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 Cp-2YMe . THF. The process provides carbobicyclics in high yields an d with excellent selectivities. The active catalyst is postulated to b e Cp-2YH . THF, which is generated in situ. A variety of alkenyl-subs tituted cyclopentane and cyclohexane substrates were examined. The hig h diastereoselectivities apparently originate fi om a preference for a chairlike transition structure that minimizes unfavorable steric inte ractions between the bulky Cp ligands of the catalyst and the preexis ting ring of the substrate. Acyclic triene precursors, 4-ethenyl-subst ituted 1,5-heptadienes and 5-ethenyl-substituted 1,8-nonadienes were a lso 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 diastereoselec tivity. 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 versat ile alcohols utilizing known oxidation methods.