Ampicillin → Penicillanic Acid

0.75 ml (about 3 mmol) of N,O-bis(trimethylsilyl) acetamide was added under anhydrous conditions, to a suspension of 1.05 g (3 mmol) of D(-) 6-α-amino-benzylcarbonamidopenicillanic acid (anhydrous D(-) ampicillin) in 50 ml of dry dichloromethane and after 30 minutes stirring at about 20°C, a clear solution was obtained. Then, a solution of 0.6 g (3 mmol) of diethoxyphosphinylisothiocyanate [(C2H5O)2P(O)NCS] in 10 ml of dichloromethane was added at 0°-5°C over a period of 10 minutes and as the reaction did not cause a rise in temperature, the reaction mixture was further stirred at 5°C for 150 minutes. A small amount of precipitate was formed and according to thin-layer chromatograms, ampicillin was no longer present in the reaction mixture. The reaction mixture was poured into 75 ml of ice-water and the pH was brought to 7.0. The layers were separated and as the organic layer did contain part of the desired penicillin, the aqueous layer was extracted once with 75 ml of ethyl acetate at pH 7.0. The organic layers were combined, concentrated to some extent in vacuo in order to remove dichloromethane, and subsequently treated with n-hexane until a precipitate was formed, which was vacuum filtered off, washed with n-hexane and dried in vacuo to constant weight to obtain 0.5 g (30%) of D-6-[α-{3-diethoxyphosphinyl)-thioureido}benzylcarbonamido]-penicillanic acid.