A variety of substituted 3-acyltetrahydrothiophenes can be prepared with high stereoselectivity in 50-70% yield by acid-promoted condensation of mercapto allylic alcohols 1 (X = S) with aldehydes and ketones. The mercapto allylic alcohol must be substituted at the internal alkene carbon and the terminal alkene carbon must be unsubstituted. This new synthesis of tetrahydrothiophenes will be most useful for preparing 3-acyltetrahydrothiophenes having cia side chains at C2 and C5. The complete preservation of enantiomeric purity in the conversion of (4R)-9 to tetrahydrothiophene 22 is consistent with a cyclization-pinacol pathway (24 --> 25 --> 26 --> 27, Scheme 8).