Intramolecular nucleophilic attack in 7α-aminophenylacetamido-cephalosporin esters

7α-Aminophenylacetamido-Δ3-cephalosporin esters cyclize to 3-dihydrothiazinyl-6-phenylpiperazine-2,5-diones by an intramolecular nucleophilic attack of the side-chain α-amino-function while under the same conditions the analogous 6α-aminophenylacetamidopenicillin ester does not cyclize, because of steric hindrance to intramolecular attack.