A solution of 1.0 g of N-2-naphthylsulfonyl-(D)-tryptophan in 20 ml of DMF is treated with 0.5 ml of ethyldiisopropylamine. After the addition of 0.6 g of t-butyl (S)-3-hydroxymethyl-1-piperidinecarboxylate (Example 3c) and 1.1 g of benzotriazol-1-yloxy-tris(dimethylamino)phosphoniumhexafluorophosphate the reaction mixture is stirred for 15 hours. The solvent is evaporated and the residue is taken up in water and extracted with ethyl acetate. The ethyl acetate phase is washed with water, dried and evaporated. Chromatography over silica gel with ethyl acetate gives 1.0 g of N-(2-naphthylsulfonyl)-D-tryptophan (S)-l-t-butyoxycarbonyl-3-piperidinylmethyl ester.
