Synthesis of (+)-Methyl Dihydropalustramate and of the Pyrido[1,2-a]azepine Core of Stemona Alkaloids

Dedicated to Professor K. Peter C. Vollhardt

Abstract

Starting from a readily available, enantiomerically pure 2,6-disubstituted piperidine the synthesis of pyrido[1,2-a]azepines was accomplished. Key reactions for the ring closure were a photochemically induced acyl radical addition or a SmI₂-promoted ketyl radical addition to an α,β-unsaturated ester. En route to the cyclization precursor an epoxidiation/ring opening sequence led to an undesired oxazolidinone which turned out to be useful for the configuration assignment. The compound was successfully converted into (+)-methyl dihydropalustramate.

Key words

alkaloids - cyclization - piperidines - radical reaction - total synthesis

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