Synthesis of ketols of the natural pyrethrins

Abstract

A stereospecific five-stage synthesis of (±)-cis-pyrethrolone is described, involving cis-octa-1,3-dien-7-one as the key intermediate. The ethylene acetal of this ketone was made by a Wittig reaction, under salt-free condiditions, with vaporised acraldehyde. The overall yield for the synthesis was 21%, and it provides highly pure (pm;)-cis-pyrethrolone for the first time. The material is spectrally identical with a sample of naturel (+)-pyrethrolone.

Improved syntheses of both (±)-cis-cinerolone and (±)-cis-jasmololone are reported. The cis-side chains are introduced by Wittig reactions or by selective hydrogenation of acetylenic intermediates.

An allenic formulation, entertained for natural pyrethrolone in the early literature, is synthesised.