A Total Synthesis of (±)-α- and (±)-β-Copaenes and Ylangenes

Syntheses of racemic α-copaene (1), α-ylangene (2), β-copaene (33), and α-ylangene (34) are described. The cis-decalin “envelope” ring component of these substances was constructed by the orientationally selective reaction of 3-carbomethoxy-2-pyrone (3) with 4-methyl-3-cyclohexenone (4) which afforded the keto ester 5. Epoxidation of the ethylene ketal of 5 using peracid afforded the α-oxide 14 which was further transformed into the dienone ketal 18, a key intermediate for the synthesis of the four title compounds. Reduction of 18 using lithium in liquid ammonia (to 19), tosylation, deketalization, and internal Sn2 cyclization then afforded the tricyclic ketone 23 which was further converted to (±)-1 and (±)-2 by a sequence which transformed the carbonyl group into >CH-i-C₃H₇. Reduction of the dienone ketal 18 using zinc-acetic acid led to the exo-methylene keto ketal 28 from which (±)-β-copaene (33) and (±)-β-ylangene (34) were obtained by a sequence paralleling that used for the synthesis of 1 and 2 from 19.