SYNTHESIS OF (5R)-(Z)-6-(1-METHYL-1,2,3-TRIAZOL-4-YLMETHYLENE)PENEM-3-CARBOXYLIC ACID, A POTENT BROAD-SPECTRUM BETA-LACTAMASE INHIBITOR, FROM 6-AMINOPENICILLANIC ACID

(5R)-(Z)-6-(1-Methyl-1,2,3-triazol-4-ylmethylene)penem-3-carboxylic acid 34 (BRL 42715) has been prepared from 6-aminopenicillanic acid 4 (6-APA) by a short, stereoselective and efficient route via the novel intermediate. p-methoxybenzyl (5R,6S)-6-bromopenem-3-carboxylate 17. Elaboration of 6-APA 4 to the azetidinone disulfide 10 by established methodology, followed by reductive formylation provided the crystalline C-4 formylthio-azetidinone derivative 29. Cyclization of the oxalimide 28, obtained by ozonolysis of the formylthio derivative 29. to the crystalline 6 alpha-bromopenem ester 17 was effected by way of the phosphite-mediated carbonyl-carbonyl coupling reaction.