A solution of 192 mg (1.0 mmol) of 3,4-methylenedioxycinnamic acid, 121 mg (0.5 mmol) of proline benzyl ester hydrochloride and 108 mg (0.5 mmol) of alanine benzyl ester hydrochloride in 6 mL of acetonitrile was treated sequentially with 0.35 mL (2.0 mmol) of diisopropylethylamine and 443 mg (1.0 mmol) of benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate. The mixture was stirred for 16 hrs then concentrated in vacuo. The residue was dissolved in 10 mL of dichloromethane and poured into 3 volumes of diethyl ether. The mixture was washed sequentially with water, 10% potassium bisulfate solution, water, saturated sodium bicarbonate solution, water and saturated sodium chloride solution. The organic layer was dried over magnesium sulfate, filtered and concentrated in vacuo. The residue was purified by silica gel column chromatography, using a stepwise gradient of 30%, 35%, 40%, and 45% ethyl acetate in hexane as eluant. Ester (9) (142 mg) was obtained as a colorless oil, Rf=0.4 (40% ethyl acetate/hexane); HPLC, Rt 12.26; 1H NMR (300 MHz) consistent with structure. Ester (10) (162 mg) was obtained as a colorless oil; TLC, Rf =0.22 (40% ethyl acetate/hexane); HPLC, Rt 11.84 min; 1H NMR (300 MHz) consistent with structure.
