Synthesis and reactivity with β-lactamases of "penicillin-like" cyclic depsipeptides

Several 7-carboxy-3-amido-3,4-dihydro-2H-1-benzopyran-2-ones have been synthesized as potential beta-lactamase substrates and/or mechanism-based inhibitors. Substituted o-tyrosine precursors were prepared by the Sorensen method and then heated in vacuo to give the lactones. These compounds are cyclic analogues of aryl phenaceturates which are known to be beta-lactamase substrates. The goal of incorporating the scissile ester group into a lactone was to retain the leaving group tethered to the acyl moiety at the acyl-enzyme stage of turnover by serine beta-lactamases, in a manner similar to that during penicillin turnover. Further, in two cases, a functionalized methylene group para to the leaving group phenoxide oxygen was incorporated. These molecules possess a latent p-quinone methide electrophile which could, in principle, be unmasked during enzymic turnover and react with an active site nucleophile. All of these compounds were found to be substrates of class A and C beta-lactamases, the first delta-lactones with such activity. Generally, k(cat) values were smaller than for the analogous acyclic depsipeptides, which suggests that the tethered leaving group may obstruct the attack of water on the acyl-enzymes. Further exploration of this structural theme might lead to quite inert acyl-enzymes and thus to significant inhibitors. Despite the apparent advantage offered by the longer-lived acyl-enzymes, the functionalized compounds were no better as irreversible inhibitors than comparable acyclic compounds [Cabaret, D.; Liu, J.; Wakselman, M.; Pratt, R. F.; Xu, Y. Bioorg. Med. Chem. 1994, 2, 757-771]. Thus, even tethered quinone methides, at least when placed as dictated by the structures of the present compounds, were unable to efficiently trap a nucleophile at serine beta-lactamase active sites.