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Nα -t-butyloxycarbonyl-glycyl-glycine benzyl ester (3.22 g, 10 mmol) was dissolved in TFAOH -trifluoracetic acid - (30 ml) at 20° C. and left to stand for 2 h. After evaporation to dryness at 20° C. under reduced pressure, the glycyl-glycine benzyl ester trifluoracetate (3.23 g, 9.6 mmol) was obtained as a solid by trituration of the residual oil in ether and was coupled to Nα -BOC-aspartic acid β-t-butyl ester (2.78 g, 9.6 mmol) according to the procedure described for the preparation of dipeptide of formula V. After the usual work-up, the product which could not be induced to crystallize, was obtained after evaporation of the solvent (ethyl acetate) and drying as a foam. Analysis: calculated for C24H35N3O8 : C, 58.40; H, 7.15; N,8.52. Pound: C, 58.20; H, 7.19; N, 8.48. [Rf : 0.52 from chloroform: methanol (20:1)]. The yield was 4.50 g (95 %).

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2. customAfter evaporation to dryness at 20° C. under reduced pressure