Hydrochloric Acid → methyl (S)-phenyl((R)-piperidin-2-yl)acetate

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As alluded to above, racemic threo methylphenidate hydrochloride is known and commercially available, as is the resolving agent employed in Steps 2 and 1A, viz., (R)-(-)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate. In the latter connection, another advantage of the process of the invention is that the resolving agent is recyclable. Thus, the (R)-(-)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate may be recovered by: 1) combining the sodium salt thereof obtained by basifying the filtrate from Step 2, collecting the solid by filtration, and extracting the solid with isopropyl acetate, with the sodium salt and aqueous layer obtained in Step 3; 2) allowing the mixture to react at reflux temperature for between 30 and 45 minutes; and 3) treating the mixture with concentrated hydrochloric acid, initially at the reflux temperature for between 30 and 60 minutes, and then at a temperature of between 15° and 30° C. for a period of between 3 and 5 hours. The recovered (R)-(-)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate is of comparable purity to that of the commercially available material. A resolution of the free base form of racemic threo methylphenidate with the recovered material yielded the desired phosphate salt enriched with the d-threo isomer of methylphenidate.

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