Cylopentannulation by an iterative process of sequential claisen rearrangement and enyne radical closure: Routes to triquinane and propellane systems and use in the synthesis of (+/-)-ceratopicanol

Cycloalkenyl acetylenes 4, which are easily prepared from allylic alcohols by Claisen rearrangement and homologation (1 --> 4), generally undergo radical cyclization (4 --> 6) on treatment with stannyl radicals. The products (6) can themselves be converted into allylic alcohols, so that the annulation sequence can then be repeated. In certain cases enyne cyclization initiated by stannyl radicals does not work, but an alternative process (cf. Scheme 9) of epoxide opening with bis(cyclopentadienyl)titanium(III) chloride can then be used. Di- and triquinanes (Scheme 3) were made by the stannyl radical approach; the epoxide route was used to prepare a simple propellane (Scheme 4) and in a synthesis of (+)-ceratopicanol (Schemes 5 and 9).