[NH:1]1[C:5](=[O:6])[CH2:4][CH2:3][C@H:2]1[C:7]([NH:9][C@H:10]([C:17]([NH:19][C@H:20]([C:31](O)=[O:32])[CH2:21][C:22]1[C:30]2[C:25](=[CH:26][CH:27]=[CH:28][CH:29]=2)[NH:24][CH:23]=1)=[O:18])[CH2:11][C:12]1[N:16]=[CH:15][NH:14][CH:13]=1)=[O:8].[CH3:34][O:35][C:36](=[O:41])[C@H:37]([CH2:39][OH:40])[NH2:38].C(N(CC)CC)C>C(#N)C>[CH3:34][O:35][C:36](=[O:41])[C@H:37]([CH2:39][OH:40])[NH:38][C:31](=[O:32])[C@H:20]([CH2:21][C:22]1[C:30]2[C:25](=[CH:26][CH:27]=[CH:28][CH:29]=2)[NH:24][CH:23]=1)[NH:19][C:17](=[O:18])[C@H:10]([CH2:11][C:12]1[N:16]=[CH:15][NH:14][CH:13]=1)[NH:9][C:7](=[O:8])[C@@H:2]1[CH2:3][CH2:4][C:5](=[O:6])[NH:1]1L-Pyroglutamyl-L-histidyl-L-tryptophan (IV) (0.452 g) and L-serine methyl ester (V) hydrochloride (0.155 g) in dry acetonitrile (50 ml) are treated with triethylamine (0.14 ml) and DCI (0.227 g) in dry acetonitrile (5 ml) at 0°C. After stirring at room temperature for 24 hours, the reaction mixture is evaporated to dryness. The residue is dissolved in methanol and the dicyclohexylurea is removed by filtration. The filtrate is concentrated to dryness to L-pyroglutamyl-L-histidyl-L-tryptophyl-L-serine methyl ester (VI).
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