Hydrochloric Acid → Focalin

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An alternative approach to synthesizing enantiomerically pure d-threo-methylphenidate hydrochloride using an enantiomerically pure starting material, d-pipecolic acid, was reported by Thai et al. in J. Med. Chem. (1998) 41, 591-601. Enantiomerically pure d-pipecolic acid was obtained in 37% yield by recrystallization of the diastereomeric tartrate salt, followed by the separation of the desired amino acid from tartaric acid by ion-exchange chromatography. d-pipecolic acid was protected with a BOC group to afford N—BOC-d-pipecolic acid in 97% yield. The key amino ketone was prepared from N—BOC-d-pipecolic acid in two steps involving its conversion to the N-methoxy-N-methyl amide, followed by a reaction with the amide with phenyllithium. The amino ketone underwent a Wittig olefination with methyltriphenylphosphonium bromide in the presence of potassium tert-butoxide to give the alkene in high yield. The transformation of the obtained alkene to the desired alcohol as a racemic mixture of threo-stereoisomers via a hydroboration/oxidation reaction was critical to introducing the second stereogenic center. Hydroboration with BH3-THF gave a 72:28 mixture of threo and erythro isomers, respectively, from which the threo alcohol was isolated in 64% yield after chromatography. Oxidation of the threo alcohol with pyridinium dichromate (PDC) in dimethylformamide (DMF) followed by esterification of the resulting acid with diazomethane, and N—BOC group deprotection with 3 N methanolic hydrochloric acid furnished d-threo-methylphenidate hydrochloride in 67% yield after recrystallization from a mixture of ethanol and ether.

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