Macrocycle-embedded ß-lactams as novel inhibitors of the Penicillin Binding Protein PBP2a from MRSA.

Assuming that bicyclic ß-lactams endowed with high conformational adaptability should more easily form acyl-enzyme complexes with PBP2a than the traditional antibiotics, we have prepared a series of bis-2-oxo-azetidinyl macrocycles as potential inhibitors. The compounds are formally "head-head" (HH) cyclodimers of 1-(ω-alkenoyl)-3-(S)-(ω'-alkenoylamino)-2-azetidinones, with various lengths of the alkene chains, obtained by two successive metathesis reactions using the Grubbs catalyst. All compounds behave as acylating inhibitors of PBP2a and one ß-lactam (5c), embedded into the largest ring (32 atoms), features an activity close to that of Ceftobiprole. Conformational analyses, theoretical reactivity models and docking experiments in PBP2a cavity allow to propose a novel pharmacophore, i.e. the 3-(S)-acylamino-1-acyl-2-azetidinone ring, with the syn-conformation of the imide function, associated to a flexible macrocycle favoring the opening of the active site.

12- to 22-membered bridged ß-lactams as potential penicillin-binding protein inhibitors.

As potential inhibitors of penicillin-binding proteins (PBPs), we focused our research on the synthesis of non-traditional 1,3-bridged ß-lactams embedded into macrocycles. We synthesized 12- to 22-membered bicyclic ß-lactams by the ring-closing metathesis (RCM) of bis-ω-alkenyl-3(S)-aminoazetidinone precursors. The reactivity of 1,3-bridged ß-lactams was estimated by the determination of the energy barrier of a concerted nucleophilic attack and lactam ring-opening process by using ab initio calculations. The results predicted that 16-membered cycles should be more reactive. Biochemical evaluations against R39 DD-peptidase and two resistant PBPs, namely, PBP2a and PBP5, revealed the inhibition effect of compound 4d, which featured a 16-membered bridge and the N-tert-butyloxycarbonyl chain at the C3 position of the ß-lactam ring. Surprisingly, the corresponding bicycle, 12d, with the PhOCH(2)CO side chain at C3 was inactive. Reaction models of the R39 active site gave a new insight into the geometric requirements of the conformation of potential ligands and their steric hindrance; this could help in the design of new compounds.

DOSY-NMR analysis of ring-closing metathesis (RCM) products from ß-lactam precursors.

The discrimination between cyclomonomers and various oligomers formed during a ring-closing metathesis (RCM) process is not an easy task. Their (1)H NMR patterns are often very similar, and the use of mass spectrometry techniques is usually recommended. Here, we show that the DOSY-NMR method is a reliable tool to help in the identification of cyclomonomers versus cyclodimers by comparing the translational diffusion coefficient of the compounds issued from RCM reactions with the diffusion coefficient of their respective precursors.