Alkaloid biosynthesis. Part XIII. The structure, stereochemistry, and biosynthesis of loganin

Abstract

Verbenalin is of established structure and configuration (6) and has been converted by steps not involving the asymmetry at C-2,3 and -7 into an olefin (12) also prepared from loganin; this proves that the structure and absolute configuration of loganin is as (1) apart from the centres at C-5 and C-1. The configuration at C-5 is determined by three independent methods and that of C-1 by n.m.r measurements. It is proved that the C-methyl group of loganin lies in the thermodynamically stable configuration and this knowledge allows a route by way of 5-dehydrologanin (24) for the preparation of [5-³H]loganin and [5-³H]epiloganin to be developed. The specific incorporation of geraniol into loganin is demonstrated.