CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from International application No.

PCT/IN2011/000054 filed on 27 January 2011, the contents of which are incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a novel stereospecific synthesis of (-)(2S,3S)-1-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol an intermediate in the synthesis of tapentadol.

BACKGROUND OF THE INVENTION

Tapentadol is an analgesic which has been approved as tapentadol hydrochloride in US and EU for the treatment of acute pain and available in the market under the brand name Nucynta in the form of tablets in dosage strengths 50, 75 and 100 mg as equivalent to base. Tapentadol hydrochloride is chemically described as 3-[(lR,2R)-3-(dimethylamino)-l-ethyl-2-methylpropyl]phenol hydrochloride (herein after referred by its generic name tapentadol) and has the following structure (I):

U.S. Patent No. USRE39593E (US6248737B1) describes tapentadol and its related compound along with their pharmaceutically acceptable salts, a pharmaceutical composition and method of treatment. The US'593 patent discloses a process for the preparation of tapentadol hydrochloride, which is illustrated by the scheme below:

US application publication US20090326271A1 describes a process for the preparation of tapentadol hydrochloride which is illustrated by the below scheme

US application publication US2010099916A1 describes a process for the preparation of tapentadol hydrochloride which is illustrated by the below scheme


Aforementioned process for the preparation of intermediate compound of formula (III) involves separation of optical isomers by chiral HPLC, which is tedious, involves the use of larger volumes of organic solvents and results in very low yields thus making the process not ecofriendly and not suitable on commercial scale. Moreover the unwanted isomers cannot be racemised and recycled.

Hence, there is a need to provide a process that would avoid the aforementioned steps like chiral HPLC and that could be well applicable on commercial scale and also the unwanted isomers can be racemised and recovered thus resulting in high yield and purity of the desired isomer.

The process of present invention allows obtaining the said intermediate (III) of tapentadol with tremendous yields, purities that too under environmentally acceptable conditions which is more convenient and more efficient than the previously known~methods.

SUMMARY OF THE INVENTION

The present invention relates to a novel stereospecific synthesis of (-)(2S,3S)-1-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol an intermediate in the synthesis of tapentadol.

In one aspect, the present invention relates to a process for the preparation of (-)(2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol of formula III reacting the compound S(+)-1,1-dimethyl amino-2-methyl pentan-3-one of formula (V)

Where X is halogen (F, Cl, Br, I)
to afford the compound (-)(2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan- 3-ol of formula (III).

In another aspect, the present invention relates to racemisation process for the conversion of (+) or (-) -l,l-dimethylamino-2-methyl pentan-3-one (V or Va) into the racemic compound comprising :

a) reacting (+) or (-) -l,l-dimethylamino-2-methyl pentan-3-one compound of formula (V or
Va) with a base optionally in the presence of a solvent or a mixture of solvents; and,
b) recovering the racemic mixture compound (VI) in pure form.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1: is a schematic representation of the process of the present invention. DETAILED

DESCRIPTION OF THE INVENTION

The present invention relates to a novel stereospecific synthesis of (-)(2S,3S)-1-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol an intermediate in the synthesis of tapentadol.

In one embodiment, the present invention provides a process for the preparation of (-) (2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol of formula III reacting the compound S(+)-1,1 -dimethyl amino-2-methyl pentan-3-one of formula (V)

Where X is halogen (F, CI, Br, I)
to afford the compound (-)(2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan- 3-ol of formula (III).

The compound of formula (V) can also be prepared by the process described in the prior art for eg. Chirality 6: 389-399 (1994).

The reaction step of conversion of formula V to the intermediate compound of formula III comprises of subjecting the compound 3-bromo anisole to Grignard reaction consisting of magnesium, catalytic amount of iodine, and solvent ether to afford compound of formula IV followed by reaction with the compound of formula V to afford the compound of formula III.

The reaction step comprising Grignard reaction followed by condensation are carried out successively in a one pot.

The solvents employed is selected from the group consisting of hydrocarbons such as toluene, xylene, n-hexane, n-heptane, cyclohexane and the like; ethers such as tetrahydrofuran (THF), 1,4-dioxane, diethyl ether, diisopropyl ether and the like; halocarbonated solvents such as methylene chloride, ethylene chloride, chloroform, carbon tetrachloride, dichlorobenzene and the like; esters such as ethyl acetate, isopropyl acetate, n-butyl acetate, tert-butyl acetate and the like; or mixtures of thereof. Preferably tetrahydrofuran (THF) is being used.

The reaction temperatures and time in step a) should be suitable to bring the reaction to completion at a minimum time, without the production of unwanted side products or impurities. The reaction temperature can be from about 0°C to about 100°C or boiling point of the solvent(s) used. Preferably at about 25°C.

The reaction time can vary in dependence on various parameters, such as, for example, temperature, pressure, nature of the compound to be reacted or the nature of the reagent and solvent(s) employed, and can be determined for the process in question by the person skilled in the art using preliminary tests.

The time period can be from about 15mins. to about 10 hours, preferably from about 30mins. to about 2 hours.

The molar equivalent of compound of formula IV and reagent used can be from about 0.25 to about 7 molar equivalents on the weight of the compound of formula V taken. Preferably 3 moles.

This reaction step is highly stereoselective, because the optical purity of the intermediate compound of formula V is greater than about 99% by chiral HPLC.

The particular intermediate compounds of formula (V) and (III) can be purified by conventional methods known to the person skilled in the art.

The intermediate compound of formula (III) obtained by the above described process of present invention can be further converted into Tapentadol hydrochloride of formula I by processes described in the art. Illustratively, by the process described in U.S. Patent No. USRE39593E which is incorporated herein by reference in its entirety.

The compound of formula (III) obtained by the process of present invention described above is useful as an intermediate in the synthesis of various active pharmaceutical ingredients. For ex. Tapendaol.

The process reported for the preparation of compound of formula (III) in the above mentioned reference U.S. Patent No. USRE39593E is by chiral HPLC separation method which is tedious and not suitable on commercial scale, moreover results in the desired" isomer with very low yield and purity.

In another embodiment, the present invention relates to racemisation process for the conversion of (+) or (-) -l,l-dimethylamino-2-methyl pentan-3-one (V or Va) into the racemic compound comprising:

a) reacting (+) or (-) -l,l-dimethylamino-2 -methyl pentan-3-one compound of formula (V or Va) with a base optionally in the presence of a solvent or a mixture of solvents; and,

b) recovering the racemic mixture compound (VI) in pure form.

The base that can be used in step a) include organic base or inorganic base. Inorganic bases such as sodium hydroxide, potassium hydroxide, potassium tert-butoxide sodium carbonate, potassium carbonate, sodium bicarbonate, aqueous ammonia and the like; The organic bases that can be used include, but are not limited to triethylamine, tripropylamine, pyridine, diisopropylamine, diisopropylethylamine and the like or mixtures thereof. Preferably aqueous sodium hydroxide is being used

The solvent that can be used optionally include but are not limited to hydrocarbons such as toluene, xylene, n-hexane, n-heptane, cyclohexane and the like; halogenated solvents such as methylene chloride, ethylene chloride, trichloroethylene, chloroform, chlorobenzene, dichlorobenzene and the like; esters such as ethyl acetate, isopropyl acetate, n-butyl acetate, tert-butyl acetate and the like; ethers such as diethyl ether, diisopropyl ether, tetrahydrofuram, 1,4-dioxane, methyl tertiary butyl ether and the like; aprotic polar solvents such as dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile or mixtures thereof. Preferably methylene chloride.

The reaction temperature range can be from about 30°C to about 100°C, or 'boiling point of the solvents used. Preferably about 30°C.

The pH in the reaction step a) can be adjusted from about 7 to about 12, preferably about 12.

Advantageously, the process of present invention described herein has simple reaction steps, produces the intermediate surprisingly in high yields and purities than the processes reported in the literature and well amenable on commercial scale.

As used herein, the term "HPLC" refers to High-performance liquid chromatography.

As used herein, the term "% area by HPLC" refers to the area in an HPLC chromatogram of one or more peaks compared to the total area of all peaks in the HPLC chromatogram expressed in percent of the total area.

In this specification the term "racemic mixture" may include mixtures of enantiomers in ratios other than, as well as, a 50:50 mixture of R:S enantiomers (for example from 99:1 to 1:99). A particular process of the invention begins with a 50:50 mixture of enantiomers. The process may involve differing mixtures of enantiomers at various stages (including, but not limited to 50:50 mixtures). The term "racemisation" covers the conversion of an unresolved enantiomer into a mixture containing the enantiomer to be resolved.

The present invention provides a simple, ecofriendly, costeffective, reproducible, robust, commercially suitable process for preparation intermediate of tapentadol hydrochloride.

The following examples are provided to enable one skilled in the art to practice the invention and are merely illustrative of the invention. The examples should not be read as limiting the scope of the invention as defined in the claims, appended herewith.

EXAMPLES Example -1: Preparation of (-) (2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol (III)

(S)-(+)-l,l-dimethylamino-2-methyl pentan-3-one (V) (42gms) (SOR: +95°) was added to 3-Methoxyphenylmagnesium bromide (IV) (prepared from 166gms of 3-Bromo anisole and 21.3 gms of magnesium in 126 ml of tetrahydrofuran) drop-wise for 90-120 min. at about 25°C and maintained for 30 min. Then the resulted solution was cooled to 5°C. Then added water slowly at 5-10°C.Added 157 ml. of acetic acid to adjust the pH=4.0 at 5-10°C. The aqueous layer was basified (PH = 8.0) with 80 ml. of aqueous sodium hydroxide solution. The organic layer was separated. The aqueous layer was extracted with (3x200 ml.) of toluene and distilled under high vacuum to obtain a residue.

Wt: 92 gms. (%Yield: 99%); Purity by GC: 78.9%.

b) To 94 gms. of the residue obtained above, added 470 ml. of water and the pH was adjusted to about 4 with 212 ml. of acetic acid. The layers were separated and the aqueous layer was washed with dichloromethane (4x50 ml.). Then, the aqueous layer PH was adjusted to 12 with 180 ml. of aqueous sodium hydroxide solution at ambient temperature and the product was extracted from aqueous layer with ethyl acetate (4x100 ml.). Finally the ethyl acetate layer was distilled to obtain the title compound as residue.
Yield : 86 gms (%Yield:94%) ; Purity by GC: 98% ; SOR= (-) 20° (C=l% CH3OH).

Example-2: Racemisation of S-(+)-l,l-dimethylamino-2-methyl pentan-3-one (V)

Taken 100 gms. of S-(+)-l,l-dimethylamino-2-methyl pentan-3-one (V) (SOR : + 98°) (Purity by GC: 98%) and 930 ml. of water and added 80 ml. of aqueous sodium hydroxide solution (50%) and maintained for 30 min. followed by extracted with dichloromethane (4 x 100 ml). The organic layer was separated and distilled to provide the compound (VI). Yield: 95 gms (% Yield: 95%); SOR: 0° (C=l% in CH3OH). Example-3: Racemisation of R-(-)-l,l-dimethylamino-2-methyl pentan-3-one (Va)

Taken 100 gms of R-(-)-l,l-dimethylamino-2-methyl pentan-3-one (Va) (SOR : - 98°) (Purity by GC: 98%) and 930 ml. of water and added 80 ml. of aqueous sodium hydroxide solution (50%) and maintained for 30min. followed by extracted with dichloromethane (4x100ml). The organic layer was separated and distilled to provide the compound (VI). Yield: 92gms (% Yield: 92%); SOR: 0° (C=l% in CH3OH).

We Claim:

1) A process for preparing (-)(2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol compound of formula III

reacting the compound S(+)-1,1 -dimethyl amino-2-methyl pentan-3-one of formula (V) with the compound of formula (IV)
Where X is halogen (F, C1 , Br, I)

to afford the compound (-)(2S,3S)-l-dimethylamino-3-(3-methoxyphenyl)-2-methyl pentan-3-ol of formula (HI).

2) The process of claim 1, wherein the solvents employed is selected from the group consisting of ethers like tetrahydrofuran (THF), 1,4-dioxane, diethyl ether, halo carbonated solvents methylene chloride, ethylene chloride, chloroform, carbon, tetrachloride, dichlorobenzene, esters such as ethyl acetate, isopropyl acetate, n-butyl acetate, tert-butyl acetate or mixtures of thereof, preferably tetrahydrofuran.

3) The process of claim 1 wherein the reaction temperature and time is from about 0°C to about 100°C or boiling point of the solvents used, preferably at about 25°C and the time period can be from about 15 mins. to about 10 hours, preferably from about 30mins to about 2 hours.

4) The process of claim 1, wherein the molar equivalent of compound of formula IV and reagent used can be from about 0.25 to about 7 molar equivalents on the weight of the compound of formula V taken, preferably 1 mole.

5) A process for the racemisation of (+) or (-) -l,l-dimethylamino-2- methyl pentan-3-one (V or
Va) into the racemic compound comprising:

a) reacting (+) or (-) -l,l-dimethylamino-2-methyl pentan-3-one compound of formula (V or Va) with a base optionally in the presence of a solvent or a mixture of solvents; and,

b) recovering the racemic mixture compound (VI) in pure form.

6) The process of claim 5, wherein the base used is selected from the group consisting of inorganic bases like sodium hydroxide , potassium hydroxide, potassium tert-butoxide sodium carbonate, potassium carbonate, sodium bicarbonate, aqueous ammonia, organic bases like triethylamine, tripropylamine, pyridine, diisopropylamine, diisopropylethylamine or mixtures thereof, preferably aqueous sodium hydroxide.

7) The process of claim 5, wherein the solvent that can be used optionally include but are not limited to halogenated solvents like methylene chloride, ethylene dichloride, chloroform, chlorobenzene, dichlorobenzene, esters such as ethyl acetate, isopropyl acetate, n-butyl acetate, tert-butyl acetate, or mixtures thereof, preferably methylene chloride.

8) The process of claim 5, wherein the reaction temperature in step a) can range from about 30°C to about 100°C, or boiling point of the solvents used, preferably about 30°C.