Disclosed are 1-substituted-pen-2-em-3-carboxylic acids (I) and their pharmaceutically acceptable salts and esters which are useful as antibiotics; such compounds are prepared by total synthesis. ##STR1## wherein R.sup.1 and R.sup.2 are, inter alia, substituted and unsubstituted alkyl, aryl, aralkyl, cycloalkyl, cycloalkylalkyl, and spirocycloalkyl.