Sulfur-free Penicillin Derivatives. II. Functionalization of the Methyl Groups

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Abstract
Allylic bromination of anhydropenicillin chlorination products by N-bromosuccinimide (NBS) is described. With two molar-equivalents of NBS both methyl groups are functionalized; with one molar-equivalent of NBS a mixture of Z and E monobrominated isomers is obtained.Nitrogen and oxygen nucleophiles attack the dibrominated derivatives at the β-lactam carbonyl group, with concomitant ring-opening. However, the mono-brominated derivatives undergo ready displacement by azide and formate ions. The mechanism of these reactions, leading to preferred formation of Z-isomer, is discussed.Cyclization of the allylic formates yields sulfur-free derivatives corresponding to the cephalosporin Cc class of compounds.

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