or complete deprotection of the phosphonate ester have been realised by either acidic hydrolysis or by treatment with trimethylsilyl bromide. Evaluation against penicillin-bindingproteins has shown that our compounds are modest inhibitors of class A beta-lactamases, but have an interesting activity against R39 (D,D-peptidase/carboxypeptidase). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim