Synthesis of Cinerone, Cinerolone and of Cinerin-I

Abstract

As part of a scheme to elucidate by synthetical methods the structures of the cinerins and pyrethrins, the important insecticidal constituents of pyrethrum flowers, one of us in 1946¹ synthesized 2-but-2'-enyl-and 2-but-3';-enyl-3-methylcyclopent-2-enone (Ia and Ib) for comparison with cinerone, which is a degradation product of cinerolone³. LaForge and Barthel³ had previously shown that dihydrocinerone was identical with 3-methyl-2-n-butylcyclopent-2-enone (Ic), so that only the position and configuration of the side-chain double bond was in question. They had assigned this to the 2’ position on the somewhat uncertain basis of terminal methyl values². This assignment accords with the ultra-violet absorption spectrum of (+)-cinerolone, which excludes the 1’ position; and the non-identity of cinerone with synthetic Ib, which excludes the 3’ position¹. We have confirmed the but-2'-enyl side-chain in (+)-cinerolone by ozonization, using a specimen prepared from the semicarbazone (kindly provided by Dr. T. F. West). This yielded acetaldehyde together with a small proportion of formaldehyde. The latter product indicates a small proportion of contaminant, probably pyrethrolone.