Abstract: A process for preparing azetidinones of the formulae 1 and 2A and the stereoisomers of the latter of formula 2B ##STR1## wherein R is an amino-protecting group commonly used in penicillin chemistry and is selected from the group consisting of H, C.sub.1 -C.sub.6 alkyl, phenyl, (optionally substituted in the o-, m-, or p- positions by CH.sub.3, F, Cl, OCH.sub.3, or a nitro group), benzyl, 2-thienylmethyl, tetrazol (1-, 2-, or 5-)-ylmethyl, 2-phenyl-5-methylisoxazol-4-yl, phenoxymethyl, and R.sup.5 O-- and R.sup.5 S, wherein R.sup.5 stands for C.sub.1 -C.sub.6 alkyl, phenyl, benzyl or trichloroethyl; it should be noted that the value of R is immaterial to the process described;X is selected from the group consisting of H, SCl, SBr, Cl, Br, and OH. In the case where X is OH the group RCX.dbd.N-- is better represented as the amide, RCONH--, group;RCX.dbd.N may also represent the phthalimido, succinimido-, or tritylamino- group;R.sup.