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Paper | Regular issue | Vol 34, No. 9, 1992, pp.1773-1782
Published online, 1st January, 1970
DOI: 10.3987/COM-92-6086
A New Short Synthesis of (±)-Eburnamonine

Esko Karvinen and Mauri Lounasmaa*

*Laboratory for Organic and Bioorganic Chemistry, Technical University of Helsinki, SF-02150 Espoo, Finland

Abstract

A new short synthesis of the indole alkaloid eburnamonine (1), starting from the easily available enamine (3) (1,12b-didehydroindolo[2,3-a]quinolizidine) is described. Pentacyclic lactam (7) (18-nor-19-ethoxycarbonyl-21-epieburnamonine) (trans D/E ring juncture), the key intermediate in this synthesis, was prepared from enamine (3) by dialkylating with ethyl iodoacetate in the presence of diisopropylethylamine (DIPEA). LiAlH4-reduction of lactam (7) gave a 2:1 mixture of compounds (10a) (21-epi-18-hydroxyeburnamine) and (10b) (16-epi-21-epi-18-hydroxyeburnamine). Wolff-Kishner reduction of the "masked" carbonyl group present at C(16) of compounds (10a) and (10b) gave the alcohol (14) (1-ethyl-1-(2-hydroxyethyl)indolo[2,3-a]quinolizidine) possessing the natural cis-relationship between C(21)-hydrogen and C(20)-ethyl group. PCC oxidation of alcohol (14), followed by cyclization, completed the synthesis of (±)-eburnamonine (1).