Diisopropylamine → 2-(1-methyl-1H-indol-3-yl)pentanoic acid

Under a nitrogen atmosphere, butyl lithium (1.6M, 10 mL, 16 mmol) was added at −78° C. to a solution of diisopropylamine (2.22 mL, 15.84 mmol) in anhydrous tetrahydrofurane. After 25 minutes, a solution of 1-methyl-3-indoleacetic acid (1 g, 5.28 mmol) in tetrahydrofurane (20 mL) was slowly added at −78° C., followed by the addition of propyliodide (1 mL, 10 mmol) after 45 minutes. The mixture was then warmed to room temperature and stirred for 2 hours. The mixture was quenched with water (2 mL) and concentrated in vacuo. The residue was diluted with water (20 mL) and washed with diethyl ether (20 mL). The organic layer was acidified with hydrochloric acid aqueous solution 1N to pH 3 and extracted with dichloromethane (3×10 mL). The organic layer was washed with brine, dried over sodium sulfate, filtered and evaporated under reduced pressure to afford the desired acid (12a) as a brown solid without further purification (1.22 g, 5.28 mmol, 100%).