Stereocontrolled preparation of spirocyclic ethers by intramolecular trapping of oxonium ions with allylsilanes

The stereoselectivity of the spontaneous intramolecular cyclization of 2-(benzenesulfonyl)-2-(4-((trimethylsilyl)methyl)-4-pentenyl)tetrahydropyrans substituted by alkyl groups at various ring positions has been examined. For the 4- and 6-methyl derivatives, formation of the spirocyclic center occurs exclusively 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 substituents act in a reinforcing manner and direct cyclization uniquely in one direction. Both the cis and trans bicyclic ethers ring close on that π-surface of the intermediate oxonium ion syn to the angular hydrogen. The results are rationalized in terms of the predilection of the associated oxonium ions for nucleophilic capture via a chairlike or twist-boat transition state.