Synthesis of cadinanolide type of tricyclic alpha-methylene-gamma-lactone using intramolecular cyclization of alpha-trimethylsisylmethyl-alpha,beta-unsaturated ester with cyclic ketone

Fluoride-promoted intramolecular cyclization of ethyl 6-(2-oxocyclohex-l-yl)-2-(trimethylsilylmethyl)hex-2-enoate afforded ethyl 2-(1-hydroxybicyclo[4.4.0]decan-2-yl)acrylate as the major product, together with tricyclic alpha-methylene-gamma-lactone, a model compound of cadinanolides. The former product was also converted to gamma-lactone. The cyclization reaction promoted by TiCl4 gave the hydroxy ester and its dehydrated product. Both Lewis acid- and fluoride-promoted cyclizations gave a trans-decaline system mainly. This stereoselectivity is completely different from that of Reformatsky cyclization observed by Dreiding and co-workers.