The synthesis and in vitro activity of several new carbapenems, 2-(3-cis-substituted cyclopentylthio)-and 2-(4-cis-substituted cyclohexylthio)-(6 S)-[(1'R)-hydroxyethyl]-(5 R)- carbapen-2-em-3-carboxylic acids 7 and 8, are described. The synthesis involves the stereospecific preparation of cis-substituted cyclopentyl-and cyclohexylthiols 2 and 3 by the hetero Diels-Alder reaction followed by some chemical manipulations.