Synthesis and Characterization of Some 2-Azetidinones and Unexpected Azet-2(1H)-ones

2-Azetidinone (2b–e) and some unexpected azet-2(1H)-one derivatives (3b–f) were synthesized in two steps from the substitution of 2-aminobenzothiazole and different substituted aromatic aldehydes. Firstly, the Schiff bases were prepared via reaction of different 2-aminobenzothiazoles with different aromatic aldehydes. Second step was the formation of corresponding 2-azetidinone and some unexpected azet-2(1H)-one analogues by cyclocondensation of the Schiff bases with chloroacetyl chloride and phenoxy acetyl chloride in the presence of triethylamine. The chemical structures of the newly synthesized compounds were confirmed by FTIR, 1H NMR, 13C NMR, HMQC, elemental analysis and mass spectroscopic analysis.


INTRODUCTION
N recent years there has been a growing interest of chemists for the synthesis of heterocyclic compounds due to their significant biological characteristics. There are lots of studies on unique structure and strong antibacterial activity of 2-azetidinones. Penicillin, cephalosporin, carbapenem, nocardicin, monobactam, clavulanic acid, sulbactam and tazobactam antibiotics containing 2-azetidinone ring system are extensively used as chemotherapeutical agents for the treatment of bacterial infections and microbial diseases. [1][2][3][4][5][6][7][8] Studies of synthesis of 2-azetidinone and investigation of its antimicrobial characteristics have been conducted since 1990s. In recent years, various compounds with β-lactam rings have been synthesized for obtaining the compounds having different pharmacological activities such as cholesterol absorption inhibition activity. [9] Staudinger's ketene-imine reaction is the most common method of 2-azetidinones synthesis. [10] Although Staudinger reaction was found a hundred years ago (1907) as the reaction between ketenes and imines, this method has still been used as a useful method for synthesis of 2azetidinones and their derivatives. This reaction is applied thermally or photochemically for ketene formation by using acid chlorides in triethylamine environment. [11] Although, it is defined as [2+2] cycloaddition, the reaction is usually described as a step by step reaction. The first step of the reaction contains nucleophilic attack of imine nitrogen to sp hybridized carbon of ketene to form zwitterion intermediate product which creates 2-azetidinone ring. The result can be cis, trans or mixture of both isomers in view of stereochemistry of 2-azetidinone. In this study, it has been observed that some unexpected azet-2(1H)-ones have been formed while synthesizing new benzothiazole derivative cis-2-azetidinones through Staudinger keteneimine reaction (Scheme 1). The mechanism proposed for the reaction is given in Scheme 2.

RESULTS AND DISCUSSION
Compound 1a was synthesized by refluxing m-methylbenzaldehyde (1 eq) and 2-amino-6-methoxybenzothiazole (1 eq) in toluene for 2 days.The other Schiff bases were synthesized by the same method (1b-g) (Scheme 1). Compound 1e was obtained with a high yield as 85 % along with the synthesized other Schiff bases (1a-g) most probably because of the electron-donating groups increasing the efficiency of forming of the Schiff base. In the second step, I 2-azetidinone compounds were synthesized according to the method described in the experimental section. This method was applied to all the Schiff bases (1a-g) and different products were obtained (Scheme 1). Various studies have reported that different ratios of cis and trans isomers of β-lactams are formed depending on the addition of reactant order. [12][13][14] Cis-β-lactam was found to be the exclusive or major product when acid chloride was added dropwise at room temperature to the solution of imines and a tertiary base. [15] In Staudinger reaction, when the ketene occurs before the cyclocondensation, the β-lactam product becomes predominantly in cis form. On the other hand, when the imine reacts directly with the acyl chloride, the subsequent intramolecular SN2 displacement determines the final trans selectivity. [16] In this study, to synthesize the cis-2-azetidinone product, ketene formation was carried out by adding acyl chloride derivative in the presence of triethylamine before forming of zwitterion. The reaction was initiated by the nucleophilic attack of an imine to a ketene, giving rise to a zwitterionic intermediate. A conrotatory electrocyclic ring-closure of the zwitterionic intermediate produces cis-2-azetidinone compounds (Scheme 2). When the concentrations of triethylamine and acyl chloride derivative were used within 2-3 eq and 1.5-3 eq respectively and dichloromethane was used as solvent, cis-2-azetidinone compounds were formed in a good yield. Compound 2c was obtained from the reaction of 6-methoxy-N-(4-methoxybenzylidene)benzo [d]thiazol-2-amine (1 eq) (1e) and chloroacetyl chloride (1.5 eq) in the presence of triethylamine (2 eq) at 0-5 °C in dry dichloromethane. The other 2-azetidinones were synthesized by the same method (2b-e) (Scheme 1). The synthesized compounds were characterized by spectral data and elemental analysis. Stereochemistry of these compounds were determined by comparing Ja,b > 4.0 Hz coupling constant for H a and Hb protons for cisstereoisomers with Ja,b < 3.0 Hz for trans-stereoisomers. [17][18][19] 3H singlets observed at 3.67 and 3.73 ppm in the 1 H NMR spectrum of compound 2d were marked as OCH3 protons of benzothiazole and phenyl, respectively. It was observed that the coupling constant of 1H doublet of Ha and Hb protons in cis-2-azetidinone ring at 5-6 ppm range is J ≥ 4 Hz as mentioned in the literature and 1H doublet at 5.07 (J = 15 Hz) and 5.37 (J = 15.5 Hz) ppm were marked as related with the Ha and Hb protons in 2-azetidinone ring. The 2H doublets at 7.15 and 7.31 ppm were ascribed to H-12, H-14 and H-11, H-15 protons, respectively. Also, 1H doublets at 7.32 and 8.00 ppm were ascribed to H-4 and H-5 protons, respectively. 6H multiplet at 6.86 ppm was resulted from 5 protons in phenoxy group and H-7 proton. 20 signals were observed in 13 C NMR spectrum. While the signals at 55.408 and 56.061 ppm were related with the methoxy carbons, the signals at 62.585 and 75.474 ppm  were signed as the carbons which are bound to Ha and Hb protons, respectively. In 2-azetidinone ring. C=O carbon signal in 2-azetidinone ring was observed at 166.610 ppm and the other signals were ascribed to aromatic carbons. The sharp signal observed at 1728.16 cm -1 in IR spectrum was attributed to the carbonyl group in 2-azetidinone ring. The similar signals in the spectrum of compound 2d were observed in 1 H NMR and 13 C NMR spectra of compound 2c. In the mass spectrum of compound 2c, molecular ion peak was observed at m / z: 374.5. 2-Azetidinone ring has two types of fragmentation at EI-MS. These are the type A fragmentation leads to formation of ketene and imine ions or the type B fragmentation leads to formation of olefin and isocyanate ions. [20] The 2-azetidinone ring in compound 2c showed type A fragmentation, resulting in peaks at m / z: 298 and m / z: 192, respectively (Scheme 3).
Reactions were carried out at different temperatures by using different equivalents, different substitute Schiff bases and different acyl chloride derivatives. Cis-2azetidinones were obtained in good yield by using phenoxyacetyl chloride (1.5-3 eq) and triethylamine (2-3 eq) concentration. However, some unexpected azet-2(1H)-ones were synthesized instead of cis-2-azetidinone product by using triethylamine (7.4-15 eq) and chloroacetyl chloride (2-3,7 eq) concentration without changing the other reaction conditions such as temperature and solvent type (Scheme 1). Actually, direct acylation of the imine with the appropriate acid chloride yields N-acyliminium chloride (I) which may be at equilibrium with chloro amide (II) (Scheme 2). The reaction with N-acyliminium chloride or chloro amide bases gives β-lactam. [21][22][23] When the imine reacted directly with the acyl chloride, the subsequent intramolecular SN2 displacement determined the final trans selectivity. [16] The nonpolar solvents can not stabilize the zwitterionic intermediates which are facilitating the direct ring closure to form cis products, while the polar solvents can stabilize the zwitterionic intermediates and increase their half-life which increases the isomerization of the imine moiety to form trans products. [24] However, as mentioned in the literature, we could not obtain trans products using triethylamine (5-15 eq) and chloroacetyl chloride (2-3,7 eq) concentration without changing the other reaction conditions such as temperature and solvent type but unexpected azet-2(1H)-ones were synthesized instead of the trans product. We conclude that the formation of azet-2(1H)-ones arised from an increase in triethylamine concentration the mechanism of which was suggested. Formation mechanism of azet-2(1H)-ones is shown in Scheme 2. In addition to 2-azetidinone 2a and azet-2(1H)-one 3a was also obtained at the rate of 1 : 1 when the reaction was carried out by using of 7.4 eq of triethylamine and 3,7 eq of chloroacetyl chloride. 3a compound was purified by column chromatography using ethylacetate:dichloromethane:hexane solvent system in a ratio of 0.1 : 2 : 1.2, but 2a compound was not purified (Scheme 1).
Compound 3e was synthesized with 75% yield by using Schiff base 1e (1 eq), triethylamine (9 eq) and chloroacetyl chloride (5 eq). In 1 H NMR spectrum of the compound 3e, at 3.80 ve 3.81 ppm, 3H singlets were observed for Bt-OCH3 and Ph-OCH3, respectively. 2H dublets at 7.03 and 8.10 ppm are related to H-11, H-15 and H-12, H-14 protons, 1H dublets at 7.14 and 8.81 ppm are belong to H-4 and H-5 protons and 1H singlet at 7.70 ppm is related to H-7 proton. At about 4-6 ppm, the signals for the protons of Ha and Hb in the 2-azetidinone ring were not observed, while the 1H singlet at 6.88 ppm was observed for the Hc proton in the azet-2(1H)-one.
In IR spectrum, stretching vibrations of C=O group in azet-2(1H)-one ring and 2-azetidinone ring were observed at 1676.08 cm -1 and 1728 cm -1 , respectively. As expected, presence of conjugation in azet-2(1H)-one ring decreased the intensity of stretching signal of C=O group. The different fragmentations were observed in the mass spectrum of compound 2c when compared with EI-MS Scheme 4. 1 H NMR and HMQC spectra of compound 2c.

EXPERIMENTAL
The chemicals and reagents used for the synthesis were obtained from commercial sources. Solvents were distilled with an appropriate drying agent.