Construction of Bridged Polycyclic Systems by Polyene Cyclization

A stereoselective construction of bridged tri- and tetracyclic systems embedded in some 3,5-dimethylorsellinic acid (DMOA)-derived meroterpenoids was achieved by exploiting a polyene cyclization of suitably functionalized epoxyallylsilanes. Both the olefinic substituent on the epoxide and allylic trimethylsilyl (TMS) group were found to play pivotal roles in the success of the present reaction. The fact that the cyclization of monocyclized byproducts did not proceed strongly suggests that the reaction could be a concerted transformation. Expansion to bridged compounds: Et₂AlCl-promoted polycyclization of epoxyallylsilanes was found to be applicable to the synthesis of bridged tri- and tetracyclic compounds. Five contiguous stereocenters, including three quaternary carbon atoms, were incorporated through internal asymmetric induction in a single operation (see scheme, TMS=trimethylsilyl, Ts=toluenesulfonyl).