Stereoselective Synthesis of Carbobicyclics via Organoyttrium-Catalyzed Sequential Cyclization/Silylation Reactions

The sequential cyclization/silylation of 1,5-dienes and 1,6-dienes was effected under mild reaction conditions using catalytic quantities of Cp₂*YMe·THF. The process provides carbobicyclics in high yields and with excellent selectivities. The active catalyst is postulated to be Cp₂*YH·THF, which is generated in situ. A variety of alkenyl-substituted cyclopentane and cyclohexane substrates were examined. The high diastereoselectivities apparently originate from a preference for a chairlike transition structure that minimizes unfavorable steric interactions between the bulky Cp* ligands of the catalyst and the preexisting ring of the substrate. Acyclic triene precursors, 4-ethenyl-substituted 1,5-heptadienes and 5-ethenyl-substituted 1,8-nonadienes were also examined. These triene substrates, when exposed to the cyclization/silylation protocol, provide the strained trans-bicyclo[3.3.0]octanes and trans-decalin systems in high yield with excellent diastereoselectivity. The high selectivity is again attributed to the preference for a chairlike transition structure. The cyclized organosilane products isolated from these reactions were easily converted to the more versatile alcohols utilizing known oxidation methods.