2. Materials and Methods
All reactions were carried out in oven-dried glassware with magnetic stirrers under an argon atmosphere. THF was dried over Na/benzophenone and DCM was dried over CaH2. Commercially available chemicals were purchased from Sigma-Aldrich and Alfa Aesar. EtOAc and pet ether were distilled before use. All melting points were taken in open capillaries and are uncorrected. Analytical thin-layer chromatography (TLC) was performed on commercially available Merck TLC Silica gel 60 F254. Silica gel column chromatography was performed on silica gel 60 (spherical 100–200 μm). FTIR spectra were recorded on Perkin-Elmer FT/IR-4000 spectrophotometer and only the characteristic peaks are reported. Mass spectra were scanned on a Shimadzu LCMS 2010 spectrometer. 1H NMR spectra were recorded on Varian-400 (400 MHz) spectrometer. Chemical shifts of 1H NMR spectra were reported relative to tetramethylsilane. 13C NMR spectra were recorded on Varian-400 (100 MHz) spectrometer. Chemical shifts of 13C NMR spectra were reported to be relative to CDCl3 (77.0). Splitting patterns were reported as s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; dd, double doublet; and br, broad.
2.1. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-((trimethylsilyl)ethynyl)phenyl)propanoate (4a)
To a solution of compound 3a (1.0 g, 2.80 mmol) in triethylamine (10 mL), PdCl2(PPh3)2 (0.098 g, 0.14 mmol), CuI (0.013 g, 0.07 mmol), and trimethylsilylacetylene (0.411 g, 4.20 mmol) were added under argon atmosphere and heated at 80°C in a sealed tube for 12 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). After completion of the reaction, the reaction mixture was filtered. The filtrate was evaporated to give the crude product which was charged on silica gel column. The column was eluted with 20% ethyl acetate/pet ether to give the compound 4a (0.800 g, 76% yield) as light yellow liquid.
IR (KBr, cm−1): 3375, 2961, 2158, 1716, 1505, 1250, 1168, 865, 843. 1H NMR (400 MHz, CDCl3): δ = 7.42–7.36 (m, 2H), 7.06 (d, J = 7.8 Hz, 2H), 4.94 (d, J = 8.1 Hz, 1H), 4.57 (d, J = 7.4 Hz, 1H), 3.69 (s, 3H), 3.09 (td, J = 14.2, 6.1 Hz, 2H), 1.42 (s, 9H), 0.2 (s, 9H). MS (EI): m/z 375 (M + 1, 100).
2.2. Experimental Procedure for the Preparation of Methyl 2-Pivalamido-3-(4-((trimethylsilyl)ethynyl)phenyl)propanoate (4b)
To a solution of compound 3b (3.5 g, 10.26 mmol) in triethylamine (20 mL), PdCl2(PPh3)2 (0.359 g, 0.51 mmol), CuI (0.048 g, 0.25 mmol), and trimethylsilylacetylene (1.20 g, 12.31 mmol) were added under argon atmosphere and heated at 90°C in a sealed tube for 12 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, the reaction mixture was filtered. The filtrate was evaporated to give the crude reaction mixture which was charged on silica gel column. The column was eluted with 20% ethyl acetate/pet ether to give the compound 4b (1.8 g, 48% yield) as off-white solid.
m.p. 143–145°C. IR (KBr, cm−1): 3328, 2958, 2158, 1751, 1638, 1205, 841. 1H NMR (300 MHz, DMSO): δ = 7.40 (dd, J = 1.7, 7.8 Hz, 2H), 7.20 (dd, J = 10.3, 8.1 Hz, 2H), 4.53–4.38 (m, 1H), 3.61 (d, J = 1.7 Hz, 3H), 3.18–2.89 (m, 2H), 1.00 (d, J = 1.6 Hz, 9H), 0.2 (s, 9H). MS (EI): m/z 360 (M + 1, 100).
2.3. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-ethynylphenyl)propanoate (5a)
To a solution of compound 4a (0.800 g, 2.13 mmol) in THF (10 mL), 1 M TBAF in THF (4.26 mL, 4.26 mmol) was added at −70°C and stirred for 2 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). After the reaction was complete, the reaction mixture was quenched with water (10 mL) and extracted with ethyl acetate thrice. The organic layers were combined and washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 5a (0.650 g, 95% yield) as light yellow solid.
m.p. 94–97°C. IR (KBr, cm−1): 3355, 2974, 2103, 1739, 1682, 1519, 1170, 826. 1H NMR (400 MHz, CDCl3): δ = 7.42 (d, J = 7.8 Hz, 2H), 7.09 (d, J = 7.8 Hz, 2H), 4.96 (s, 1H), 4.58 (d, J = 7.8 Hz, 1H), 3.71 (d, J = 1.0 Hz, 3H), 3.21–2.94 (m, 3H), 1.42 (s, 9H). MS (EI): m/z 303 (M + 1, 100).
2.4. Experimental Procedure for the Preparation of Methyl 3-(4-Ethynylphenyl)-2-pivalamidopropanoate (5b)
To a solution of compound 4b (1.0 g, 3.84 mmol) in THF (20 mL), 1 M TBAF in THF (3.8 mL, 7.66 mmol) was added at −70°C and stirred for 2 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). After the reaction was complete, the reaction mixture was quenched with water (20 mL) and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 5b (0.650 g, 82% yield) as off-white solid.
m.p. 65–68°C. IR (KBr, cm−1): 3326, 2957, 1750, 1737, 1637, 1522, 1202, 1116. 1H NMR (400 MHz, DMSO): δ = 7.39–7.33 (m, 2H), 7.27–7.20 (m, 2H), 4.46 (m, 1H), 4.12 (s, 1H), 3.61 (s, 3H), 3.18–2.90 (m, 2H), 1.00 (d, J = 2.2 Hz, 9H). MS (EI): m/z 288 (M + 1, 100).
2.5. Experimental Procedure for the Preparation of 2,5-Dimethoxybenzaldehyde Oxime (7a)
To a solution of compound 6a (1 g, 6.02 mmol) in MeOH (10 mL), NaOAc (0.98 g, 12.04 mmol) and NH2OH·HCl (0.62 g, 9.03 mmol) were added under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 2 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). After completion of the reaction, the solvent was evaporated, quenched with water (20 mL), and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 7a (1.0 g, 91% yield) as off-white solid.
m.p. 105–107°C. IR (KBr, cm−1): 3245, 2838, 1577, 1503, 1277, 1232, 1038, 970. 1H NMR (400 MHz, CDCl3): δ = 8.5 (s, 1H), 7.6–8.0 (br s, 1H), 7.3 (m, 1H), 6.9 (m, 1H), 6.8 (m, 1H), 3.8 (s, 3H), 3.7 (s, 3H). MS (EI): m/z 181 (M+, 100).
2.6. Experimental Procedure for the Preparation of 2,5-Dimethoxy-4-methylbenzaldehyde Oxime (7b)
To a solution of compound 6b (3.0 g, 16.6 mmol) in MeOH (30 mL), NaOAc (2.73 g, 33.3 mmol) and NH2OH·HCl (1.73 g, 24.9 mmol) were added under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 2 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). After completion of the reaction, the solvent was evaporated, quenched with water (20 mL), and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 7b (3.05 g, 93% yield) as off-white solid.
m.p. 125–129°C. IR (KBr, cm−1): 3187, 2995, 1613, 1405, 1212, 1045. 1H NMR (400 MHz, CDCl3): δ = 11.10 (s, 1H), 8.20 (s, 1H), 7.15 (s, 1H), 6.9 (s, 1H), 3.75 (s, 6H), 2.2 (s, 3H). MS (EI): m/z 195 (M+, 100).
2.7. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-(3-(2,5-dimethoxyphenyl)isoxazol-5-yl)phenyl)propanoate (8a)
To a solution of compound 7a (0.150 g, 0.83 mmol) in dichloromethane (8 mL), compound 5a (0.27 g, 0.91 mmol), triethylamine (0.12 g, 1.24 mmol), and NaOCl (9–12% in H2O, 8 mL) were added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 12 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, the solvent was evaporated, quenched with water (20 mL), and extracted with dichloromethane. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 8a (0.28 g, 71% yield) as light yellow liquid.
IR (KBr, cm−1): 3304, 2970, 2939, 1712, 1627, 1500, 1276, 1222, 1170, 1045. 1H NMR (300 MHz, CDCl3): δ = 7.78 (d, J = 8.0 Hz, 2H), 7.59–7.48 (m, 1H), 7.24 (s, 2H), 7.04 (s, 1H), 6.98 (t, J = 1.8 Hz, 2H), 5.00 (s, 1H), 4.62 (s, 1H), 3.97–3.63 (m, 9H), 3.22 (d, J = 3.1 Hz, 2H), 1.42 (s, 9H). MS (EI): m/z 482 (M + 1, 100).
2.8. Experimental Procedure for the Preparation of Methyl 3-(4-(3-(2,5-Dimethoxyphenyl)isoxazol-5-yl)phenyl)-2-pivalamidopropanoate (8b)
To a solution of compound 7a (0.2 g, 1.11 mmol) in dichloromethane (10 mL), compound 5b (0.35 g, 1.22 mmol), triethylamine (0.16 g, 1.66 mmol), and NaOCl (9–12% in water, 10 mL) were added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 12 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, the solvent was evaporated, quenched with water (20 mL), and extracted with dichloromethane. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gives the crude reaction mixture which was charged on silica gel column. The column was eluted with 25% ethyl acetate/pet ether to give the compound 8b (0.3 g, 58% yield) as light yellow liquid.
IR (KBr, cm−1): 3437, 2926, 1623, 1275, 1260, 764, 750. 1H NMR (300 MHz, CDCl3): δ = 7.82–7.73 (m, 2H), 7.55–7.47 (m, 1H), 7.24–7.16 (m, 2H), 7.05 (s, 1H), 6.98 (t, J = 1.9 Hz, 2H), 6.12 (d, J = 7.5 Hz, 1H), 4.90 (dt, J = 7.5, 5.8 Hz, 1H), 3.89 (s, 3H), 3.84 (s, 3H), 3.81–3.74 (m, 3H), 3.33–3.07 (m, 2H), 1.17 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
177.9, 172.0, 169.0, 160.4, 153.6, 151.6, 138.0, 129.8, 126.6, 125.8, 118.4, 117.3, 114.9, 113.5, 113.0, 112.4, 100.9, 56.2, 55.8, 52.7, 52.4, 42.7, 38.7, 38.6, 37.7, 29.6. HRMS (ESI): Calcd for C26H31N2O6 [M + H]+: 467.2182; Found: 467.2455.
2.9. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-(3-(2,5-dimethoxy-4-methylphenyl)isoxazol-5-yl)phenyl)propanoate (8c)
To a solution of compound 7b (0.2 g, 1.02 mmol) in dichloromethane (10 mL), compound 5a (0.34 g, 1.12 mmol), triethylamine (0.15 g, 1.53 mmol), and NaOCl (9–12% in H2O, 10 mL) were added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 12 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (20 mL) was added and extracted with dichloromethane thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. The solvent was evaporated to give the crude reaction mixture which was charged on silica gel column. Elution of the column with 20% ethyl acetate/pet ether gave the compound 8c (0.41 g, 80% yield) as off-white solid.
m.p 147–150°C. IR (KBr, cm−1): 3373, 3149, 2932, 1742, 1702, 1520, 1216, 1044. 1H NMR (300 MHz, CDCl3): δ = 7.78 (d, J = 7.8 Hz, 2H), 7.45 (s, 1H), 7.26 (d, J = 1.4 Hz, 2H), 7.06 (s, 1H), 6.85 (s, 1H), 5.03 (d, J = 8.4 Hz, 1H), 4.63 (q, J = 6.7 Hz, 1H), 3.87 (d, J = 4.6 Hz, 6H), 3.73 (s, 3H), 3.17 (td, J = 16.2, 15.0, 5.8 Hz, 2H), 2.28 (s, 3H), 1.42 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
172.0, 168.9, 160.5, 155.0, 151.9, 151.1, 138.1, 130.0, 129.8, 126.6, 125.9, 115.3, 115.0, 110.3, 100.9, 80.0, 56.3, 55.9, 54.2, 52.3, 38.3, 28.2, 16.6. MS (EI) m/z 496 (M + 1, 100). HRMS (ESI): Calcd for C27H33N2O7 [M + H]+: 497.1882; Found: 497.2288.
2.10. Experimental Procedure for the Preparation of Methyl 3-(4-(3-(2,5-Dimethoxy-4-methylphenyl)isoxazol-5-yl)phenyl)-2-pivalamidopropanoate (8d)
To a solution of compound 7b (0.15 g, 0.76 mmol) in dichloromethane (10 mL), compound 5b (0.24 g, 0.84 mmol), triethylamine (0.116 g, 1.15 mmol), and NaOCl (9–12% in water, 10 mL) were added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 12 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (10 mL) was added and the reaction mixture was extracted with dichloromethane thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. The solvent was evaporated to give the crude reaction which was charged on silica gel column. Elution of the column with 25% ethyl acetate/pet ether gave the compound 8d (0.31 g, 83% yield) as light yellow liquid.
IR (KBr, cm−1): 3444, 2929, 1637, 1473, 1275, 1261, 1215, 764. 1H NMR (300 MHz, DMSO): δ = 7.81 (dd, J = 13.0, 8.0 Hz, 3H), 7.41 (d, J = 8.1 Hz, 2H), 7.32 (d, J = 3.3 Hz, 2H), 7.07 (s, 1H), 4.51 (s, 1H), 3.85 (s, 3H), 3.80 (s, 3H), 3.64 (s, 3H), 3.24–2.96 (m, 2H), 2.20 (d, J = 19.4 Hz, 3H), 1.02 (s, 9H). MS (EI): m/z 480 (M + 1, 100).
2.11. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-(3-(3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)phenyl)propanoate (9a)
To a solution of compound 8a (0.15 g, 0.31 mmol) in acetonitrile (6 mL) and H2O (1 mL), CAN (0.511 g, 0.93 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (10 mL) was added and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 9a (0.12 g, 85% yield) as yellow solid.
m.p. 125–127°C. IR (KBr, cm−1): 3355, 2979, 1743, 1720, 1654, 1522, 1288, 1251, 1167. 1H NMR (300 MHz, CDCl3): δ = 7.77 (d, J = 7.9 Hz, 2H), 7.48 (d, J = 1.9 Hz, 1H), 7.26 (d, J = 0.9 Hz, 2H), 7.12 (d, J = 0.9 Hz, 1H), 6.96–6.86 (m, 2H), 5.02 (s, 1H), 4.63 (s, 1H), 3.74 (d, J = 1.0 Hz, 3H), 3.29–3.02 (m, 2H), 1.42 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
186.8, 185.2, 171.9, 170.7, 156.5, 154.9, 139.0, 136.8, 136.6, 134.4, 133.3, 130.0, 126.0, 125.7, 100.7, 80.1, 54.2, 52.3, 38.3, 28.2. HRMS (ESI): Calcd for C24H24N2O7 [M + H]+: 453.1662; Found: 453.1640.
2.12. Experimental Procedure for the Preparation of Methyl 3-(4-(3-(3,6-Dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)phenyl)-2-pivalamidopropanoate (9b)
To a solution of compound 8b (0.2 g, 0.42 mmol) in acetonitrile (8 mL) and H2O (2 mL), CAN (0.705 g, 1.28 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (20 mL) was added and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 9b (0.136 g, 72% yield) as yellow solid.
m.p. 115–117°C. IR (KBr, cm−1): 3321, 2958, 1749, 1656, 1640, 1531, 1286, 1199, 1107. 1H NMR (300 MHz, CDCl3) δ = 7.76 (d, J = 7.9 Hz, 2H), 7.47 (d, J = 2.2 Hz, 1H), 7.23 (d, J = 7.9 Hz, 2H), 7.12 (s, 1H), 6.90 (d, J = 2.7 Hz, 2H), 6.13 (d, J = 7.7 Hz, 1H), 4.90 (q, J = 6.1 Hz, 1H), 3.77 (s, 3H), 3.37–3.04 (m, 2H), 1.17 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
186.8, 185.1, 177.9, 172.0, 170.6, 156.5, 138.9, 136.8, 136.6, 134.4, 133.3, 130.0, 125.9, 125.7, 100.7, 52.7, 52.4, 38.6, 37.7, 27.3. HRMS (ESI): Calcd for C24H25N2O6 [M + H]+: 437.1713; Found: 437.1879.
2.13. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(4-(3-(4-methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)phenyl)propanoate (9c)
To a solution of compound 8c (0.23 g, 0.46 mmol) in acetonitrile (10 mL) and H2O (2 mL), CAN (0.76 g, 1.39 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (20 mL) was added and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 9c (0.205 g, 95% yield) as yellow solid.
m.p. 153–155°C. IR (KBr, cm−1): 3364, 2979, 1732, 1693, 1660, 1524, 1252, 1170, 1020. 1H NMR (300 MHz, CDCl3): δ = 7.83–7.69 (m, 2H), 7.45 (s, 1H), 7.34–7.20 (m, 2H), 7.11 (s, 1H), 6.74 (q, J = 1.5 Hz, 1H), 5.04 (d, J = 8.1 Hz, 1H), 4.63 (q, J = 6.7 Hz, 1H), 3.74 (s, 3H), 3.29–3.00 (m, 2H), 2.13 (d, J = 1.6 Hz, 3H), 1.42 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
187.4, 185.4, 171.9, 170.5, 156.5, 154.9, 146.2, 138.9, 134.3, 133.5, 133.4, 130.0, 126.0, 125.7, 100.8, 80.1, 54.2, 52.3, 38.3, 28.2, 15.5. HRMS (ESI): Calcd for C25H27N2O7 [M + H]+: 467.1818; Found: 467.1611.
2.14. Experimental Procedure for the Preparation of Methyl 3-(4-(3-(4-Methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)phenyl)-2-pivalamidopropanoate (9d)
To a solution of compound 8d (0.3 g, 0.62 mmol) in acetonitrile (12 mL) and H2O (3 mL), CAN (1.02 g, 1.86 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (20 mL) was added and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 9d (0.25 g, 89% yield) as yellow solid.
m.p. 130–132°C. IR (KBr, cm−1): 3379, 2959, 2924, 1734, 1657, 1237, 1020, 807. 1H NMR (300 MHz, CDCl3): δ = 7.82–7.68 (m, 2H), 7.45 (d, J = 1.3 Hz, 1H), 7.22 (d, J = 7.9 Hz, 2H), 7.12 (d, J = 1.0 Hz, 1H), 6.74 (q, J = 1.4 Hz, 1H), 6.12 (d, J = 7.4 Hz, 1H), 4.90 (q, J = 6.2 Hz, 1H), 3.76 (d, J = 0.9 Hz, 3H), 3.34–3.05 (m, 2H), 2.18–2.07 (m, 3H), 1.17 (d, J = 0.9 Hz, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
187.3, 185.4, 177.9, 172.0, 170.4, 156.5, 146.2, 138.8, 134.2, 133.5, 133.4, 130.0, 125.9, 125.8, 100.9, 52.7, 52.4, 38.6, 37.7, 27.3, 15.5. HRMS (ESI): Calcd for C25H27N2O7 [M + H]+: 467.1818; Found: 467.1611.
2.15. Experimental Procedure for the Preparation of (3-(2,5-Dimethoxyphenyl)isoxazol-5-yl)methanol (15a)
To a solution of compound 7a (2 g, 11.11 mmol) in ethyl acetate (20 mL), compound 14 (2.01 g, 16.66 mmol), N-chlorosuccinamide (2.21 g, 16.66 mmol), and NaHCO3 (1.86 g, 22.22 mmol) were added and the reaction mixture was refluxed for 16 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (20 mL) was added and the reaction mixture was extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude product which was purified by column chromatography to give the compound 15a (2.1 g, 80% yield) as white solid.
m.p. 69–73°C. IR (KBr, cm−1): 3330, 2943, 1709, 1510, 1295, 1225, 1036. 1H NMR (400 MHz, CDCl3): δ = 7.45 (d, J = 2.9 Hz, 1H), 7.01–6.90 (m, 2H), 6.79 (s, 1H), 4.82 (d, J = 6.3 Hz, 2H), 3.83 (d, J = 12.0 Hz, 6H), 2.19 (t, J = 6.5 Hz, 1H). MS (EI): m/z 235 (M + 1, 100).
2.16. Experimental Procedure for the Preparation of (3-(2,5-Dimethoxy-4-methylphenyl)isoxazol-5-yl)methanol (15b)
To a solution of compound 7b (1.0 g, 5.12 mmol) in ethyl acetate (20 mL), compound 14 (0.93 g, 7.69 mmol), N-chlorosuccinamide (1.02 g, 7.69 mmol), and NaHCO3 (0.861 g, 10.2 mmol) were added and refluxed for 16 hr. The progress of the reaction was monitored by TLC analysis (30% Ethyl acetate/pet ether). Then, water (20 mL) was added and the reaction mixture was extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude product which was purified by column chromatography to give the compound 15b (1.0 g, 90% yield) as off-white solid.
m.p. 58–62°C. IR (KBr, cm−1): 3426, 2940, 2129, 1715, 1216, 1038. 1H NMR (300 MHz, DMSO): δ = 11.05 (s, 1H), 7.27 (s, 1H), 7.05 (s, 1H), 6.74 (s, 1H), 4.60 (d, J = 0.8 Hz, 2H), 3.80 (d, J = 10.2 Hz, 6H), 2.21 (s, 3H). MS (EI): m/z 250 (M + 1, 100).
2.17. Experimental Procedure for the Preparation of 5-(Bromomethyl)-3-(2,5-dimethoxyphenyl)isoxazole (16a)
To a solution of compound 15a (1.5 g, 6.38 mmol) in dichloromethane (15 mL), phosphorous tribromide (2.59 g, 9.57 mmol) was added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. The solvent was evaporated and the crude reaction mixture was purified by column chromatography to give the compound 16a (1.2 g, 63% yield) as white solid.
m.p. 71–75°C. IR (KBr, cm−1): 3432, 2925, 1852, 1603, 1465, 1270, 1021. 1H NMR (400 MHz, CDCl3): δ = 7.47 (d, J = 3.0 Hz, 1H), 7.02–6.91 (m, 2H), 6.87 (s, 1H), 4.52 (s, 2H), 3.84 (d, J = 15.1 Hz, 6H). MS (EI): m/z 297 (M + 1, 100).
2.18. Experimental Procedure for the Preparation of 5-(Bromomethyl)-3-(2,5-dimethoxy-4-methylphenyl)isoxazole (16b)
To a solution of compound 15b (1.0 g, 4.0 mmol) in dichloromethane (20 mL), phosphorous tribromide (1.62 g, 6.0 mmol) was added at 0°C under nitrogen atmosphere. Then the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (20 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. The solvent was evaporated and the crude reaction mixture was purified by column chromatography to give the compound 16b (1.0 g, 80% yield) as brown solid.
m.p. 65–68°C. IR (KBr, cm−1): 3445, 2936, 1716, 1471, 1285, 1218, 1042. 1H NMR (400 MHz, CDCl3): δ = 7.39 (s, 1H), 6.82 (d, J = 6.4 Hz, 2H), 4.82 (dd, J = 6.5 Hz, 2H), 3.85 (s, 6H), 2.27 (s, 3H). MS (EI): m/z 311 (M + 1, 100).
2.19. Experimental Procedure for the Preparation of Methyl 3-(3-(2,5-Dimethoxyphenyl)isoxazol-5-yl)-2-((diphenylmethylene)amino)propanoate (17a)
To a solution of compound 10 (1.12 g, 4.44 mmol) in acetonitrile (20 mL), K2CO3 (2.78 g, 20.2 mmol) was added under nitrogen atmosphere and stirred at RT for 1 hr. Then compound 16a (1.2 g, 4.04 mmol) was added and the reaction mixture was refluxed for 16 hr. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). Then, reaction mixture was filtered and filtrate was evaporated. The crude reaction mixture was purified by column chromatography to give the compound 17a (1.5 g, 74% yield) as light yellow liquid.
IR (KBr, cm−1): 3292, 2952, 1740, 1510, 1276, 1227, 1043. 1H NMR (300 MHz, CDCl3): δ = 7.63 (d, J = 6.9 Hz, 2H), 7.44–7.28 (m, 7H), 7.04–6.83 (m, 4H), 6.52 (s, 1H), 4.47 (dd, J = 7.5, 5.8 Hz, 1H), 3.78 (d, J = 8.5 Hz, 6H), 3.59 (s, 3H), 3.53–3.41 (m, 2H). MS (EI): m/z 470 (M + 1, 100).
2.20. Experimental Procedure for the Preparation of Methyl 3-(3-(2,5-Dimethoxy-4-methylphenyl)isoxazol-5-yl)-2-((diphenylmethylene)amino)propanoate (17b)
To a solution of compound 10 (0.9 g, 3.55 mmol) in acetonitrile (20 mL), K2CO3 (2.45 g, 17.7 mmol) was added under nitrogen atmosphere and stirred at RT for 1 hr. Then compound 16b (1.21 g, 3.91 mmol) was added and the reaction mixture was refluxed for 16 hr. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/pet ether). Then, reaction mixture was filtered and filtrate was evaporated. The crude reaction mixture was purified by column chromatography to give the compound 17b (1.1 g, 69% yield) as off-white solid.
m.p. 142–146°C. IR (KBr, cm−1): 3447, 2949, 1736, 1284, 1213, 1041. 1H NMR (300 MHz, CDCl3): δ = 7.63 (d, J = 8.2 Hz, 2H), 7.49–7.24 (m, 7H), 7.04–6.92 (m, 2H), 6.75 (s, 1H), 6.53 (s, 1H), 4.61–4.36 (m, 1H), 3.79 (dd, J = 16.5 Hz, 6H), 3.59 (d, J = 0.9 Hz, 3H), 3.54–3.35 (m, 2H), 2.24 (s, 3H). MS (EI): m/z 484 (M + 1, 100).
2.21. Experimental Procedure for the Preparation of Methyl 2-Amino-3-(3-(2,5-dimethoxyphenyl)isoxazol-5-yl)propanoate (18a)
To a solution of compound 17a (1.5 g, 3.19 mmol) in diethyl ether (20 mL), 1 M HCl (20 mL) was added at 0°C. Then the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (10% methanol/chloroform). The layers were separated and the aqueous layer was basified with aqueous ammonia until PH 10 and extracted with ethyl acetate. The combined organic layers were washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the compound 18a (0.850 g, 87% yield) as pale yellow liquid.
IR (KBr, cm−1): 3383, 2953, 1738, 1602, 1471, 1227, 1042. 1H NMR (300 MHz, DMSO): δ = 7.27 (d, J = 3.0 Hz, 1H), 7.17–6.97 (m, 2H), 6.67 (s, 1H), 3.78 (d, J = 17.1 Hz, 7H), 3.64 (s, 3H), 3.07 (qd, J = 15.1, 6.6 Hz, 2H), 1.99 (s, 2H). MS (EI): m/z 306 (M + 1, 100).
2.22. Experimental Procedure for the Preparation of Methyl 2-Amino-3-(3-(2,5-dimethoxy-4-methylphenyl)isoxazol-5-yl)propanoate (18b)
To a solution of compound 17b (0.5 g, 1.03 mmol) in diethyl ether (10 mL), 1 M HCl (10 mL) was added at 0°C. Then the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (10% methanol/chloroform). The layers were separated and the aqueous layer was basified with aqueous ammonia until PH 10 and extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the compound 18b (0.27 g, 82% yield) as off-white solid.
m.p. 218–221°C. IR (KBr, cm−1): 3468, 2838, 1741, 1472, 1250, 1220, 1041. 1H NMR (300 MHz, DMSO): δ = 8.58 (s, 2H), 7.28 (s, 1H), 7.06 (s, 1H), 6.82 (s, 1H), 4.53 (t, J = 6.1 Hz, 1H), 3.93–3.65 (m, 9H), 3.41 (d, J = 6.2 Hz, 2H), 2.22 (s, 3H). MS (EI): m/z 320 (M + 1, 100).
2.23. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(3-(2,5-dimethoxyphenyl)isoxazol-5-yl)propanoate (19a)
To a solution of compound 18a (0.300 g, 0.98 mmol) in dichloromethane (15 mL), triethylamine (0.19 g, 1.98 mmol) was added. Then (Boc)2O (0.23 g, 1.07 mmol) was added and the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude compound which was purified by column chromatography to give the compound 19a (0.380 g, 95% yield) as light brown solid.
m.p. 110–113°C. IR (KBr, cm−1): 3372, 2948, 1742, 1690, 1524, 1274, 1220, 1025. 1H NMR (300 MHz, CDCl3): δ = 7.44 (d, J = 2.8 Hz, 1H), 7.01–6.88 (m, 2H), 6.62 (s, 1H), 5.24 (s, 1H), 4.70 (s, 1H), 3.90–3.72 (m, 9H), 3.37 (d, J = 6.0 Hz, 2H), 1.44 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
171.2, 167.4, 159.9, 155.0, 153.6, 151.5, 118.2, 117.3, 113.4, 113.0, 104.5, 80.2, 56.1, 55.8, 52.6, 52.1, 29.7, 28.2. MS (EI): m/z 406 (M + 1, 100). HRMS (ESI) Calcd for C20H26N2O7 [M + H]+: 407.1818; Found: 407.1743.
2.24. Experimental Procedure for the Preparation of Methyl 3-(3-(2,5-Dimethoxyphenyl)isoxazol-5-yl)-2-pivalamidopropanoate (19b)
To a solution of compound 18a (0.300 g, 0.98 mmol) in DCM (15 mL), dimethylaminopyridine (0.012 g, 0.098 mmol) was added. Then pivaloyl chloride (0.356 g, 2.94 mmol) was added and the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude compound which was purified by column chromatography to give the compound 19b (0.300 g, 78% yield) as light yellow liquid.
IR (KBr, cm−1): 3366, 2959, 1745, 1651, 1511, 1267, 1227, 1043. 1H NMR (300 MHz, CDCl3): δ = 7.44 (d, J = 2.8 Hz, 1H), 7.03–6.85 (m, 2H), 6.59 (d, J = 2.3 Hz, 1H), 6.38 (d, J = 7.0 Hz, 1H), 5.00–4.80 (m, 1H), 3.95–3.70 (m, 9H), 3.56–3.28 (m, 2H), 1.22 (d, J = 2.3 Hz, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
178.2, 171.1, 167.3, 159.9, 153.6, 151.5, 118.1, 117.4, 113.3, 112.9, 104.7, 56.0, 55.8, 52.8, 50.8, 38.6, 28.9, 27.3, 27.0. MS (EI): m/z 390 (M + 1, 100).
2.25. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(3-(2,5-dimethoxy-4-methylphenyl)isoxazol-5-yl)propanoate (19c)
To a solution of compound 18b (0.2 g, 0.62 mmol) in dichloromethane (10 mL), triethyl amine (0.12 g, 1.25 mmol) was added. Then (Boc)2O (0.15 g, 0.68 mmol) was added and the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude compound which was purified by column chromatography to give the compound 19c (0.25 g, 95% yield) as off-white solid.
m.p. 103–107°C. IR (KBr, cm−1): 3344, 2928, 2846, 1733, 1677, 1526, 1219, 1048. 1H NMR (300 MHz, DMSO): δ = 7.47 (d, J = 8.3 Hz, 1H), 7.26 (s, 1H), 7.04 (s, 1H), 6.68 (d, J = 4.3 Hz, 1H), 4.39 (t, J = 9.2 Hz, 1H), 3.91–3.73 (m, 6H), 3.65 (d, J = 10.3 Hz, 3H), 3.28–3.04 (m, 2H), 2.21 (s, 3H), 1.35 (s, 9H). 13C NMR (100 MHz, DMSO-d6):
δ
C
171.9, 168.7, 159.2, 155.2, 151.2, 150.7, 129.3, 115.3, 114.6, 109.6, 109.5, 103.6, 78.5, 56.1, 55.5, 52.1, 51.9, 28.0, 26.8, 16.2. MS (EI): m/z 420 (M + 1, 100). HRMS (ESI) Calcd for C21H29N2O7 [M + H]+: 421.1975; Found: 421.1988.
2.26. Experimental Procedure for the Preparation of Methyl 3-(3-(2,5-Dimethoxy-4-methylphenyl)isoxazol-5-yl)-2-pivalamidopropanoate (19d)
To a solution of compound 18b (0.3 g, 0.93 mmol) in dichloromethane (10 mL), dimethylaminopyridine (0.011 g, 0.093 mmol) was added. Then pivaloyl chloride (0.34 g, 2.81 mmol) was added and the reaction mixture was stirred at RT for 16 hr. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude compound which was purified by column chromatography to give the compound 19d (0.31 g, 82% yield) as off-white solid.
m.p. 116–120°C. IR (KBr, cm−1): 3323, 2963, 1741, 1531, 1431, 1217, 1041. 1H NMR (300 MHz, DMSO): δ = 7.98 (d, J = 8.0 Hz, 1H), 7.25 (s, 1H), 7.03 (s, 1H), 6.65 (s, 1H), 4.61 (td, J = 8.2, 6.6 Hz, 1H), 3.78 (d, J = 5.9 Hz, 6H), 3.66 (s, 3H), 3.38–3.24 (m, 2H), 2.21 (s, 3H), 1.07 (s, 9H). 13C NMR (100 MHz, DMSO-d6):
δ
C
177.5, 171.2, 169.0, 159.2, 151.2, 150.7, 129.2, 115.3, 114.7, 109.6, 103.6, 56.0, 55.5, 52.1, 50.4, 37.8, 27.5, 27.0, 16.2. MS (EI): m/z 404 (M + 1, 100). HRMS (ESI): Calcd for C21H29N2O6 [M + H]+: 405.2026; Found: 405.1703.
2.27. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(3-(3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)propanoate (20a)
To a solution of compound 19a (0.250 g, 0.61 mmol) in acetonitrile (10 mL) and H2O (2 mL), CAN (1.01 g, 1.84 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (20% ethyl acetate/Pet ether). Then, water (10 mL) was added and extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 20a (0.150 g, 64% yield) as yellow solid.
m.p. 110–112°C. IR (KBr, cm−1): 3359, 2979, 2955, 1749, 1688, 1524, 1283, 1163. 1H NMR (300 MHz, CDCl3): δ = 7.44–7.36 (m, 1H), 6.87 (d, J = 1.5 Hz, 2H), 6.69 (s, 1H), 5.22 (s, 1H), 4.68 (s, 1H), 3.80 (s, 3H), 3.53–3.25 (m, 2H), 1.44 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
186.7, 185.0, 170.9, 169.3, 156.1, 154.9, 136.7, 136.5, 134.4, 133.3, 104.4, 80.4, 52.8, 52.0, 29.8, 28.2. MS (EI): m/z 376 (M + 1, 100). HRMS (ESI): Calcd for C18H20N2O7 [M + H]+: 377.1349; Found: 377.0531.
2.28. Experimental Procedure for the Preparation of Methyl 3-(3-(3,6-Dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)-2-pivalamidopropanoate (20b)
To a solution of compound 19b (0.2 g, 0.51 mmol) in acetonitrile (10 mL) and H2O (2 mL), CAN (0.560 g, 1.025 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 20b (0.160 g, 86% yield) as yellow solid.
m.p. 107–109°C. IR (KBr, cm−1): 3312, 2961, 2872, 1752, 1662, 1639, 1534, 1287, 1206, 1093. 1H NMR (400 MHz, CDCl3): δ = 7.39 (s, 1H), 6.87 (s, 2H), 6.65 (s, 1H), 6.36 (d, J = 7.1 Hz, 1H), 4.90 (q, J = 5.7 Hz, 1H), 3.82 (s, 3H), 3.53 (dd, J = 15.4, 5.4 Hz, 1H), 3.37 (dd, J = 15.3, 5.3 Hz, 1H), 1.22 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
186.7, 184.9, 178.2, 170.9, 169.2, 156.0, 136.7, 136.5, 134.3, 133.3, 104.5, 52.9, 50.8, 38.7, 29.0, 27.3. MS (EI): m/z 360 (M + 1, 100). HRMS (ESI): Calcd for C18H21N2O6 [M + H]+: 361.1400; Found: 361.1388.
2.29. Experimental Procedure for the Preparation of Methyl 2-((tert-Butoxycarbonyl)amino)-3-(3-(4-methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)propanoate (20c)
To a solution of compound 19c (0.2 g, 0.47 mmol) in acetonitrile (10 mL) and H2O (2 mL), CAN (0.52 g, 0.95 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). Then, water (10 mL) was added and extracted with ethyl acetate. The combined organic layer was washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 20c (0.185 g, 86% yield) as yellow solid.
m.p. 89–93°C. IR (KBr, cm−1): 3379, 2977, 1742, 1695, 1654, 1524, 1239, 1170, 1040. 1H NMR (300 MHz, CDCl3): δ = 7.36 (s, 1H), 6.74–6.62 (m, 2H), 5.23 (d, J = 6.3 Hz, 1H), 4.68 (s, 1H), 3.80 (s, 3H), 3.39 (m, 2H), 2.25–1.99 (m, 3H), 1.44 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
187.3, 185.2, 170.9, 169.1, 156.1, 154.9, 146.2, 134.2, 133.5, 133.4, 109.9, 104.5, 52.8, 52.0, 29.8, 28.2, 15.5. MS (EI): m/z 390 (M + 1, 100). HRMS (ESI): Calcd for C19H23N2O7 [M + H]+: 391.1505; Found: 391.1601.
2.30. Experimental Procedure for the Preparation of Methyl 3-(3-(4-Methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)isoxazol-5-yl)-2-pivalamidopropanoate (20d)
To a solution of compound 19d (0.2 g, 0.49 mmol) in acetonitrile (10 mL) and H2O (2 mL), CAN (0.54 g, 0.99 mmol) was added and the reaction mixture was stirred at RT for 1 h. The progress of the reaction was monitored by TLC analysis (30% ethyl acetate/pet ether). After completion of the reaction, water (10 mL) was added and extracted with ethyl acetate thrice. The organic layers were combined, washed with water, brined, and dried over anhydrous Na2SO4. Evaporation of the solvent in high vacuum gave the compound 20d (0.175 g, 94% yield) as yellow solid.
m.p.118–120°C. IR (KBr, cm−1): 3351, 2958, 2872, 1735, 1657, 1524, 1230, 1042. 1H NMR (300 MHz, CDCl3): δ =7.36 (s, 1H), 6.70 (q, J = 1.6 Hz, 1H), 6.68 (s, 1H), 6.35 (d, J = 7.3 Hz, 1H), 4.89 (dt, J = 7.2, 5.2 Hz, 1H), 3.82 (s, 3H), 3.59–3.44 (m, 1H), 3.36 (dd, J = 15.2, 5.2 Hz, 1H), 2.11 (d, J = 1.7 Hz, 3H), 1.21 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ
C
187.3, 185.2, 178.2, 171.0, 169.0, 156.1, 146.2, 134.1, 133.5, 133.4, 104.6, 52.9, 50.8, 38.7, 29.0, 27.3, 15.5. MS (EI): m/z 374 (M + 1, 100). HRMS (ESI): Calcd for C19H23N2O6 [M + H]+: 375.1556; Found: 375.1468.