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1. Phenoxymethylpenicillin sulphoxide 4-methoxybenzyl ester was labelled with 3H in its 2-beta-methyl group. Its specific radioactivity was 362 mCi/mmol. 2. Removal of the side chain of this compound yielded the corresponding ester of 6-aminopenicillanic acid sulphoxide and coupling of the latter with the appropriate protected alpha-aminoadipic acid gave 4-methoxybenzyloxycarbonylisopenicillin N sulphoxide di-4-methoxybenzyl ester or the corresponding derivative of penicillin N. 3. Removal of the protective groups by hydrogenolysis and reduction of the sulphoxide group yielded 3H-labelled isopenicillin N or penicillin N. 4. 3H-labelled phenoxymethylpenicillin sulphoxide was obtained by hydrogenolysis from its 4-methoxybenzyl ester. Reduction of its sulphoxide group and subsequent removal of the side chain gave 3H-labelled 6-aminopenicillanic acid.

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