A stereoselective synthesis of corynantheine-type alkaloids via enamine annulation. Total synthesis of (±)-dihydrocorynantheol and formal total synthesis of (±)-corynantheine and (±)-ajmalicine

Abstract

Enamine annulation between 3,4-dihydro-1-methyl-β-carboline (4) and dimethyl 3-methoxyallylidenemalonate (5) yielded 2-(2,2-dimethoxyethyl)-2,3,4,6,7,12-hexahydro-3-methoxycarbonyl-4-oxoindolo [2,3-a]quinolizine (6), which was transformed into (±)-dihydrocorynantheol (1). (±)-Corynantheal (19), which is convertible into (±)-corynantheine (2) and (±)-ajmalicine (3), was stereoselectively synthesised from (6)via empimerisations at the angular position using Adams catalyst.