Abstract:
Processes and intermediates for preparing 2-substituted piperidines such as 2-substituted d-threo piperidines are provided, including processes and intermediates for resolution of piperidyl acetamide stereoisomers.

Description:
FIELD OF THE INVENTION 
     This invention is directed to novel processes for resolution of piperidyl acetamide stereoisomers. The invention additionally is directed to synthetic intermediates and reaction products useful in such processes. 
     BACKGROUND OF THE INVENTION 
     Substituted piperidines have found use in the treatment of many nervous system disorders. For example, methylphenidate has been used to treat Attention Deficit Disorder (ADD), Attention Deficit Hyperactivity Disorder (ADHD) and cognitive decline in Acquired Immunodeficiency Syndrome (AIDS) and AIDS Related Complex (ARC) patients. (See, e.g., Greenhill, Child &amp; Adol. Psych. Clin. NA., 1995, 4, 123, and Brown, Intl. J Psychl. Med., 1995, 25, 21). 
     Many currently available synthetic routes to methylphenidate and other substituted piperidines involve preparation of racemic mixtures. (See, e.g., U.S. Pat. No. 2,507,631, to Hartmann, et al., and U.S. Pat. No. 2,957,880, to Rometsch, et al.). There are, however, a number of disadvantages associated with racemic mixtures of such drugs. Current administration of racemic methylphenidate often results in notable side effects such as anorexia, weight loss, insomnia, dizziness and dysphoria. Additionally, racemic methylphenidate produces a euphoric effect when administered intravenously or through inhalation, and thus carries a high potential for substance abuse in patients. 
     U.S. Pat. No. Nos. 2,507,631 and 2,957,880 disclose synthetic procedures wherein methylphenidate, alternatively known as methyl α-piperid-2-ylphenylacetate, is prepared through a multi-step process in which 2-chloropyridine and phenylacetonitrile initially are coupled to form α-pyrid-2-ylphenylacetonitrile. The resulting α-pyrid-2-ylphenylacetonitrile then is hydrated in the presence of acid to yield α-pyrid-2-ylphenylacetamide which, in turn, is either: (a) catalytically hydrogenated to yield α-piperid-2-ylphenylacetamide and then converted to methyl α-piperid-2-ylphenylacetate, or (b) converted to methyl α-pyrid-2-ylphenylacetate which, in turn, is hydrogenated to yield methyl α-piperid-2-ylphenylacetate. 
     In the first embodiment of U.S. Pat. No. 2,507,631 and in the process described in U.S. Pat. No. No. 2,957,880, α-piperid-2-ylphenylacetamide is first separated into the threo and erythro diastereomeric racemates. This is accomplished through evaporation of the solvent utilized in the hydrogenation (i.e., acetic acid), addition of sodium hydroxide to precipitate the α-piperid-2-ylphenylacetamide free base, recrystallization of this amide from ethyl acetate, and preferential crystallization of the erythro form by passing gaseous hydrogen chloride through an ethanolic solution of the amide. 
     The isolated erythro racemate then is resolved through formation of the l-tartrate salt, repeated recrystallizations of this salt from 96% ethanol are performed until a constant rotation is obtained, and the l-erythro form of α-piperid-2-ylphenylacetamide is precipitated with sodium hydroxide. The l-erythro form of α-piperid-2-ylphenylacetamide thus obtained is said to be subjected to epimerization to yield the desired d-threo diastereomer of α-piperid-2-ylphenylacetamide through treatment with 6M potassium hydroxide. According to the disclosed procedure, the α-piperid-2-ylphenylacetamide thus obtained is converted to d-threo methyl α-piperid-2-ylphenylacetate through hydrolysis and esterification. 
     Some in the art have raised doubts as to whether the procedures disclosed in U.S. Pat. Nos. 2,507,631 and 2,957,880 do, in fact, produce the desired d-threo isomer. Indeed, J. R. Soares, &#34;Stereochemical Studies On Potential Central Nervous System Active Agents and Studies On The Chemistry Of Some 3-Benzoylpiperidines,&#34; 1971, Columbia University Ph.D. dissertation, p. 115, discloses that &#34;all attempts to epimerize the resolved erythro-amides to the corresponding threo-amides by the procedure outlined in  U.S. Pat. No. 2,957,880! failed completely.&#34; 
     In any event, the synthetic procedure described in U.S. Pat. Nos. 2,507,631 and 2,957,880 is wasteful in that it involves discarding the threo α-piperid-2-ylphenylacetamide racemate which is isolated following the recrystallization step and which typically represents approximately 25% of the acetamide product obtained via hydrogenation. 
     Consequently, there remains a need in the art for alternative synthetic procedures for the preparation of methylphenidate and other substituted piperidines. In particular, there is a need for synthetic procedures that do not require separating and discarding threo stereoisomers from the hydrogenation reaction product. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide processes for the preparation of substituted piperidines. 
     It is another object of this invention to provide processes that provide synthetic intermediates and, hence, products having high optical purity. 
     It is yet another object to provide processes that proceed more efficiently than the processes disclosed by the prior art. 
     SUMMARY OF THE INVENTION 
     These and other objects are satisfied by the present invention, which provides processes and intermediates for preparing piperidyl acetamides. In preferred embodiments, the processes of the invention comprise reacting d,l-threo piperidyl acetamide stereoisomers having formulas IIa and IIb: ##STR1## (R 1  =aryl having about 6 to about 28 carbon atoms) with an acid resolving agent in an organic solvent, thereby forming acid salts of the d-threo stereoisomers preferentially with respect to the l-threo stereoisomers. The resulting acid salts then are reacted with aqueous base to form the corresponding piperidyl acetamide, which subsequently is converted to a corresponding ester. 
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides novel processes for stereoselective synthesis of a variety 2-substituted piperidine stereoisomers. In one aspect, the invention is directed to synthetic methods involving hydrogenation of pyridines having formula I: ##STR2## wherein R 1  is aryl having about 6 to about 28 carbon atoms. Aryl groups, as used herein, are aromatic groups containing a delocalized π-electron cloud. Such aromatic groups can be substituted with one or more substituents, such as, for example, halo, alkyl, aryl, hydroxy, alkoxy, carboxy, and cycloalkyl. Exemplary aryl groups include phcnyl, naphthyl, xylyl, chlorophenyl, fluorophenyl, trifluoromethylphenyl, and bromophenyl. Phenyl groups are preferred. 
     This hydrogenation can be effected by any of the numerous techniques known in the art. One preferred hydrogenation technique involves reacting the pyridine with hydrogen gas in the presence of a suitable catalyst in an alkanoic acid having 1 to about 10 carbon atoms. The hydrogenation preferably run at 25° C. and 40 psi. Representative catalysts contain platinum, with platinum oxide being particularly preferred. One preferred alkanoic acid is acetic acid. 
     Hydrogenation of pyridine I provides a mixture of piperidine diastereomers IIa-d: ##STR3## In accordance with the present invention, this mixture is treated with an organic base in an organic solvent to epimerize the erythro stereoisomers into threo forms. The epimerization can, for example, be effected in an aromatic hydrocarbon solvent such as toluene using an alkali metal alkoxide such as potassium tert-butoxide. In preferred embodiments, the epimerization is effected at 70° C. in an aromatic hydrocarbon solvent such as toluene using two equivalents of an alkali metal alkoxide such as potassium tert-butoxide. 
     The resulting composition, which should consist predominantly of d,l-threo piperidyl acetamide stereoisomers, is reacted with an acid resolving agent in an organic solvent, thereby forming acid salts of the d-threo stereoisomers preferentially with respect to the l-threo stereoisomers. Alkyl groups according to the invention are hydrocarbons which are straight, branched, or cyclic. Such hydrocarbons can be substituted with one or more substituents, such as, for example, halo, hydroxy, alkoxy, and carboxy groups. Exemplary alkyl groups include methyl, ethyl, isopropyl, n-butyl, t-butyl, n-pentyl, acetyl, trifluoromethyl, chloromethyl, and hexyl groups. Representative solvents include alcohols, alkyl alkanoates (e.g., ethyl acetate), ketones (e.g., acetone), and ethers (e.g., tetrahydrofuran, dioxane). Preferred solvents are alcohols having 1 to about 5 carbon atoms, include branched and straight chain compounds such as ethyl, propyl and tert-butyl alcohol, with isopropanol being particularly preferred. The reaction of piperidyl acetamide stereoisomers with acid resolving agents preferably is performed with stirring at room temperature. 
     Representative acid resolving agents include L-(+)- or D-(-)- tartaric acid, dipivaloyl-D-tartaric acid, (1S)-(+)-10-camphorsulphonic acid, L-(-)-malic acid, (S)-(+)-mandelic acid, N-acetyl-l-aspartic acid (and other N-protected amino acids), (R)-(+)- 1,1&#39;-bi-s-napthol, (+)-camphoric acid, D-glucuronic acid, and derivatives thereof. Those believed to be useful for forming d-threo stereoisomers preferentially with respect to l-threo isomers include (+)-dibenzoyl-D-tartaric acid. Derivatives of D-(-)-tartaric acid are preferred, including those having formula (III): 
     
         HO.sub.2 CCH OC(O)R.sub.3 !CH OC(O)R.sub.3 !CO.sub.2 H     (III) 
    
     where each R 3 , independently, is aryl having 6 to about 28 carbon atoms or aralkyl having 7 to about 28 carbon atoms. Aralkyl groups according to the invention are those (such as, for example, benzyl groups, which both aryl and alkyl portions and are covalently bound to a core molecule (such as the above-noted carbonyl-functionalized tartaric acid) through the alkyl portions thereof. 
     In certain alternative embodiments of the invention, the piperidyl acetamide stereoisomers having formulas IIa and IIb are reacted with an acid resolving agent in an organic solvent to form acid salts of the l-threo stereoisomers preferentially with respect to the d-threo stereoisomers. Resolving agents believed to be useful for forming l-threo stereoisomers preferentially with respect to d-threo isomers include (-)-dibenzoyl-L-tartaric acid. Derivatives of L-(-)-tartaric acid are preferred, including those having formula (III). Crystallization preferably is performed at ambient temperature. 
     The acid salts obtained via resolution typically are dissolved in water and treated with an aqueous base such as a carbonate, bicarbonate, or hydroxide to precipitate the corresponding piperidyl amide free base in substantially pure form. They then can be reacted with an alcohol having, for example, 1 to about 5 carbon atoms in the presence of acid to form the corresponding ester. 
    
    
     Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting. 
     EXAMPLE 1 
     Preparation of d-Threo-methylphenidate Hydrochloride Via Diastereomeric Separation and Resolution of d,l-erythro-Amide (Comparative Example) 
     A. α-Phenyl-α-pyridyl-(2)-acetonitrile 
     
         ______________________________________Materials:______________________________________2-Chloropyridine (99%)               286 g (2.50 moles)Benzyl cyanide (98%)               314 g (2.62 moles)Sodium amide (90%)  217 g (5.00 moles)Toluene             0.98 + 0.17 LWater               0.87 LEthyl acetate       0.43 LHexanes             1.56 + 1.95 LBrine               0.43 L______________________________________ 
    
     Procedure: 
     A 5 L multi-neck glass reactor was charged with 2-chloropyridine, benzyl cyanide, and toluene (0.98 L). Sodium amide powder was added over a 1 h period via a solid-addition finnel, keeping the reaction temperature below 30° C. The reaction mixture was stirred for 16 h at ambient temperature. The reaction was then cooled to ˜10° C., and quenched with 0.87 L water. Ethyl acetate (0.43 L) was added to solubilize the precipitated product. The organic layer was separated and washed once with 0.43 L brine. Solvent was removed from the organic layer on a rotovap, and toluene (0.17 L), followed by hexanes (1.56 L), were added to the resulting residue. The resulting slurry was filtered. The filter cake was washed with hexanes (1.95 L), and dried to give 441 g of α-phenyl-α-pyridyl-(2)-acetonitrile as light brown crystals (90% yield based on 2-chloropyridine). 
     B. α-Phenyl-α-pyridyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetonitrile                 441 g (2.27 moles)Conc. H.sub.2 SO.sub.4                 0.55 LWater                 1.63 L50% NaOH              1.27 L______________________________________ 
    
     Procedure: 
     The reactor was charged with conc. H 2  SO 4 , and cooled to ˜10° C. α-Phenyl-α-pyridyl-(2)-acetonitrile (from Example 1.A) was added portionwise, keeping the temperature below 30° C. The reaction was stirred at ambient temperature for 16 h. The reaction mixture then was cooled to 10° C., at which point water was added. The NaOH then was added to a pH of 12, keeping the temperature below 30° C. The resulting crystals were filtered, and the filter cake was washed with water and dried under vacuum to give 482 g (100%) of α-phenyl-α-pyridyl-(2)-acetamide. 
     NH 4  OH can be substituted for NaOH to adjust the pH to 12. One advantage of using NH 4  OH is that the by-product that is formed (ammonium sulfate) is more soluble in water then the by-product (sodium sulfate) formed when NaOH is used as the base. Thus, the product crystals are less likely to be contaminated with inorganic salts. 
     C. d,l-erythro-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetamide                482 g (2.27 moles)Platinum oxide (PtO.sub.2)                8.06 gAcetic acid          1.68 + 4.13 LCelite               500 + 250 gEthyl acetate        3.10 + 0.62 + 2.07 +                2.07 + 4.13 + 0.21 LWater                4.13 + 1.03 + 2.07 L50% NaOH             0.56 L______________________________________ 
    
     Procedure: 
     The reactor was charged with α-phenyl-α-pyridyl-(2)-acetamide (from Example 1.B), acetic acid (1.68 L), and PtO 2 . The reactor then was filled with hydrogen gas, and pressurized to 60 psi. The reaction mixture was hydrogenated at room temperature for 16 h. The reaction mixture was filtered through a pad of Celite (500 g) to remove catalyst, and the Celite pad washed with acetic acid (4.13 L). The filtrate was concentrated under reduced pressure. Ethyl acetate (3.10 L) was added to the residue and stirred for 2 h. The resulting crystals (first crop) were filtered, washed with ethyl acetate (0.62 L), and dried. The filtrate was concentrated under reduced pressure. Ethyl acetate (2.07 L) was added to the residue and stirred for 2 h. The resulting crystals (second crop) were filtered, washed with ethyl acetate (2.07 L), and dried. The crystals from first and second crops were combined and dissolved in water (4.13 L), filtered through a pad of Celite (250 g), and the Celite pad was washed with water (1.03 L). The resulting filtrate was cooled to 10° C., followed by addition of 50% NaOH until the pH of the mixture was 13 and the free amine crystallized out. The crystals were filtered, washed with water (2.07 L), and dried to give 297 g (60%) of d,l-erythro-α-phenyl-α-piperidyl-(2)-acetamide. 
     D. l-erythro-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________d,l-erythro-α-phenyl-α               297.2 g (1.361 moles)piperidyl-(2)-acetamideD-(-)-Tartaric acid 204.3 g (1.361 moles)Methanol            7.13 + 7.13 LWater               2.0 L50% NaOH            0.1 L______________________________________ 
    
     Procedure: 
     D-(-)-Tartaric acid dissolved in methanol (7.13 L) was added to a stirred solution of d,l-erythro-α-phenyl-d-piperidyl-(2)-acetamide (from Example 1.C), dissolved in methanol (7.13 L). The resulting clear solution was stirred for 16 h, whereby the tartrate salt of l-erythro-acetamide crystallized out. The crystals were filtered, washed with methanol and dried. This tartrate salt was dissolved in water and 50% NaOH was added to a pH of 12, whereby the free base precipitated out. The precipitated crystals were filtered, washed with water and dried to give 119 g (40%) of l-erythro-α-phenyl-α-piperidyl-(2)-acetamide. 
     E. d-threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________l-erythro-α-phenyl-α-piperidyl-(2)-                119 g (0.544 moles)acetamidePotassium t-butoxide (95%)                141.5 g (1.198 moles)Toluene              3.57 LWater                0.60 + 0.30 + 1.20 LConc. HCl            0.24 + 0.12 L50% NaOH             0.14 L______________________________________ 
    
     Procedure: 
     A mixture of l-erythro-α-phenyl-α-piperidyl-(2)- acetamide (from Example 1.D), potassium t-butoxide, and toluene was heated to 70° C. and stirred for 16 h. The reaction mixture was cooled to room temperature, followed by slow addition of water (0.60 L). Conc. HCl (0.24 L) was added to this resulting mixture, and stirred for 0.5 h. The layers were separated, and the top organic layer was washed with a prepared solution of conc. HCl (0.12 L) and water (0.30 L). The combined aqueous layers were cooled to 10° C., and 50% NaOH was added to a pH of 12, whereby the free base precipitated out. The crystals were filtered, washed with water (1.20 L), and dried to give 101 g (85%) of d-threo-α-phenyl-α-piperidyl-(2)-acetamide. 
     F. d-threo-Methylphenidate Hydrochloride 
     
         ______________________________________Materials:______________________________________d-threo-α-phenyl-α-              101 g (0.46 moles)piperidyl-(2)-acetamideConc. H.sub.2 SO.sub.4              121 mLMethanol           1.1 LWater              0.81 L50% NaOH           175 mLDiethyl ether      1.0 + 1.0 + 1.0 + 1.0 LMagnesium sulfate  20 gHCl gas            As needed.______________________________________ 
    
     Procedure: 
     A solution of d-threo-α-phenyl-α-piperidyl-(2)-acetamide (from Example 1.E) and conc. H 2  SO 4  in methanol was heated to reflux and stirred for 2 days. The reaction mixture was cooled to room temperature and concentrated under reduced pressure. Water (0.81 L) and ether (1.0 L) were added to the residue. NaOH was added to a pH of 12, and the layers ere separated. The aqueous layer was extracted with ether (1.0 L). MgSO 4  was added to he combined ether layers, filtered, and washed with ether (1.0 L). HCl gas was passed through the filtrate with stirring, whereby white crystals of d-threo-methylphcnidate hydrochloride precipitated out. The crystals were filtered, washed with ether (1.0 L), and dried to give 100 g (80%) of d-threo-methylphenidate hydrochloride. 
     The overall yield for Example 1 was 14.7%. 
     EXAMPLE 2 
     Preparation of d-Threo-methylphenidate Hydrochloride Via Epimerization and Resolution of d,l-Threo-amide Enantiomers 
     A. α-Phenyl-α-pyridyl-2-acetonitrile 
     
         ______________________________________Materials:______________________________________2-Chloropyridine (99%)               172 g (1.50 moles)Benzyl cyanide (98%)               188 g (1.576 moles)Sodium amide (90%)  130 g (3.00 moles)Toluene             0.59 + 0.10 LWater               0.52 LEthyl acetate       0.26 LHexanes             0.94 + 1.17 LBrine               0.26 L______________________________________ 
    
     Procedure: 
     The reactor was charged with 2-chloropyridine, benzyl cyanide, and toluene (0.59 L). Sodium amide powder was added over a 1 h period via a solid-addition funnel, keeping the reaction temperature below 300° C. The reaction mixture was stirred for 16 h at ambient temperature. The reaction was cooled to ˜10° C., and quenched with 0.52 L water. Ethyl acetate (0.26 L) was added to solubilize the precipitated product. The organic layer was separated and washed once with 0.26 L brine, and solvent was removed from the organic layer on a rotovap. Toluene (0.10 L), followed by hexanes (0.94 L) were added to the resulting residue. The resulting slurry was filtered, and the filter cake was washed with hexanes (1.17 L), and dried to give 265 g of α-phenyl-α-pyridyl-(2)-acetonitrile as light brown crystals (90% yield based on 2-chloropyridine). 
     B. α-Phenyl-α-pyridyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetonitrile                 264 g (1.362 moles)Conc. H.sub.2 SO.sub.4                 0.33 L (6.226 moles)Water                 0.98 L50% NaOH              0.77 L______________________________________ 
    
     Procedure: 
     The reactor was charged with conc. H 2  SO 4 , and cooled to ˜10° C. α-Phenyl-α-pyridyl-(2)-acetonitrile (from Example 2.A) was added portionwise, keeping the temperature below 30° C. The reaction was stirred at ambient temperature for 16 h. The reaction mixture then was cooled to 10° C, the water was added, and the NaOH was added to a pH of 12, keeping the temperature below 30° C. The resulting crystals were filtered, the filter cake was washed with water, and dried under vacuum to give 289 g (100%) of α-phenyl-α-pyridyl-(2)-acetamide. 
     C. d,l-erythro/threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetamide                289 g (1.365 moles)Platinum oxide (PtO.sub.2)                4.84 gAcetic acid          1.01 + 2.48 LCelite               300 + 150 gWater                2.48 + 0.62 + 1.24 L50% NaOH             0.33 L______________________________________ 
    
     Procedure: 
     The reactor was charged with α-phenyl-α-pyridyl-(2)-acetamide (from Example 2.B), acetic acid (1.01 L), and PtO 2 . The reactor then was filled with hydrogen gas, pressurized to 60 psi, and the mixture hydrogenated at room temperature for 16 h. The reaction mixture then was filtered through a pad of Celite (300 g) to remove the catalyst, and the Celite pad is washed with acetic acid (2.48 L). The filtrate was concentrated under reduced pressure. The resulting residue was dissolved in water (2.48 L), filtered through a pad of Celite (150 g), and the Celite pad was washed with water (0.62 L). The resulting filtrate was cooled to 10° C., followed by addition of 50% NaOH until the pH of the mixture was 13 and the free amine crystallized out. The crystals were filtered, washed with water (1.24 L), and dried to give 297 g (100%) of a 4:1 mixture of d,l-erythro-α-phenyl-α-piperidyl-(2)-acetamide and d,l-threo-α-phenyl-α-piperidyl-(2)-acetamide. 
     D. d,l-threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________Mixture of d,l-erythro-acetamide                297 g (1.36 moles)and d,l-threo-acetamidePotassium t-butoxide (95%)                354 g (2.996 moles)Toluene              8.92 LWater                1.49 + 0.74 + 3.00 LConc. HCl            0.59 + 0.30 L50% NaOH             0.36 L______________________________________ 
    
     Procedure: 
     A mixture of d,l-erythro-acetamide and d,l -threo-acetamide (from Example 2.C), potassium t-butoxide, and toluene was heated to 70° C. and stirred for 16 h. The reaction mixture was cooled to room temperature, followed by slow addition of water (1.49 L). Conc. HCl (0.59 L) was added to this resulting mixture, which was stirred for 0.5 h. The layers were separated, and the top organic layer was then washed with a prepared solution of conc. HCl (0.30 L) and water (0.74 L). The combined aqueous layers were cooled to 10° C., and 50% NaOH was added to a pH of 12 whereby the free base precipitated out. The crystals were filtered, washed with water (3.00 L), and dried to give 253 g (85%) of d,l-threo-α-phenyl-α-piperidyl-(2)-acetamide. 
     E. d-threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________d,l-threo-α-phenyl-a-piperidyl-                253 g (1.159 moles)(2)-acetamideDibenzoyl-D-tartaric acid                415 g (1.159 moles)Isopropanol          8.11 L6N HCl (aqueous)     1.67 LWater                1.0 LSolid NaCl           290 g50% NaOH (aqueous)   1.0 L______________________________________ 
    
     Procedure: 
     Dibenzoyl-D-tartaric acid and d,l-ihreo-α-phenyl-α-piperidyl-(2)-acetamide (from Example 2.D) were dissolved in isopropanol by warming the reaction mixture to ˜50° C. The resulting clear solution was stirred at ambient temperature for 16 h, whereby the tartrate salt of d-threo-acetamide crystallized out. The crystals were filtered, and the filter cake was washed with isopropanol and dried in a vacuum oven at 40° C. This tartrate salt was added in portions to a stirred solution of 6N aq. HCl, and the resultant slurry was stirred at ambient temperature for 4 h. The slurry was filtered, and the filter cake (containing free dibenzoyl-D-tartaric acid) was washed with water. Solid NaCl was added to the filtrate (which contained d-threo-acetamide hydrochloride salt) and the mixture was cooled to ˜10° C. The NaOH was added to this mixture to a pH of 12, whereby the free base of d-threo-acetamide precipitated out. The precipitated crystals were filtered, washed with water and dried to give 101 g (40%) of d-threo-α-phenyl-α-piperidyl-(2)- acetamide. 
     F. d-threo-Methylphenidate Hydrochloride 
     
         ______________________________________Materials:______________________________________d-threo-α-phenyl-α-piperidyl-               101 g (0.46 moles)(2)-acetamideConc. H.sub.2 SO.sub.4               121 mLMethanol            1.1 LWater               0.81 L50% NaOH            175 mLDiethyl ether       1.0 + 1.0 + 1.0 + 1.0 LMagnesium sulfate   20 gHCl gas             As needed.______________________________________ 
    
     Procedure: 
     A solution of d-threo-α-phenyl-α-piperidyl-(2)- acetamide (from Example 2.E) and conc. H 2  SO 4  in methanol was heated to reflux and stirred for 2 days. The reaction mixture was cooled to room temperature and concentrated under reduced pressure. Water (0.81 L) and ether (1.0 L) were added to the residue. The NaOH was added to a pH of 12, and the layers were separated. The aqueous layer was extracted with ether (1.0 L). MgSO 4  was added to the combined ether layers, filtered, and washed with ether (1.0 L). HCl gas was passed through the filtrate with stirring, whereby white crystals of d-threo-methylphenidate hydrochloride precipitated out. The crystals were filtered, washed with ether (1.0 L), and dried to give 100 g (80%) of d-threo-methylphenidate hydrochloride. 
     In contrast to Example 1, the overall yield for Example 2 was 24.5%, an increase of over 66%. 
     EXAMPLE 3 
     Preparation of l-Threo-methylphenidate Hydrochloride Via Epimerization and Resolution of d,l-Threo-amide Enantiomers 
     A. α-Phenyl-α-pyridyl-2-acetonitrile 
     
         ______________________________________Materials:______________________________________2-Chloropyridine (99%)               172 g (1.50 moles)Benzyl cyanide (98%)               188 g (1.576 moles)Sodium amide (90%)  130 g (3.00 moles)Toluene             0.59 + 0.10 LWater               0.52 LEthyl acetate       0.26 LHexanes             0.94 + 1.17 LBrine               0.26 L______________________________________ 
    
     Procedure: 
     The reactor was charged with 2-chloropyridine, benzyl cyanide, and toluene (0.59 L). Sodium amide powder was added over a lh period via a solid-addition funnel, keeping the reaction temperature below 300° C. The reaction mixture was stirred for 16 h at ambient temperature. The reaction was cooled to ˜10° C., and quenched with 0.52 L water. Ethyl acetate (0.26 L) was added to solubilize the precipitated product. The organic layer was separated and washed once with 0.26 L brine, and solvent was removed from the organic layer on a rotovap. Toluene (0.10 L), followed by hexanes (0.94 L) were added to the resulting residue. The resulting slurry was filtered, and the filter cake was washed with hexanes (1.17 L), and dried to give 265 g of α-phenyl-α-pyridyl-(2)-acetonitrile as light brown crystals (90% yield based on 2-chloropyridine). 
     B. α-Phenyl-α-pyridyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetonitrile                264 g (1.362 moles)Conc. H.sub.2 SO.sub.4                0.33 L (6.226 moles)Water                0.98 L50% NaOH             0.77 L______________________________________ 
    
     Procedure: 
     The reactor was charged with conc. H 2  SO 4 , and cooled to ˜10° C. α-Phenyl-α-pyridyl-(2)-acetonitrile (from Example 3.A) was added portionwise, keeping the temperature below 30° C. The reaction was stirred at ambient temperature for 16 h. The reaction mixture then was cooled to 10° C., the water was added, and the NaOH was added to a pH of 12, keeping the temperature below 30° C. The resulting crystals were filtered, the filter cake was washed with water, and dried under vacuum to give 289 g (100%) of α-phenyl-α-pyridyl-(2)-acetamide. 
     C. d,l-erythro/threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________α-Phenyl-α-pyridyl-(2)-acetamide                289 g (1.365 moles)Platinum oxide (PtO.sub.2)                4.84 gAcetic acid          1.01 + 2.48 LCelite               300 + 150 gWater                2.48 + 0.62 + 1.24 L50% NaOH             0.33 L______________________________________ 
    
     Procedure: 
     The reactor was charged with α-phenyl-α-pyridyl-(2)-acetamide (from Example 3.B), acetic acid (1.01 L), and PtO 2 . The reactor then was filled with hydrogen gas, pressurized to 60 psi, and the mixture hydrogenated at room temperature for 16 h. The reaction mixture then was filtered through a pad of Celite (300 g) to remove the catalyst, and the Celite pad is washed with acetic acid (2.48 L). The filtrate was concentrated under reduced pressure. The resulting residue was dissolved in water (2.48 L), filtered through a pad of Celite (150 g), and the Celite pad was washed with water (0.62 L). The resulting filtrate was cooled to 10° C., followed by addition of 50% NaOH until the pH of the mixture was 13 and the free amine crystallized out. The crystals were filtered, washed with water (1.24 L), and dried to give 297 g (100%) of a 4:1 mixture of d,l-erythro-α-phenyl-α-piperidyl-(2)-acetamide and d,l-threo-α-phenyl-α-piperidyl-(2)-acetamide. 
     D. d,l-threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________Mixture of d,l-erythro-acetamide                297 g (1.36 moles)and d,l-threo-acetamidePotassium t-butoxide (95%)                354 g (2.996 moles)Toluene              8.92 LWater                1.49 + 0.74 + 3.00 LConc. HCl            0.59 + 0.30 L50% NaOH             0.36 L______________________________________ 
    
     Procedure: 
     A mixture of d,l-erythro-acetamide and d,l-threo-acetamide (from Example 3.C), potassium t-butoxide, and toluene was heated to 70° C. and stirred for 16 h. The reaction mixture was cooled to room temperature, followed by slow addition of water (1.49 L). Conc. HCl (0.59 L) was added to this resulting mixture, which was stirred for 0.5 h. The layers were separated, and the top organic layer was then washed with a prepared solution of conc. HCl (0.30 L) and water (0.74 L). The combined aqueous layers were cooled to 10° C., and 50% NaOH was added to a pH of 12 whereby the free base precipitated out. The crystals were filtered, washed with water (3.00 L), and dried to give 253 g (85%) of d,l-threo-α-phenyl-α-piperidyl-(2)-acetamide. 
     E. l-threo-α-Phenyl-α-piperidyl-(2)-acetamide 
     
         ______________________________________Materials:______________________________________d,l-threo-α-phenyl-α-piperidyl-                253 g (1.159 moles)(2)-acetamideDibenzoyl-L-tartaric acid                415 g (1.159 moles)Isopropanol          8.11 L6N HCl (aqueous)     1.67 LWater                1.0 LSolid NaCl           290 g50% NaOH (aqueous)   1.0 L______________________________________ 
    
     Procedure: 
     Dibenzoyl-L-tartaric acid and d,l-threo-α-phenyl-α-piperidyl-(2)-acetamide (from Example 3.D) is dissolved in isopropanol by warming the reaction mixture to ˜50° C. The resulting clear solution is stirred at ambient temperature for 16 h, whereby the tartrate salt of l-threo-acetamide crystallizes out. The crystals are filtered, and the filter cake washed with isopropanol and dried in a vacuum oven at 40° C. This tartrate salt is added in portions to a stirred solution of 6N aq. HCl, and the resultant slurry is stirred at ambient temperature for 4 h. The slurry is filtered, and the filter cake (containing free dibenzoyl-L-tartaric acid) is washed with water. Solid NaCl is added to the filtrate (which contains l-threo-acetamide hydrochloride salt) and the mixture is cooled to ˜10° C. The NaOH is added to this mixture to a pH of 12, whereby the free base of l-threo-acetamide precipitates out. The precipitated crystals are filtered, washed with water and dried to give l-threo-α-phenyl-α-piperidyl-(2)- acetamide. 
     F. l-threo-Methylphenidate Hydrochloride 
     
         ______________________________________Materials:______________________________________l-threo-α-phenyl-α-piperidyl-               101 g (0.46 moles)(2)-acetamideConc. H.sub.2 SO.sub.4               121 mLMethanol            1.1 LWater               0.81 L50% NaOH            175 mLDiethyl ether       1.0 + 1.0 + 1.0 + 1.0 LMagnesium sulfate   20 gHCl gas             As needed.______________________________________ 
    
     Procedure: 
     A solution of l-threo-α-phenyl-α-piperidyl-(2)- acetamide (from Example 3.E) and conc. H 2  SO 4  in methanol is heated to reflux and stirred for 2 days. The reaction mixture is cooled to room temperature and concentrated under reduced pressure. Water (0.81 L) and ether (1.0 L) are added to the residue. The NaOH is added to a pH of 12, and the layers are separated. The aqueous layer is extracted with ether (1.0 L). MgSO 4  is added to the combined ether layers, filtered, and washed with ether (1.0 L). HCl gas is passed through the filtrate with stirring, whereby white crystals of l-threo-methylphenidate hydrochloride precipitate out. The crystals are filtered, washed with ether (1.0 L), and dried to give l-threo-methylphenidate hydrochloride. 
     Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the present invention and that such changes and modifications may be made without departing from the spirit of the invention. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.