Diels−Alder Reactions of 4-Alkenylthiazoles: A New Approach to Thiazole Functionalization

Somewhat unexpectedly, the computed highest occupied molecular orbital (HOMO) energies of some
4-alkenylthiazoles afforded values close to those calculated for the Danishefsky−Kitahara and Rawal
dienes. In fact, 4-alkenylthiazoles behave as all-carbon dienes in Diels−Alder reactions with the
participation of the formal C−C double bond of the thiazole ring and the side-chain double bond. The
reactions with N-substituted maleimides, maleic anhydride, and naphthoquinone take place with high
levels of stereocontrol to give the corresponding endo-cycloadducts in good to excellent yields. Depending
on the dienophile, the cycloadduct further transforms under the reaction conditions through either a 1,3-hydrogen shift, dehydrogenation, or an ene reaction or Michael addition with another molecule of
dienophile. These unprecedented results open new synthetic perspectives for the functionalization of the
thiazole ring.