Use of the kinetically controlled Pictet-Spengler reaction in the asymmetric synthesis of indole alkaloids: Formal syntheses of (-)-ajmaline, (-)-koumine, (-)-taberpsychine, (-)-koumidine and (-)-suavoline

By employing the kinetically controlled Pictet-Spengler reaction L-tryptophan was used as the chiral starting material for the synthesis of the cis-1,3-disubstituted tetrahydro-ß-carboline 14a. Protection of the two nitrogens and subsequent cyclisation/decarboxylation led to generation of the bridged ketone (-)-(7c), which was shown to be optically pure within our detection limits (ee >95%). Simple protecting group modifications gave access to (-)-7a and (-)-7b, and constituted formal synthesis of (-)-ajmaline, (-)-koumine, (-)-taberpsychine, (-)-koumidine and (-)-suavoline. These results demonstrate that such alkaloids with the correct absolute configuration are accessible in a stereocontrolled manner from L-tryptophan.