Structure of oxolinic acid, a potent antibacterial agent. 1‐Ethyl‐1,4‐dihydro‐6,7‐methylenedioxy‐4‐oxo‐3‐quinolinecarboxylic acid, C13H11NO5

M r = 2 6 1 . 2 , monoclinic, P21/c, a= 7.182(2), b = 1 0 . 5 7 5 ( 2 ) , c = 1 4 . 7 5 8 ( 2 ) A , r = 94.26 (1) °, V = 1117.8/k 3, Z = 4, D x = 1.552 Mg m -3, Cu Ka, it = 1.5418/k, /z -0.98 mm -l, F(000) = 544, T = 294 K, final R = 0.038 for 1565 observed reflections. The molecule is planar within +0 .12/k except for the terminal carbon atom of the N-ethyl group, which is displaced 1.36A from the mean plane. There is considerable double-bond localization in the benzene-ring moiety, and a significant difference between the two N--C bond lengths in the pyridine ring, the bond adjacent to the benzene ring being longer by 0.058 (2)A. The crystal structure features a bifurcated hydrogen bond with a strong intramolecular component from the carboxylic acid group to the neighbouring carbonyl oxygen and a weak intermolecular component from the same donor to 0(9) in the dioxole ring. highly active antibacterial agents. These compounds cause a preferential and reversible inhibition of DNA synthesis by inhibiting DNA gyrase (Sugino, Peebles, Kreuzer & Cozzarelli, 1977; Gellert, Mizuuchi, O'Dea, Itoh & Tomizawa, 1977). The crystal structure of nalidixic acid was determined by Achari & Neidle (1976) from photographic data and refined subsequently from more precise diffractometric data (Huber, Sake Gowda & Acharya, 1980). The crystal structure of 5-aminooxolinic acid (3), which shows some antibacterial activity as well, has also been investigated (Czugler, Argay, Frank, M6szhros, Kutschabsky & Reck, 1976). It is of some interest to compare the structure of oxolinic acid with these, and with the molecular structures of some other closely related antibacterial agents for which no X-ray data are yet available. Introduetlon. Oxolinic acid (1) and the less potent but closely related nalidixic acid (2) belong to a group of * Issued as NRCC No. 24238. t Author to whom correspondence should be addressed. 0108-2701/85/071052-04501.50 0 0 NH 2 0 o ooo