Thiopyrans from acetylenes

The first approach to 2//-thiopyrans from acetylenes resembles the synthesis of oxa analogs 92a,b. In contrast to the latter, intermediates like 100 have been directly used as starting compounds. 

Cycloadditions of dimethyl acetylenedicarboxylate (DMAD) to some thiacylmethylenethiazolines 107 were discovered to be the second approach to thiopyrans from acetylenic precursors.150 The reaction was observed to... 

Selnick and Brookes prepared a series of thiopyrans as carbonic anhydrase 11 inhibitors via intramolecular oxazole-alkyne cycloadditions. The starting 4-methyl-2-thiooxazole-alkyne 215 was readily constructed from 2-mercapto-4-methyloxazole (Fig. 3.62). Refluxing 215 in toluene afforded an 88% yield of the furanothiopyran 216. Varying the length of the oxazole-acetylene linker provided furans fused to 5- and 7-membered rings as well. 

Adducts of type (39 X = CO-Ph or C02Me), containing the AH-thiopyran structure, are formed from l,2-dithiole-3-thiones and acetylenes ... 

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