Triethylamine → [3-(1-tert-butoxycarbonyl-2-n-butylimidazol-4-yl)-2-(3,4-methylenedioxyphenyl)propionyl]-L-tryptophan benzyl ester

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To a solution of 3-(1-tert-butoxycarbonyl-2-n-butylimidazol-4-yl)-2-(3,4-methylenedioxyphenyl)propionic acid (250 mg), L-tryptophan benzyl ester hydrochloride (209 mg) and triethylamine (128 mg) in N,N-dimethylformamide (5 ml) was added diphenylphosphoryl azide (198 mg) and the mixture was stirred at ambient temperature for 5.5 hours. The resulting mixture was diluted with ethyl acetate and the organic layer was washed successively with 0.5N hydrogen chloride aqueous solution, saturated sodium bicarbonate aqueous solution and brine. Drying, filtering and removal of the solvents afforded a crude product. The crude product was chromatographed on silica gel (eluent; n-hexane:ethyl acetate=1:2) to give [3-(1-tert-butoxycarbonyl-2-n-butylimidazol-4-yl)-2-(3,4-methylenedioxyphenyl)propionyl]-L-tryptophan benzyl ester (125 mg) as a yellow oil.

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