[NH:1](C(OC(C)(C)C)=O)[C@H:2]([C:10]([OH:12])=[O:11])[CH2:3][CH2:4][CH2:5][NH:6][C:7](=[NH:9])[NH2:8].C([O-])([O-])=O.[Cs+].[Cs+].[Cs].[CH2:27](Br)[C:28]1[CH:33]=[CH:32][CH:31]=[CH:30][CH:29]=1>CO.O.CN(C=O)C>[NH2:1][C@H:2]([C:10]([O:12][CH2:27][C:28]1[CH:33]=[CH:32][CH:31]=[CH:30][CH:29]=1)=[O:11])[CH2:3][CH2:4][CH2:5][NH:6][C:7](=[NH:9])[NH2:8]Water (2 ml) was added to a solution of Boc-Arg-OH (2.5 mmol) in methanol (20 ml). The solution was neutralised with a 20% solution of Cs2CO3 in water and then evaporated in vacuo to dryness. Residual water was removed by repeated addition and evaporation of toluene. The solid cesium salt of Boc-arginine was treated with DMF (25 ml) and benzyl bromide (3 mmol) and stirred at room temperature for 6 h. The DMF was removed in vacuo and the product was dissolved in acetone and filtered. The filtrate was evaporated in vacuo and the product was treated with 95% trifluoroactetic acid (TFA) (4 ml). The resulting product H-Arg-OBzl was isolated by tituration by diethyl ether. The salt of H-Arg-OBzl was isolated by treating the product with para-toluenesulfonic acid (5 mmol) in ether.
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