Piperidine → 3-[2-(5-Chloro-2-oxo-1,2-dihydroindol-3-ylidenemethyl)-4,5,6,7-tetrahydro-1H-indol-3-yl]-propionic acid

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3-(2-Formyl-4,5,6,7-tetrahydro-1H-indol-3-yl)-propionic acid (5.4 g), 3.7 g of 5-chloro-2-oxindole and 2.7 g of piperidine (or 2.2 g of pyrrolidine) in 25 mL of ethanol was refluxed for 4 hours. Slow addition of acetic acid (8 mL) resulted in a precipitate. The mixture was refluxed for 5 minutes and cooled to ambient temperature. The precipitate was collected by vacuum filtration and washed with 20 mL of ethanol. The solids were slurry-washed by refluxing in 30 mL of ethanol, cooled, collected by vacuum filtration, washed with 30 mL of ethanol and dried under high vacuum to give 6.5 g (80% yield) 3-[2-(5-chloro-2-oxo-1,2-dihydroindol-3-ylidenemethyl)-4,5,6,7-tetrahydro-1-H-indol-3-yl]-propionic acid as an orange solid: mp 370-384° C.

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