Beta-lactamase inhibiting compounds

The invention provides compounds of formula (I) and (IV), wherein R1-R11 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 (IV), and novel intermediates useful for the synthesis of compounds of formula (I) and (IV).