The invention provides compounds of formula (I): ##STR00001## wherein: R.sub.1-R.sub.4 and A have any of the values defined in the specification, and their pharmaceutically acceptable salts, are useful for inhibiting .beta.-lactamase enzymes, for enhancing the activity of .beta.-lactam antibiotics, and for treating .beta.-lactam resistant bacterial infections in a mammal. The invention also provides pharmaceutical compositions, processes for preparing compounds of formula (I), and intermediates useful for the synthesis of compounds of formula (I).