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923
8.98 (br, s, 2H, NH2), 8.13 (s, 1H, H2-quinoline), 7.99 (d, 1H,
H5-quinoline, J = 13.17 Hz), 7.25 (d, 1H, H8-quinoline,
J = 7.31 Hz), 4.55 (q, 2H, NCH2CH3, J = 6.96 Hz), 3.61–
3.60 (m, 4H, piperazine), 3.33–3.00 (m, 4H, piperazine),
1.41 ppm (t, 3H, NCH2CH3, J = 6.95 Hz). 13C-NMR (DMSO-
d6, 100 MHz) δ: 205.22, 176.18, 165.96, 161.01, 148.52,
141.16, 138.20, 118.51, 111.34, 111.17, 106.48, 105.65,
50.38, 49.92, 44.81, 14.31 ppm. Anal. C18H19FN6O5S2 (C,
H, N). (ClogP: Vb = –0.9712; VIIb = 0.1567).
(m, 4H, cyclopropyl). MS (FAB) m/z: 554 [M + 1]. Anal.
C21H23F2N7O5S2 (C, H, N). (ClogP: sparfloxacin = –1.0256;
Vc = –1.3413; VIIc = 0.1023).
5.2.4. 7-[4-(5-Amino-1,3,4 thiadiazole-2-sulfonyl)]-3′-
methylpiperazinyl-1 cyclopropyl-6-fluoro-8-methoxy-4-oxo-1,4-
dihydro-quinoline-3-carboxylic acid (VIId)
The compound VIId was synthesized by reacting a mixture
of gatifloxacin Vd (1.20 g, 32 mmol) and 5-acetylamino-[1,3,4]
thiadiazole-2-sulfonyl chloride IV (0.845 g, 35 mmol) using the
general procedure as described above. The intermediate 7-[4-(5-
acetylamino-[1,3,4]thiadiazole-2-sulfonyl)-3′-methylpiperazin-
1-yl]-1-cyclopropyl-6-fluoro-8-methoxy-4-oxo-1,4-dihydro-qui-
noline-3-carboxylic acid VId was collected, washed with cold
acetone, recrystallized from aqueous DMF and obtained as pale
yellow solid in 72% yield. M.p. 204–207 °C (dec.); IR (KBr)
5.2.3. 5-Amino-7-[4-(5-amino-[1,3,4]-thiadiazole-2-sulfonyl)-
3′,5′-dimethylpiperazin-1 -yl]-1-cyclopropyl-6,8-difluoro-4-
oxo-1,4-dihydro-quinoline-3-carboxylic acid (VIIc)
The starting material 5-acetylamino-1-cyclopropyl-7-(3′,5′-
dimethyl-piperazin-1-yl)-6,8-difluoro-4-oxo-1,4-dihydro-qui-
noline-3-carboxylic acid Vc used in the synthesis of title com-
pound VIIc was prepared by refluxing a mixture of sparflox-
acin (3.92, 0.01 mol), acetic anhydride (10 ml) and pyridine
(5 ml) on a steam bath for 4 h, left over night, the crystalline
mass separated was filtered, recrystallized from aqueous DMF
and obtained as yellow crystals in 70% yield. M.p. 212–214 °C
(dec.); IR (KBr) nmax: 3420, 3312, 2977, 2845, 1721, 1659,
1618, 1444, 1320, 1234, 1046, 927, 773 cm−1. 1H-NMR
(DMSO-d6, 400 MHz) δ: 14.71 (s, 1H, COOH), 9.70 (d, 1H,
NH, piperazine and 1H, NH of NHCOCH3), 7.48 (s, 1H, H2-
quinoline), 3.97 (s, 3H, CH3 of NHCOCH3), 3.24 (m, 6H,
3′,5′-dimethyl of piperazine), 3.22–2.68 (m, 6H, piperazine
and 1H cyclopropyl), 1.35–1.01 ppm (m, 4H, cyclopropyl).
EIMS m/z: found 434.24 [M + 1]; calcd. 434.17. Anal
C21H24F2N4O4.
The compound VIIc was synthesized by reacting a mixture
of compound Vc (1.38 g, 32 mmol) and 5-acetylamino-[1,3,4]
thiadiazole-2-sulfonyl chloride IV (0.845 g, 35 mmol) using the
general procedure. The intermediate 5-acetylamino-7-[4-(5-
acetylamino-[1,3,4] thiadiazole-2-sulfonyl)-3′,5′-dimethyl-pi-
perazin-1-yl]-1-cyclopropyl-6,8-difluoro-4-oxo-1,4-dihydro-
quinoline-3-carboxylic acid VIc was obtained as yellow crystals
in 63% yield. M.p. 231–232 °C (dec.); IR (KBr) nmax: 3410,
3250, 3018, 2948, 2856, 1724, 1680, 1620, 1458, 1334, 1230,
1163, 1060, 940, 739 cm−1. 1H-NMR (DMSO-d6, 400 MHz) δ:
14.60 (s, 1H, COOH), 9.82, 3.81 (d, 2H, NH, NH; d, 6H, CH3,
CH3 of NHCOCH3 and NHCOCH3), 8.42 (s, 1H, H2-quino-
line), 3.35 (m, 6H, 3′,5′-dimethyl piperazine), 3.14–2.18 (m,
6H, piperazine and 1H cyclopropyl), 1.43–1.01 ppm (m, 4H,
cyclopropyl). 13C-NMR (DMSO-d6, 100 MHz) δ: 181.60,
176.02, 165.96, 163.20, 160.97, 153.93, 151.46, 148.31,
145.16, 137.03, 129.28, 128.52, 119.50, 111.26, 48.41, 47.71,
35.72, 28.94, 21.23, 9.13 ppm. Anal. C25H27F2N7O6S2 (C, H,
N). Hydrolysis of VIc yielded the title compound VIIc, which
was purified from column chromatography, recrystallized from
aqueous DMF and obtained as yellow crystals in 70% yield. M.
p. 208–210 °C (dec.); IR (KBr) nmax: 3468, 3401, 3122, 2950,
2868, 1730, 1619, 1461, 1334, 1158, 1060, 972, 740 cm−1. 1H-
NMR (DMSO-d6, 400 MHz) δ: 14.84 (s, 1H, COOH), 10.15 (s,
2H, NH2), 8.82 (s, 1H, H2-quinoline), 4.25 (br, s, 2H, NH2, C5-
quinoline), 3.63 (m, 6H, 3′,5′-dimethyl of piperazine), 3.34–
2.12 (m, 6H, piperazine and 1H cyclopropyl), 1.45–1.16 ppm
n
max: 3450, 3265, 3168, 2940, 2794, 1721, 1683, 1617, 1515,
1448, 1303, 1173, 1035, 947, 743 cm−1. Hydrolysis of later
yielded title compound VIId, which was purified from column
chromatography, recrystallized from aqueous DMF and ob-
tained as colorless crystals in 82% yield. M.p. 170–172 °C
(dec.). IR (KBr) nmax: 3435, 3258, 3158, 2914, 2779, 1697,
1629, 1550, 1430, 1370, 1228, 1102, 1063, 962, 741 cm−1. H
1
-NMR (DMSO-d6, 400 MHz) δ: 14.84 (s, 1H, COOH), 8.99
(br, s, 2H, NH2), 8.74 (s, 1H, H2-quinoline), 7.84 (d, 1H, H5-
quinoline, J = 12.01 Hz), 3.81 (s, 3H, OCH3), 3.45–3.26 (m,
7H, piperazine and 1H, CH, cyclopropyl), 1.28 (m, 3H, 3′-
methylpiperazine), 1.26–1.05 ppm (m, 4H, cyclopropyl). 13C-
NMR (DMSO-d6, 100 MHz) δ: 201.33, 177.43, 167.06,
155.28, 151.06, 146.65, 141.77, 137.45, 118.77, 111.32,
109.10, 105.69, 46.34, 42.49, 35.94, 22.98, 16.22, 9.61 ppm.
Anal. C21H23FN6O6S2 (C, H, N). (ClogP: Vd = 0.6868;
VIId = 0.4411).
5.3. Biological assay
5.3.1. In vitro evaluation of antibacterial activity
The MIC determination of the tested compounds was inves-
tigated in side-by-side comparison with ciprofloxacin, norflox-
acin, sparfloxacin and gatifloxacin against Gram-positive (Cor-
ynebacterium, S. aureus, E. faecelis, S. epidermidis, Bacillus
sp.) and Gram-negative bacteria (P. aeruginosa, E. coli, Kleb-
siella sp., C. freundi, Proteus sp.) by broth microdilution meth-
od [41,42]. Serial dilutions of the test compounds and refer-
ence drugs were prepared in Mueller–Hinton agar. Drugs
(6.4 mg) were dissolved in dimethylsulfoxide (DMSO, 1 ml)
and the solution was diluted with distilled water (9 ml). Further
progressive dilutions with melted Mueller–Hinton agar were
performed to obtain the required concentrations of 1, 5, 10,
20 and 50 μg ml–1. The petri dishes were inoculated with 1–
5 × 104 cfu ml–1 and incubated at 37 °C for 18 h. The MIC
was the lowest concentration of the tested compound that yield
no visible growth on the plate. To ensure that the solvent had
no effect on the bacterial growth, a control was performed with
the test medium supplemented with DMSO at the same dilu-
tions as used in the experiments.