Reaction SMILES: C([N:8]([C:20]([O:22][CH2:23][C:24]1[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=1)=[O:21])[CH2:9][C:10]([N:12]1[CH2:19][CH2:18][CH2:17][C@H:13]1[C:14]([OH:16])=O)=[O:11])C1C=CC=CC=1.C(N(C(C)C)CC)(C)C.ClC(OCC)=O.C1(C)C=CC(S(O)(=O)=O)=CC=1.[CH2:56]([O:63][C:64](=[O:70])[C@H:65]([CH2:67][CH2:68][CH3:69])[NH2:66])[C:57]1[CH:62]=[CH:61][CH:60]=[CH:59][CH:58]=1>ClCCl>[CH2:56]([O:63][C:64](=[O:70])[C@H:65]([CH2:67][CH2:68][CH3:69])[NH:66][C:14](=[O:16])[C@@H:13]1[CH2:17][CH2:18][CH2:19][N:12]1[C:10](=[O:11])[CH2:9][NH:8][C:20]([O:22][CH2:23][C:24]1[CH:25]=[CH:26][CH:27]=[CH:28][CH:29]=1)=[O:21])[C:57]1[CH:62]=[CH:61][CH:60]=[CH:59][CH:58]=1
Reactant: O=C(O)[C@@H]1CCCN1C(=O)CN(Cc1ccccc1)C(=O)OCc1ccccc1
Reactant: CCC[C@H](N)C(=O)OCc1ccccc1
Agent: CCOC(=O)Cl
Agent: Cc1ccc(S(=O)(=O)O)cc1
Solvent: CCN(C(C)C)C(C)C
Solvent: ClCCl
Product: CCC[C@H](NC(=O)[C@@H]1CCCN1C(=O)CNC(=O)OCc1ccccc1)C(=O)OCc1ccccc1
Temperature: 0 °C
Reaction time: 0.08 h
Yield: 77.9 %

Procedure:
To a stirred solution of benzyl N-benzyloxycarbonyl-glycyl-L-proline 3 (0.4 g, 1.3 mmol) under nitrogen in dry dichloromethane (20 cm3) was added diisopropylethylamine (0.31 cm3, 1.76 mmol). The suspension was stirred for 5 min (a solution was obtained), cooled to 0° C. and ethyl chloroformate (0.166 cm3, 1.73 mol) was added. After 45 min a solution of diisopropylethylamine (0.31 cm3, 1.76 mmol) and p-toluenesulphonate 2 (0.644 g, 1.70 mmol) in dry dichloromethane (20 cm3) was added dropwise over 5 min. The cooling bath was removed and the resultant solution was stirred overnight. The solvent was removed in vacuo and the residue was chromatographed (silica gel, 50-100% ethyl acetate-hexane) to give fully protected tripeptide 4 (0.502 g, 78%) as a colourless oil. Tripeptide 4 was shown to be a 84:16 trans.-cis mixture of conformers by 13C NMR analysis (the ratio was estimated from the integration of the ψEβ-C carbons at δ 34.0 and 33.6 and the Eγ-C carbons at δ 18.5 and 18.8 of the major and minor conformers respectively): [α]D −70.3 (c 0.8 in dichloromethane); δH (300 MHz; CDCl3; Me4Si) 0.87 (3H, d, J 6.0, Eγ-CH3), 1.26-1.33 (2H, m, Eγ-H2), 1.58-1.68 (1H, m, Eβ-HAHB), 1.70-2.26 (5H, m, Eβ-HAHB, Proβ-H2, and Proγ-H2), 3.38 (0.84H, d, J 7.6, Proδ-HAHB), 3.51-3.56 (0.84H, m, Proδ-HAHB), 3.60-3.65* (0.32H, m, Proδ-H2), 3.76* (0.16H, d, J 16.5, Glyα-HAHB), 3.91 (1.84H, m, Glyα-H2), 4.37* (0.16H, br s, Proα-H), 4.49-4.62 (2H, m, Proα-H, Eα-H), 5.09-5.21 (4H, m, 2×OCH2Ph), 5.85 (1H, br s, Gly-NH), 7.2 (1H, d, J 5.5, Eα-NH) and 7.34 (10H, s, 2×Ph); δC (75 MHz; CDCl3) 13.4 (CH3, Eγ-CH3), 13.5* (CH3, Eγ-CH3), 18.5 (CH2, Eγ-C), 18.8* (CH2, Eγ-C), 22.2* (CH2, Proγ-C), 24.7 (CH2, Proγ-C), 27.7, (CH2, Proβ-C) 31.9* (CH2, Proβ-C) 33.6* (CH2, Eβ-C), 34.0 (CH2, Eβ-C), 43.3 (CH2, Glyα-C), 46.2 (CH, Proδ-C), 47.0* (CH, Proδ-C), 52.1* (CH, Eα-C), 52.3 (CH, Eα-C), 59.9 (CH, Proα-C), 66.8 (CH2, OCH2Ph), 67.1* (CH2, OCH2Ph), 127.9 (CH, Ph), 127.99 (CH, Ph), 128.0 (CH, Ph), 128.1* (CH, Ph), 128.2 (CH, Ph), 128.4 (CH, Ph), 128.5 (CH, Ph), 135.4 (quat., Ph), 136.3 (quat., Ph), 156.2 (quat., NCO2), 156.4* (quat., NCO2), 167.9 (quat., Gly-CO), 168.2* (quat., Gly-CO), 170.7 (quat., Pro-CON), 171.1* (quat., Pro-CON) and 172.1 (quat., Eα-CO); m/z (FAB+) 495.2381 (MH+.C27H33N3O6 requires 495.2370). ψE refers to the proline analog portion in question* Denotes resonances assigned to minor conformer.