Cycloaddition reactions of 4-sulfur-substituted dihydro-2-pyridones and 2-pyridones with conjugated dienes

Cycloaddition reactions of sulfoxide- and sulfone-substituted dihydro-2-pyridones and 2-pyridones with electron-rich dienes gave new bicyclic and tricyclic products in good to fair yields. The reactivity, regioselectivity, and stereoselectivity of these reactions were compared with semi-empirical theoretical calculations. Evidence is provided for rather unusual two parallel reaction pathways for the cross-Diels–Alder reaction of a sulfone-substituted 2-pyridone with cyclopentadiene.