214 mg (1 mmol) of 5-(2-methoxycarbonyl-ethyl)-thiophene-2-carboxylic acid (example 1, step b) were dissolved in 5 ml of anhydrous dimethylformamide. 265 mg (1 mmol) of (3S)-3-amino-3-(benzo[1,3]dioxol-5-yl)-propionic acid tert-butyl ester and 328 mg (1 mmol) of TOTU were added, and 3 mmol (0.51 ml) of N,N-diisopropyl-N-ethylamine were added to keep the pH of the mixture at 7.5 to 8. The reaction mixture was stirred for 4.5 hours at room temperature. Solvents were removed under reduced pressure to give a dark brown oil. The residue was dissolved in dichloromethane and washed with saturated sodium bicarbonate solution, 10% citric acid and with water. Then the organic phase was dried with anhydrous magnesium sulfate. After filtration the solvents were removed in vacuo to give 600 mg of a brownish oil which was purified by chromatography (silica gel; ethyl acetate/heptane (1/2, v/v)). 425 mg of a resin were obtained.
