Dicyclohexylcarbodiimide → Nalpha-Benzyloxycarbonyl-L-tryptophyl-O-benzyl-D-tyrosyl-O-benzyl-L-seryl-D-alanine methyl ester

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Reaction Steps

Nα-t-Butoxycarbonyl-O-benzyl-O-tyrosyl-O-benzyl-L-seryl-D-alanine resin (Example 11), 6.4 g., 5.1 mmol, is coupled with 2.23 g., 6.3 mmol, of Nα-benzyloxycarbonyl-L-tryptophan and 1.3 g., 6.3 mmol, of dicyclohexylcarbodiimide according to the general procedure of Example 1. The resulting Nα-benzyloxy-carbonyl-L-tryptophyl-O-benzyl-D-tyrosyl-O-benzyl-L-seryl-D-alanine resin is mixed with 20 ml. of triethylamine, 150 ml. of methanol and 50 ml. of dimethylformamide and let stand overnight. After filtration and evaporation, the crude product is triturated with either-petroleum ether and crystallized from isopropanol; 1.75 g.; [α]D 23 -5° (c. 1.01, DMF).

Provenance & Evidence