To a solution of 6.3 g of Z-Thr(tBu)-OH and 2.34 ml of N-methylmorpholine in 100 ml of tetrahydrofuran are added at -17°C 3.0 ml of chloroformic acid-isobutylester, the batch stirred for 20 minutes at this temperature, then reacted with 6.7 g of H-Pro-OtBu in 100 ml of tetrahydrofuran. The batch is allowed to stand at room temperature overnight, then filtered, the filtrate evaporated to dryness under vacuum, the residue dissolved in ethyl acetate and extracted several times with dilute aqueous citric acid solution, dilute aqueous sodium carbonate solution, and saturated sodium chloride solution. The resulting oily compound shows in the thin-layer chromatogram on silica gel:
