The synthesis of benzylpenicillin and 6-aminopenicillanic acid, labeled with tritium in the β-methyl group, is described. Benzylpenicillin S-sulfoxide benzyl ester is refluxed in benzene and tritiated water and is successively debenzylated and deoxygenated to (β-methyl-3H)-benzylpenicillin. Removal of the side chain with a bacterial acylase gives (β-methyl-3H)-6-aminopenicillanic acid.
