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Abstract 


Various 7-chloro-6-fluoro-2-arylidenylaminobenzo(1,3)thiazole (2a-h) have been synthesized by the condensation of 7-chloro-6-fluoro-2-aminobenzo(1,3)thiazole (1) with different aromatic aldehydes. The Schiff's bases on reaction with acetyl chloride, chloroacetyl chloride and phenyl acetyl chloride yielded 1-(7-chloro-6-fluorobenzothiazol-2-yl)-3,4-substituted-aryl-azetidin-2-ones (3a-x). Similarly, cyclization of Schiff's base with thioglycolic acid furnished 3-(7-chloro-6-fluoro-benzothiazol-2-yl)-2-substituted-arylthiazolidin-4-ones (4a-h). The structures of the newly synthesized compounds have been established on the basis of their spectral data and elemental analysis. Some selected compounds were evaluated for antiinflammatory, analgesic, CNS depressant and skeletal muscle relaxant activity.

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Indian J Pharm Sci. 2008 Sep-Oct; 70(5): 572–577.
PMCID: PMC3038279
PMID: 21394251

Synthesis and Pharmacological Evaluation of Azetidin-2-ones and Thiazolidin-4-ones Encompassing Benzothiazole

Abstract

Various 7-chloro-6-fluoro-2-arylidenylaminobenzo(1,3)thiazole (2a-h) have been synthesized by the condensation of 7-chloro-6-fluoro-2-aminobenzo(1,3)thiazole (1) with different aromatic aldehydes. The Schiff's bases on reaction with acetyl chloride, chloroacetyl chloride and phenyl acetyl chloride yielded 1-(7-chloro-6-fluorobenzothiazol-2-yl)-3,4-substituted-aryl-azetidin-2-ones (3a-x). Similarly, cyclization of Schiff's base with thioglycolic acid furnished 3-(7-chloro-6-fluoro-benzothiazol-2-yl)-2-substituted–arylthiazolidin-4-ones (4a-h). The structures of the newly synthesized compounds have been established on the basis of their spectral data and elemental analysis. Some selected compounds were evaluated for antiinflammatory, analgesic, CNS depressant and skeletal muscle relaxant activity.

Keywords: Benzothiazole, azetidinones, thiazolidinones, pharmacological activity

The β-lactam antibiotics are extensively used for bacterial infections. The cephalosporins1 have withstood the onslaught of microorganisms and have come to be physician's arsenal in combating a wide range of microbial infections. Moreover various β-lactams are associated with antitumor2, antitubercular3, antiinflammatory4 activities. Similarly, thiazolidinones have attracted considerable attention as they are also enrolled with wide range of pharmacological activities like anticonvulsant5, analgesic6 and antiinflammatory7 activities. In continuation of our studies on benzothiazole8,9, we have synthesized benzothiazole moiety linked to bioactive β-lactam and thiazolidinone rings, to analyse their biological profile.

The starting material for the synthesis of desired compounds is 7-chloro-6-fluoro-2-aminobenzo(1,3)thiazole10 (1), which on treatment with different aromatic aldehydes in concentrated sulphuric acid yields the respective Schiff bases (2a-h). The Schiff bases were separately reacted with substituted acetyl chloride and mercaptoacetic acid produced 1-(7-chloro-6-fluorobenzothiazol-2-yl)-3,4-substituted-arylazetidin-2-ones (3a-x) and 3-(7-chloro-6-fluoro-benzothiazol-2-yl)-2-substituted-arylthiazolidin-4-ones (4a-h) respectively (Scheme 1). The newly synthesized compounds were characterized by spectroscopic data and elemental analysis and were screened for their antinflammatory, analgesic, CNS depressant and skeletal muscle relaxant activities.

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Synthetic scheme of Schiff's bases, azetidine-2-ones and thiazolidin-4-ones

R= C6H5, C6H4-4-OCH3, C6H4-2-OH, C6H4-3-OCH3, C6H4-4-N(CH3)2, C6H4-2-NO2, C6H4-3Cl and C4H3O (2-furyl). R' = H, Cl and C6H5.

MATERIALS AND METHODS

Melting points were determined in open capillaries and are uncorrected. IR spectra were recorded in KBr on FTIR Shimadzu 1400S and NMR spectra were recorded on AMX-400 in CDCl3/DMSO-d6 using TMS as internal standard (chemical shifts in δ ppm). Mass spectra were recorded on FT VG-7070H Mass spectrophotometer using the EI technique at 70 eV. Satisfactory elemental analyses were obtained for all the compounds and were within ±0.4% of the theoretical values. The reactions were monitored on TLC with solvents of varying polarity and the spots were located by iodine vapors. For antiinflammatory and analgesic activities, adult healthy rats of Wistar strain of either sex weighing between 150-200 g were used. For CNS depressant and skeletal muscle relaxant activity studies, adult, healthy mice of Wistar strain of either sex weighing 20-25 g were used. All animals were maintained under standard conditions and had access to pelletted animal feed and water ad libitum. The study protocols were carried out as approved by the Institutional Ethics Committee (CPCSEA Reg. No. 144).

To a mixture of 7-chloro-6-fluoro-2-aminobenzo(1,3)thiazole (1) (0.1 mol) and benzaldehyde (0.1 mol), was added two drops of concentrated sulphuric acid and the reaction mixture was refluxed in ethanol (25 ml) for 3 h. The contents were poured into cold water. The Schiff's base (2a) thus formed was filtered off and recrystallised from hot ethanol to give 7-chloro-6-fluoro-(2-hydroxy-benzylidine)-benzo(1,3)thiazole. IR (Vmax): 1650(C=N) and 3480 (Ar-OH). 1H NMR(CDCl3): 9.25 (s, 1H, -N=CH), 12.1 (s, 1H, Ar-OH), 7.0-8.0 (m, 6H, Ar-H). Mass: m/z 306 and fragment ion peaks at 289, 202, 175 and 140. Similarly, the other Schiff's bases (2b-h) were prepared.

The mixture of Schiff's base (2a) (2.90 g, 0.01 mol) and triethylamine (1.02 ml, 0.01 mol) was dissolved in dioxane (40 ml) and kept in an ice bath. To this, cold solution of acetyl chloride (0.72 ml, 0.01 mol) was added slowly at 0°, stirred for 10-12 h and left over night. The precipitated triethylammonium chloride was filtered off and dioxane was removed by distillation. Residue was poured into cold water; the resulting solid was dried and crystallized from ethanol to give 3a. The Schiff's bases (2b-h) were treated separately with acetyl chloride to get 3b-h. Similarly, 3i-p and 3q-x were prepared by treating 2a-h with chloroacetyl chloride and phenyl acetyl chloride separately. 3i IR (Vmax): 1650(C=O). 1H NMR(CDCl3):3.7(d, 1H, -NCH), 3.9 (d, 1H, CHCl), 7.2-7.9 (m, 6H, Ar-H); 3q IR(Vmax): 1660 (C=O stretch) azetidinone ring, 1H NMR(CDCl3): 3.1 (d, 1H, -NCH), 3.7 (d, 1H, CH-Ar), 7.0-7.8 (m, 12H, Ar-H).

A mixture of Schiff's base 2a (2.90 g, 0.01 mol) and mercaptoaceticacid (1.19 ml, 0.01 mol) was dissolved in dioxane (20 ml). A pinch of anhydrous zinc chloride was added and then refluxed for 8 h. Separated solid was filtered, washed with sodium bicarbonate solution and then recrystallised from ethanol. Similarly, the other compounds (4b-h) were prepared. 4a IR (Vmax): 1660 (C=O) thiazolidine ring. 1H NMR(CDCl3): 3.8 (s, 2H, CH2), 3.6 (s, 1H, -NCH), 7.2-7.6 (m, 7H, Ar-H). Mass: m/z 364 and fragment ions peak at 289, 202, 175, 101, 81 and 69. Physical data of the compounds is given in Table 1.

TABLE 1

PHYSICAL DATA OF THE SYNTHESIZED COMPOUNDS

Compd NoRR1Mp (°)Yield (%)Mol. formula*
2a--16578C14H8ClFN2S
2b--16076C15H10ClFN2OS
2c--17581C14H7ClFN2OS
2d--15569C15H10ClFN2O2S
2e--18075C16H13ClFN3S
2f--16872C14H7ClFN3O2S
2g--17068C14H7Cl2FN2S
2h--19870C12H6ClFN2OS
3aC6H5H18850C16H10ClFN2OS
3bC6H4 -4-OCH3H18563C17H12ClFN2O2S
3cC6H4-2-OHH19058C16H10ClFN2O2S
3dC6H3-4-OH, 3-OCH3H17063C17H12ClFN2O3S
3eC6H4-4-N(CH3)2H19570C18H15ClFN3OS
3fC6H4-2-NO2H17980C16H9ClFN3O3S
3gC6H4-3-ClH18073C16H9Cl2FN2OS
3hC4H3O (2-furyl)H20568C15H8ClFN2O2S
3iC6H5Cl15072C15H9ClFN2OS
3jC6H4-4-OCH3Cl20069C15H11Cl2FN2OS
3kC6H4-2-OHCl19455C16H9Cl2FN2O2S
3lC6H3-4-OH, 3-OCH3Cl18762C17H11Cl2FN2O3S
3mC6H4-4-N(CH3)2Cl19865C18H14Cl2FN3OS
3nC6H4-2-NO2Cl20271C16H8Cl2FN3OS
3oC6H4-3-ClCl17082C16H8Cl3FN2OS
3pC4H3O (2-furyl)Cl21072C14H7Cl2FN2O2S
3qC6H5C6H521582C22H14ClFN2O2S
3rC6H4-4-OCH3C6H521673C23H16Cl2FN2O2S
3sC6H4-2-OHC6H521054C22H16ClFN2O3S
3tC6H3-4-OH, 3-OCH3C6H523563C22H16ClFN2O3 S
3uC6H4-4-N(CH3)2C6H520868C24H19ClFN3O3S
3vC6H4-2-NO2C6H519375C22H13ClFN3O3S
3wC6H4-3-ClC6H521870C22H13Cl2FN2OS
3xC4H3O (2-furyl)C6H521973C20H12ClFN2O2S
4aC6H5-19182C16H10ClFN2OS2
4bC6H4-4-OCH3-11885C17H12ClFN2O2S2
4cC6H4-2-OH-18080C16H9ClFN2O2S2
4dC6H3-4-OH, 3-OCH3-15784C17H12ClFN2O3S2
4eC6H4-4-N(CH3)2-14278C18H15ClFN3OS2
4fC6H4-2-NO2-17574C16H9ClFN3OS2
4gC6H4-3-Cl-17070C16H9Cl2FN2OS2
4hC4H3O (2-furyl)-18665C14H8ClFN2O2S2
*C, H, N and S are within the limit of ± 0.3%

Antiinflammatory activity:

Antiinflammatory activity of some selected compounds was evaluated using carrageenan induced rat hind paw oedema method11. The animals were divided into control, standard and test groups, each consisting of six animals. The first group was treated with Tween-80 (1%) suspension which served as control, second group was administered with a dose of 20 mg/kg suspension of diclofenac sodium intraperitoneally which served as standard and other groups were treated with 30 mg/kg of suspension of test compounds in Tween-80. After 30 min, the rats were injected with 0.1 ml of carrageenan (1% w/v) to the sub plantar region of left paw of the rats. The volume of paw was measured using potassium permanganate solution displacement technique with the help of plethysmograph both in control and animals treated with standard and test compounds at 0, 1, 2 and 3 h after injection of carrageenan. The percentage inhibition of oedema was calculated by using formula, percent inhibition = (1–Vt/Vc)×100, where Vt is the mean paw volume of the test drug, Vc is the mean paw volume of the control. The results are recorded in Table 2.

TABLE 2

RESULTS OF ANTIINFLAMMATORY ACTIVITY OF THE SYNTHESIZED COMPOUNDS

Compound No.Paw volumes ±SEM and% reduction

1 h2 h3 h



Mean±SEM%RPEVMean±SEM%RPEVMean±SEM%RPEV
3a1.50±0.0714.601.74±0.0717.141.87±0.06*22.08
3c1.43±0.0319.661.53±0.0527.141.70±0.04*29.16
3f1.40±0.0521.341.45±0.05*30.951.63±0.04*32.08
3i1.42±0.0320.221.57±0.1125.231.63±0.10*32.08
3k1.38±0.0522.471.40±0.0633.331.43±0.05*40.41
3n1.32±0.0725.841.35±0.0535.711.35±0.04*43.75
3q1.50±0.0415.731.78±0.0515.231.90±0.0220.83
3s1.38±0.0522.471.70±0.0514.041.80±0.05*25.00
3v1.30±0.0326.961.65±0.0321.421.75±0.06*27.08
4a1.48±0.0422.001.57±0.05*25.231.65±0.06*31.25
4c1.45±0.0418.501.53±0.0427.141.63±0.0632.08
4f1.28±0.0328.081.42±0.06*32.381.48±0.04*38.30
Control1.78±0.05-2.10±0.05-2.4±0.03-
Diclofenac Sodium0.99±0.0543.241.11±0.03*46.001.11±0.05*54.00

RPEV is reduction in paw edema volume

*Indicates significant difference at p<0.001 when compared to control

Analgesic activity:

The animals were divided into four groups of six animals each. The animals, which showed reaction time of 2-3 s, were selected for experiment and analgesic activity of some selected compounds was studied by tail flick method12. The tail received radiant heat from a wire, which is heated by passing a current of 6 mA. The time taken for the withdrawal of tail was recorded before the administration of the compounds and for 30, 60, 90 and 120 min after administration of compounds. The cut off time for determination of latent period was taken as 40 s to avoid the injury to the skin and based on our pilot studies. One group served as a standard (pentazocine hydrochloride) with dose of 10 mg/kg body weight and another group served as control (1% Tween-80) and rest of the groups used for the test drugs. The test compounds and pentazocine hydrochloride were suspended in 1% Tween-80 which was used as vehicle for the control group. The tested compounds were administered at the dose of 30 mg/kg in the form of suspension and administered intraperitoneally. The results of analgesic activity are shown in Table 3.

TABLE 3

RESULTS OF ANALGESIC ACTIVITY OF THE SYNTHESIZED COMPOUNDS

Compound No.ABRT in secABRT in sec after treatment

Pre treatment ‘0’ min15 min30 min45 min60 min
3a3.60±0.084.5±0.06*6.53±0.04*7.03±0.13*11.20 ±0.2*
3c3.80±0.024.5±0.06*7.05±0.08*11.40±0.2*11.40 ±0.2*
3f3.60±0.024.62±0.10*4.67±0.06*5.50±0.14*5.30±0.03*
3i3.20±0.045.5±0.22*5.5±0.224.4±0.203.40±0.37
3k3.40±0.103.83±0.316.83±0.48*6.00±0.37*6.50±0.22*
3n2.62±0.042.67±0.213.33±0.214.67±0.214.00± 0.37
3q3.60±0.104.17±0.31*5.17±0.318.0±0.269.83±0.31*
3s3.20±0.154.00±0.26*5.5±0.346.17±0.31*6.67±0.21*
3v3.50±0.025.33±0.33*8.83±0.78*5.67±0.21*5.33±0.21*
4a3.10±0.067.17±0.48*11.17±0.40*10.67±0.5*9.67±0.21*
4c3.60±0.066.33±0.80*7.76±0.0410.33±0.12*9.25±0.08
4f3.60±0.203.67±0.21*3.83±0.314.33±0.224.33±0.42
Control3.60±0.063.60±0.10*3.50±0.153.60±0.153.40±0.06
PentazocineHCl3.60±0.084.93±0.18*8.90±0.15*10.10±0.2*12.50±0.1*

ABRT: Average Basal Reaction Time

*Indicates significant difference at p<0.001 when compared to control

CNS depressant activity:

The CNS depressant activity13 of the compounds was studied on mice using actophotometer. Animals of either sex weighing 25-30 g were divided into groups of six each. The actophotometer counts for 10 min were recorded by placing the animals in the actophotometer, which gives the initial reading. The compounds were administered intraperitonially in the form of suspension prepared in 1% Tween-80 at a dose of 30 mg/kg body weight. Each group served as its own control. Diazepam was administered as standard to one of the test group at a dose of 5 mg/kg body weight. After 30 min and 60 min of administration of test compound, the actophotometer counts were noted for 10 min. Decrease in the number of counts for each group was recorded and finally, the percentage CNS depressant activity was determined. The results of CNS depressant activity are shown in Table 4.

TABLE 4

RESULTS OF CNS DEPRESSANT ACTIVITY OF THE SYNTHESIZED COMPOUNDS

CompoundBefore drug (Mean±SEM)After drug (Mean±SEM)CNS depressant activity (%)


30 min60 min30 min60 min
3a835.91±03.63438.00±07.53621.50±09.4546.5023.60
3c830.90±03.63113.17±07.00*354.33±04.12*86.2056.40
3f804.65±02.60272.33±14.55*503.83±04.9467.0038.00
3i877.65±02.50324.17±05.30*386.83±06.43*60.4052.50
3k915.63±02.30384.17±14.28*453.33±13.24*53.1044.30
3n858.13±05.36360.50±18.36*466.17±12.23*56.0042.70
3q766.18±07.21477.17±07.82*655.83±12.0841.7019.30
3s811.55±02.20252.50±16.80*347.00±17.80*69.2057.30
3v823.37±01.45608.33±02.23725.33±02.9425.7010.80
4a811.50±02.00624.00±07.13733.50±03.4523.8009.80
4c825.53±02.14215.00±15.92*405.00±08.37*73.7050.20
4f815.49±01.93217.33±08.73*385.33±12.12*73.50 52.60
Control820.23±06.88818.67±05.93813.17±07.01--
Diazepam840.64±09.2682.17±04.04*112.50±06.55*90.0086.20
*Indicates significant difference at p<0.001 when compared to control

Muscle relaxant activity:

Some of the synthesized compounds were also tested for muscle relaxant activity14 (muscle grip strength) in mice using rotorod apparatus. The animals were trained to remain on the rotating rod (moving at speed of 25 rpm) for 5 min. The animals which were able to remain on the rotating rod for 5 min and above were selected for study and were divided into groups of six each. The compounds were administered intraperitonially at a dose of 30 mg/kg body weight in the form of suspension in 1% Tween-80 while one group received diazepam as a standard drug at a dose of 4 mg/kg body weight. After 30 min, the mice were placed again on the rotating rod and animal's fall off time was recorded. The percentage muscle relaxant activity was calculated and the results are shown in Table 5.

TABLE 5

RESULTS OF SKELETAL MUSCLE RELAXANT ACTIVITY OF SYNTHESIZED COMPOUNDS

CompoundFall of time% Decrease in time

Before the drug treatmentAfter the drug treatment
3a43.00±1.3715.50±0.12*64.00
3c23.50±1.5214.17±1.01*39.70
3f26.17±1.1916.00±0.68*38.86
3i38.50±1.3822.50±1.38*41.55
3k26.83±1.4720.67±0.9522.95
3n34.83±1.3516.67±0.80*52.13
3q44.00±1.6533.67±0.3323.50
3s35.67±1.2530.62±1.0114.15
3v42.33±0.7638.17±1.2809.30
4a32.67±0.8415.33±1.05*53.07
4c26.17±1.0810.83±1.38*58.61
4f30.17±0.9512.00±1.10*60.22
Control34.83±1.2534.67±1.33-
Diazepam28.32 ± 0.205.75 ± 0.07*79.69*
*Indicates significant difference at p<0.001 when compared to control

RESULTS AND DISCUSSION

Two new series of compounds namely substituted azetidinones (3a-x) and thiazolidinones (4a-h) possessing fluoro-benzothiazoles have been synthesized by using experimental protocol as shown in Scheme 1. All the derivatives were supported by spectral data. The IR, 1H-NMR and Mass spectra are in agreement with the proposed structures.

The result of the antiinflammatory activity summarized in Table 2 indicated that 3-chloro-azetidinones showed better activity among the tested compounds. Further the presence of ortho substituted aryl ring at 4th position of azetidinones moiety increased the anti inflammatory activity. Substitution of chloro group with phenyl ring at 3rd position of the azetidinone generally causes decrease in the activity. Among the thiazolidinones, 2-nitro phenyl attached to 5th position of thiazolidinone ring showed maximum activity.

The analgesic activity summarized in Table 3, indicated that 3a and 3c showed maximum analgesic activity. The compounds 4a and 4c exhibited higher analgesic activity than the standard pentazocine at 15, 30 and 45 min interval. However the activity decreases after 60 min suggesting shorter duration of activity for the same.

The CNS depressant activity (Table 4) reveals that 2-hydroxyl phenyl substituted azetidinone (3c) and thiazolidinone (4c) showed better depressant activity. However activity decreases after 60 min indicating that the molecules to be short acting CNS depressants. The skeletal muscle relaxant activity (Table 5) indicated that the thiazolidinones (4a, 4c and 4f) and azetidinone (3a) showed significant muscle relaxant activity.

Acknowledgments

Authors are thankful to the Professor and Chairman, Department of Studies in Biochemistry, for providing laboratory facilities and the Board of members, National Education Society and National College of Pharmacy, Shimoga, for providing facilities to conduct pharmacological activities.

Footnotes

Gopal, et al.: Pharmacological Evaluation of Azetidinones/thiazolidinones of Benzothiazole

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