Compounds of the formula ##STR00001## wherein R.sub.a is H; substituted or unsubstituted alkyl (1-10C); substituted or unsubstituted alkenyl (2-10C); substituted or unsubstituted alkynyl (2-10C); substituted or unsubstituted aryl (4-14C); substituted or unsubstituted arylalkyl (5-20C); or OR.sub.a is replaced by H; R.sub.b is H or halogen; R.sub.c is H or a protecting group; R.sub.d is methyl, unsubstituted alkyl (3-10C); substituted alkyl (1-10C); substituted or unsubstituted alkynyl (2-10C); substituted or unsubstituted aryl (4-14C); substituted or unsubstituted arylalkyl (5-20C); substituted or unsubstituted arylalkenyl (5-20C); substituted or unsubstituted arylalkynyl (5-20C); substituted or unsubstituted amidoarylalkyl (5-20C); substituted or unsubstituted amidoarylalkenyl (5-20C); or substituted or unsubstituted amidoarylalkynyl (5-20C); R.sub.e is H or a protecting group; L is methylene or carbonyl; T is --O--, --N(R)--, --N(OR)--, --N(NHCOR)--, --N(N.dbd.CHR)--, or --N(NHR)-- wherein R is H or R.sub.a as defined above, with the proviso that when L is methylene, T is --O--; one of Z and Y is H and the other is OH, protected OH, or amino, mono- or dialkylamino, protected amino, or an amino heterocycle or Z and Y together are .dbd.O, .dbd.NOH or a derivatized oxime; including any pharmaceutically acceptable salts thereof and any stereoisomeric forms and mixtures of stereoisomeric forms thereof, are antimicrobial agents.