Formal Synthesis of (±)-Guanacastepene A

A 17 step synthesis of 55, a late intermediate in Danishefsky's guanacastepene A synthesis, has been completed in 4% overall yield. Key features include the use of vinylmagnesium bromide in the Pd-catalyzed coupling with triflate 13 to give triene 16 without the formation of Heck products, a novel extension of the Stork-Jung vinylsilane Robinson annulation that provides tricyclic 2-hydroxymethylcyclohexenone 42 from 23b in four steps and 51% yield, the ability to obtain almost exclusively alpha'-alkylation of 35ba by the proper choice of protecting groups, and the ability to obtain the desired beta-alcohol selectively by reduction of keto alcohol 42 rather than keto ester 53.