Diels-Alder reaction of pyran-2(H)-ones: Part VI-Preparation and investigation of the Diels-Alder reaction of 4,6-disubstituted thiopyran-2(H)-ones with 1,4-naphthoquinone and N-phenylmaleimide

4,6-Disubstituted pyran-2(H)-ones 1 when treated with P 2 S 5 the corresponding pyran-2(H)-thiones 2 are obtained. 2 on boiling in aq. methanol in the presence of Na 2 CO 3 give the corresponding thiopyran-2(H)-ones 3. The mechanism for this reaction has been proposed, which is supported by the AMI calculation results and the spectral data. The Diels-Alder reactions of 2 and 3 with N-phenylmaleimide and 1,4-naphthoquinone have been studied.