The invention provides a process for preparing a compound of the formula: ##STR1## or a base salt thereof, wherein R.sup.2 is hydrogen or C.sub.1 -C.sub.6 alkyl optionally substituted by up to 3 substituents each independently selected from the group consisting of C.sub.1 -C.sub.6 alkoxy and C.sub.1 -C.sub.6 alkoxy(C.sub.1 -C.sub.6 alkoxy)-; and R.sup.3 is C.sub.1 -C.sub.6 alkyl or benzyl, said benzyl group being optionally ring-substituted by up to 2 nitro or C.sub.1 -C.sub.4 alkoxy substituents comprising reacting a compound of the formula: ##STR2## wherein R.sup.1 is C.sub.1 -C.sub.4 alkyl, phenyl or benzyl or C.sub.1 -C.sub.4 alkoxy; and R.sup.2 and R.sup.3 are as previously defined for a compound of the formula (I), with hydrogen peroxide or a source of peroxide ions: said process being optionally followed by conversion of the compound of the formula (I) to a base salt thereof. The present invention also relates to novel compounds of the formula (II).