A number of versatile sulfonium and pyridinium salts reacted stereoselectiv
ely with some arylidenecyanothioacetamides under mild heating to give trans
-4,5-dihydrothiophenes. The stereochemistry of the products was established
on the basis of X-ray analysis. Repeating the same reactions at high tempe
rature furnished a mixture of trans-4,5-dihydrothiophenes and thiazoles.