A partial solution of 1.32 g (5.74 mmole) of (S)-1-(tert-butyloxycarbonyl)piperazine-2-carboxylic acid in 25 ml of dried DMF was treated dropwise over 5 min with 1.48 g (11.47 mmole of diisopropylethyl amine at 25° under nitrogen with stirring. The clear solution which resulted was treated dropwise over 15 min with a solution of 1.26 g (5.74 mmole) of dipentylcarbamoyl chloride in 5 ml of DMF and the mixture was stirred under nitrogen at 25° for 16 hr. The solution was concentrated in vacuo and the residue was dissolved in 100 ml of dichloromethane which was extracted with 2×100 ml of 0.5M citric acid and 2×25 ml of water. Without drying, the solution was concentrated in vacuo to 1.85g (78%) of medium yellow oil which was homogeneous by the (silica gel, 1:1:1:1 ethylacetate: n-butanol: acetic acid: water, Rf =0.50).
