Abstract: A compound of formula (I), or a pharmaceutically acceptable salt thereof: ##STR1## wherein R.sub.1 is hydroxy or amino;R.sub.2 is hydrogen or amino;R.sub.3 is hydrogen, hydroxymethyl or acyloxymethyl;R.sub.4 is hydrogen or (when R.sub.3 =H and Z is a bond or CH.sub.2) hydroxy, acyloxy, hydroxymethyl or acyloxymethyl;Z is a bond, or a group CHR.sub.8 wherein R.sub.8 is hydrogen, or (when R.sub.3 =R.sub.4 =H), R.sub.8 is hydroxy, acyloxy, hydroxymethyl or acyloxymethyl;R.sub.5 is a group of formula: ##STR2## wherein R.sub.6 and R.sub.7 are independently selected from hydrogen,C.sub.1-6 alkyl and optionally substituted phenyl.