Asymmetric synthesis of indole alkaloids from (L)-tryptophan: formal syntheses of (-)-koumine, (-)-taberpsychine & (-)-koumidine

The reaction of (L)-tryptophan methyl ester with methyl 4-oxobutanoate under conditions of kinetic control gave a high yield of the cis-1,3-disubstituted tetrahydro-β-carboline (6); a simple five step procedure allowed this to be transformed into the optically pure (e.e. greater 95%) bridged ketone (−)-(17), whose (+) isomer has previously been used in the syntheses of (+)-koumine (1), (+)-taberpsychine (2) & (+)- koumidine (3).