Synthesis and antibacterial activity of cephalosporins having hydroxamic acid at C-7 position

Total synthesis of (+)-sieboldine a: evolution of a pinacol-terminated cyclization strategy

This article describes synthetic studies that culminated in the first total synthesis of the Lycopodium alkaloid sieboldine A. During this study, a number of pinacol-terminated cationic cyclizations were examined to form the cis-hydrindanone core of sieboldine A. Of these, a mild Au(I)-promoted 1,6-enyne cyclization that was terminated by a semipinacol rearrangement proved to be most efficient. Fashioning the unprecedented N-hydroxyazacyclononane ring embedded within the bicyclo[5.2.1]decane-N,O-acetal moiety of sieboldine A was a formidable challenge. Ultimately, the enantioselective total synthesis of (+)-sieboldine A was completed by forming this ring in good yield by cyclization of a protected-hydroxylamine thioglycoside precursor.

Vanilloid and isovanilloid analogues as inhibitors of methionyl-tRNA and isoleucyl-tRNA synthetases

As aminoacyl adenylate surrogates, a series of methionyl and isoleucyl phenolic analogues containing bioisosteric linkers mimicking ribose have been investigated. Inhibition of synthesized compounds to the aminoacylation reaction by the corresponding Escherichia coli methionyl-tRNA and isoleucyl-tRNA synthetases indicated that 18 was found to be a potent inhibitor of isoleucyl-tRNA synthetase. A molecular modeling study demonstrated that in 18, isovanillate and hydroxamate served as proper surrogates for adenine and ribose in isoleucyl adenylate, respectively.

Antimicrobial effects of novel siderophores linked to beta-lactam antibiotics

As a strategy to increase the penetration of antibiotic drugs through the outer membrane of gram-negative pathogens, facilitated transport through siderophore receptors has been frequently exploited. Hydroxamic acids, catechols, or very close isosteres of catechols, which are mimics of naturally occurring siderophores, have been used successfully as covalently linked escorting moieties, but a much wider diversity of iron binding motifs exists. This observation, coupled to the relative lack of specificity of siderophore receptors, prompted us to initiate a program to identify novel, noncatechol siderophoric structures. We screened over 300 compounds for their ability to (1) support growth in low iron medium of a Pseudomonas aeruginosa siderophore biosynthesis deletion mutant, or (2) compete with a bactericidal siderophore-antibiotic conjugate for siderophore receptor access. From these assays we identified a set of small molecules that fulfilled one or both of these criteria. We then synthesized these compounds with functional groups suitable for attachment to both monobactam and cephalosporin core structures. Siderophore-beta-lactam conjugates then were tested against a panel of Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus strains. Although several of the resultant chimeric compounds had antimicrobial activity approaching that of ceftazidime, and most compounds demonstrated very potent activity against their cellular targets, only a single compound was obtained that had enhanced, siderophore-mediated antibacterial activity. Results with tonB mutants frequently showed increased rather than decreased susceptibilities. suggesting that multiple factors influenced the intracellular concentration of the drugs.