Citric Acid → 4-(1H-indol-3-yl)piperidine-1-carboxylic acid-(4-dimethylamino-4-phenylcyclohexylmethyl)-amide hemicitrate

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The non-polar diastereoisomer of 4-(1H-indol-3-yl)piperidine-1-carboxylic acid-(4-dimethylamino-4-phenylcyclohexylmethyl)-amide (280 mg, 0.61 mmole) was dissolved in abs. ethanol (16 ml) and DCM (6 ml). Citric acid (118 mg, 0.616 mmole) was added in one portion at ca. 40° C. while stirring. After 20 minutes a colourless precipitate began to form. The suspension was stirred for 24 hours at RT. Following this the precipitate was filtered off and washed with cold ethanol (2×1 ml) and diethyl ether (4×2 ml). The non-polar diastereoisomer of 4-(1H-indol-3-yl)piperidine-1-carboxylic acid-(4-dimethylamino-4-phenylcyclohexylmethyl)-amide hemicitrate (241 mg, m.p. 165°-168° C., 61%, Example 16) was obtained as a colourless solid.

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