Reaction Steps
N-tert-butoxycarbonyl O-allyl L-seryl L-valyl L-valyl O-allyl L-seryl glycine methyl ester 22 (SEQ ID NO: 1) (0.734 g, 1.14 mmol) was dissolved in a 50% (v/v) solution of TFA in CH2Cl2 (16 mL). The reaction mixture was stirred for 1 h 30 min before the solvent and bulk of excess TFA were evaporated affording an oil. Addition of Et2O (25 mL) resulted in the precipitation of a white solid. The mixture was centrifuged and the Et2O decanted off. The residue was washed with an additional 2×25 mL Et2O and dried under reduced pressure overnight affording a fine white powder (0.719 g, 96%); δH (300 MHz; d6-DMSO) 8.48 (1H, d, J 9, NH(Valx)), 8.41 (1H, t, J 6, NH(Gly)), 8.24 (3H, br s, NH3+), 8.01-7.98 (2H, m, NH(Valy)/NH(Ser(All)2)), 5.90-5.77 (2H, m, CH═CH2), 5.30-5.10 (4H, m, CH═CH2), 4.56-4.50 (1H, m, CαH(Ser(All)2)), 4.32 (1H, dd, J 7 and 9, CαH(Valx)), 4.24 (1H, dd, J 7 and 8, CαH(Valy)), 4.10 (1H, br s, CαH(Ser(All)1)), 3.98-3.93 (4H, m, CH2CH═CH2), 3.86-3.84 (2H, m, CαH2(Gly)), 3.72-3.52 (4H, m, CH2(Ser(All)1)/CH2(Ser(All)2)), 3.61 (3H, s, OCH3), 2.08-1.89 (2H, m, CH(CH3)2), 0.88-0.80 (12H, m, CH3); δC (75 MHz; d6-DMSO) 170.6, 170.4, 169.9, 169.8, 166.2, 158.3 (q, JCF 37) 134.8, 134.4, 117.1, 116.9 (q, JCF 293), 116.5, 71.4, 71.0, 69.5, 68.3, 57.9, 57.6, 52.4, 52.2, 51.6, 40.6, 30.6, 30.4, 19.1, 18.1, 17.9; m/z (ESI) 542.3181 (M+; C25H44N5O8 requires 542.3189)
