A cephem compound of the formula: ##STR1## wherein R.sup.1 is a group of the formula: ##STR2## in which R.sup.5 is amino or protected amino, R.sup.6 is hydrogen, lower alkyl, lower alkenyl, lower alkynyl, carboxy(lower) alkyl, protected carboxy(lower) alkyl, cyclo(lower) alkyl or cyclo (lower)alkenyl, and Z is CH or N; R.sup.2 is hydrogen, phenyl, pyridyl which may have a lower alkyl group, or cyano; and R.sup.3 is carboxy or protected carboxy; and pharmaceutically acceptable salt thereof.
In the reaction of 2-ethoxy-1,3-oxathiolane with carbonylcompounds in the presence of ZnCl2 or HgCl2, it has been found that only the breaking of the endocyclic bond (C–O or C–S bond) occurs, while the breaking of the exocyclic C–O bond to give the 1,3-oxathiolan-2-ium ion is unfavorable. This behavior is different from that of 2-ethoxy-1,3-dithiolane, in which the breaking of the endocyclic C–S bond
3-oxathiolane was deprotonated at the 4-position, followed by the cycloelimination of the resultant anion, leading to ethyl formate and the ethenethiolate anion. The anion was trapped by several chloromethyl alkyl ethers and sulfides to afford alkoxy(vinylthio)methanes and alkylthio(vinylthio)methanes respectively.