Cationic Cyclizations and Rearrangements as Models for Strobane and Hispanane Biogenesis

Article abstract of DOI:10.1021/jo00149a017

In contrast to methyl 15(R)-hydroxypimar-8(14)-en-oate (5a) the 15(S)-epimer 9a gave a stable toluenesulfonate.Solvolysis of the latter gave, depending upon the conditions, the 15(R)-epimer 5a, the methoxy derivative 9d, or, with rearrangement, the strobane derivative 10a and the cyclopropane resin acid ester 11a.Treatment of methyl 15-hydroxy-16-nor-8(14)-pimaren-18-oate (9c) with toluenesulfonyl chloride-pyridine resulted, as in the case of 5a, in spontaneous rearrangement to 10b and 11b.The difference in behavior of 5a and 9e on the one hand and 9a on the other is attributed to different spatial demands in the homoallylic cations produced from the starting materials.The reactions provide laboratory analogies for postulated biogenetic pathways to diterpenoids of the strobane and hispanane families.In an effort to test an alternative proposal for biogenesis of the strobanes, the epimeric labda-8(17),13-dien-12(S)-and -12(R)-ols 22 and 23 were synthesized and subjected to reagents known to effect cationic cyclizations.These experiments failed to provide compounds with the strobane carbon skeleton.Formic acid converted 23, but not 22, to the tricyclic hydrocarbon 31.