6-[D (-)-α-(Acylamino)-4-hydroxyphenylacetamido] penicillanic acids (1-12) were prepared by reacting amoxicillin with the active esters of nitrogen-containing heterocyclic compounds (11). These penicillins were tested for in vitro activities against S. aureus, E. coli, K. pneumoniae, P. mirabilis, and P. aeruginosa. Structure-activity relationships were discussed. The 4-hydroxyquinoline derivative (5) and the 4-hydroxy-1, 5-naphthyridine derivatives (8-11) were found to have potent antibacterial activity against E. coli, K. pneumoniae, P. mirabilis, and P. aeruginosa. Among the penicillins tested, sodium 6-[D (-)-α-(4-hydroxy-1, 5-naphthyridine-3-carboxamido)-4-hydroxyphenylacetamido] penicillanate (8) exhibited the highest activity against P. aeruginosa, its MIC being 1.56μg/ml.
