Reaction Steps
To a suspension of 4-hydroxybenzoxazole (2.00 g, 14.8 mmol) (Musser et al., J. Med. Chem., 30, pp. 62-67 (1987)) in acetone (80.0 ml) was added dried K2CO3 (2.25 g, 16.3 mmol) followed by iodomethane (1.38 ml, 22.2 mmol). The reaction was heated under reflux under N2 for 4.5 h, then filtered and reduced in vacuo to afford the crude product. The resulting residue was purified by flash chromatography (25:75 ethyl acetate/hexane) to give 2.0 g (91) of the title compound as a white crystalline solid: m.p. 72-74° C.; IR (KBr) 3089, 1619, 1610, 1503, 1496, 1322, 1275, 1090, 1071, 780, 741; 1H NMR (CDCl3) δ 8.02 (1H, s), 7.32 (1H, t, J=8.0), 7.18 (1H, d, J=8.0), 6.81 (1H, d, J=8.0), 4.04 (3H, s). Anal. Calcd. for C8H7NO2: C, 64.42; H, 4.73; N, 9.39. Found: C, 64.40; H, 4.84; N, 9.31; m/z (EI) 149 (M++1, 100%). (3S,4R,S) t-Butyl N-(allyloxycarbonyl)-3-amino-4-hydroxy-4-(2-(7-methoxybenzoxazolyl))butanoate (66a). To a stirred solution of 7-methoxybenzoxazole 65a (548.6 mg, 3.68 mmol) in anhydrous THF (18.5 ml) at −78° C. under N2 was added 1.56M n-butyl lithium in hexanes (2.47 ml, 3.86 mmol) dropwise, to produce a yellow colored solution. After stirring at −78° C. for 20 min, dry MgBr2OEt2 (1.045 g, 4.05 mmol) was added as a solid. The resulting heterogeneous mixture was warmed to −45° C. and stirred for 15 min. The reaction mixture was then recooled to −78° C. and a solution of (S)-Alloc-Asp(t-Bu)H1b (946.4 mg, 3.68 mmol) in THF (18.5 ml) was added dropwise. The reaction was stirred at −78° C. for 30 min, warmed to 0° C. and stirred for 1 h. The resulting homogeneous reaction was warmed to room temperature and stirred for 16 h. The reaction was quenched with 5% sodium bicarbonate (3.5 ml) then THF was removed in vacuo. The resulting aqueous residue was extracted with methylene chloride (x6). The combined extracts were washed with brine, dried (MgSO4), filtered and reduced in vacuo to give 1.8 g of crude product. Flash chromatography (40:60 ethyl acetate/hexane) gave 1.21 g (81%) of the title compound, an oil, as a mixture of diastereoisomers at C-4: IR (CH2Cl2) 3425, 2983, 1725, 1504, 1290, 1157, 1101; 1H NMR (CDCl3) δ 7.35-7.19 (2H, m), 6.89-6.81 (1H, m), 6.00-5.57 (2H, m), 5.32-5.05 (3H, m), 4.68-4.35 (3H, m), 4.01 (3H, s), 2.86-2.59 (2H, m), 1.45 (9H, s), 1.41 (9H, S); 13C NMR (CDCl3) δ 171.18, 171.09, 165.80, 165.30, 156.71, 156.60, 145.65, 142.76, 142.71, 140.82, 140.72, 133.23, 125.81, 125.72, 118.41, 118.21, 113.07, 112.87, 108.95, 82.16, 70.28, 69.98, 66.52, 66.39, 57.03, 52.57, 52.29, 37.83, 36.86, 28.65. Anal. Calcd. for C20H26N2O7. 0.6H2O: C, 57.57; H, 6.57; N, 6.72. Found: C, 57.49, H, 6.34, N, 6.60. M.S. (+FAB); 407 (M++1); 351, 307, 154. (3S,4R,S) t-Butyl N-(allyloxycarbonyl)-3-amino-4-hydroxy-4-(2-(4-methoxybenzoxazolyl))butanoate (66b), was prepared according to the method described for 66a which afforded 1.29 g (26%, 68% based on recovered starting material) of the title compound as an oil and as a mixture of diastereoisomers at C-4: IR (CH2Cl2) 3400, 1725, 1625, 1505, 1369, 1354, 1281, 1263, 1226, 1158, 1092, 1048; 1H NMR (CDCl3) δ 7.34-7.24 (1H, m), 7.16 (1H, d, J=8.2), 6.79 (1H, d, J=7.9), 6.00-5.50 (2H, m), 5.30-5.05 (3H, m), 4.70-4.35 (4H, m), 4.02 (3H, s), 2.90-2.45 (2H, m), 1.45-1.41 (9H, 2×s). Anal. Calcd. for C20H26N2O7. 0.4H2O: C, 58.07; H, 6.53; N, 6.77. Found: C, 58.09; H, 6.41; N, 6.63. M.S. (+FAB); 407 (M++1, 88%); 351 (100). (3S,4R,S) t-Butyl N-(N-acetyl-(S)-(O-tert-butyl-tyrosinyl)-(S)-valinyl-(S)-alaninyl)-3-amino-4-hydroxy-4-(2-(7-methoxybenzoxazolyl))butanoate (67a). To a stirred solution of the benzoxazole 66a (481.9 mg, 1.19 mmol) and Ac-Tyr(tBu)-Val-Ala-OH (586.3 mg, 1.30 mmol) in methylene chloride (3.5 ml) and DMF (3.5 ml) was added bis(triphenylphosphine)palladium (II) chloride (18.0 mg), followed by tributyltinhydride (0.80 ml, 2.96 mmol) dropwise. Hydroxybenzotriazole (320.4 mg, 2.37 mmol) was added and the mixture cooled to 0° C. 1-Ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (278.2 mg, 1.42 mmol) was added and the mixture was allowed to warm to room temperature and stirred for 16.5 h. The reaction was diluted with ethyl acetate and washed twice with 1M sodium hydrogensulphate, twice with saturated sodium bicarbonate, water, and brine. The organic layer was dried (MgSO4), filtered and reduced in vacuo to yield 2.0 g of crude product. Flash chromatography (95:5 methylene chloride/methanol) gave 844.0 mg (94%) of the title compound as a white solid: m.p. 205° C.; IR (KBr) 3399, 3304, 2977, 1729, 1643, 1506, 1367, 1290, 1161;
