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Synlett 2006(15): 2383-2386  
DOI: 10.1055/s-2006-949622
LETTER
© Georg Thieme Verlag Stuttgart · New York

Syntheses of Highly Substituted Furan and Pyrrole Derivatives via Lithiated 3-Aryl-1-methoxyallenes: Application to the Synthesis of Codonopsinine

Morshed Alam Chowdhury, Hans-Ulrich Reissig*
Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany
Fax: +49(30)83855367; e-Mail: hans.reissig@chemie.fu-berlin.de;
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Publication History

Received 16 June 2006
Publication Date:
08 September 2006 (online)

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Abstract

Lithiated 1-methoxy-3-phenylallenes were generated in situ from a phenyl-substituted propargylic ether. They smoothly combined with aldehydes, ketones, or imines to give allenyl adducts, which cyclized to highly substituted heterocycles either under basic conditions or with silver nitrate assistance. Analogously, a dihydropyrrole derivative was obtained by the addition of ­anisyl-substituted lithiated methoxyallene derivative to an N-tosyl imine and subsequent cyclization of the intermediate. The two diaste­reomers obtained were subsequently transformed into the alkaloid (±)-codonopsinine and one of its epimers. The key steps of these ­sequences are highly diastereoselective hydroborations of silyl enol ethers quantitatively leading to hydroxylated intermediates.

Key words

methoxyallene - organo lithium compounds - pyrroles - hydroboration - alkaloids

11

Conversion of 9 into 13; Typical Procedure To a solution of 1-methoxy-4-(3-methoxy-1-propynyl)ben-zene (9; 1.20 g, 6.82 mmol) in Et2O (15 mL) n-BuLi (2.5 M in hexane, 2.73 mL, 6.82 mmol) was added at -78 °C and stirred for 1 h. MeOH (276 µL, 6.82 mmol) was added slowly to the mixture and the resulting solution was warmed with stirring to r.t. (0.5 h). The mixture was cooled again to -78 °C and n-BuLi (2.73 mL, 6.82 mmol) was added slowly and the resulting solution was stirred for 0.5 h. Imine 11 (1.34 g, 6.82 mmol) dissolved in Et2O (15 mL) was slowly transferred to the reaction flask by syringe. After stirring for 1.5 h at -78 °C to -20 °C, the mixture was quenched with H2O and the aqueous phase was extracted with Et2O (3 × 25 mL). The combined organic phases were washed with brine (25 mL) and dried with MgSO4. Filtration and evaporation of solvents in vacuo at r.t. afforded allenyl amine 12 (3.17 g, dr ca. 50:50), which was used for the subsequent cyclization without purification. The crude product was dissolved in anhyd MeCN (45 mL); K2CO3 (2.35 g, 17.0 mmol) followed by AgNO3 (289 mg, 1.70 mmol) were added to the solution. The resulting mixture was stirred for 16 h in the dark under argon, then filtered through a pad of celite, washed with EtOAc, and the filtrate was concentrated in vacuo at r.t. The resulting product was purified by column chromatography on silica gel (hexane-EtOAc, 4:1) to give 13 (1.98 g, 78% from 9) as a mixture of diastereomers (trans/cis 45:55). Isomers (2,5-trans)-13 (650 mg, 25%) and (2,5-cis)-13 (1.12 g, 44%) were separated by fractional crystallization.
3-Methoxy-5-(4-methoxyphenyl)-2-methyl-1-tosyl-2,5-dihydro-1 H -pyrrole (2,5- trans )-13. Colorless crystals; mp 138-140 °C. IR (KBr): 3100-3000 (=CH), 3000-2840 (CH), 1670 (C=C), 1610 (CN), 1340, 1160 (RSO2N) cm-1. 1H NMR (500 MHz, CDCl3): δ = 1.63 (d, J = 6.3 Hz, 3 H, Me), 2.31 (s, 3 H, Ts-Me), 3.62, 3.77 (2 s, 3 H each, OMe), 4.40-4.52 (m, 2 H, 2-H, 5-H), 5.50 (dd, J = 1.9, 4.7 Hz, 1 H, 4-H), 6.55-6.65, 6.90-7.00, 7.00-7.10 (3 m, 2 H, 4 H, 2 H, Ar). 13C NMR (126 MHz): δ = 20.9, 21.4 (2 q, Me), 55.4, 57.1 (2 q, OMe), 60.2 (d, C-2), 67.5 (d, C-5), 94.6 (d, C-4), 113.3, 126.7, 128.8, 129.9 (4 d, Ar), 132.1, 138.4, 141.9, 159.4 (4 s, Ar), 157.8 (s, C-3). MS (EI, 80 eV, 30 °C):
m/z (%) = 373 (30) [M]+, 358 (13) [M - CH3]+, 266 (26) [M - C7H7O]+, 218 (100) [M - C7H7SO2]+, 155 (40) [C7H7SO2]+. HRMS (EI, 80 eV, 30 °C): m/z calcd for C20H23NO4S: 373.13478; found: 373.13450. Anal. Calcd for C20H23NO4S (373.5): C, 64.32; H, 6.21; N, 3.75. Found: C, 64.29; H, 6.32; N, 3.69.
(2,5- cis )-13. Brownish liquid. IR (KBr): 3100-3000 (=CH), 3000-2840 (CH), 1665 (C=C), 1610 (CN), 1350, 1160 (RSO2N) cm-1. 1H NMR (500 MHz, CDCl3): δ = 1.50 (d, J = 6.3 Hz, 3 H, Me), 2.39 (s, 3 H, Ts-Me), 3.54, 3.78 (2 s, 3 H each, OMe), 4.30-4.42 (m, 2 H, 2-H, 5-H), 5.35 (m, 1 H, 4-H), 6.80-6.90, 7.20-7.35, 7.60-7.65 (3 m, 2 H, 4 H, 2 H, Ar). 13C NMR (126 MHz): δ = 21.6, 21.8 (2 q, Me), 55.4, 57.3 (2 q, OMe), 60.3 (d, C-2), 67.2 (d, C-5), 94.0 (d, C-4), 113.8, 127.7, 128.4, 129.6 (4 d, each, Ar), 134.9, 135.5, 143.4, 159.2 (4 s, Ar), 157.3 (s, C-3). MS (EI, 80 eV, 30 °C): m/z (%) = 373 (8) [M]+, 358 (2) [M - CH3]+, 266 (11) [M - C7H7O]+, 218 (48) [M - C7H7SO2]+, 217 (100), 155 (9) [C7H7SO2]+. HRMS (EI, 80 eV, 30 °C): m/z calcd for C20H23NO4S: 373.13478; found: 373.13432.