Novel 7-azido-3-cephem compounds are prepared via .alpha.-amino-phosphonoacetate esters. The cephem compounds are intermediates for the preparation of novel and known useful antibiotic cephalosporins.
Cephalosporin antibiotics of the formula: ##STR1## are produced by cycloaddition of a glycine derivative of the formula ##STR2## with a thiazine derivative of the formula ##STR3##