Disclosed are 6- and 6,6-disubstituted-2-substituted-oxapen-2-em-3-carboxylic acids of the following structure: ##STR1## wherein: R.sup.1 and R.sup.2 are independently selected from hydrogen, alkyl, alkoxyl, aralkyl, aryl, heterocyclyl and heterocyclylalkyl; R is selected from hydrogen, --OR, --SR, --NR.sub.2, alkyl, aryl, aralkyl, heterocyclyl, or heterocyclylalkyl. Such compounds and their pharmaceutically acceptable salt, and ester derivatives are useful as antibiotics. Also disclosed are processes for the preparation of such compounds, pharmaceutical compositions comprising such compounds and methods of treatment comprising administering such compounds and compositions when an antibiotic effect is indicated.