PBu3-Mediated Vinylogous Wittig Reaction of α-Methyl Allenoates with Aldehydes and Mechanistic Investigations

A highly stereoselective PBu3-mediated vinylogous Wittig olefination between α-methyl allenoates and a variety of aldehydes is presented as the first example of a practical and synthetically useful vinylogous Wittig reaction. Mechanistic experiments including deuterium-labeling, intermediate entrapment, and NMR monitoring have been deliberately conducted. On the basis of mechanistic investigations, a reliable mechanism for the vinylogous Wittig reaction is proposed, which features a water/phosphine-coassisted allylic phosphorus ylide 1,3-rearrangement pathway, rather than previous retro-Diels–Alder ones. It is noteworthy that mechanistic findings in this work also provide supportive evidence for typical mechanisms of important phosphine-mediated reactions of allenoates.