Direct synthesis of 5-cycloalkene-substituted γ-butenolides via [2,3]-Wittig rearrangement of vinylogous urethanes

A direct synthesis of 5-cycloalkene-substituted γ-butenolides via an tandem [2,3]-Wittig rearrangement/lactonization of γ-allyloxy vinylogous urethane enolates are described. The reaction accommodates diverse cycloalkenyl substrates and affords the corresponding γ-butenolides in good yields with diastereoselectivity that depends on the substrate. The observed stereochemical outcomes, as established by X-ray crystallographic analysis, can be rationalized by simple transition-state models, and the method offers an efficient approach to γ-butenolide frameworks relevant to sesquiterpene lactones.