Synthesis, Crystal Structure and Antitumor Activity of N-(4-tert-butyl-5-(4-chlorobenzyl)thiazol-2-yl)-2,6-difluorobenzamide①

SHEN Kun PENG Jun-Mei LI Wan

YAN Xiao-Wei HU Ai-Xi②

(College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China)

1 INTRODUCTION

Recently, thiazole derivatives have aroused great interest for their excellent biological activity, such as antibacterial[1-4], antitumor[5], anti-imflammatory[6,7],insecticidal[8-10]and weeding activities[11]. Schiff base has been studied for a long time, and some of them had been synthesized and proved to have good anticancer activity[12-14]. However, as for compound 1[15-17], it is not stable and could easily change into compound 2 and aromatic aldehydes. According to our previous studies[16,17], we choose the acylamino group instead of imino to design and synthesize the title compound. Such groups are widely used on the development of anticancer drugs. In this paper, we report the synthesis, characterization and antitumor activity of the new compound N-(4-tert-butyl-5-(4-chlorobenzyl)thiazol-2-yl)-2,6-difluorobenzamide 3 (Scheme 1), which was also studied by singlecrystal X-ray structure analysis.

Scheme 1. Design and synthetic route of the title compound (3)

2 EXPERIMENTAL

2. 1 Instruments and general methods

All solvents were of reagent grade. All chemicals were of analytical reagent grade and used directly without further purif i cation. Melting point was measured on an X-4 electrothermal digital melting point apparatus and uncorrected. NMR spectra were recorded on a Bruker advanced instrument with TMS as internal standard at 400 MHz (proton)and 100 MHz (carbon)with chemical shifts (δ)expressed in ppm.

2. 2 Synthesis of the title compound 3

The synthesis procedure is shown in Scheme 1. A mixture of 4-tert-butyl-5-(4-chlorobenzyl)-2-aminothiazol (1.5 mmol)and 2,6-difluorobenzoic acid (1.6 mmol)was dissolved in methylene dichloride (40 mL)at room temperature. 4-dimethylaminopyridine(0.15 mmol)was added into the mixture and stirred for half an hour. N,N?-dicyclohexyl carbodiimide(DCC, 1.6 mmol)was then added quickly into the solution. The reaction was monitored by TLC until the reaction was completed. The resulting mixture was filtered and the filtrate was concentrated, and then purified by column chromatography on silica gel to afford the desired compound 3. Yield: 84.1%.m.p.: 130～132 ℃.1H NMR(CDCl3, 400 MHz), δ:1.35(s, 9H, 3×CH3), 4.24(s, 2H, CH2), 7.02～7.03(m,2H, C6H33,5-H), 7.15(d, J = 8.4 Hz, 2H, C6H42,6-H), 7.28(d, J = 8.4 Hz, 2H, C6H43,5-H), 7.43～7.51(m, 1H, C6H34-H), 9.22(br, 1H, NHCO).13C NMR(CDCl3, 100 MHz), δ: 30.81(3C), 32.18, 35.70,111.92, 112.30, 112.56, 123.77, 128.73(2C),129.62(2C), 132.39, 133.20, 138.78, 152.46, 153.73,157.30, 159.10, 161.59.

2. 3 X-ray structure determination

The crystal of the title compound suitable for X-ray structure determination was obtained by slowly evaporating an ethanol solution for about 15 days at room temperature with dimensions of 0.43mm × 0.41mm × 0.26mm. X-ray intensity data were measured at 173(2)K on a Bruker AXS SMART 1000 CCD diffractometer equipped with a graphite-monochromatic MoKα (λ = 0.71073 ?)radiation. Corrections for incident and diffracted beam absorption effects were applied using SADABS[18]. The structure was solved by direct methods with SHELXS-97[19]and expanded by difference Fourier techniques. The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were added according to theoretical models.The structure was refined by full-matrix leastsquares techniques on F2with SHELXL-97[20]. A total of 15736 reflections were collected in the range of 1.83＜θ＜26.00°, of which 7779 were independent(Rint= 0.0186)and 6101 were observed with I ＞2s(I). The final R = 0.0365 and wR = 0.0880 (w =1/[σ2(Fo2)+ (0.0486P)2+ 0.073P], where P = (Fo2+2Fc2)/3). S = 1.02, (Δ/σ)max= 0.001, (Δρ)max= 0.37 and (Δρ)min= –0.22 e·??3. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?)and Bond Angles (°)

?

2. 4 Bioassay of the antitumor activities

Antitumor activity of the title compound was tested by the typical MTT method according to the experimental methods in literature[21].

3 RESULTS AND DISCUSSION

The NMR data for the product are in good agreement with the structure of 3, and the structure with atomic numbering scheme is shown in Fig. 1.

Fig. 1. X-ray crystal structure of the title compound

In the crystal structure of the title compound, the molecule is made up of three planes including the 2,6-difluorophenyl ring (Plane 1), thiazole ring(Plane 2), and 4-chlorphenyl ring (Plane 3). According to Fig. 1 and Table 1, the bond C(3)–N(1)in 1.303(2)? belongs to the typical C=N bond[22].Compared to the normal C–N bond distance of 1.47(?), the C(3)–N(2)and C(15)–N(2)bond distances are 1.387(2)and 1.364(2)?, and they are shorter than the typical bond distance due to the effect of conjugation[22], and longer than the typical C=N bond. Similarly, the bond distance of C(15)–C(16)(1.493(2)?)is shorter than the typical value of 1.54 ?[23]. The dihedral angle between the 2,6-difluorophenyl and thiazole rings is 42.68(6)°, while the other one between the thiazole and 4-chlorphenyl rings is 84.26(7)°, almost perpendicular.

As shown in Figs. 1 and 2, the intermolecular hydrogen bonds of N(2)–H(2)··N(3)and N(4)–H(4)··N(1)take participate in generating an octatomic ring between the adjacent molecules (Table 2),which is effective in stabilizing the structure.

The antitumor activity test in vitro showed that the title compound exhibits good antitumor activity with the IC50of 0.046 μmol/mL against Hela. The result of preliminary bioassay indicated that further researches on the title compound are of great significance.

Fig. 2. Crystal packing of the title compound

Table 2. Hydrogen Bond Lengths (?)and Bond Angles (°)for the Title Compound

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