To a cooled solution (0° C.) of indole-3-propionic acid methyl ester 4 (0.797 g, 3.42 mmol) in DMF (20 mL) was added sodium hydride (60%; 0.25 g; 0.0625 mol) was added in one portion and stirred for 30 min followed by the addition of 4-methoxybenzenesulfonyl chloride (1.3 g; 6.31 mmol). The reaction was allowed to warm up to room temperature and stirred for 16 h, subjected to aqueous work up, and product was extracted with ethyl acetate. The ethyl acetate layer was washed with brine, dried over anhydrous sodium sulfate, evaporated under reduced pressure, and purified by flash-chromatography (silica gel; 80% n-hexane-20% ethyl acetate) to afford the ester 5 as a white solid (0.83 g; 61% yield; M+1=404.1).
