Compounds of general formula I ##STR1## (wherein R represents NH.sub.2 -- or an acylated or silylated amino group;R.sup.2 represents a hydrogen, halogen, alkyl, aryl, carboxyl or lower alkoxycarbonyl group;R.sup.3 is hydrogen or a carboxyl blocking group;B is >Sor>S.fwdarw.O (.alpha.- or .beta.-); and the dotted line represents .DELTA..sup.2 or .DELTA..sup.3 unsaturation) and salts, solvates and esters thereof are described.Compounds where R.sup.3 is hydrogen, the dotted line represents .DELTA..sup.3 unsaturation and R is a group of formula ##STR2## (where R.sup.a is an optionally substituted heterocyclic aryl group having one or more hetero atoms selected from S, N and O in the ring, R.sup.b is an optionally substituted aryl group, R.sup.c is hydrogen, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, aryl or aralkyl, R.sup.d is as defined for R.sup.a or is an optionally substituted group and X is amino, hydroxyl, acylated hydroxyl, carboxyl or esterified carboxyl)and especially the compounds in which B is >S, have valuable antibiotic activities.Compounds in which the --CH.dbd.CH--C.tbd.C--R.sup.2 group is in the trans configuration have been found to have good oral absorption.The remaining compounds of formula (I) are valuable as intermediates in processes for the preparation of the above active antibiotics, which processes are also described.