Abstract: There is described the stereoselective total synthesis of novel .DELTA..sup.2,3 -1,4-morpholine-2-carboxylic acids possessing a fused .beta.-lactam ring in the 1,6-position and carrying a substituent cis to carbon 5 in the 7-position of the fused ring system represented by the general formula ##STR1## wherein Z is halo, hydroxyl, etherified hydroxyl or hydroxyl esterified with a carboxylic acid residue or a sulfonic acid residue and X is amino, azido or acylamino. Also included in the invention are compounds of formula I in which the carboxyl group at the 2-position is protected as by an easily cleavable ester group and salts of both the free acids and carboxyl-protected compounds of formula I. The compounds of formula I are potent antibacterial agents or are of use as intermediates in the preparation of such antibacterial agents.