The synthesis and antimicrobial activity of the methylenedioxy positional isomers, l-ethyl-1, 4- dihydro-5, 6-methylenedioxy-4-oxo-3-quinolinecarboxylic acid (9) and l-ethyl-l, 4-dihydro-7, 8-methylenedioxy-4-oxo-3-quino-linecarboxylic acid (17), of oxolinic acid (18) have been accomplished. Isomer 9 was prepared by the reaction of N-ethyl-6, 7-methylenedioxyisatoic anhydride with sodioethyl formylacetate [LA Mitscher, HE Gracey, GW Clark 111, and T. ...