Facile synthesis of highly functionalized six-membered heterocycles via PPh3-catalyzed [4+2] annulations of activated terminal alkynes and hetero-dienes: scope, mechanism, and application

A novel [4+2] annulation between activated terminal alkynes and aza-dienes or oxo-dienes has been developed with the use of triphenylphosphine catalyst (20 mol %), which provides a facile method for synthesis of the corresponding highly functionalized dihydropyridines or dihydropyrans in good to excellent yields. The reaction mechanism has also been established, consisting formal hetero-Diels–Alder reaction catalyzed by PPh3 and [1,3]-proton transfer, which exhibits a large isotopic effect.