Compounds of the formula ##STR1## where Z, A, and R are as disclosed herein and pharmaceutically compatible, readily hydrolyzable esters and salts of these compounds are disclosed. The compounds have .beta.-lactamase inhibiting properties and are useful in the control of .beta.-lactamase-forming pathogens in combination with .beta.-lactam antibiotics. They also exhibit antibacterial activity of their own and can accordingly be used themselves in the control or treatment of infectious diseases.