A mixture of (2'S)-2-(N-tert-butoxycarbonylpyrrolidin-2-yl)-2-oxo-ethyl phenyl sulfone (6) (5.51g, 15.6mmol), N-bromoacetylglycine methyl ester (10) (2.96g, 14.1mmol) and finely ground, dried potassium carbonate (5.09g, 36.8mmol) was dissolved in dry N,N-dimethylformamide (50ml), placed under an atmosphere of argon, and stirred for 6h. Ethyl acetate (100ml) was added, and the mixture washed sequentially with 2M hydrochloric acid (100ml) and water (3x100ml), then dried over magnesium sulphate and concentrated under reduced pressure. The residue was purified by chromatography on silica gel (petroleum ether/ethyl acetate 1:1) to give (2'S, 3S/R)-4-(N-tert-butoxycarbonylpyrrolidin-2-yl)-4-oxo-3-phenylsulfonyl-butnoylglycylglycine methyl ester (12) (5.21g, 76%) as a pale yellow solid (ca. 1:1 mixture of diastereoisomers), m.p. 62degC.

Proton Nmr (ca. 1:1 mixture of diastereoisomers) deltaH (300MHz, CDCl3) 1.43(4.5H, s, OC(CH3)3 diastereoisomer), 1.46(4.5H, s, OC(CH3)3 diastereoisomer), 2.50-1.53(4H, m, 2xCH2), 3.31-2.68(2H, m, CH2CO), 3.48-3.28(2H, m, CH2NBoc), 3.67(3H, s, OCH3), 4.09-3.75(2H, m, CH2CO2Me), 4.81(0.5H, dd, J=2.5, 11.0Hz, diastereoisomer), 4.88(0.5H, dd, J=3.5, 11.0Hz, CHNBoc diastereoisomer), 5.21(1H, m, CHSO2Ph), 6.29(1H, br. s, NH), 8.01-7.46(5H, m, Ar-H); Vmax: 3316, 1754, 1683cm^-1; m/z(NH3, CI) 500(M+NH4⁺, 18%), 483(M+H⁺, 5%), 360(100%), 343(55%), 304(28%), 177(25%), 160(72%). Found [M+H]⁺ 483.1795, C22H31N2O8S requires 483.1801.

Preparation of (2'S, 3S/R)-4-(N-tert-Butoxycarbonylpyrrolidin-2-yl)-4-oxo-3-phenylsulfonylbutaoylglycyl-L-alanine Methyl Ester (13)

A mixture of (2'S)-2-(N-tert-butoxycarbonylpyrrolidin-2-yl)-2-oxo-ethyl phenyl sulfone (6) (3.19g, 9.03mmol), N-bromoacetyl-L-alanine methyl ester (11) (1.56g, 6.98mmol) and finely ground anhydrous potassium carbonate (3.28g, 23.70mmol) was dissolved in dry N,N-dimethylformamide (20ml). The mixture was then placed under an atmosphere of argon, and stirred at room temperature for 6.5h. Ethyl acetate (120ml) was then added, the mixture washed sequentially with 2M hydrochloric acid (60ml) and water (3x150ml), then dried over magnesium sulphate and concentrated under reduced pressure. The residue was purified by chromatography on silica gel (petroleum ether/ethyl acetate 1:1) to give (2'S, 3S/R)-4-(N-tert-butoxycarbonylpyrrolidin-2-yl)-4-oxo-3-phenylsulfonyl-butnoyl-glycyl-L-alanine methyl ester (57b) (2.56g, 74%) as a pale yellow solid (13) (ca. 2:1 mixture of diastereoisomers), m.p. 60degC.

Proton Nmr (ca. 2:1 mixture of diastereoisomers) deltaH (300MHz, CDCl3) 3.10-1.20(18H, m) 3.42-3.29(2H, m, CH2NBoc), 3.68(1H, s, OCH3 minor diastereoisomer), 3.69(3H, s, OCH3 minor diastereoisomer), 4.42-4.32(1H, m, CHMe), 4.80(0.3H, dd, J=2.5, 11.5Hz, CHNBoc minor diastereoisomer), 4.88(0.6H, dd, J=3.0, 11.0Hz, CHNBoc major diastereoisomer), 5.27-5.23(1H, m, CHSO2Ph), 6.10(0.3H, br. d, J=7.5Hz, NH minor diastereoisomer), 6.23(0.6H, br. d, J=7.0Hz, NH minor diastereoisomer), 8.02-7.46(5H, m, Ar-H); Vmax: 3321, 1747, 1733, 1688, 1683cm^-1; m/z (NH3, CI) 514(M+NH4⁺, 43%), 374(100%), 357(23%), 177(36%), 160(55%). Found [M+NH4]⁺ 514.2220, C23H36N3O8S requires 514.2223.

Preparation of (2'S, 3R/S)-4-(N-tert-Butoxycarbonyl-L-alanylpyrrolidin-2-yl)-4-oxo-3-phenylsulonylbutanoylglycyl-L-alanine Methyl Ester (14)

A mixture of (2'S)-2-(N-tert-butoxycarbonyl-L-alanylpyrrolidin-2-yl)-2-oxo-ethyl phenyl sulfone (7) (20mg, 0.05mmol), N-bromoacetyl-L-alanine methyl ester (11) (12mg, 0.05mmol) and finely ground anhydrous potassium carbonate (7mg, 0.05mmol) in dry N, N-dimethylformamide (5ml) was stirred at room temperature for 5.5h under argon. Ethyl acetate (30ml) was then added, and the mixture washed sequentially with 2M hydrochloric acid (30ml) and water (3x50ml), then dried over magnesium sulphate and concentrated under reduced pressure. The residue was purified by chromatography on silica gel (petroleum ether/ethyl acetate 1:1) to give (2'S, 3R/S)-4-(N-tert-butoxycarbonyl-L-alanylpyrrolidin-2-yl)-4-oxo-3-phenylsulonyl-butanoylglycyl-L-alanine methyl ester (14) (18mg, 62%) as a colourless oil (2:1 mixture of diastereoisomers).

Proton Nmr (ca. 2:1 mixture of diastereoisomers) deltaH (300MHz, CDCl3) 1.22-1.45(15H, m, 2xCH3 + O(CH3)3), 1.70-2.55(4H, m, 2xCH2), 2.66(0.4H, dd, J=5.0, 15.5Hz, CHaHbCO, minor diastereoisomer), 2.67(0.6H, dd, J=9.0, 15.5Hz, CHaHbCO, major diastereoisomer), 2.84(0.6H, dd, J=3.0, 15.5Hz, CHaHbCO, major diastereoisomer), 2.89(0.4H, dd, J=7.5, 15.5Hz, CHaHbCO, minor diastereoisomer), 3.47-3.74(2H, m, CH2NBoc), 3.68(1H, s, OCH3 minor diastereoisomer), 3.69(2H, s, OCH3 major diastereoisomer), 4.34-4.52(2H, m, 2xCHMe), 4.81(0.6H, dd, J=3.0, 11.0Hz, CHNBoc major diastereoisomer), 4.88(0.4H, dd, J=5.5, 13.0Hz, CHNBoc minor diastereoisomer), 5.13(0.4H, dd, J=5.0, 7.5Hz, CHSO2Ph minor diastereoisomer), 5.30(0.4H, br. d, J=8.0Hz, NHBoc minor diastereoisomer), 5.51(0.6H, dd, J=3.0, 9.0Hz, CHSO2Ph major diastereoisomer), 5.55(0.6H, br. d, J=7.5Hz, NHBoc major diastereoisomer), 6.24(0.6H, d, J=7.0Hz, NH major diastereoisomer), 6.64(0.4H, d, J=7.0Hz, NH minor 7.49-7.70(3H, m, p-Ar-H + m-Ar-H), 7.88(0.7H, d, J=7.5Hz, o-Ar-H minor diastereoisomer), 7.96(1.3H, d, J=7.5Hz, o-Ar-H major diastereoisomer); Vmax: 3322, 1738, 1704, 1683, 1650cm^-1; m/z (NH3, CI) 585(M+NH4⁺, 0.3%), 568(M+H⁺, 1%), 308(59%), 160(100%), 104(31%).