STEREOCONTROLLED PREPARATION OF SPIROCYCLIC ETHERS BY INTRAMOLECULAR TRAPPING OF OXONIUM IONS WITH ALLYLSILANES

The stereoselectivity of the spontaneous intramolecular cyclization of trimethylsilyl)methyl)-4-pentenyl)tetrahydropyrans substituted by alk yl groups at various ring positions has been examined. For the 4- and 6-methyl derivatives, formation of the spirocyclic center occurs exclu sively anti to the methyl. The outcome in the 5-methyl example is a 3. 7:1 syn/anti split. For the trans-4,6-dimethyl derivative, the substit uents act in a reinforcing manner and direct cyclization uniquely in o ne direction. Both the cis and trans bicyclic ethers ring close on tha t pi-surface of the intermediate oxonium ion syn to the angular hydrog en. The results are rationalized in terms of the predilection of the a ssociated oxonium ions for nucleophilic capture via a chairlike or twi st-boat transition state.