Process for the preparation of optically active acyloxyazetidinones

A method of using a compound of the formula IIa ##STR15## wherein R.sub.1 is hydrogen or lower alkyl;

• R.sub.2 is hydrogen or a hydroxy protecting group R.sub.2 ';
• and
• R.sub.3 and R.sub.4 are identical to each other and are selected from the group consisting of
• (i) phenyl,
• (ii) phenyl substituted by lower alkoxy, lower alkyl, and/or halogen,
• (iii) 4-nitrophenyl, and
• (iv) lower alkyl;
• which comprises
• (a) treating said compound of formula IIa with a peracid to result in a compound of formula III ##STR16## (b) replacing the radical of the formula R.sub.4 --C(.dbd.O)-- in formula III by hydrogen by enzymatic ester cleavage to obtain the compound of formula IV ##STR17## and (c) removing from the compound of formula IV the hydroxymethyl group in the 1-position, if appropriate after conversion into a formyl group, to result in a compound of formula I ##STR18## and, if desired, in a compound of formula I thus obtained in which R.sub.2 is a protective group R.sub.2 ', converting the protected hydroxyl group into the free hydroxyl group, and/or if desired in a compound of formula I thus obtained in which R.sub.2 is a hydrogen, converting the free hydroxyl group into a protected hydroxyl group.