Diels–Alder reaction of benzylidene(cyano)methyl-1,3-benzoxa/thiazoles as stable 1-azabuta-1,3-dienes

Diels–Alder reactions of benzylidene(cyano)methyl-1,3-benzothiazoles 2 and -1,3-benzoxazoles 3 as 1-azabuta-1,3-dienes are described. The dienes 2, 3, featuring stabilized imine moieties in the form of heteroaromatic rings, react with both electron-deficient and electron-rich dienophiles 7–9 to give corresponding cycloadducts 10–15 regioselectively. Cycloadditions of the intramolecular systems 18c, d and 19c, d proceed smoothly via an exo-transition state, stereoselectively affording polycyclic compounds 20c, d and 21c, d in good to excellent yields. X-Ray crystallographic studies of compounds 12e, 13a and 14b are also reported.