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Selectivity Control in Terpene Rearrangements: A Biomimetic Synthesis of the Halimanic Bicyclic Core

The bicyclic core of the halimanic framework is synthesized in optically active form by an acid-induced rearrangement of a homo­drimanic epoxide. The substrate can follow two different pathways under acidic treatment. Using fluorosulfonic acid as a promoter at low temperature favors ring contraction to a perhydrindanic structure. In contrast, milder acids at higher temperatures bring about predominantly an angular methyl migration and formation of the halimanic bicyclic system. In particular, an acidic pillared clay selectively promoted this transformation.