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Article

Fused Heterocycles: Synthesis of Some New Imidazo[1,2-a]- pyridine Derivatives

Istanbul University, Faculty of Pharmacy, Department of Pharmaceutical Chemistry, 30416, Istanbul, Turkey
*
Author to whom correspondence should be addressed.
Molecules 2004, 9(10), 894-901; https://doi.org/10.3390/91000894
Submission received: 4 August 2004 / Accepted: 13 September 2004 / Published: 30 September 2004

Abstract

:
Some new thiazolidines and spirothiazolidines derived from hydrazones of 2-methylimidazo[1,2-a]pyridine-3-carboxylic acid hydrazide, a bioisosteric derivative of isoniazid, were synthesized and characterized by analytical, IR, 1H- and 13C-NMR and mass spectral data. Some of the newly synthesized compounds were screened for their antimycobacterial activities. None of the tested compounds showed significant in vitro antituberculous activity at 6.25 μg/mL (MIC rifampin 0.031 μg/mL).

Introduction

Mycobacterium tuberculosis infects over one-third of the world’s population and causes almost three million deaths every year [1]. Isonicotinic acid hydrazide (isonazid) is one of the primary drugs used in combination with ethambutol, rifampin, streptomycin and pyrazinamide to treat tuberculosis, but the treatment of this disease is still a major health problem due to multi-drug resistant bacterial strains and new antimycobacterial agents, different from available first-line drugs, are urgently needed. As part of our studies on imidazo[1,2-a]pyridine we have recently reported the synthesis of some imidazo[1,2-a]pyridine-3-carboxylic acid hydrazides and related compounds and their antimyco-bacterial activities [2]. Continuing our search for new antimycobacterial agents we have now synthesized some new ketone-hydrazones 3a-c, thiazolidines 4a-c and spiro compounds 4d-g incorporating an imidazo[1,2-a]pyridine moiety. These compounds were characterized by their elemental and spectral analyses (IR, 1H-NMR, 13C-NMR and mass spectra).

Results and Discussion

The synthetic pathway followed in the preparation of the compounds is outlined in Scheme 1. The starting materials, ethyl 2-methylimidazo[1,2-a]pyridine-3-carboxylate (1) and 2-methylimidazo[1,2-a]pyridine-3-carboxylic acid hydrazide (2), were obtained by previously described methods [3,4].
Scheme 1.
Scheme 1.
Molecules 09 00894 g001
Condensation of 2 with the appropriate ketones in ethanol yielded the corresponding ketone-hydrazones 3. The hydrazones were reacted with mercaptoacetic acid in dry benzene (Method A) to give cyclocondensation products 4b,d and e in 69.8-72.3 % yields. On the other hand, refluxing a mixture of 2 and the appropriate ketone together with mercaptoacetic acid in dry benzene (Method B) also produced the target compounds 4 but in higher yields (69.7-99.1 %), except in the case of 4b (55.5 %). All the compounds were characterized by their physical data and elemental analyses (Table 1), IR, 1H- and 13C-NMR and EI mass spectra.
Table 1. Some physical and analytical data of compounds 3 and 4
Table 1. Some physical and analytical data of compounds 3 and 4
Comp.RR1R2nM.p.
(oC)
Yield
%
Formula
(molecular weigh)
Analysis
(calcd./found)(%)
CHN
3aCH3C2H5--120-575.8C13H16N4O
(244.30)
63.916.6022.94
63.816.9622.55
3b---1162-662.1C14H16N4O.1.5H2O
(283.61)
59.356.7619.78
60.846.9619.70
3c---276-863.8C15H18N4O.2H2O
(306.33)
58.817.2418.29
58.947.5618.21
4aCH3CH3--222-587.3
(Method B)
C14H16N4O2S.H2O
(322.38)
52.165.6317.38
52.706.0417.30
4bCH3C2H5--138-4369.8
(Method A)
55.5
(Method B)
C15H18N4O2S.H2O
(336.39)
53.565.9916.65
53.456.1016.83
4d---1137-4375.5
(Method A)
80.0
(Method B)
C16H18N4O2S.H2O
(348.42)
55.155.7916.08
55.105.8215.92
4e-- -2258-6577.3
(Method A)
99.1
(Method B)
C17H20N4O2S
(344.43)
59.285.8516.27
58.975.7716.10
4f--CH32154-672.3
(Method B)
C18H22N4O2S.0.5H2O
(367.46)
58.856.3115.26
58.647.2615.42
4g--C2H52142-681.7
(Method B)
C19H24N4O2S.2H2O
(408.52)
55.866.9113.71
55.446.5612.09
The IR spectra of the starting materials 3 showed C=O bands in the 1654-1679 cm-1 region. A new strong band at 1690-1710 cm-1 in the spectra of 4 provided firm support for ring closure. The most significant evidence for the reaction was the presence of two doublets (dd, 2H, J=16 Hz) at about 3.61 and 3.68 in the 1H-NMR spectrum of 4b [6]. In the spectra of 4a,c-g, the same protons were observed as singlets (2H) at about 3.40-3.72 ppm due to the lack of chirality. 13C-NMR and DEPT (135) spectra of the prototypes (4b,d and, e) were also studied and are detailed. Signals at about 71.44-76.59 ppm, which are not seen in DEPT spectra, were assigned to the quarternary (spiro) carbon atoms. According to the data obtained from DEPT and HETCOR experiments the signals at about 28.80-29.72 ppm were assigned to the CH2 group located in the thiazolidine moiety [7]. The mass spectra of all the compounds were relatively simple and showed (except for 4g) the peaks due to molecular ions.

Antituberculous Activity

Primary screening was conducted at 6.25 μg/mL against M. tuberculosis H37Rv. The M. tuberculosis H37Rv was grown in a medium containing a radiolabeled substrate. Labeled CO2 produced was detected and quantitated with a BACTEC 460 automatic radiometric system. Compounds giving inhibitions < 90 % (MIC > 6.25 μg/mL, MIC rifampin 0.031 μg/mL) were not evaluated further [5]. None of the compounds showed antituberculous activity at the tested concentration.

Experimental

General

Melting points determined with a Buchi 530 melting point apparatus in open capillaries and are uncorrected. IR (KBr disks) and 1H- and 13C-NMR spectra (DMSO-d6) were recorded on Perkin Elmer Model 1600 and Bruker AC 200 and DPX 400 instruments, respectively. Microanalyses were carried out on a Carlo Erba 1106 elemental analyzer. All starting materials were purchased E. Merck (Darmstadt, Germany).

Ethyl 2-methyimidazo[1,2-a]pyridine-3-carboxylate (1) [3].

2-Aminopyridine (0.01 mol) was heated under reflux with ethyl 2-chloroacetoacetate (0.1 mol) in 96 % C2H5OH (25 mL) for 6h and then cooled. Excess C2H5OH was evaporated in vacuo. The residual red oil was partitioned between ether-water. After drying, the ether extracts were evaporated and the residual oil was allowed to crystallize. M.p. 69 °C, yield 45.05%.

2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid hydrazide (2) [4].

Ethyl 2-methylimidazo[1,2-a]pyridine-3-carboxylate (0.01 mol) was heated under reflux with H2NNH2 (0.1 mol) in 96% C2H5OH (15 mL) for 5h and then cooled. The crystals formed were washed with H2O, dried and recrystallized from C2H5OH (96 %). M.p.180 °C, yield 27.16 %.

General procedure for preparation of 2-methylimidazo[1,2-a]pyridine-3-carboxylic acid (alkylidene / cycloalkylidene) hydrazides 3a-c.

2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid hydrazide (2, 0.01 mol), the appropriate ketone (0.011 mol), a drop of conc. H2SO4 and 96 % C2H5OH (20 mL) were heated under reflux for 6h. The crude products which precipitated on cooling were filtered and recrystallized from an C2H5OH-H2O mixture.
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid sec-butylidenehydrazide (3a): IR: 1654 (C=O) cm-1; 1H-NMR: δ (ppm) = 1.04 (3H, t, CH2CH3), 1.98 (3H, s, CH3), 2.28 (2H, q, CH2CH3), 2.53 (3H, s, 2-CH3), 7.01 (1H, t, 6-H), 7.38 (1H, t, 7-H), 7.58 (1H, d, 8-H), 8.88 (1H, d, 5-H), 10.03 (1H, s, CONH); EIMS (%) = 244 (M+., 38), 159 (100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid cyclopentylidenhydrazide (3b): IR: 1670 (C=O) cm-1; 1H-NMR: δ (ppm) = 1.68-1.83 (4H, m, cyclopentylidene-3H,4H), 2.34-2.49 (4H, m, cyclo-pentylidene-2H,5H), 2.54 (3H, s, 2-CH3), 7.00 (1H, t, 6-H), 7.40 (1H, t, 7-H), 7.58 (1H, d, 8-H), 8.89 (1H, d, 5-H), 9.91 (1H, s, CONH); EIMS (%) = 256 (M+., 100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid cyclohexylidenhydrazide (3c): IR: 1679 (C=O) cm-1; 1H-NMR: δ (ppm) = 1.4-1.78 (6H, m, cyclohexylidene 3H,4H,5H), 2.21-2.31 (2H, m, cyclo-hexylidene-2H,6H, axial), 2.33-2.60 (2H, m, cyclohexylidene-2H,6H, equatorial), 2.52 (3H, s, 2-CH3), 7.01 (1H, t, 6-H), 7.37 (1H, t, 7-H), 7.56 (1H, d, 8-H), 8.86 (1H, d, 5-H), 10.28 (1H, s, CONH); EIMS (%) = 270 (M+., 72), 78 (100).

General procedures for preparation of 2-methylimidazo[1,2-a]pyridine-3-carboxylic acid amides 4 a-g.

Method A

A mixture of 3a-c (0.01 mol) and HSCH2COOH (0.15 mol) was heated under reflux for 6h in dry benzene (30 mL) using a Dean-Stark trap for removal of water of condensation. Excess benzene was evaporated in vacuo. The residue was triturated with saturated NaHCO3 until CO2 evaluation ceased and then allowed to stand overnight. The solid thus obtained was filtered, washed with H2O and recrystallized from an C2H5OH-H2O mixture.

Method B

The appropriate ketone (0.011 mol) was added to a solution of 2 (0.01 mol) in dry benzene (30 mL) and the mixture was heated under reflux for 1.5h using a Dean-Stark trap. After cooling HSCH2COOH (0.15 mol) was added dropwise to the solution and the resulting mixture was refluxed for 6h. The compounds were purified using the procedure described under Method A.
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (2,2-dimethyl-4-oxo-1,3-thiazolidin-3-yl)amide (4a): IR: 1662 (CONH), 1690 (thiazolidine C=O) cm-1; 1H-NMR: δ (ppm) = 1.36 (6H, s, -C(CH3)2), 2.44 (3H, s, 2-CH3), 3.52 (2H, s, CH2S), 6.88 (1H, t, 6-H), 7.25 (1H, t, 7-H), 7.42 (1H, d, 8-H), 8.65 (1H, d, 5-H), 9.81 (1H, s, CONH); EIMS (%) = 304 (M+., 3), 156 (100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (2-ethyl-2-methyl-4-oxo-1,3-thiazolidin-3-yl)amide (4b): IR: 1662 (CONH), 1690 (thiazolidine C=O) cm-1; 1H-NMR (CDCl3): δ (ppm) = 1.04 (3H, t, CH2CH3), 1.66 (3H, s, C-CH3), 1.76-1.84,1.92-1.99 (1H, 1H, 2m, CH2CH3), 2.60 (3H, s, 2-CH3), 3.61, 3.68 (1H, 1H, dd, J=16 Hz, CH2S), 6.93 (1H, t, 6-H), 7.34 (1H, t, 7-H), 7.46 (1H, d, 8-H), 9.22 (1H, d, 5-H), 7.93 (1H, s, CONH); 13C-NMR δ(ppm) = 168.67/161.73 (thiazolidine CO and CONH), 148.19/146.57 (imidazopyridine C2 and C8a), 128.19 (imidazopyridine C5), 127.80 (imidazopyridine C7), 117.14 (imidazopyridine C8), 114.33 (imidazopyridine C3), 71.44 (thiazolidine C2), 34.72 (CH2CH3), 29.72 (thiazolidine C3), 28.32 (CH3), 16,73 (2-CH3), 9.53 (CH2CH3); EIMS (%) = 318 (M+.,100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (2,2-diethyl-4-oxo-1,3-thiazolidin-3-yl)amide (4c): IR: 1662 (CONH), 1690 (thiazolidine C=O) cm-1; 1H-NMR: δ (ppm) = 0.8 (6H, t, CH2CH3), 1.50-1.65 (4H, m, CH2CH3), 2.40 (3H, s, 2-CH3), 3.40 (2H, s, CH2S), 6.64 (1H, t, 6-H), 7.22 (1H, t, 7-H), 7.40 (1H, d, 8-H), 8.66 (1H, d, 5-H), 9.72 (1H, s, CONH); EIMS (%) = 332 (M+., 4.5), 46 (100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (3-oxo-1-thia-4-azaspiro[4.4]non-4-yl)amide (4d): IR: 1662 (CONH), 1691 (spiro[4.4]nonane C=O) cm-1; 1H-NMR: δ (ppm) = 1.67-1.97 (4H, m, spiro-7H,8H), 2.15-2.21 (2H, m, spiro-6H,9H axial), 2.23-2.40 (2H, m, spiro-6H,9H equatorial), 2.64 (3H, s, 2-CH3), 3.72 (2H, s, CH2S), 7.05 (1H, t, 6-H), 7.46 (1H, t, 7-H), 7.62 (1H, d, 8-H), 8.90 (1H, d, 5-H), 9.98 (1H, s, CONH);13C-NMR δ (ppm) = 168.67/161.73 (spiro[4.4]nonane C3 and CONH), 148.05/146.62 (imidazopyridine C2 and C8a) , 128.25 (imidazopyridine C5), 127.85 (imidazopyridine C7), 117.12 (imidazopyridine C8), 114.74 (imidazopyridine C3), 114.34 (imidazopyridine C6), 76.79 (C5), 39.22 (spiro[4.4]nonane C6 and C9), 29.72 (spiro[4.4]nonane C2), 23.62 (spiro[4.4]nonane C7 and C8), 16.75 (2-CH3); EIMS (%)= 330 (M+., 66.45), 90 (100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (3-oxo-1-thia-4-azaspiro[4.5]dec-4-yl)amide (4e): IR: 1673 (CONH), 1709 (spiro[4.5]decane C=O) cm-1; 1H-NMR: δ (ppm) = 1.05-2.54 (10H, m, spiro-6H,7H,8H,9H,10H), 2.67 (3H, s, 2-CH3), 3.64 (2H, s, CH2S), 7.07 (1H, t, 6-H), 7.44 (1H, t, 7-H), 7.62 (1H, d, 8-H), 8.90 (1H, d, 5-H), 9.93 (1H, s, CONH); 13C-NMR δ (ppm) = 168.67/161.73 (spiro[4.5]decane C3 and CONH), 148.00/146.00 (imidazopyridine C2 and C8a), 128.29 (imidazo-pyridine C5), 127.84 (imidazopyridine C7), 117.11 (imidazopyridine C8), 114.80 (imidazopyridine C3), 114.37 (imidazopyridine C6), 73.04 (spiro[4.5]decane C5), 28.80 (spiro[4.5]decane C2), 24.90 (spiro[4.5]decane C8), 23.76 (spiro[4.5]decane C6 and C9), 23.62 (spiro[4.5]decane C6 and C10), 16.78 (2-CH3); EIMS (%) = 344 (M+., 92.4), 160 (100 ).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (8-methyl-3-oxo-1-thia-4-azaspiro[4.5]dec-4-yl) amide (4f): IR: 1662 (CONH), 1693 (spiro[4.5]decane C=O) cm-1; 1H-NMR: δ (ppm) = 0.67 (3H, s, CH3), 1.28-1.63 (9H, m, spiro-6H,7H,8H,9H,10H), 2.43 (3H, s, 2-CH3), 3.43 (2H, s, CH2S), 6.85 (1H, t, 6-H), 7.22 (1H, t, 7-H), 7.40 (1H, d, 8-H), 8.67 (1H, d, 5-H), 9.79 (1H, s, CONH); EIMS (%) = 358 (M+., 4), 46 (100).
2-Methylimidazo[1,2-a]pyridine-3-carboxylic acid (8-ethyl-3-oxo-1-thia-4-azaspiro[4.5]dec-4-yl) amide (4g): IR: 1672 (CONH), 1710 (spiro[4.5]decane C=O) cm-1; 1H-NMR: δ (ppm) = 0.84 (3H, s, CH2CH3), 1.05-1.98 (11H, m, spiro-6H,7H,8H,9H,10H, CH2CH3), 2.64 (3H, s, 2-CH3), 3.64 (2H, s, CH2S), 6.99 (1H, t, 6-H), 7.37 (1H, t, 7-H), 7.67 (1H, d, 8-H), 8.86 (1H, d, 5-H), 9.99 (1H, s, CONH); EIMS (%) = 46 (100).

In vitro evaluation of antituberculous activity [5]

A primary screen was conducted at 6.25 μg/mL against M. tuberculosis H37Rv in BACTEC 12B medium using a BACTEC 460 radiometric system. Compounds 3a-c, 4b,d-e, chosen as prototypes, did not show in vitro antituberculous activity at 6.25 μg/mL (MIC rifampin 0.031 μg/mL).

Acknowledgements

We thank Dr. Joseph A. Maddry from the Tuberculosis Antimicrobial Acquisition and Coordination Facility (TAACF), National Institute of Allergy and Infectious Diseases Southern Research Institute, Birmingham, AL (USA) for the in vitro evaluation of antituberculous activity. This work was supported by Istanbul University Research Fund Project No. T-452/071197.

References

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  • Samples Availability: Available from the authors.

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MDPI and ACS Style

Kasimogullari, B.O.; Cesur, Z. Fused Heterocycles: Synthesis of Some New Imidazo[1,2-a]- pyridine Derivatives. Molecules 2004, 9, 894-901. https://doi.org/10.3390/91000894

AMA Style

Kasimogullari BO, Cesur Z. Fused Heterocycles: Synthesis of Some New Imidazo[1,2-a]- pyridine Derivatives. Molecules. 2004; 9(10):894-901. https://doi.org/10.3390/91000894

Chicago/Turabian Style

Kasimogullari, Birgul Ozden, and Zafer Cesur. 2004. "Fused Heterocycles: Synthesis of Some New Imidazo[1,2-a]- pyridine Derivatives" Molecules 9, no. 10: 894-901. https://doi.org/10.3390/91000894

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