FIELD OF THE INVENTION

The present invention relates to an improved process for O-demethylating methoxy substituted morphinan-6-one derivatives using AlCl₃as a demethylating agent in a reaction-inert solvent having a water content ranging from about 0.1% wt to about 0.8% wt.

BACKGROUND OF THE INVENTION

The morphinan compounds are a group of structurally related alkaloids that can act as opiate receptor agonists or opiate receptor antagonists. Opiate receptor agonists such as e.g. morphine, codeine, hydrocodone, oxycodone, hydromorphone, oxymorphone, and nalbuphine, are of use as analgesics for pain relief. Compounds such as nalmefene, naltrexone and naloxone are opiate receptor antagonists useful for the treatment of substance abuse or to reverse the effects of opiate agonists.

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A process for preparing 14-hydroxymorphinone by O-demethylating 14-hydroxy-codeinone is described by Ulrich Weiss in Journal of Organic Chemistry, p. 1505-1508 (1957), by brief treatment of 14-hydroxycodeinone with a concentrated aqueous solution of hydrobromic acid at 120Â° C., removal of non-phenolic material by extraction with chloroform from alkaline medium, and extraction of the phenolic reaction products with chloroform or chloroform-ethanol at pH=8-9. The reported yields are modest and variable ranging between 35% and 52%. Also undesired hydration of the double bond in 14-hydroxymorphinone was also observed.

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Hence there is a need for a more efficient process with a higher yield and little or no formation of the undesired 8,14-dihydroxydihydromorphinone.

Another process for process for preparing 14-hydroxymorphinone by O-demethylating 14-hydroxy-codeinone is described by Zhang A. et al. in Organic Letters, vol. 7, no. 15, 3239-3242 (2005) by treatment of 14-hydroxycodeinone with BBr₃in anhydrous dichloromethane. According to the Supporting Material of the Zhang paper 14-hydroxymorphinone was obtained as a pure white solid with a 70% yield. A feasibility study was done to confirm the yield of said described procedure and to check the quality of the obtained material. The procedure in the Zhang paper could not be reproduced successfully. It was found that full conversion to 14-hydroxymorphinone could not be found even after elongated stirring up to 4 days and chromatographic analysis showed large amounts of different byproducts. Following the reported quench procedure and subsequent extraction procedure in accordance with the Zhang paper, the yield was only 15-25% crude product with -85% purity that contained up to 10% unreacted 14-hydroxycodeinone. LC-MS analysis of the reaction mixture showed that during the aqueous work-up procedure HBr was formed when unreacted BBr₃was hydrolysed resulting in a side reaction of HBr addition to the double bond.

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Better results were achieved by quenching the reaction mixture into an aqueous borate buffer solution at pH 8.5 to 9.0, which was held in the appropriate pH-range by addition of an aqueous ammonia solution. This procedure delivered crude yields of 72% to 81%, however also under these conditions purities were only between 75% and 85% and still 3.5% to 11% unreacted 14-hydroxycodeinone was present in the end-product.

Hence there is a need for a more efficient process with a higher yield whereby little or no unreacted starting material is found after work-up procedures. Additionaly the use of the highly corrosive BBr₃, which releases HBr fumes very easily upon contact with water, should be avoided.

The use of AlCl₃as a demethylation agent is described by Asnawi A. et al. in ITB J. Sci., vol. 43a, no. 1, 43-50 (2011) and by Burwell R. L. in Chem. Rev., 615-685 (1954).

It has now been found that O-demethylation of methoxy substituted morphinan-6-one derivatives of formula (II), e.g. 14-hydroxycodeinone, can be performed with a higher yield, higher purity, and little or no undesired hydration of the double bond, using AlCl₃as the demethylating agent in a reaction-inert solvent having a water content ranging from 0.1% wt to 0.8% wt.

DESCRIPTION OF THE INVENTION

In one aspect, the present invention relates to a process for preparing a compound of formula (I), which is characterized by the steps of

O-demethylating a compound of formula (II) in a reaction-inert solvent having a water content ranging from 0.1% wt to 0.8% wt using a demethylating agent wherein said demethylating agent is AlCl₃,

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- wherein
- R¹is hydrogen, C_1-6alkyl, C_2-6alkenyl, or CH₂substituted with C_3-6cycloalkyl; and
- R²is hydrogen, C_1-4alkyl, C_1-4alkylcarbonyl, C_1-4alkyloxycarbonyl, benzyl, or benzoyl.

As used in the foregoing definitions:

- C_1-4alkyl defines straight and branched chain saturated hydrocarbon radicals having from 1 to 4 carbon atoms such as, for example, methyl, ethyl, propyl, butyl, 1-methylethyl, 2-methylpropyl and the like;
- C_1-6alkyl is meant to include C_1-4alkyl and the higher homologues thereof having 5 or 6 carbon atoms, such as, for example, 2-methylbutyl, pentyl, hexyl and the like;
- C_3-6cycloalkyl is generic to cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl;
- C_2-6alkenyl defines straight and branched chain unsaturated hydrocarbon radicals having from 2 to 6 carbon atoms, such as ethenyl, propenyl, butenyl, pentenyl or hexenyl.

To provide a more concise description, some of the quantitative expressions given herein are not qualified with the term â€œaboutâ€ . It is understood that whether the term â€œaboutâ€ is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value.

To provide a more concise description, some of the quantitative expressions herein are recited as a range from about amount X to about amount Y. It is understood that where a range is recited, the range is not limited to the recited upper and lower bounds, but rather includes the full range from about amount X through about amount Y, or any amount or range therein.

Interesting processes of the present invention are those wherein one or more of the following restrictions apply to the compounds of formula (II):

a) R¹is hydrogen; or

b) R¹is C_1-4alkyl; or

c) R¹is methyl; or

d) R²is hydrogen; or

e) R¹is methyl and R²is hydrogen; or

f) R¹is hydrogen and R²is hydrogen; or

g) R¹is CH₂-cyclopropyl and R²is hydrogen; or

h) R¹is â€”CH₂â€”CH=CH₂and R²is hydrogen.

The reaction-inert solvent can be any solvent such as e.g. toluene, fluorobenzene, trifluorotoluene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichloro-benzene, benzene, tetrachloromethane, trichloromethane, dichloromethane, nitrobenzene, 1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, 1-chloro-4-nitrobenzene, tetraalkylammonium chloride, tetraalkyl-ammonium bromide, or a mixture thereof In an embodiment the reaction-inert solvent is preferably nitrobenzene.

It has been found that the presence of a small amount of water in the reaction-inert solvent reduces the formation of chlorinated side-products. In practice the reaction-inert solvent comprises an amount of water ranging from about 0.1% wt to about 0.8% wt.

When the solvent is nitrobenzene, the amount of water typically ranges from about 0.2% wt to about 0.3% wt.

The process of the present invention can be performed at a temperature between 20Â° C. and the reflux temperature of the reaction-inert solvent. When nitrobenzene is used as the reaction-inert solvent, the process is performed at a temperature between about 20Â° C. and about 150Â° C. In an embodiment the temperature can range between about 60Â° C. and about 90Â° C., or between about 70Â° C. and about 80Â° C.

The amount of demethylating agent AlCl₃ranges from about 2 mol to about 4 mol with respect to the compound of formula (II). If less AlCl₃is used the reaction takes longer and may not run to completion. If more AlCl₃is used, more side-products are formed. In an embodiment the amount of AlCl₃is preferably about 3.2 mol with respect to the compound of formula (II).

Experimental Part

EXAMPLE 1Preparation of Compounds of Formula (I)

A 350 ml 4-neck flask was equipped with an overhead stirrer, thermostat, thermometer and dropping funnel. Under a nitrogen atmosphere AlCl₃(26.8 g, 201 mmol) was added in small portions to nitrobenzene (110.1 g, with a water content of 0.27 wt %) and the resulting mixture was heated up to 50Â° C. until a clear solution was obtained (50 minutes) and was then allowed to cool to a temperature of 25Â° C.

14-Hydroxycodeinone (20 g, 62.8 mmol) was slowly added in portions over a period of 30 minutes. The resulting mixture was heated up to 70Â° C. and stirred until complete (14 hours). The reaction mixture was cooled down to 25Â° C. over 30 minutes and an aqueous mixture of acetonitrile (10% wt in water, 150 ml) was added, while stirring for a further 60 minutes. The resulting suspension was filtered and the filtration cake was washed with methylisobutylketone.

The crude product was suspended in methanol and heated up to reflux. The mixture was cooled and the solid residue was isolated by filtration at 0Â° C. and washed with acetone.

The obtained product was suspended in water, titrated with triethylamine to a pH value of 8.8-9.2 at a temperature of 20Â° C. to 25Â° C. After filtration the solid product was washed with water and dried in vacuo to obtain 14-hydroxymorphinone with a yield of 72%. HPLC analysis of the isolated 14-hydroxymorphinone was done using the following analytical procedure.

Analytical Method:

Instrument : Agilent 11011200 series HPLC or equivalent

- DAD detector or equivalent

Sample solvent: 10 g of H3PO4 (85 wt %) in 1000 g of water

Column: Interchim Uptisphere Strategy RP, 3 Î¼m, 250Ã—4.6 mm,

- Serial No. 1217919 or equivalent

Cycle time : 86 minutes

- Run time (Analysis time) 70 minutes
- Equilibration time (Post time) 15 minutes

Injection volume : 5 Î¼l, with needle wash, wash solution: acetonitrile/water (1/1)

Mobile phases: A: water (HPLC grade)

- B: acetonitrile
- C: 50 g of H₃PO₄(85 wt %)/1000g of water

Gradient:

Time (minutes)% A% B% C073.53.023.5473.53.023.51071.55.023.51766.510.023.53261.515.023.55051.525.023.57013.063.523.570.173.53.023.58573.53.023.5

Flow rate: 0.600 ml/minute

Column Temperature: 30Â° C.

Detection: UV (205 nm)

HPLC purity of obtained 14-hydroxymorphinone: 99.6%

No formation of 8,14-dihydroxydihydromorphinone was observed.