Preparation and Reactions of 5-Alkylpentachloro-1,3-Cyclopentadienes. Application to Sesquiterpene Synthesis

A variety of 5-alkyl-1,2,3,4,5-pentachloro-1,3-cyclopentadienes(4a-d) were prepared. The key step in these syntheses involved the reaction of a phosphite with hexachlorocyclopentadiene. Attempts were made to effect intramolecular Diels-Alder cyclization of these compounds without rearrangement in an effort to synthesize longifolene. Thermal cyclization occurred in one case, that of 4b, ir, low yield, but rearrangement preceded cyclization. The molecular structure of this cyclization product was unequivocally elucidated as 5a by X-ray crystallographic analysis. Consideration of this structure, which requires reduction as well as rearrangement, suggested a more efficient synthesis. Reduction of (E)-4b with lithium aluminum hydride followed by thermal cyclization gave 5a in good yield. Benzyl ether 5b was similarly prepared. Both 5a and 5b are potential intermediates in a synthesis of isolongifolene.