(-)-Mandelic acid → (s)-(+)-3-(4-Fluorobenzyl)piperidine mandelic acid salt

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The crude racemic 3-(4-fluorobenzyl)piperidine (2.0 g) was dissolved in 25 mL acetonitrile and heated to reflux. The solution was hazy. To this was added 1.56 g (1 equivalent) of (R)-(−) mandelic acid dissolved in 15 mL acetonitrile. Some initial precipitation occurred when the cooler solution was added but it redissolved when refluxing resumed. The heat was turned off and small amounts of enantiomerically pure salt were added as the temperature dropped. At first the seed crystals dissolved, but when the temperature dropped to 75° C., they remained suspended in the stirred solution. After a few more degrees of cooling, crystal growth was obvious. Cooling was continued at the rate of 1 degree/min. At 50° C., the solution was filtered to recover 0.9 g of salt, which melted at 164° C. It was recrystallized from acetonitrile twice to give (S)-(+)-3-(4-fluorobenzyl)piperidine mandelic acid salt in 98% enantiomeric excess, mp 168-171° C.

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