An Approach to the Skeleton of Yohimbine-Type Alkaloids via Mannich-Michael Reactions with Indolylethylimines

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Abstract
Schiff bases derived from tryptophane methyl ester and tryptamine react with Danishefsky's diene [1-methoxy-3-(trimethylsiloxy)-1,3-butadiene] in the presence of zinc(II) chloride to give enaminones (1-[2-(indol-3-yl)ethyl]-4-oxo-1,2,3,4-tetrahydropyridines), which are subsequently transformed into indolo[2,3-a]quinolizidin-2-ones (1,2,3,4,6,7,12,12b-octahydro-2-oxoindolo[2,3-a]quinolizines) by acid-catalyzed cyclization.