HETEROCYCLES

Short Paper | Regular issue | Vol. 83, No. 4, 2011, pp. 867-874
Received, 13th January, 2011, Accepted, 21st February, 2011, Published online, 2nd March, 2011.
DOI: 10.3987/COM-11-12138

■ Synthesis of Caboxamycin and Its Derivatives Using Eco-Friendly Oxidation

Yoshinobu Tagawa,* Hiroki Koba, Kazuhiro Tomoike, and Kunihiro Sumoto

Faculty of Pharmaceutical Sciences, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Abstract

The reaction of 3-hydroxyanthranilic acid or methyl 3-hydroxyanthranilate with O-benzylsalicylaldehyde in xylenes gave benzoxazole derivatives, which lead to a novel benzoxazole antibiotic, caboxamycin via debenzylation or demethylation in good yield, in the presence of dry activated carbon and bubbling molecular oxygen. The present reaction involves the simple procedure, easy workup and environmentally benign materials such as reusable activated carbon and molecular oxygen.

Caboxamycin is a new antibiotic of the benzoxazole family isolated from deep-sea sediment in Canary Basin.1 It showed inhibitory activity against Gram-positive bacteria, selected human tumor cell lines and enzyme phosphodiesterase. The structure of caboxamycin was determined by the various spectral data and x-ray analysis, but the synthesis is officially still not reported.1,2 It is of great interest to prepare caboxamycin and the derivatives to examine various biological activities. We have already reported the preparation of benzimidazole derivatives by eco-friendly oxidation of 1,2-phenylenediamine with a variety of aromatic aldehydes in the presence of environmentally benign materials, dry activated carbon and bubbling molecular oxygen.3 Applying this synthetic method, we investigated the preparation of caboxamycin and its derivatives, and finally obtained caboxamycin and its derivatives in satisfactory yields. At the first stage we have tried the direct reaction of 3-hydroxyanthranilic acid with salicyl aldehyde in xylenes in the presence of activated carbon and molecular oxygen to obtain caboxamycin in one step reaction, but no reaction proceeded. The reaction of 2-amino-m-cresol (1) with salicyl aldehyde (2) in xylenes in the presence of activated carbon and molecular oxygen gave compounds (4) and (5) in 58% and 7% yields, respectively. Compound 5 was oxidized to 4 in 39% yield under the same reaction conditions above mentioned. The oxidation of methyl group on 4 to carboxylic acid will give caboxamycin (6). So, a variety of oxidations, i.e., t-BuOK/O2/DMF,4 crown ether/KMnO4/PhH,5 Co(OAc)2/NHPI/O2/AcOH,6 Ni(bpy)2Cl2/NaOCl/MeCN,7 KMnO4/H2O,8 in order to oxidize methyl group on 4 to carboxylic acid was examined to proceed in vain. The compound (7) prepared from 4 also showed the resistance to oxidation of methyl group to carboxylic acid by Co(OAc)2/NHPI/O2/AcOH6 (Scheme 1).

Reaction of 3-hydroxyanthranilic acid9 (8), which was prepared via 5 step reactions from m-cresol, with 2-benzyloxybenzaldehyde10 (3) or o-anisaldehyde11 (9) in xylenes in the presence of activated carbon and molecular oxygen gave benzyloxyphenyl benzoxazole derivative (10) or methoxyphenyl benzoxazole derivative (11) in 82% or 53% yields, respectively. Compound 10 was reductively debenzylated by Pd/C/H212 to give 6 in 64% yield, whose spectral data such as 1H and 13C-NMR spectra were completely coincided with those reported on the original journal.1 On the other hand, 11 was not demethylated by Pd/C/H2, p-TsOH/PhMe12 or piperazine/DMA13 to give 6 (Scheme 2).

Methyl 3-hydroxyanthranilate14 (12) reacted with 2,4-dichlorobenzaldehyde in xylenes in the presence of activated carbon and molecular oxygen to give dichlorophenyl benzoxazole ester (13) in 23% yield, but reaction of 12 with 2 did not proceed to the desired cyclic compound (14). So, we carried out the reaction of 12 with 3 and 9, which have no phenolic hydroxygroup, under the same reaction conditions aforementioned to give benzyloxyphenyl benzoxazole ester (15) and methoxyphenyl benzoxazole ester (16) in 69% and 77% yields, respectively. While 15 was debenzylated by Pd/C/H2 to result in hydroxyphenyl benzoxazole ester (17) in 31% yield, which was hydrolyzed with 5N-NaOH aqueous solution12 to give 6 in 40% yield, 16 was hydrolyzed with 5N-NaOH aqueous solution to give 11 in 60% yield (Scheme 2).

In conclusion, we synthesized caboxamycin with 2 routes via debezylation and demethylation under eco-friendly and simple reaction conditions composed of activated carbon and molecular oxygen in moderate yield, and the present report is officially the first synthesis of caboxamycin. It is possible to prepare a variety of derivatives of caboxamycin by changing functional group and functional group position or using various aromatic benzaldehydes. From these standpoints, our investigation is currently under way to prepare such derivatives of caboxamycin which are expected to have any valuable biological activities.

EXPERIMENTAL
Melting points were measured on a Yanagimoto micro melting points apparatus and are uncorrected. Spectral data were recorded on the following spectrometers: IR spectra, JASCO FT/IR-4100; 1H NMR spectra, JEOL GX-400 (400 MHz) and JEOL A-500 (500 MHz); 13C NMR spectra, JEOL GX-400 (100 MHz) and JEOL A-500 (125MHz); mass spectra, JEOL JMS-DX300 for EI-ms and JMS-HX110 for FAB-ms. The HH-COSY, CH-COSY, and DEPT experiments were also used for the assignments of the structures. The chemical shifts are given on the δscale (ppm) using TMS as the internal standard. Elemental analyses were performed on Yanaco MT-6 instrument. Medium pressure liquid chromatography (mplc) was carried out with a Yamazen 540 FMI-C pump and Wakogel FC-40 (20-40 μm, Wako). Column chromatography was carried out with Kieselgel 60 (70-230 mesh, Merck). Activated carbon (Darco® KB) was purchased from Aldrich Chemical Company and used after drying in vacuo with heating.

General Procedure for reaction of 2-aminophenol derivatives with aromatic aldehyde in the presence of activated carbon and molecular oxygen
A mixture of 2-aminophenol derivative (5.00 mmol), aromatic aldehyde (5.00 mmol) and activated carbon (50 weight% of 2-aminophenol derivative) was heated at 110-115 ℃ for the time shown in Schemes 1 and 2 in xylenes (40 mL) with stirring under bubbling of molecular oxygen into xylenes. The resulting solution was filtered and the activated carbon was washed with MeOH. After removal of the solvent, the residue was worked up in the manner as shown below.

The residue of the reaction of 1 with 2 was chromatographed with chloroform to give 0.65 g (58%) of 4 and 0.08 g (7%) of 5, respectively.

2-(4-Methylbenzo[d]oxazol-2-yl)phenol (4)
This compound was recrystallized from 99% EtOH to give colorless prisms, mp 113-114 ℃.
1H NMR(CDCl3) δ(ppm): 2.62 (3H, s, Me), 6.98-7.01 (1H, m), 7.10-7.12 (1H, m), 7.15 -7.17 (1H, m), 7.24-7.27 (1H, m), 7.40-7.44 (2H, m), 8.00 (1H, dd, J=7.6 and 1.5 Hz), 11.55 (1H, s, OH). 13C NMR (CDCl3) δ(ppm): 16.4 (q, CH3), 107.9 (d), 110.8 (s), 117.4 (d), 119.5 (d), 125.1 (d), 125.5 (d), 127.0 (d), 129.8 (s), 133.3 (d), 139.3 (s), 148.9 (s), 158.7 (s), 162.2 (s). MS (FAB) m/z 226 (MH+). Anal. Calcd for C14H11NO2: C, 74.65; H, 4.92; N, 6.22. Found: C 74.68 ; H, 5.00 ; N, 6.20. IR (KBr, cm-1): 3030, 1629, 1591, 1549, 1488, 1417, 1249, 1071, 753, 714.

2-(4-Methyl-2,3-dihydrobenzo[d]oxazol-2-yl)phenol (5)
This compound was recrystallized from 99% EtOH to give pale yellow prisms, mp 215-217 ℃.
1H NMR(DMSO-d6) δ(ppm): 2.22 (3H, s, Me), 6.75 (1H, d, J=7.3 Hz), 6.84 (1H, d, J=8.2 Hz), 6.93-6.98 (3H, m), 7.39-7.42 (1H, m), 7.57 (1H, dd, J=7.9 and 1.8 Hz), 8.95 (1H, s, NHCH), 9.65 (1H, s, NH), 13.52 (1H, s, OH). 13C NMR(DMSO-d6) δ(ppm): 18.3 (q, CH3), 114.3 (d), 116.6 (d), 118.9 (d), 119.3 (s), 121.1 (d), 126.0 (d), 132.1 (s), 132.3 (d), 132.8 (d), 134.4 (s), 148.5 (s), 160.5 (s), 167.1 (d, NH-CH). MS (FAB) m/z 228 (MH+). Anal. Calcd for C14H13NO2 : C, 73.99; H, 5.77; N, 6.16. Found: C, 73.94; H, 5.89; N, 6.13. IR (KBr, cm-1): 1615, 1536, 1456, 1382, 1272, 1218, 1140, 1111, 561, 490.

Transformation of 5 into 4
A mixture of 5 (0.15 g, 0.66 mmol) and activated carbon (0.075 g) was heated at 110-115 ℃ for 3 h in xylenes (30 mL) with stirring under bubbling of molecular oxygen into xylenes. The resulting solution was filtered and the activated carbon was washed with MeOH. After removal of the solvent, the residue was chromatographed with chloroform to give 0.058 g (39%) of 4.

2-(2-(Benzyloxy)phenyl)-4-methylbenzo[d]oxazole (7)
The residue was chromatographed with CHCl3 to give 1.06 g (67%) of 7. This compound was recrystallized from 95% EtOH to give colorless needles, mp 92-93 ℃. 1H NMR(CDCl3) δ(ppm): 2.71 (3H, s, Me), 5.28 (2H, s, CH2), 7.10-7.15 (3H, m), 7.23 (1H, dd, J=8.1 and 8.1 Hz), 7.29-7.32 (1H, m), 7.37-7.40 (3H, m), 7.45-7.49 (1H, m), 7.67-7.69 (2H, m), 8.17 (1H, dd, J=7.6 and 1.8 Hz).
13C NMR(CDCl3) δ(ppm): 16.6 (q, CH3), 70.7 (t, CH2), 107.7 (d), 113.9 (d), 117.4 (s), 121.2 (d), 124.6 (d), 124.8 (d), 126.8 (d), 127.7 (d), 128.4 (d), 130.6 (s), 131.4 (d), 132.4 (d), 136.9 (s), 141.5 (s), 150.3 (s), 157.5 (s), 160.9 (s). MS (FAB) m/z 316 (MH+). Anal. Calcd for C21H17NO2: C, 79.98; H, 5.43; N, 4.44. Found: C, 80.16; H, 5.57; N, 4.44. IR (KBr, cm-1): 1610, 1499, 1443, 1292, 1266, 1239, 1022, 744, 730, 696.

Preparation10 of 7 from 4
To a solution of 4 (0.3 g, 1.33 mmol) in dry acetone (30 mL) was added anhydrous K2CO3 (0.28 g, 2 mmol) and benzyl bromide (0.26 g, 1.53 mmol), and the resulting mixture was refluxed for 4 h. After removal of the solvent, the residue was extracted with EtOAc. After evaporation of the solvent, the residue was purified by mplc (hexane : EtOAc = 10 : 1) to give 0.1 g (23%) of 7.

2-(2-(Benzyloxy)phenyl)benzo[d]oxazole-4-carboxylic acid (10)
The residue was chromatographed with CHCl3 to give 1.41 g (82%) of 10. This compound was recrystallized from 2-propanol – H2O to give colorless prisms, mp 68-69 ℃. 1H NMR(CDCl3) δ(ppm): 5.29 (2H, s, CH2), 7.15 (1H, dd, J=7.5 and 7.5 Hz), 7.18 (1H, d, J=8.2 Hz), 7.35 (1H, dd, J=7.3 and 7.3 Hz), 7.42 (1H, dd, J=7.5 and 7.9 Hz), 7.48 (1H, dd, J=7.9 and 7.9 Hz), 7.51-7.59 (4H, m), 7.76 (1H, dd, J=7.9 and 0.9 Hz), 8.13 (1H, dd, J=7.6 and 0.9 Hz), 8.21 (1H, dd, J=7.9 and 1.4 Hz), 11.83 (1H, brs, CO2H). 13C NMR(CDCl3) δ(ppm): 71.0 (t, CH2), 113.8 (d, Ar), 114.8 (s, Ar), 115.1 (d, Ar), 120.2 (s, Ar), 121.2 (d, Ar), 125.4 (d, Ar), 127.0 (d, Ar), 127.2 (d, Ar), 128.3 (d, Ar), 128.8 (d, Ar), 131.6 (d, Ar), 134.3 (d, Ar), 136.0 (s, Ar), 141.1 (s, Ar), 149.9 (s, Ar), 158.2 (s, Ar), 163.1 (s, Ar), 164.8 (s, C=O). MS (FAB) m/z 346 (MH+). Anal. Calcd for C21H15NO4 + 1.2 H2O: C, 68.73; H, 4.78; N, 3.82. Found: C, 68.71; H, 4.73; N, 3.79. IR (KBr, cm-1): 3421, 1747, 1704, 1605, 1541, 1496, 1432, 1290, 1253, 751, 697.

2-(2-Methoxyphenyl)benzo[d]oxazole-4-carboxylic acid (11)
The residue was chromatographed with CHCl3 to give 0.71 g (53%) of 11. This compound was recrystallized from 2-propanol to give pale yellow cottony crystals, mp 192-194 ℃. 1H NMR(CDCl3) δ(ppm): 4.03 (3H, s, Me), 7.11-7.16 (2H, m), 7.50 (1H, dd, J=8.2 and 7.9 Hz), 7.57-7.60 (1H, m), 7.82 (1H, d, J=8.2 Hz), 8.14 (1H, d, J=7.9 Hz), 8.19 (1H, dd, J=7.9 and 1.8 Hz), 11.97 (1H, brs, CO2H). 13C NMR(CDCl3) δ(ppm): 56.1 (q, CH3), 112.4 (d), 114.3 (s), 115.2 (d), 120.1 (s), 120.9 (d), 125.3 (d), 127.0 (d), 131.5 (d), 134.3 (d), 141.1 (s), 149.9 (s), 159.2 (s), 163.2 (s), 164.8 (s, C=O). MS (FAB) m/z 270(MH+). Anal. Calcd for C15H11NO4: C, 66.91; H, 4.12; N, 5.20. Found: C, 66.86; H, 4.22; N, 5.23. IR (KBr, cm-1): 3211, 1753, 1604, 1529, 1477, 1430, 1404, 1267, 1250, 1168, 754.

Methyl 2-(2,4-dichlorophenyl)benzo[d]oxazole-4-carboxylate (13)
The residue was purified by mplc (hexane : EtOAc = 10 : 1) to give 0.37 g (23%) of 13.
This compound was recrystallized from hexane to give yellow curdy (cotton) crystal, mp 129-130 ℃.
1H NMR(CDCl3) δ(ppm): 4.05 (3H, s, Me), 7.41 (1H, dd, J=8.4 and 2.0 Hz), 7.47 (1H, dd, J=7.9 and 7.9 Hz), 7.59 (1H, d, J=2.1 Hz), 7.81 (1H, dd, J=8.2 and 0.9 Hz), 8.07 (1H, dd, J=7.8 and 1.1 Hz), 8.22 (1H, d, J=8.5 Hz). 13C NMR(CDCl3) δ(ppm): 52.5 (q, CH3), 115.1 (d), 122.7 (s), 124.5 (s), 125.1 (d), 127.3 (d), 127.4 (d), 131.3 (d), 133.2 (d), 134.7 (s), 138.1 (s), 141.0 (s), 151.4 (s), 162.0 (s), 165.6 (s, C=O). MS (FAB) m/z 322(MH+). Anal. Calcd for C15H9NO3Cl2: C, 55.93; H, 2.82; N, 4.35. Found: C, 55.96; H, 2.87; N, 4.32. IR (KBr, cm-1): 1710, 1561, 1460, 1392, 1308, 1291, 1137, 1098, 788, 754.

Methyl 2-(2-(benzyloxy)phenyl)benzo[d]oxazole-4-carboxylate (15)
The residue was chromatographed with CH2Cl2 to give 1.24 g (69%) of 15.
This compound was recrystallized from hexane to give colorless prisms, mp 106-107 ℃.
1H NMR(CDCl3) δ(ppm): 4.01 (3H, s, Me), 5.30 (2H, s, CH2), 7.11-7.14 (2H, m), 7.30-7.33 (1H, m), 7.38-7.41 (3H, m), 7.48-7.51 (1H, m), 7.63-7.65 (2H, m), 7.74 (1H, dd, J=8.2 and 1.1 Hz), 8.03 (1H, dd, J=7.9 and 1.1 Hz), 8.27 (1H, dd, J=7.6 and 1.8 Hz). 13C NMR(CDCl3) δ(ppm): 52.4 (q, CH3), 70.8 (t, CH2), 113.9 (d), 114.7 (d), 116.6 (s), 121.1 (d), 122.2 (s), 124.1 (d), 126.8 (d), 126.9 (d), 127.7 (d), 128.5 (d), 132.0 (d), 133.2 (d), 136.8 (s), 141.6 (s), 151.4 (s), 157.9 (s), 163.7 (s), 166.0 (s, C=O). MS (FAB) m/z 360(MH+). Anal. Calcd for C22H17NO4: C, 73.53; H, 4.77; N, 3.90. Found: C, 73.75; H, 4.92; N, 3.91. IR (KBr, cm-1): 1705, 1603, 1533, 1449, 1421, 1318, 1293, 1269, 1242, 1126, 752.

Methyl 2-(2-methoxyphenyl)benzo[d]oxazole-4-carboxylate (16)
The residue was purified by mplc (hexane : EtOAc = 10 : 1) to give 1.09 g (77%) of 16.
This compound was recrystallized from hexane to give pale yellow crystals, mp 110-111 ℃.
1H NMR(CDCl3) δ(ppm): 4.02 (3H, s, OMe), 4.06 (3H, s, COOMe), 7.07-7.12 (2H, m), 7.40 (1H, dd, J=7.9 and 7.9 Hz), 7.50-7.53 (1H, m), 7.78 (1H, dd, J=8.1 and 1.1 Hz), 8.03 (1H, dd, J=7.9 and 1.2 Hz), 8.24 (1H, dd, J=7.9 and 1.8 Hz). 13C NMR(CDCl3) δ(ppm): 52.4 (q, COOMe), 56.1 (q, OMe), 112.1 (d), 114.8 (d), 115.9 (s), 120.7 (d), 122.1 (s), 124.2 (d), 126.9 (d), 131.9 (d), 133.3 (d), 141.6 (s), 151.3 (s), 158.9 (s), 163.7 (s), 166.0 (s, C=O). MS (FAB) m/z 284(MH+). Anal. Calcd for C16H13NO4: C, 67.84; H, 4.63; N, 4.94. Found: C, 67.82; H, 4.68; N, 4.89. IR (KBr, cm-1): 1706, 1607, 1482, 1422, 1314, 1302, 1271, 1239, 1196, 739.

General Procedure for reduction of compounds 10 and 15 by hydrogenation on Pd/C
A mixture of 10 or 15 (0.5 mmol) and Pd/C (20 mg, 10% of Pd on charcoal) in MeOH (30 mL) under hydrogen atomosphere (3.3 kgf/cm2) was hydrogenated for 3 h. After the catalyst was removed by filtration, the filtrate was worked up in the manner as shown below.

2-(2-Hydroxyphenyl)benzo[d]oxazole-4-carboxylic acid (Caboxamycin) (6)
The residue was recrystallized from 2-propanol to give white powder. Yield 64%. mp 237-239 ℃.
1H NMR(DMSO-d6) δ(ppm): 0.50-2.00 (0.5H, brs, OH), 7.10-7.16 (2H, m), 7.55-7.60 (2H, m), 8.00 (1H, dd, J=7.8 and 1.1 Hz), 8.05 (1H, dd, J=7.9 and 1.5 Hz), 8.10 (1H, dd, J=8.2 and 0.9 Hz), 11.79 (1H, s, COOH). 13C NMR(DMSO-d6) δ(ppm): 109.7 (s), 115.3 (d), 117.3 (d), 120.0 (d), 121.8 (s), 125.3 (d), 127.1 (d), 127.4 (d), 134.4 (d), 138.7 (s), 149.5 (s), 158.4 (s), 163.6 (s), 165.5 (s, C=O). MS (FAB) m/z 256(MH+). Anal. Calcd for C14H9NO4: C, 65.88; H, 3.55; N, 5.49. Found: C, 65.86; H, 3.71; N, 5.48. IR (KBr, cm-1): 3011, 1703, 1631, 1546, 1484, 1432, 1302, 1258, 1245, 755.

Methyl 2-(2-hydroxyphenyl)benzo[d]oxazole-4-carboxylate (17)
The residue was chromatographed with CHCl3 to give 0.04 g (31%) of 17.
This compound was recrystallized from hexane to give colorless crystals, mp 136 ℃.
1H NMR(CDCl3) δ(ppm): 1.66 (1H, brs, OH), 4.06 (3H, s, Me), 7.02 (1H, dd, J=7.5 and 7.5 Hz), 7.15 (1H, d, J=8.5 Hz), 7.43-7.49 (2H, m), 7.80 (1H, d, J=7.6 Hz), 8.03 (1H, dd, J=7.6 and 1.5 Hz), 8.07 (1H, d, J=7.6 Hz). 13C NMR(CDCl3) δ(ppm): 52.4 (q, CH3), 110.0 (s), 114.9 (d), 117.8 (d), 119.6 (d), 121.5 (s), 124.8 (d), 127.2 (d), 127.5 (d), 134.3 (d), 139.5 (s), 149.8 (s), 159.5 (s), 164.4 (s), 165.6 (s, C=O). MS (FAB) m/z 270(MH+). Anal. Calcd for C15H11NO4 + 0.2 H2O: C, 66.03; H, 4.21; N, 5.13. Found: C, 66.00; H, 4.06; N, 5.11. IR (KBr, cm-1): 1718, 1633, 1550, 1489, 1422, 1304, 1244, 1061, 748, 708.

General Procedure for saponification of compounds 16 and 17 by NaOH
To a solution of compounds 16 or 17 (0.5 mmol) in THF (10 mL) was added 5 N NaOH solution (10 mL), and the solution was refluxed for 6.5 h. After removal of the solvent, the residue was acidified by 10% HCl solution and resulting solution was extracted with CHCl3 or EtOAc. The CHCl3 or EtOAc solution was worked up in the manner as shown below.

2-(2-Hydroxyphenyl)benzo[d]oxazole-4-carboxylic acid (Caboxamycin) (6)
After removal of the solvent, the residue was recrystallized from 2-propanol to give 0.05 g (40%) of 6.

2-(2-Methoxyphenyl)benzo[d]oxazole-4-carboxylic acid (11)
After removal of the solvent, the residue was recrystallized from 2-propanol to give pale yellow cottony crystals, 0.08 g (60%).

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