Starting reagents were purchased from Sigma Aldrich (St. Louis, Missouri, USA). USA) and were used without any purification. 1H-NMR were recorded on Bruker DPX-300 (Billerica, MA, USA) instrument (300 MHz) and Bruker 500 MHz-Avance III (500 MHz) spectrometer (Billerica, MA, USA), while 13C-NMR spectra were recorded on Bruker 500 MHz-Avance III (at 125 MHz) at room temperature using TMS as reference. The solvent used for NMR was DMSO-d6. Chemical shifts are reported in parts per million (δ/ppm). Coupling constants are reported in Hertz (J/Hz). The peak patterns are indicated as follows: s, singlet; d, doublet; t, triplet; q, quadruplet; m, multiplet; dd, doublet of doublets, br/m, broad multiplet and br/s, broad singlet. Infrared spectra were measured on an Shimadzu Prestige-21 FT-IR instrument (Shimadzu Corporation, Kyoto, Japan). The signals are reported in reciprocal centimeters (cm−1). Mass spectra were recorded with LC-MS/MS-4500Qtrap Triple quadrupole liquid chromatograph mass-spectrometer from ABSciex USA (ESI) (AB Sciex LLC, Framingham, MA, USA) using methanol and 1% formic acid as mobile phase for infusion. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer model 2400 analyzer (PerkinElmer, Shelton, CT, USA). Microwave-assisted synthesis was performed on Biotage® Initiator+ Alstra™, which is a fully automated microwave plus peptide synthesizer (Biotage®, Uppsala, Sweden). The melting points were determined in open capillary and are uncorrected.
General Procedure for the Synthesis of Schiff Bases of Isoniazid
(a) Microwave-assisted synthesis. Various aldehydes (1mM) each in ethanol were added to isoniazid (1mM) in ethanol with intermittent shaking separately. To this mixture, glacial acetic acid (GAA, 2 drops) was added dropwise with shaking. The microwave glass tubes were closed with the help of Teflon septum and aluminum caps. The microwave synthesis was performed by irradiating the reaction mixture under microwave (5–10 W, 0–1 atm. pressure), at 85 °C until the completion of the reaction (10 to 20 min.). TLC was used to monitor the progress of the reaction.
(b) Conventional method. Various aldehydes (2mM) each in ethanol were added to isoniazid (2mM) in ethanol with intermittent shaking separately. To this mixture, glacial acetic acid (GAA) was added dropwise with shaking and then refluxed for 6 to 8 h. The completion of reaction was monitored by thin layer chromatography using chloroform:methanol:acetic acid (90:10:0.5 v/v/v) as solvent system. The reaction mixture was concentrated, and the residue obtained washed with water and dried. The crude product obtained was recrystallized using ethyl alcohol.
(E)-N’-((5-(2-Nitrophenyl)furan-2-yl)methylene)isonicotinohydrazide (1): (yield, time (a) 99%, 15 min. (b) 95%, 6h, yellow crystalline), m.p. 180–182 °C (decomposed). 1H-NMR (300 MHz, DMSO-d6): δ: 12.04 (s, 1H, NH-N=); 8.76–8.74 (br/m, 2H, H2′ and H6′ of pyr); 8.35 (s, 1H, N=CH-); 7.94–7.91(br/m, 2H, H3′’ and H6′’ of ArH); 7.72–7.85 (m, 3H, H3′, H5′ of pyr and H5′’ of ArH); 7.58–7.63 (br, 1H, H4′’ of ArH); 7.03 (d, 1H, J = 12 Hz, H4 of furan), 6.99 (d, 1H, J = 12 Hz, H3 of furan). LCMS/MS for C17H12N4O4 (ESI+ion): m/z = 337.6 [M + 1], 336.6[M+], 319.2, 216, 148, 121, 104.8, 80, 79. IR (KBr, cm−1): 3442.9 (N-H), 3209.6 (C-H, arom.), 1660.7 (C=O), 1616.4 (C=C), 1533.4 (N-H), 1410.0 (C-N), and 690.5 (C-H, arom.). Elemental Analysis C17H12N4O4, Calculated C, 60.71; H, 3.60; N, 16.66; Found C, 60.49; H, 3.59; N 16.62. Rf value = 0.69 (chloroform:methanol:acetic acid: 90:10:0.5 v/v/v).
(E)-N’-(4-Fluoro-3-nitrobenzylidene)isonicotinohydrazide (2): (yield, time (a) 98%, 10 min. (b) 95%, 6 h, brown crystals), m.p. 200–205 °C (decomposed). 1H-NMR (500 MHz, DMSO-d6): δ: 12.32 (1H; s; NH-N=); 8.81(d, 2H, J = 5.0 Hz, H-2′ and H-6′, pyr); 8.54 (1H, s, N=C-H), 8.51 (d, 1H, J = 6.8 Hz, H-2), 8.19 (d, 1H, J = 7.0 Hz, H-6), 7.83 (d, 2H, J = 5.0 Hz, H-3′ and H-5′, pyr), 7.69 (dd, 1H, 3JH-F = 10.5 Hz, H-5); 13C-NMR (125 MHz, DMSO-d6): δ 162.38 (C=O), 155.77 (d, 1JC-F = 263 Hz, C-4), 150.84 (C-2′ and C-6′, pyr), 146.15 (C′′), 140.64 (C-4′), 137.78 (d, 2JC-F = 7.50 Hz, C-2), 134.95 (d, 2JC-F = 9.5 Hz, C-3), 131.98 (C-1), 124.80 (C-6), 122.02 (C-3′ and C-5′, pyr), 119.74 (d, 2JC-F = 21.50 Hz, C-5). LC-MS/MS for C13H9FN4O3 (ESI+ion): m/z = 289.6 [M+1], 288.6, 242.8, 123, 121, 104, 80, 79. IR (KBr, cm−1): 3446.8 (N-H), 3201.8, 3118.9, 3086.1 (C-H, arom.), 1674.2 (C=O), 1618.3 (C=C), 1577.8 (N-H), 1537.3 (C=C), 1411.9 (C-N), 1159.2 (C-F), and 752.2 and 694.4 (C-H, arom.). Elemental Analysis C13H9FN4O3, Calculated C, 54.17; H, 3.15; N, 19.44; Found C, 54.29; H, 3.16; N 19.51. Rf value = 0.52 (chloroform: methanol: acetic acid:90:10:0.5 v/v/v).
(E)-N’-(2-Fluoro-5-nitrobenzylidene)isonicotinohydrazide (3): (yield, time (a) 98.5%, 20 min. (b) 95.0%, 8 h, off white, fine crystals), m.p. 208–210 °C; 1H-NMR (300 MHz, DMSO-d6): δ: 12.38 (s, 1H, NH-N=C); 8.82(d, J = 5.1 Hz, 2H, H-2′ and H-6′, pyr), 8.70 (s/br, N=C-H, overlapped with 1H, H-6), 8.35-8.37 (m/br, 1H, H-4), 7.85 (d, J = 5.1 Hz, 2H, H-3′ and H-5′, pyr), 7.64 (dd, 1H, 3JH-F =10.5 Hz, H-3). LCMS/MS for C13H9FN4O3 (ESI+ion): m/z = 289.8 [M + 1], 288.8, 242.8, 126, 123, 121, 105, 80, 79. IR (KBr, cm−1): 3427.5 (N-H), 3228.8, 3203.8, 3074.5 (C-H, arom.), 1656.9(C=O), 1626.0 (C=C), 1602.9 (C=C), 1556.6 (N-H), 1525.7 (C=C), 1423.5 (C-N), 1143.8 (C-F), 746.5 (C-H, arom.), and 682.8 (C-H, arom.); Elemental Analysis C13H9FN4O3, Calculated: C, 54.17; H, 3.15; N, 19.44; Found C, 54.32; H, 3.14; N 19.56. Rf value = 0.63 (chloroform: methanol: acetic acid:90:10:0.5 v/v/v).