Compounds having the formula:
wherein n is 0, 1 or 2;
X is OH or OCOR2m wherein R2 is hydrogen or (C,-C.) alkyl;
R is hydrogen, a radical group forming an ester hydrolyzable under physiological conditions, or an acyloxymethyl or 1-(acyloxy)ethyl radical derived from a convential beta-lactam antibiotic; and
R' is selected from a wide range of alkyl, cycloalkyl, aryl, aralkyl, heterocyclyl or heterocyclylalkyl groups
and their pharmaceutically acceptable salts.
These compounds are useful as antibacterials and/or beta-lactamase inhibitors.