Efficient methods for the preparation of N-protected 3-phenylthio-2 (1H)-dihydropyridinones from 2-piperidone and their Diels-Alder reaction with 2-trimetylsilyloxybutadiene are described. The role of the electron-withdrawing N-protecting group in the dienophiles is rationalized in terms of the lowered energy level in LUMO.