Efficient Synthesis of Functionalized 2H-Thiopyrans via Hetero-Diels-Alder Reactions of an Enaminothione with Electrophilic Olefins

Summary.  The hetero-Diels-Alder reaction of 3-dimethylamino-1-(2-thienyl)-2-propene-1-thione (diene) with substituted β-nitrostyrenes, as well as maleic and fumaric acids (dienophiles) yielded 3,4-dihydro-2H-thiopyran derivatives. The treatment of some of those cycloadducts with acetic acid caused elimination of dimethylamine, affording stable 2H-thiopyrans. A reaction of the diene with maleic anhydride furnished a cycloadduct which underwent spontaneous rearrangement to form an N,N-dimethylamide derivative. Cycloadditions of the diene to maleimide, N-phenylmaleimide, maleic acid monoanilide, diethyl maleate, fumarate, and butenolide carried out in the presence of acetic anhydride were followed by elimination of dimethylamine under formation of stable 2H-thiopyran derivatives. Received February 19, 2001. Accepted (revised) March 12, 2001