Synthesis of seven-membered cyclic enol ether derivatives from the reaction of a cyclic phosphonium ylide with alpha,beta-unsaturated esters

The tandem Michael-intramolecular Wittig reactions of a five-membered cyclic phosphonium ylide (2) with alpha,beta-unsaturated esters afforded seven-membered cyclic enol ether derivatives 4a-e in 37-73% yield. The reaction proceeded via a rigid phosphabicyclic intermediate and supplied the enol ether derivatives with high stereoselectivity. On the other hand, the reaction using ethyl acrylate as a substrate gave the 1:3 adduct 15 of the ylide and the enoate via the repetition of the Michael-type addition and regeneration of the ylide followed by the intramolecular Wittig reaction.