[0001] This invention relates to an economic process for the manufacture of a single isomer of a precursor to d-threo-methylphenidate.

[0002] Methylphenidate is a therapeutic agent that is widely used in the treatment of attention-deficient hyperactivity disorder. It is a controlled substance.

[0003] Methylphenidate was first prepared as a mixture of the erythro [R*S*] and threo [R*R*] racemates. U.S. Pat. No. 2,957,880 discloses studies upon the two racemic mixtures, which revealed that the therapeutic activity resides in the threo diastereoisomer. It is now considered that it is the d-threo [or (R,R)] enantiomer that has the preferred therapeutic activity. Uses of this enantiomer are disclosed in WO-A-9703671, WO-A-9703672 and WO-A-9703673, the contents of which are incorporated herein by reference.

[0004] The resolution of threo methylphenidate can be achieved using the expensive resolving agent 1,1′-binaphthyl-2,2′-diyl hydrogen phosphate, a process first reported by Patrick et al, The Journal of Pharmacology and Experimental Therapeutics, 241:152-158 (1987). More efficient resolutions, using a O,O′-diaroyltartaric acid or menthoxyacetic acid, are disclosed in WO-A-9727176 and in PCT/GB97/00643, the contents of which are incorporated by reference; in particular, the use of O,O′-di-p-toluoyltartaric acid allows the diastereoisomeric salts to be very readily separated, to give the desired enantiomer in high enantiomeric excess and high chemical purity.

[0005] In an alternative approach, disclosed in U.S. Pat. No. 2,957,880, the amide of erythro methylphenidate (i.e. as —CONH₂ instead of —CO₂Me) is resolved using tartaric acid. However, this resolution must be followed by amide hydrolysis, and equilibration at the benzylic centre, to give the threo isomer of the carboxylic acid which is esterified.

[0006] It would be desirable to find a satisfactory substrate for resolution that did not involve handling the active drug. Ritalinic acid might be a target, and is a common intermediate, in threo form, in synthesis preceding or following the two respective resolutions described above.

[0007] U.S. Pat. No. 2,957,880 discloses single isomer ritalinic acid hydrochloride. It is prepared (see Example 6) from the corresponding acid amide.

[0008] The present invention is based on the surprising discovery that, although ritalinic acid will not undergo any effective degree of resolution with any of a wide range of resolving agents, a salt thereof is an effective substrate for resolution, e.g. with a chiral base. In a particular preferred embodiment of the invention, threo-ritalinic acid hydrochloride is resolved with (−)-1-phenylethyl amine. The chiral base may form a novel double salt.

[0009] For the purposes of illustration at least, the salt that is the substrate for resolution according to this invention may be prepared by base hydrolysis of methylphenidate, using NaOH or another hydroxide (MOH). A suitable acid salt may then be prepared by adding an acid (HX) that releases M from the resultant salt (e.g. a metal or ammonium salt) of ritalinic acid. On passing the isoelectric point, it appears that the piperidine N atom is protonated. Alternatively, preparation of salts may be via acid hydrolysis of methylphenidate.

[0010] The resolution is conducted using conditions that are generally known in the art. Examples of suitable chiral bases are 1-phenylethylamine, and also 1-(1-naphthyl) ethylamine, cinchonine, cinchonidine and N-methyl-D-glucamine. The use of, say, (−)-1-phenylethylamine gives the preferred d-threo-enantiomer of ritalinic acid salt. That can be converted to d-threo-methylphenidate hydrochloride by reaction with methanol and HCl, with heating.

[0011] Salts that are substrates for resolution according to this invention have good or at least adequate solubility in various solvents, especially polar solvents, including aqueous systems. Adjustment of pH, e.g. by adding acid (which may be ritalinic acid), can enhance solubility.

[0012] The following Example illustrates the invention.