Allenic and acetylenic intermediates in organic synthesis

Abstract

Dihalocarbenes were added to allenes to form dihalo-methylene- cyclopropanes which were then converted by the n-butyl lithium method to cumulenes. Both dichloro- and dibroraocarbenes were found to add to the allene double bond with the highest number of alkyl substituents. Propargyl alkenylmalonates were prepared in good yields and pyrolised under carefully controlled conditions to give allenic alkylidenemalonates in fair yields by Cope Rearrangement, but gave fission products under more extreme conditions. The position of the alkyl substituents on the alkenylmalonate group seemed to influence the ease of the rearrangement. Cyclopropanes were prepared in good yields from different types of x, b-unsaturated esters by the dimethylsulphozonium methylide reaction in dimethyl sulphoxide and dimethylformamide solvents. An Allentc ester, on the other hand, gave an unstable complex postulated as dimethylsulphoxonium 3-ethozycarbonyl 2-methylallylide. The syntheses of the only two naturally-occurring unconjugated straight-chain allenic acids, octadeca-5,6-dienoic acid ("Laballenicacid") and 8-hydro2y-5,6-dienoic acid, were attempted. Laballenic acid was synthesised and its absolute configuration determined via the lithium aluminium hydride reduction of hexadec-2-en-4-yn-l-oland malonate condensation of the bromide from hexadeca-3,4-dien-l-ol.A similar reduction of l-tetrahydropyranylosy-hez-2-yn-4-en-6-ol,however, yielded a mixture of hexa-4,5-dien-l-ol and hex-4-yn-l-olotydue to the elimination of the tetrahydropyranyl oxy group.