Stereoselective synthesis of optically active bicyclic β-lactam carboxylic acids that target pilus biogenesis in pathogenic bacteria

Optically active bicyclic beta-lactams were synthesized, starting from 2-H-Delta(2)-thiazolines and Meldrum's acid derivatives. Several methods to accomplish an ester hydrolysis without damaging the beta-lactam framework were investigated. A rapid CsOH saponification of the beta-lactam methyl esters was developed and protonation of the Cs-carboxylates by Amberlite (IR-120 H+) afforded a series of bicyclic beta-lactam carboxylic acids. Moreover, a convenient method for the synthesis of 2-H-Delta(2)-thiazolinecarboxylic acid methyl ester 2 was developed. Bicyclic beta-lactam carboxylic acids 7a-g and aldehydes 4a-d were screened for their affinity to the bacterial periplasmic chaperone PapD using a surface plasmon resonance technique. beta-Lactams substituted with large acyl substituents showed better binding to the chaperone than the native C-terminal peptide PapG 8, demonstrating that bicyclic beta-lactams constitute a new class of potential bacterial chaperone inhibitors.