Concise syntheses of cembrenes based on radical-mediated vinylcyclopropane ring-opening reactions in casbene

Abstract

Treatment of the bicyclo[12.1.0] pentadecatriene casbene 1 with ethanethiol radical (EtSH in C₆H₆, heat) results in regiospecific 1,4-addition to the vinylcyclopropane ring system, producing the cembrene sulfide 10. In similar reactions chrysanthemyl alcohol 11 and carene 13 lead exclusively to the corresponding sulfides 12 and 14, respectively, whereas irradiation of methyl chrysanthemate 15 with ethanethiol produces a 1 : 1 mixture of the isomeric sulfides 16 and 17. Oxidation of the cembrene sulfide 10, followed by thermolysis of the resulting sulfoxide then produces isocembrene 20, identical with natural material from Pinus sibirica. When a solution of casbene 1 in CCl₄ was heated in the presence of NBS and AIBN, it underwent smooth oxidative rearrangement, via 21 and 22, to cembrenene 23, identical with the natural product isolated from the soft coral Sinularia maji.