HETEROCYCLES

Note | Regular issue | Vol. 78, No. 4, 2009, pp. 1031-1039
Received, 24th October, 2008, Accepted, 8th December, 2008, Published online, 9th December, 2008.
DOI: 10.3987/COM-08-11586

■ A Convenient One-Pot Synthesis of Arene-Centered Tris(thiazoline) Compounds

Xuhong Lu, Qingqing Qi, Yumei Xiao, Nan Li, and Bin Fu*

Department of Applied Chemistry, College of Science, China Agriculural University, Beijing, 100094, China

Abstract

A simple and practical one-pot synthesis of novel enantiopure tris(thiazoline) compounds was documented. The desired products were obtained in moderate to good yields through three steps from commercially available 1, 3, 5-benzenetricarboxylic acid or 2, 4, 6-pyridinetricarboxylic acid, and chiral amino alcohols. Only one column chromatographic purification was needed for the three steps.

Thiazoline rings have been found in a large number of biological active natural products, such as thiangazole,1 curacin A,2 and lissoclinamides.3 Moreover, thiazoline compounds have been widely applied as food additives,4 agrochemicals,5 chiral ligands,6 and so on. In view of the versatile properties of thiazoline compounds, their synthesis attracts the attention from many researchers. To date, many methods have been developed for the construction of thiazoline ring system. One major type of the methods is the condensation of 2-aminoethanethiol with carboxylic acids or their derivatives, such as nitriles, esters, and iminoesters.7 Another type is the cyclization of β-hydroxythioamides.8 Besides of the two types of methods, some other methods have also been developed with both advantages and deficients.9 In recent years, direct cyclization of β-hydroxyamide has been employed as one convenient method for its simplicity and practicability. Some mono- and bis-thiazoline compounds have been synthesized with Lawesson reagent or phosphorus pentasulfide (P2S5) as reagent.10 However, the generality of this method has not been comfirmed. Thus, our effort was focused on the development of efficient and general route towards arene-centered tris(thiazoline) compounds, analogue of arene-centered tris(oxazoline) compounds with versatile properties.11 Herein, we would like to document the efficient one-pot preparation of tris(thiazoline) compounds 1 and 2 with potent applications in molecular recognition and self-assembly.

The synthetic route is illustrated in Scheme 1. The starting material benzenetricarboxylic acid was refluxed in SOCl2 for 12 h, and the excess SOCl2 was removed in vacuo. The crude triacyl chlorides were dissolved in CH2Cl2 and added dropwise to the solution of amino alcohols and Et3N in CH2Cl2 at 0 °C. After being stirring for 4 h, the solvent was removed in vacuo, and the crude intermediate tris(β-hydroxyamides) were used in the cyclization step without further purification. The mixture of P2S5 with tris(β-hydroxyamides) was refluxed in toluene for 6~8 h in the presence of Et3N. The desired tris(thiazoline) compounds were obtained in 58~82%.yield after purification, as listed in Table 1. During the whole synthetic course, only one column chromatographic purification was needed. The addition of Et3N and high temperature were crucial, while refluxing only in toluene or Et3N gave no product (Entries 2 and 3). Alternatively, pyridine can be used instead of toluene/Et3N in this reaction, though the yield decreased significantly (Entry 4). The cheap reagent P2S5 was quite effective in the cyclization step. On the contrary, using Lawesson reagent12 in the final step the tris(thiazoline) 1a was only obtained in 20% yield. For different chiral amino alcohols, the corresponding tris(thiazoline) compounds was afforded in moderate to good yields (Entries 5~9). The structure of the novel tris(thiazoline) compounds were thoroughly characterized by spectroscopic methods. In addition, the single crystal of tris(thiazoline) (S)-1e was cultivated and detected by X-Ray crystal diffraction analysis, as illustrated in Figure 1.13

With the optimized reaction conditions in hand, we further investigated the synthesis of pyridine-centered tris(thiazoline) compounds starting from 2,4,6-pyridinetricarboxylic acid14 (Scheme 2). The reaction results were listed in Table 2. Generally, the yields in these cases were lower than corresponding benzene-centered tris(thiazoline) compounds 1 (Entries 10~13). When R was phenyl, no expected tris(thiazoline) compound was obtained (Entries 14 and 15), but complex mixture indicated by TLC analysis. Similar result was obtained when purified tris(β-hydroxyamide) was used. Such a phenomenon can be attributed to the nucleophilic nitrogen atom in the pyridine unit, which may react with the intermediates in the ring-closing step.

On the basis of our experimental results and relevant reports,10, 12 we deduced that the simple reagent combination of P2S5 and Et3N is not only suitable for the synthesis of mono- and bis(thiazoline) compounds, but also suitable for complex tris(thiazoline) compounds. The thiazolines with aryl or tertiary alkyl group at 2-position can be synthesized efficiently, while those with α-hydrogen at 2-position are not suitable target, an alternative route to those complex thiazoline compounds was developed by our group.15 The mechanism of the cyclization step is not clear now, it is hypothesized that the carbonyl group is thionated, and the hydroxyl group is transformed to a leaving group by P2S5 before the intramolecular nucleophilic substitution is occurred.


In conclusion, two series of chiral tris(thiazoline) compounds with benzene and pyridine centers were synthesized in one pot through a continuous 3-step sequence with P2S5/Et3N mediated cyclization of corresponding tris(β-hydroxyamides) as key step. The desired products can be obtained in moderate to good overall yields with only one column chromatographic purification. The mild condition and easy purification indicate the possibility of its application in large scale synthesis of fine chemicals. Although bis(thiazoline) with benzene and pyridine skeleton have been synthesized, this paper afford further extension for the synthesis of complex thiazoline compounds based on previous report.16, 6d The application of the products in molecular recognition and self-assembly are under going in our laboratory.

EXPERIMENTAL
General. Melting points were measured on an XT-4 melting point apparatus without correction. NMR spectra were recorded on a Bruker Avance DPX300 spectrometer with tetramethylsilane as internal standard. Infrared spectra were obtained on a Nicolet AVATAR 330 FT-IR spectrometer. Optical rotations were measured on a Perkin–Elmer 341 LC polarimeter. Mass spectra were carried out using a VG Autospec instrument. Elemental analyses were carried out on an Elementar Vario EL instrument. Solvents were purified and dried following standard procedures.

General procedure for the synthesis of tris(thiazoline) 1a~f
The 1, 3, 5-benzenetricarboxylic acid (0.42 g, 2.0 mmol) was refluxed with SOCl2 (3.0 mL) for 12 h, then the excess SOCl2 was removed in vacuo. Benzene (5 mL) was added and the solvent was removed again to dryness to remove the trace amount of SOCl2 and afford the triacyl trichloride. The triacyl trichloride in CH2Cl2 (20 mL) was added dropwise to a solution of amino alcohol (6.40 mmol) and Et3N (4 mL, 28.9 mmol) in CH2Cl2 (20 mL) at 0 °C and stirred at rt for 4~6 h. The reaction mixture was evaporated to remove the solvent in vacuo, and toluene (20 mL) and Et3N (6 mL, 43.4 mmol) were added to the crude trihydroxyl triamide, P2S5 (2.0 g, 9.0 mmol) was added under refluxing in 3 portions within 1 h, and the suspension was continued to reflux for another 6~8 h. After being cooled to room temperature, the solution was washed with H2O (5 mL×2), dried over anhydrous Na2SO4 and concentrated to give the crude product. Column chromatographic purification on silica gel (40% ethyl acetate in petroleum ether) afforded the tris(thiazoline) compounds 1a~f.

1, 3, 5-Tris[2-(4S)-4-benzyl-1, 3-thiazolin-2-yl]benzene (1a)
Pale oil, [α]D25 -84.4 (c 0.15 in CH2Cl2). IR (cm-1): 3025, 2923, 1610, 1584, 1494, 1189, 741, 699. 1H NMR (CDCl3): δ 8.37 (s, 3H，ArH), 7.29 (m, 15H, ArH), 5.00~4.91 (m, 3H, CHN=), 3.41~3.31 (m, 6H, CH2S), 3.20 (dd, J = 6.90, 11.4 Hz, 3H), 2.84 (dd, J = 9.30, 13.5 Hz 3H). 13C NMR (CDCl3): δ 165.9, 138.3, 134.1, 130.4, 129.3, 128.5, 126.5, 78.8, 40.1, 37.6. MS: m/z 604 (M+1). Anal. Calcd for C36H33N3S3: C, 71.60; H, 5.53; N, 6.96. Found: C, 71.85; H, 5.41; N, 6.78.

1, 3, 5-Tris[2-(4S)-4-methyl-1, 3-thiazolin-2-yl]benzene (1b)
Mp 155~157 °C. [α]D25 -64.0 (c 0.15 in CH2Cl2). IR (cm-1): 2971, 2927, 1610, 1587, 1433, 1372, 1188, 923, 715. 1H NMR (CDCl3): δ 8.47 (s, 3H, ArH), 4.80~4.77 (m, CHN=, 3H), 3.57 (dd, J = 8.10, 10.50 Hz, 3H), 3.08 (dd, J = 8.10, 10.50 Hz, 3H). 13C NMR (CDCl3): δ 165.0, 134.1, 130.2, 73.1, 40.3, 20.3. MS: m/z 376 (M+1). Anal. Calcd for C18H21N3S3: C, 57.58; H, 5.64; N, 11.19. Found: C, 57.85; H, 5.61; N, 11.28.

1, 3, 5-Tris[2-(4S)-4-i-propyl-1, 3-thiazolin-2-yl]benzene (1c)
Mp 159~161 °C. [α]D25 -49.7 (c 0.10 in CH2Cl2). IR (cm-1): 2958, 2870, 1613, 1588, 1384, 1366, 1192, 1019, 722. 1H NMR (CDCl3): δ 8.31 (s, 3H, ArH), 4.49~4.08 (m, 3H, CHN=), 3.43 (dd, J = 8.70, 11.1Hz 3H), 3.18 (dd, J = 9.6, 11.1 Hz, 3H), 2.14~2.08 (m, 3H, CH), 1.13 (d, J = 6.60 Hz, 9H), 1.02 (d, J = 6.60 Hz, 9H). 13C NMR (CDCl3): δ 164.8, 134.1, 130.2, 84.3, 35.7, 33.2, 19.8, 18.9. MS: m/z 460 (M+1). Anal. Calcd for C27H39N3S3: C, 62.70; H, 7.34; N, 9.14. Found: C, 62.85; H, 7.41; N, 9.05.

1, 3, 5-Tris[2-(4S)-iso-butyl-1, 3-thiazolin-2-yl]benzene (1d)
Pale oil, [α]D25 -55.4 (c 0.10 in CH2Cl2). IR (cm-1): 2954, 2868, 1612, 1587, 1433, 1384, 1366, 1186, 718. 1H NMR (CDCl3): δ 8.30 (s, 3H, ArH), 4.75~4.65 (m, 3H, CHN=), 3.52 (dd, J = 8.10, 10.8 Hz, 3H, CH2S), 3.08 (dd, J = 8.1, 10.8 Hz, 3H, CH2S), 1.94~1.80 (m, 6H, CH2), 1.53~1.44 (m, 3H, CH), 1.01 (d, J = 6.60 Hz, 9H), 1.00(d, J = 6.60 Hz, 9H); 13C NMR (CDCl3): δ 164.8, 134.2, 130.2, 44.1, 38.9, 25.8, 22.9, 22.5. MS: m/z; 502 (M+1). Anal. Calcd for C27H39N3S3: C, 64.62; H, 7.83; N, 8.37. Found: C, 64.82; H, 7.60; N, 8.28.

1, 3, 5-Tris[2-(4S)-4-phenyl-1, 3-thiazolin-2-yl]benzene (1e)
Mp 139~140 °C. [α]D25 -80.8 (c 0.15 in CH2Cl2). IR (cm-1)：3026, 1610, 1580, 1492, 1179, 738, 697. 1H NMR (CDCl3): δ 8.55 (s, 3H), 7.42~7.25 (m, 15H, ArH), 5.73 (t, J = 9.0 Hz, 3H, CHN=), 3.86 (dd, J = 8.7, 11.40 Hz, 3H), 3.37 (dd, J = 9.30, 11.1 Hz, 3H). 13C NMR (CDCl3): δ 167.1, 141.7, 134.1, 130.9, 128.7, 127.7, 126.6, 80.9, 41.4. MS: m/z 562 (M+1). Anal. Calcd for C33H27N3S3: C, 70.55; H, 4.84; N, 9.14. Found: C, 70.43; H, 5.11; N, 9.02.

1, 3, 5-Tris[2-(4R)-4-phenyl-1, 3-thiazolin-2-yl]benzene (1f)
Mp 139~140 °C. [α]D25 +81.4 (c 0.15 in CH2Cl2). IR (cm-1): 3029, 1610, 1580, 1492, 1179, 737, 697. 1H NMR (CDCl3): δ 8.55 (s, 3H), 7.36 (m, 15H), 5.73 (t, 3H), 3.86 (dd, 3H), 3.37 (dd, 3H). 13C NMR (CDCl3): δ 167.1, 141.7, 134.1, 130.9, 128.7, 127.7, 126.6, 80.9, 41.4. MS: m/z 562 (M+1). Anal. Calcd for C33H27N3S3: C, 70.55; H, 4.84; N, 9.14. Found: C, 70.65; H, 4.98; N, 9.00.

General procedure for the synthesis of tris(thiazoline) 2a~d
The 2, 4, 6-pyridinetricarboxylic acid (0.422 g, 2.0 mmol) was refluxed with SOCl2 (3.0 mL) for 8 h, then the excess SOCl2 was removed in vacuo. Benzene (5 mL) was added and the solvent was removed again to dryness to remove the trace amount of SOCl2 and afforded the triacyl trichloride. The triacyl trichloride in CH2Cl2 (20 mL) was added dropwise to a solution of amino alcohol (6.40 mmol) and Et3N (4 mL, 28.9 mmol) in CH2Cl2 (20 mL) at 0 °C and stirred at rt for 4~6 h. The reaction mixture was evaporated to remove the solvent in vacuo, and toluene (20 mL) and Et3N (6 mL, 43.4 mmol) were added to the crude trihydroxyl triamide, P2S5 (2.0 g, 9.0 mmol) was added under refluxing in three portions within 1 h, and the suspension was continued to reflux for another 6~8 h. After being cooled to rt, the solution was washed with H2O (5 mL×2), dried over anhydrous Na2SO4 and concentrated to give the crude product. Column chromatographic purification on silica gel (50% EtOAc in petroleum ether) afforded the tris(thiazoline) compounds 2a~d.

2, 4, 6-Tris[2-(4S)-4-benzyl-1, 3-thiazolin-2-yl]pyridine (2a)
Mp 152~154 °C. [α]D25 -76.4 (c 0.10 in CH2Cl2). MS: m/z 605(M+1); IR (cm-1): 3026, 2937, 1601, 1494, 1185, 722, 700; 1H NMR (CDCl3): δ 8.53 (s, 2 H, ArH), 7.63~7.25 (m, 15 H, ArH), 5.08~4.93 (m, 3 H, CHN=), 3.45~3.30 (m, 6 H, CH2S), 3.16~3.10 (m, 3 H, CH2Ph), 2.84 (m, 3 H, CH2Ph). 13C NMR (CDCl3): δ 168.9, 165.0, 151.3, 141.9, 138.3, 138.0, 129.7, 129.3, 128.6, 128.5, 126.6, 126.5, 121.3, 79.5, 79.0, 40.3, 40.9, 37.8, 36.2. Anal. Calcd for C35H327N4S3: C, 69.50; H, 5.33; N, 9.26. Found: C, 69.63; H, 5.21; N, 9.21.

2, 4, 6-Tris[2-(4S)-4-methyl-1, 3-thiazolin-2-yl]pyridine (2b)
Mp 162~163 °C. [α]D25 -42.6 (c 0.10 in CH2Cl2). IR (cm-1): 2967, 2925, 1603, 1450, 1374, 1183, 1025, 923, 715. 1H NMR (CDCl3): δ 8.47(s, 2H), 4.90~4.77 (m, 3H), 3.63~3.47 (m, 3H), 3.14~2.98 (m, 3H), 1.46 (t, J = 6.6 Hz, 9H). 13C NMR (CDCl3): δ 168.0, 164.0, 151.3, 141.9, 121.2, 73.7, 73.2, 40.4, 38.8, 20.5, 20.2. MS: m/z 377 (M+1). Anal. Calcd for C17H20N4S3: C, 54.22; H, 3.50; N, 14.88. Found: C, 54.29; H, 3.61; N, 14.94.

2, 4, 6-Tris[2-(4R)-4-methyl-1, 3-thiazolin-2-yl]pyridine (2c)
Mp 162~163 °C. [α]D25 +44.1 (c 0.10 in CH2Cl2). IR (cm-1): 2966, 2926, 1603, 1552, 1185 1022, 923, 713; 1H NMR (CDCl3): δ 8.48 (s, 2H), 4.91~4.77 (m, 3H, CHN=), 3.63~3.48 (m, 3H, CH2S), 3.14~2.98 (m, 3H, CH2S), 1.46 (t, J = 6.6 Hz, 9H). 13C NMR (CDCl3): δ 168.0, 164.0, 151.3, 141.9, 121.2, 73.7, 73.2, 40.4, 38.8, 20.5, 20.2. Anal. Calcd for C17H20N4S3: C, 54.22; H, 3.50; N, 14.88. Found: C, 54.37; H, 3.57; N, 14.80.

2, 4, 6-Tris[2-(4S)-4-iso-propyl-1, 3-thiazolin-2-yl] pyridine (2d)
Mp 132~134 °C. [α]D25 -48.2 (c 0.10 in CH2Cl2). IR (cm-1): 2958, 2870, 1609, 1552, 1175, 1018, 932, 723. 1H NMR (CDCl3): δ 8.47 (s, 2H, ArH), 4.59~4.47 (m, 3H, CHN=), 3.50~3.37 (m, 3H, CH2), 3.25~3.08 (m, 3H, CH2), 2.16~2.09 (m, 3H, CH), 1.13~1.01 (m, 18 H, CH3). 13C NMR (CDCl3): δ 168.0, 164.0, 151.2, 141.8, 121.1, 84.9, 84.5, 35.7, 34.1, 33.4, 33.2, 19.7, 18.9. MS: m/z 461(M+1). Anal. Calcd for C23H32N4S3: C, 59.96; H, 7.00; N, 12.16. Found: C, 59.83; H, 7.16; N, 12.21.

ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Science and Technology of China (No. 2006BAE01A01) and China Agricultural University.

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13. Intensity data for the title complex were collected at 293(2)K on a Rigaku AFC6S diffractometerr. Crystallographic data: C33H27N3S3, M = 561.76, monoclinic, space group C2, a = 30.595(6) Å, b = 6.0296(12)Å, c = 19.425(4) Å, α = 90.00°, β = 126.86 (3)°, γ = 90.00°, V = 2867.0(10)Å3, Z = 4, D = 1.301 g.cm-3; MoKα (λ = 0.71073 Å), T = 293(2)K, µ = 0.286 mm-1, crystal size (mm) 0.50×0.18.×0.18. Area detector data collected on a Rigaku AFC6S diffractometer. A total of 6187 reflections were collected (2.10< θ <25.03). Structure solution by direct method (SHELXS-97), refinement by full-matrix least-squares using all reflections, R1 = 0.0793, wR2 = 0.0812, GOF = 0.913. Crystallographic data for the structure have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-701494. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk) .
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