The present invention relates to a novel process for preparing narcotic analgesics such as hydrocodone and hydromorphone using catalytic amounts of homogeneous organometallic complexes.

Hydrocodone and hydromorphone are both semi-synthetic opioid analgesics with multiple actions qualitatively similar to those of codeine and morphine, involving the central nervous system and smooth muscle. The precise mechanism of action of these opiates is not known, although it is believed to relate to the existence of opiate receptors in the central nervous svstem.

Both hydrocodone and hydromorphone have been prepared in the past by a variety of processes including hydrogenation of codeinone (.Axch. Pharm. ( 1 20), 2__8, 295). oxidation of dihydrocodeine (DE 415097: US 2715626), oxidation of dihydromorphine (J. Org. Chem. (1950) 15, 1 103, US 2628962: US 2654756; US 2649454), by electrolytic reduction of morphine (J. Pharm. Soc. Japan (1936), ≤6, 44 and (1942), 6 _. 2, 347) or by catalytic rearrangement of either codeine or morphine (DE 623821, Appl. Radiat. Isot. (1987) 18, 651 ). However, all these processes involve a two-step process which is not cost-effective.

Catalytic rearrangement of morphine has been described using palladium black. It is not possible, however, to carry out this process on a large scale that would be suitable for manufacturing since the procedure affords 30-35% of the undesired o-desmethylthebainone along with the desired product. Isolation of the pure product is very tedious and requires extensive purification.

The present inventors have now found a new one-step route to hydrocodone and hydromorphone giving greater than 80% yield requiring minimal purification by simple methods.

Accordingly, the present invention provides a novel process for preparing a compound of formula (I)

wherein R ¹ is hydrogen, methyl or a protecting group typically used to protect phenols or alcohols; R ² is hydrogen, methyl, allyl, cyclobutylmethyl, benzyl, trialkylsilyl or an amine protecting group; R ³ is hydrogen, hydroxyl or an amino group;

comprising reacting a compound of formula (II)

wherein R ¹ , R ² and R ³ are as hereinbefore defined with an organometallic complex of formula (III)

wherein M is a metal; each Ar is an aryl group, n is from 2 to 5, X is BF ₄ ^" or ClO ₄ ^* and solv is methanol, methvlene chloride/methanol. tetrahvdrofuran or ethanol.

Examples of alcohol or phenol protecting groups within the definition of R ¹ include ethers (such as methoxymethyl, benzyloxymethyl, 2-(trimethylsilyl)-ethoxymethyl ether, tetrahydropyranyl, phenacyl, allyl, triethylsilyl, t-butyldimethylsilyl), esters (such as acetate, pivaloatc. benzoate). carbonates (such as benzyl, methyl) and sulphonates (such as methanesulphonate, toluenesulphonate).

Examples of amine protecting groups within the definition of R ² include carbamates (such as methyl, 2,2,2-trichloroethyl, 2-methylsilyl, triethylsilyl, t-butyl, benzyl), amides (such as formyl, acetyl, benzoyl, cyclobutyl), sulphonates and amino acetal derivatives.

Suitably, R ¹ is hydrogen or methyl.

Suitably, R ² is hydrogen or methyl, preferably hydrogen.

Suitably, R ³ is hydrogen.

Suitably, M is rhodium, palladium, platinum, iridium or iron, preferably rhodium.

Suitably, Ar is a phenyl group or a cyclohexyl group. Preferably Ar is a phenyl group.

Preferably, n is 4.

The reaction is suitably carried out in a polar solvent or mixture of solvents, for example methanol, methanol/methylene chloride, tetrahydrofuran, acetone or ethanol. Preferably the reaction is carried out in a mixture of methanol/methylene chloride.

Suitably the reaction is carried out at a non-extreme temperature of from 5°C to 50 ^C C. Preferably the reaction is carried out at ambient temperature, for example 25 °C to 27°C.

Compounds of formula (II) are commercially available and compounds of formula (III) may be prepared by reaction of an organometallic compound with a compound of formula (IV)

using an analogous procedure to that described in Bosnich et al. J. Am. Chem. Soc. (1991 ). 1 13. 958 and Organometallics ( 1988), 7, 936.

The present invention will now be described by way of example only and is not intended to be limiting thereof.

EXAMPLE 1 Large Scale Preparation of Hydrocodone

A 2 litre four-necked round -bottomed flask equipped with a mechanical stirrer, gas sparging tube, gas outlet and a thermometer was flushed with nitrogen and charged with anhydrous methanol (500ml). The solvent was de-oxygenated by bubbling nitrogen through it for 10 to 15 minutes. Bis(bicyclo[2.2.1.]hepta-2,5-diene)rhodium(I) tetrafluoroborate ( 1.88g, 0.005M) and 1 ,4-bis(diphenylphosphino)butane (2.17g, 0.0051 M) were added under nitrogen and the solution was stirred for 30 minutes at ambient temperature. The solution was hydrogenated by sparging hydrogen gas for 30 minutes; at this point, the colour of the solution changed from orange to tan-yellow.

Excess hydrogen was removed by bubbling nitrogen through the solution for 10 minutes. Next, codeine (150g, 0.5M) was added to the stirred solution under a stream of nitrogen. After stirring the reaction mixture for 5 to 15 minutes, hydrocodone started to precipitate out as fine crystals, methylene chloride (300ml) was added to dissolve all the

solids. The dark-red coloured homogeneous solution was stirred for 1 hour. After completion of the reaction, the solution was concentrated under vacuum at ambient temperature to about half of its original volume. The precipitated product was filtered, slurried on the filter with cold methanol ( 100ml), washed with methanol ( l xl 00ml), and dried under vacuum for 16 hours at 60 to 70°C to afford 124.5g (83%) of hydrocodone as a free base.

'H NMR (CDC1 ₃ ): δl.26 (qd, J = 4.3 Hz and 13.0 Hz, 1H); 1.75-1.90 (m, 2H); 2.06 (td, J - 4.6 Hz and 12.0 Hz, 1H); 2.20 (td, J = 3.4 Hz and 12.0 Hz, 1H); 2.30 (dd, J = 6.0 Hz and 18.5 Hz. 1H); 2.36 (td, J = 4.7 Hz and 13.7 Hz, 1H); 2.38-2.48 (m. 1 H); 2.43 (s. 3H); 2.51 - 2.61 (m. 2H); 3.03 (d. J = 18.5 Hz, 1H); 3.17 (dd. J = 2.8 and 5.4 Hz. 1H); 3.91 (s. 3H); 4.65 (s, 1H): 6.63 (d, J = 8.2 Hz, 1 H); 6.70 (d, J = 8.2 Hz. 1H).

^{I 3} C NMR (CDCI ₃ ): δ 19.8; 25.3; 35.3; 40.0; 42.4; 42.7; 46.58: 46.65: 56.6: 58.9: 91.2; 1 14.5; 1 19.5; 126.2; 127.2; 142.5: 145.2; 207.5.

EXAMPLE 2 Preparation of Hydromorphone

A 100ml three-necked round-bottomed flask equipped with a magnetic stirrer, gas inlet, gas outlet and a thermometer was flushed with nitrogen and charged with anhydrous methanol (10ml) and methylene chloride (5ml). The solvent was de-oxygenated by bubbling dry nitrogen through it for 10 minutes. Under a stream of nitrogen. bis(bicyclo[2.2.1.]hepta- 2,5-diene)rhodium(I)tetrafluoroborate (112mg, 0.0003M) and 1 ,4-bis(diphenyl- phosphino)butane ( 130mg, 0.00031 M) were added and the orange solution was stirred for 15 minutes at ambient temperature. Next, the solution was hydrogenated by bubbling hydrogen gas for 10 minutes. Excess hydrogen was removed by bubbling nitrogen through the solution for 5 minutes. The solution of the catalyst was warmed to 40°C and morphine (2.14g. 0.0075M) was added under a moderate stream of nitrogen. The dark coloured solution was stirred for 4 hours at 40°C. The Η NMR of the crude indicated 88% of hydromorphone. 8% of unreacted morphine and 3 to 4% of an unidentified by-product.

After removal of the solvents under vacuum, the crude hydromoφhone was isolated by flash chromatography (ethyl acetate/methanol 3:1 ) as a brown solid (1.21g, 56%) which was purified by recrystallisation from ethanol (25ml) to give >98% pure hydromoφhone (0.75g, 35%).

'H NMR (CDCl _r CD ₃ OD -10:1 ): δ 1.25 (qd, J = 4.5 Hz and 13.0 Hz, 1 H); 1.79 (ddd, J = 1.8, 3.4 and 12.3H, 1H); 1.88 (dq, J = 4.1 and 13.0 Hz, 1 H); 2.09 (td, J = 4.7 Hz and 12.3 Hz, 1H) 2.25 (td, J = 3.5 Hz and 12.1 Hz, 1H); 2.35 (dd, J - 5.6 Hz and 18.6 Hz. 1H); 2.38- 2.53 (m. 2H); 2.44 (s, 3H); 2.55-2.64 (m, 2H); 3.03 (d, J = 18.6 Hz, 1H); 3.19 (dd, J = 2.7 and 5.6 Hz, l H); 4.67 (s, 1 H); 6.61 (d, J = 8.1 Hz, 1H); 6.70 (d, J = 8.1 Hz, 1H).

^{, 3} C NMR (d6-DMSO): δ 19.5; 25.0; 34.8; 39.6; 41.4; 42.5; 46.2; 46.3; 58.3; 90.3; 1 16.9; 1 19.1 ; 124.4; 127.3; 139.2; 143.9; 208.6.