A Simple, Rapid and Efficient One-Pot Protocol for the Synthesis of 2-Substituted Benzothiazole Derivatives and Their Antimicrobial Screening

RESEARCH ARTICLE A. Rauf, S. Gangal, S. Sharma and M. Zahin, 63 S. Afr. J. Chem., 2008, 61, 63–67, <http://journals.sabinet.co.za/sajchem/>. A Simple, Rapid and Efficient One-pot Protocol for the Synthesis of 2-substituted Benzothiazole Derivatives and their Antimicrobial Screening Abdul Rauf,a* Saloni Gangal,a Shweta Sharmaa and Maryam Zahinb aDepartment of Chemistry, Aligarh Muslim University, Aligarh 2002002, India. bDepartment of Agricultural Microbiology, Aligarh Muslim University, Aligarh 2002002, India. Received 30 July 2007, revised 31 August 2007, accepted 14 May 2008. ABSTRACT A rapid and efficient condensation reaction of 2-aminothiophenol with various fatty acids in solvent-free conditions with or without microwave irradiation was carried out to afford the corresponding 2-substituted benzothiazole derivatives in good to excellent yields. The structures of the new products were established by elemental analyses, IR, 1H NMR, 13C NMR and mass spectral data. All the title compounds were screened for their antibacterial and antifungal activity. Most of the compounds exhibited good antimicrobial activity. KEYWORDS Fatty acids, 2-substituted benzothiazole, 2-aminothiophenol, antimicrobial activity, microwave irradiation. 1. Introduction in the presence of nitrobenzene/SiO2 , or nitrobenzene/montmo- 2-Substituted benzothiazoles are an important group of rillonite K10,18 or carboxylic acids.19 heterocyclic compounds that have widespread applications in To the best of our knowledge 2-substituted benzothiazole pharmaceutical and industrial research. The benzothiazoyl derivatives of fatty acids have not yet been reported. The present moiety is a structural element of compounds with potent and work is a continuation of our study on the derivatization of fatty selective antitumour activity,1 antiviral,2 anticonvulsant,3 neuro- acids. Tetrazoles,20,21 pyrazolines,22 tetrazine,23,24 spiro[oxa- protective4 and immunosuppressive properties.5 The benzo- thiolane-2,2'-dihydrotetrazoles]25 and aziridines26 have been thiazole nucleus is a key element in some thermally stable rigid- previously prepared in our laboratory. Cyanoethoxy and rod polymers possessing high tensile strength and in modulus morpholine derivatives of hydroxy fatty acids27 and fatty esters28 6 manufacturing. The availability of 2-substituted benzothiazoles showed significant antifungal and antibacterial activity, respec- depends on the preparative routes in which the fused thiazole tively. Thus, keeping in view the practical synthetic applications ring is constructed from acyclic reactants. Methods for the of fatty acid derivatives, we have synthesized 2-substituted preparation of 2-substituted benzothiazoles have been exten- benzothiazoles from saturated and olefinic (internal and terminal) sively studied. The most common direct method is the conden- fatty acids and the new compounds were screened for antibacte- sation of 2-aminothiophenol with the substituted aromatic rial and antifungal activity. aldehydes7 and carboxylic acids or their derivatives in polyphos- phoric acid (PPA),8 polyphosphate ester,9 or a mixture of 2. Results and Discussion methane sulphonic acid and phosphorus pentoxide.10 Other The reactions described in this paper have a remarkable methods include potassium ferricyanide cyclization of synthetic utility because they make compounds available in one thiobenzanilides,11 the reaction of α,α,α-trihalomethyl aromatic step from fatty acids in a short time with or without microwave compounds with 2-aminothiophenol in PPA,12 palladium- irradiation. catalyzed reaction of aryl halides with o-aminothiophenol in presence of carbon monoxide,13 the reaction of 2-aminothio- We now report the synthesis of 2-substituted benzothiazoles phenol with acid chlorides,14 and the ceric ammonium nitrate by the condensation of 2-aminothiophenol with various fatty mediated reaction of thiophenols and aromatic nitriles.15 How- acid chlorides (Scheme 1) (method 1). At first we focused on the ever, some of these methods suffer from one or more of the reaction of fatty acids with thionyl chloride. Since fatty acid disadvantages such as high thermal conditions and long reaction chlorides are not commercially available we planned to synthe- times, while sometimes excess reagents are required and the use size the latter in situ. In a typical procedure, the fatty acid of toxic metallic compounds results in waste stream problems. (2.5 mmol) 1a–f was treated with thionyl chloride (3 mmol) Microwave-assisted reactions in solvent or solvent-free condi- under neat conditions at 80 °C for 2 h, to yield the corresponding tions have gained popularity because of rapid reaction rate and fatty acid chloride 2a–f. The complete conversion of the fatty ease of manipulation.16 Recently, some methods use microwave acid chloride 2a–f was monitored by taking an aliquot portion of heating for the synthesis of 2-substituted benzothiazoles such as the reaction mixture with a few drops of methanol followed by condensation of aromatic or aliphatic aldehydes with 2-amino- TLC analysis. The appearance of a new spot of methyl ester and 17 the disappearance of the fatty acid spot showed the complete thiophenol on SiO2 , aromatic aldehydes with 2-aminothiophenol conversion to the corresponding acid chloride, otherwise forma- * To whom correspondence should be addressed. E-mail: [email protected] tion of acid chloride would not be observed on the TLC plate due RESEARCH ARTICLE A. Rauf, S. Gangal, S. Sharma and M. Zahin, 64 S. Afr. J. Chem., 2008, 61, 63–67, <http://journals.sabinet.co.za/sajchem/>. Scheme 1 Synthesis of 2-substituted benzothiazoles from fatty acids. to the trailing effect of acid chloride. Acid chlorides of saturated, Table 1 2-Substituted benzothiazoles. olefinic and hydroxy fatty acids were formed with sufficient ease. In the next step excess of thionyl chloride was distilled off Compound R Product yield/% b and 2-aminothiophenol (3 mmol) was added. Since the addition 4a–f of 3 with acid chloride is an exothermic reaction, mixing was carried out at 0–5 °C. Excellent yields were obtained in all cases 1a a, 2a, 4a 98 (see Table 1). Our investigations showed that 2-aminothiophenol reacts smoothly with various saturated, olefinic and hydroxy fatty acid 1b a, 2b, 4b 98 chlorides in a short time. The corresponding 2-substituted benzothiazoles were obtained in excellent yields. Double bonds, hydroxy groups or chain length do not affect the yield of the 2-substituted benzothiazoles. 1c a, 2c, 4c 97 We have also prepared the title compounds under microwave-assisted, solvent-free conditions by the reaction of 2-aminothiophenol 3 with fatty acids (Scheme 2) (method 2). Multimode microwave irradiation at full power was used. The 1d a, 2d, 4d 98 optimization results are summarized in Table 2. As shown in Table 2, 1 eq of fatty acid reacts with 1.2 eq of 2-aminothiophenol to yield the corresponding 2-substituted benzothiazoles. When the reactions were performed below 50 % power, no reaction took place. To carry out the reaction power 1e a, 2e, 4e 95 was increased up to 80 % as a result of which 2-substituted benzothiazoles were obtained but the yield was low at about 35–40 %. In order to increase the yield full power was used. Our results also indicate that the yields were significantly improved 1f a, 2f, 4a 90 when the title compounds 4a–f were prepared by method 1 rather than method 2. The title compounds were identified on the basis of IR, a The fatty acid (2.5 mmol) was treated with thionyl chloride (3 mmol) at 80 °C for 1H NMR and mass spectra. 1H NMR spectra of 2-(dec-9-enyl)- 2 h, followed by the addition of 3 (2.5 mmol) at room temperature in toluene for 1.5 h. benzothiazole 4c showed characteristic signals of two sets of b 1 13 δ δ All compounds were characterized by IR, HNMR, C NMR, MS and elemental doublets at 7.96 and 7.84 ppm and two sets of triplets at 7.44 analysis. and 7.34 ppm for a total of four aromatic protons. A triplet for two hydrogen atoms was observed at δ 3.11 ppm for methylene was purchased from Sigma-Aldrich (Germany). Thionyl chlo- protons alpha to the thiazole moiety. A methine proton of C-10 ride was obtained from Merck (Mumbai, India) and was further appeared at δ 5.82 ppm. The C-11 methylene protons designated distilled off before use. General (GR) grade solvents were δ as HE and HZ displayed two distinct -values due to coupling employed for the extraction purposes and when required sol- with the adjacent C-10 methine proton. Thus, the 1H NMR spec- vents were dried and distilled before use. Homogeneity of the trum showed two doublets of doublets at δ 5.02 and 4.90 ppm for products was confirmed using TLC. 1H NMR spectra were HZ and HE protons, respectively. The structure of 4c was further recorded in CDCl3 on a Bruker DRX-300 instrument. The chemi- supported by its mass spectral results, which showed a molecu- cal shifts (δ) were measured relative to internal TMS. Coupling lar ion peak at m/z 273 consistent with its molecular formula constants were expressed in Hz. Mass spectra were obtained on 13 C17H23NS. The base peak appears at m/z 134. C NMR values are a Jeol SX-102 (FAB) spectrometer. IR spectra were obtained on a summarized in Table 4. Detailed spectra of the title compounds Shimadzu 8201 PC FT-IR using KBr pellets. The micromave irra- are given in Tables 3 and 4. diations were carried out using an unmodified domestic oven (LG, Model MC-808WAR, 1.35 kW, 2450 MHz). 3. Experimental Undec-10-enoic, (Z)-octadec-9-enoic, stearic and palmitic 3.1. General Procedure for the Preparation of 2-Substituted acids were obtained commercially from Fluka chemicals Benzothiazoles (Method 1) (Switzerland). (9Z,12R)-12-Hydroxyoctadec-9-enoic (ricinoleic) Thionyl chloride (3 mmol) was added to the fatty acid and (9R,12Z)-9-hydroxyoctadec-12-enoic (isoricinoleic) acids (2.5 mmol) at 80 °C for about 2 h to form the corresponding acid were isolated from the natural sources, i.e.

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