Reductive, Radical-Induced Cyclizations of 5-Hexenals as a Biomimetic Model of the Chemistry of Secologanin Formation

Article abstract of DOI:10.1021/jo00225a042

A model reaction for studying the mechanism of the biological conversion of the iridoid loganin (1) to the secoiridoid secologanin (2) is the reductive, radical-induced cyclization of secologanin tetraacetate (4) to loganin tetraacetate.Treatment of 4 with Mg and Me3SiCl in THF at room temperature gives a mixture of the four possible C-6 and C-7 epimers of loganin tetraacetate in which the natural stereochemistry predominates.This result suggests that the biochemical event, which involves cleavage of the C-6 and C-7 bond of 1 in forming 2, may be a homolytic process that is initiated by formation of a carbon radical at C-8.The scope of the cyclization reaction, which formally occurred in a 5-hexanal moiety in 4, is defined by studies of the cyclization of six other δ,ε-unsaturated aldehydes.The results illustrate a new way for ring annulation through the reductive, radical-induced cyclization of δ,ε-unsaturated aldehydes.