L-isomers of cephalosporins of formula (I) are new: (where R1 = phenyl, thienyl or furyl; R3 = H, alkyl, phenylalkyl, diphenylalkyl, trialkylsilyl, trialkylsannyl, salt-forming cations, substd. phenylalkyl or CH2OCOR. R = alkyl, phenyl or phenylalkyl, the last 2 opt. substd. on the ring by halo, alkyl or alkoxy. R4 is 1-R5-1H-tetrazol-5-yl, 2-R5-1,3,4-thiadiazol-5-yl, 2-R5-1,3,4-oxadiazol-5-yl, 3-R5-1,2,4-thiadiazol-5-yl, 1,2,3,4-thiatriazol-5-yl, R5-substd. isothiazol-5-yl, R5 - substd. isoxazol-5-yl or N-oxypyrid-2-yl. R5 = H or 1-4C alkyl. All 'alkyl' etc. are '-ower'). They are prepd. e.g. by reacting the appropriate alpha-aminoacetamido cpd. with an alkali(ne earth) cyanate. (I) are broad-spectrum antibacterials effective against Gram negative and positive species. They are more effective than the corresp. D-isomers of D,L-racemates against bacteria which produce beta-lactamase e.g. Enterobacter, indole-positive Proteus, resistant E.coli and Serratia. A typical cpd. is 7beta- (aminocarbonyl)amino (L-2-thienyl)acetyl amino -3- (1-m- ethyl-1H-tetrazol-5-yl)thio methyl -8-oxo-5-thia-1-azabicyclo 4,2- ,0 oct-2-ene-2-carboxylic acid.
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