Martin L. Gilpin

Identification of a Series of Tricyclic Natural Products as Potent Broad-Spectrum Inhibitors of Metallo-β-Lactamases

ABSTRACT This work describes the discovery and characterization of a novel series of tricyclic natural product-derived metallo-β-lactamase inhibitors. Natural product screening of the Bacillus cereus II enzyme identified an extract from a strain of Chaetomium funicola with inhibitory activity against metallo-β-lactamases. SB236050, SB238569, and SB236049 were successfully extracted and purified from this extract. The most active of these compounds was SB238569, which possessed Ki values of 79, 17, and 3.4 μM for the Bacillus cereus II, Pseudomonas aeruginosa IMP-1, and Bacteroides fragilis CfiA metallo-β-lactamases, respectively, yet none of the compounds exhibited any inhibitory activity against the Stenotrophomonas maltophilia L-1 metallo-β-lactamase (50% inhibitory concentration > 1,000 μM). The lack of activity against angiotensin-converting enzyme and serine β-lactamases demonstrated the selective nature of these compounds. The crystal structure of SB236050 complexed in the active site of CfiA has been obtained to a resolution of 2.5 Å. SB236050 exhibits key polar interactions with Lys184, Asn193, and His162 and a stacking interaction with the indole ring of Trp49 in the flap, which is in the closed conformation over the active site groove. SB236050 and SB238569 also demonstrate good antibacterial synergy with meropenem. Eight micrograms of SB236050 per ml gave rise to an eightfold drop in the MIC of meropenem for two clinical isolates of B. fragilis producing CfiA, making these strains sensitive to meropenem (MIC ≤ 4 μg/ml). Consequently, this series of metallo-β-lactamase inhibitors exhibit the most promising antibacterial synergy activity so far observed against organisms producing metallo-β-lactamases.

Total synthesis of (±)-clavulanic acid

A formal total synthesis of racemic clavulanic acid has been achieved commencing with (±)-4-methylthioazetidin-2-one (3).

Total synthesis of β-lactamase inhibitors related to clavulanic acid

(E)-3-Methoxycarbonylmethylene-7-oxo-4-oxa-1-azabicyclo[3.2.0]heptane (6), its geometric isomer (7), and a related vinyl chloride (8) have been obtained by a three-step synthesis from 4-methylthioazetidin-2-one.

Wittig reactions with β-lactam carbonyls: a convenient means of protection. X-Ray crystal structure of p-nitrobenzyl-(2R,5R)-Z-7-methoxycarbonylmethylene-Z-3-(β-phthalimidoethylidene)-4-oxa-1-azabicyclo[3.2.0]heptane-2-carboxylate

Clavulanic acid derivatives and penicillin Vesters undergo Wittig reactions at the β-lactam carbonyl and the β-lactam can be regenerated by low-temperature ozonolysis; the crystal structure of one of the products is reported.

Wittig reactions with β-lactam carbonyl functions. The effect of C-7 substitution on the chemistry of penicillins and clavulanic acid derivatives

The Wittig reaction of phosphoranes and phosphonates with the carbonyl function of mono- and bicyclic β-lactams has been studied and the influence of phosphorane and β-lactam reactivities on the outcome of the reaction noted. The utility and general chemistry of the Wittig products from penicillins and clavulanic acid esters is also reported.

Total synthesis of clavulanic acid analogues via isomerisation of 7-oxo-4-oxa-1-azabicyclo[3.2.0]hept-2-enes

Sodium Z-3-benzylidene-7-oxo-4-oxa-1-azabicyclo[3.2.0]heptane-2-carboxylate (1b), a novel clavulanic acid analogue, has been prepared via the isomerisation of the bicycloheptane (2a).