To an ice cold solution of 1-(2-t-butyloxycarbonyl-3-dibenzylaminopropyl)-1-cyclopentane carboxylic acid (12.7 g, 27 mmole) in dry dichloromethane (100 ml) was added 1-hydroxybenztriazole (4.2 g,.31 mmole), and 1-ethyl-3-(dimethylaminopropyl)-carbodiimide (7 g, 36 mmole) and the resulting solution stirred at 0° C. for 30 minutes. To this solution was added 0-t-butyltyrosine t-butyl ester (8.4 g, 28.6 mmole) and N-methylmorpholine (5.25 g, 52 mmole) and the solution allowed to stand overnight at room temperature. The solvent was evaporated under reduced pressure and the resultant mobile oil was dissolved in methylene chloride and washed with water (2 x ), 2M hydrochloric acid and saturated aqueous sodium bicarbonate (1 x) dried (MgSO4), and the solution filtered and evaporated to yield the crude product as a gum. Recrystallization from n-hexane gave the title compound as a solid (13 g, 69%), m.p. 82-87° C. A further batch of material was obtained by evaporation of the supernatant liquors and further recrystallisation. Found: C,74.12; H,8.69; N,3.87. C45H62N2O6 requires C,74.34; H,8.59; N,3.85%.
