ATTEMPTED SIMMONS-SMITH REACTION ON ALPHA-OXOKETENE DITHIOACETALS - A NEW GENERAL-ROUTE TO 3,4-SUBSTITUTED AND ANNELATED THIOPHENES

A new general synthesis of 3- and 3,4-substituted 4a-q, 3,4-annelated 7a-d and condensed tricyclic 9a-e thiophenes has been developed through Simmons-Smith reaction on the respective alpha-oxoketene dithioacetals. Extension of the reaction to alpha-cinnamoylketene dithioacetals 10a-e and its higher enyl analogs 13a-d, 15a-c gave the corresponding 3-styryl 11a-e and 3-di-and trienyl 14a-d,16a-c thiophenes in highly regioselectivemanner. A probable mechanism involving an unusual intramolecular aldol condensation of sulphonium ylid formed through carbenoid methylene insertion on divalent sulfur has been suggested.