotary evaporated, and the crude product was purified by column chromatography (petroleum ether/EtOAc, 1 : 3, v/v and then CH₂Cl₂–EtOAc, 1 : 8, v/v) to give 0.34 g (85%) of 11a as a light-yellow solid. mp 221 °C dec; [α]²⁰_D −85.6 (c 1, DMF); ¹H NMR (400 MHz, CDCl₃): δ 7.76 (s, 1 H, Ar–H), 7.74 (s, 1 H, Ar–H), 7.55 (d, J = 8.8 Hz, 1 H, Ar–H), 7.10 (d, J = 8.4 Hz, 1 H, Ar–H), 7.04 (s, 1 H, Ar–H), 6.94 (s, 1 H, OH), 5.30 (d, J = 17.2 Hz, 1 H, 9-H), 4.47 (d, J = 17.2 Hz, 1 H, 9-H), 4.06 (s, 3 H, OMe), 4.01 (s, 3 H,OMe), 3.93–3.94 (m, 1H, 13a-H), 3.30–3.34 (m, 1H, 14-H), 2.53–2.78 (m, 4H, 14-H, 12-H, 13-H), 2.00–2.09 (m, 1H, 13-H); ¹³C NMR (100 MHz, DMSO-d₆) δ 173.1, 155.6, 149.0, 148.2, 130.0, 125.7, 124.0, 123.4, 123.2, 122.8, 121.6, 116.5, 106.5, 104.1, 103.9, 55.4, 55.3, 52.3, 40.1, 32.1, 29.4, 24.4. HRMS (ESI): calcd. for C₂₂H₂₁NO₄ [M + H]⁺ 364.1543; found 364.1540

(−)-6-O-desmethylantofine (A). A mixture of 11a (0.18 g, 0.50 mmol), lithium aluminium hydride (0.04 g, 1.00 mmol) and tetrahydrofuran (25 mL) was heated at reflux under nitrogen for 0.5 h. The solution was cooled to 0 °C, at which point EtOAc (10 mL) was added dropwise, followed by water (20 ml). Then dichloromethane (100 mL) and methanol (50 mL) was added, then the water phase was extracted with dichloromethane (20 mL × 3). The combined organic phase was dried over sodium sulfate, filtered and evaporated, then the product was placed under vacuum in the dark for 2 h to give 0.16 g (94%) of A as a white solid. mp: 217–218 °C (lit.² 226–228 °C); [α]²⁰_D −51.2 (c 0.25, MeOH) (lit.⁴ [α]²⁰_D −54.5 (c 0.88, MeOH)); 90% ee [flow rate 1.0 mL min⁻¹, n-hexane/2-propanol 85 : 15 and 0.1% triethylamine, 254 nm, t_R (major) = 22.75 min, t_R (minor) = 25.14 min]. ¹H NMR (400 MHz, DMSO-d6): δ 9.69 (s,1 H, OH), 7.95 (s, 1 H, Ar–H), 7.92 (s, 1 H, Ar–H), 7.73 (d, J = 8.8 Hz, 1 H, Ar–H), 7.31 (s, 1 H, Ar–H), 7.09 (d, J = 9.2 Hz, 1 H, Ar–H), 4.55 (d, J = 15.2 Hz, 1 H, 9-H), 3.99 (s, 3 H, OMe), 3.94 (s, 3 H, OMe), 3.53 (d, J = 14.8 Hz, 1 H, 9-H), 3.30–3.35 (m, 2 H, 13a-H, 11-H), 2.72–2.78 (m, 1 H, 11-H), 2.32–2.38 (m, 2 H, 14-H), 2.12–2.18 (m, 1 H, 13-H), 1.82–1.90 (m, 2 H, 12-H), 1.59–1.69 (m, 1 H, 13-H); ¹³C NMR (100 MHz, DMSO-d₆) δ 155.3, 149.0, 148.0, 129.9, 126.3, 126.1, 124.4, 124.0, 122.6, 122.2, 116.2, 106.4, 104.0, 103.9, 59.8, 55.4, 55.3, 54.4, 53.2, 32.9, 30.8, 21.1. HRMS (ESI): calcd. for C₂₂H₂₃NO₃ [M + H]⁺ 350.1751; found 350.1753

(S)-1-((6-Benzyloxy-2,3-dimethoxyphenanthren-9-yl)methyl)-5-oxo-pyrrolidine-2-carboxylic acid methyl ester (8b). With the L-Glutamic acid dimethyl ester hydrochloride as a material, a procedure analogous to the preparation of 8a was used, 7 (8.52 g, 22.78 mmol) give 8b (6.70 g, 59%) as a white solid: mp 182–183 °C; [α]²⁰_D 52.4 (c 1, CH₂Cl₂); other data are the same as those for 8.

(S)-1-((6-Benzyloxy-2,3-dimethoxyphenanthren-9-yl)methyl)-5-oxo-pyrrolidine-2-carboxylic acid (9b). A procedure analogous to the preparation of 9a was used, 8b (3.06 g, 6.12 mmol) gave 9b (2.91 g, 98%) as a white solid: mp 277–279 °C; [α]²⁰_D 40.8 (c 0.5, DMF); other data are the same as those for 9.

(S)-6-Hydroxy-2,3-dimethoxyphenanthro[9,10-b]-11,14-indolizidinedione(10b). A procedure analogous to the preparation of 10a was used, 9b (2.00 g, 4.12 mmol) gave 10b (0.80 g, 52%) as a light-yellow solid. mp 253–255 °C; [α]²⁰_D 63.4 (c 1, DMF); other data are the same as those for 10a.

(S)-6-Hydroxy-2,3-dimethoxyphenanthro[9,10-b]-11-quinolizidinone(11b). A procedure analogous to the preparation of 11a was used, 10b (0.52 g, 1.38 mmol) gave 11b (0.44 g, 88%) as a light-yellow solid. mp 230 °C dec; [α]²⁰_D 56.2 (c 1,DMF); other data are the same as those for 11a.

(+)-6-O-desmethylantofine (B). A procedure analogous to the preparation of A was used, 11a (0.20 g, 0.55 mmol) gave 12a (0.18 g, 95%) as a white solid. mp: 219–220 °C; [α]²⁰_D 66.4 (c 0.25, Methanol); 91% ee [flow rate 1.0 mL min⁻¹, n-hexane/2-propanol 85 : 15 and 0.1% triethylamine, 254 nm, t_R (major) = 25.02 min, t_R (minor) = 22.80 min]. other data are the same as those for B.

Acknowledgements

We are grateful to the National Key Project for Basic Research (2010CB126100) and the National Natural Science Foundation of China (20872072, 30890143) for generous financial support for our programs

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C NMR and HRMS spectra for the compounds 1–7, 8a–11a, A. (To avoid repetition, the NMR and HRMS spectra for 8b–11b, B are not given.) See DOI: 10.1039/c0ob00287a

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