Kinetics of Action of a Two-Stage Pro-Inhibitor of Serine β-Lactamases
摘要:
beta-Lactamase inhibitors are important in medicine in the protection of beta-lactam antibiotics from beta-lactamase-catalyzed destruction. The most effective inhibitors of serine beta-lactamases covalently modify the enzyme active site. We have recently studied O-acyl and O-phosphyl hydroxamates as a new class of such inhibitors. In this paper, we describe our studies of the N-acyl derivatives of a cyclic O-acyl hydroxamic acid, 3H-benzo[d][1,2]oxazine-1,4-dione, and, in particular, the N-tert-butoxycarbonyl derivative. This compound is not a beta-lactamase inhibitor itself but undergoes spontaneous hydrolysis in aqueous solution, yielding an O-phthaloyl hydroxamic acid, which is a beta-lactamase inhibitor. This compound spontaneously, but reversibly, cyclizes in solution to form phthalic anhydride, which is also a beta-lactamase inhibitor. Both inhibitors react to form the same transiently stable phthaloyl-enzyme complex. Thus, we have a two-step cascade, beginning with a pro-inhibitor, in which each step leads to a different inhibitor, presumably with different enzyme specificities. The kinetics of these transformations have been elucidated in detail. The phthaloyl derivatives, where the free carboxylate is important for facile reaction with the enzyme, represent a new lead for serine beta-lactamase inhibitors. Analogues can be conveniently constructed in situ by reaction of nucleophiles with phthalic anhydrides and then screened for activity. Active hits may then become new leads.
Syntheses of structurally diverse amino acids, including δ-hydroxylysine, using the acyl nitroso Diels–Alder reaction
作者:Lee Bollans、John Bacsa、Daniel A. O’Farrell、Scott Waterson、Andrew V. Stachulski
DOI:10.1016/j.tetlet.2010.02.076
日期:2010.4
with fully controlled relative stereochemistry, the acyl nitroso Diels–Alder (ANDA) reaction is ideally suited to the synthesis of structurally diverse, including hydroxylated, amino acids. The major issue to be tackled is that of regiochemistry in the ANDA addition to unsymmetrical dienes. The transformation of three diverse types of ANDA adducts into amino acids is described, in particular, the synthesis
The mechanism of amino-migration of O-phenylhydroxylamine (1a) was studied. It was found that 1 rearranges to give 2-aminophenol (50%) and 4-aminophenol (7%) in trifluoroacetic acid (TFA). The predominance of the ortho rearrangement of 1 clearly distinguishes this process from the Bamberger rearrangement. From cross-coupling experiments employing stable isotopes, it was clarified that the ortho rearrangement proceeds intramolecularly and the para rearrangement involves both intra- and intermolecular processes. Good first-order kinetics were obtained for the rearrangement. The Hammett plot (sigma+) with a large negative slope (rho = -7.8) indicates that initial heterolytic N-O bond cleavage of 1 occurs and generates a positive charge on the oxygen atom with considerable delocalization into the aromatic ring. An ion-molecule pair involving a phenoxenium ion and an ammonia molecule as an intermediate rationalizes all of the results. In this pair, intramolecular combination to the ortho position proceeds preferentially over that to the para position. Formation of catechol and hydroquinone can be explained in terms of nucleophilic attack of TFA on the phenoxenium ion in a solvent-separated pair.