WO1994005686A1

WO1994005686A1 - Anomerisation process

Info

Publication number: WO1994005686A1
Application number: PCT/GB1993/001857
Authority: WO
Inventors: Richard Thomas Walker; Paul Leslie Coe; David Ian Ward
Original assignee: University Of Birmingham
Priority date: 1992-09-04
Filing date: 1993-09-03
Publication date: 1994-03-17
Also published as: AU4973293A

Abstract

A process for producing the α- or β-anomer of a nucleoside, which process comprises anomerisation of the β- or α-anomer of a nucleoside using an anhydride and a strong acid.

Description

ANOMERISATION PROCESS

This invention relates to a process for the

production of the α- or ß-anomers of nucleosides, including 4'-thio-sugar nucleosides.

Nucleosides have two forms which differ in the steric configuration at the carbon-1' atom of the sugar. These forms are known as α- and ß-aanomers. The two anomers usually differ in their biological activity and naturally occurring nucleosides are generally ß-anomers.

Nucleosides and thionucleosides are known to be useful as, for example, anti-cancer or anti-viral agents. For example, EP-A-272065 describes a number of 2'-deoxy-5-substituted pyrimidine compounds for use as anti-viral agents. EP-A-409575 describes 2^,-deoxy-pyrimidine-4'-thionucleosides of formula (A) for use as anti-viral agents:

wherein Y is hydroxy or amino, and X is chloro, bromo, iodo, trifluoromethyl, C_2-6 alkyl, C_2-6 alkenyl, C_2-6 haloalkyl or C_2-6 alkynyl and physiologically functional derivatives thereof.

Further pyrimidine 4^,-thionucleosides and their uses are described in EP-A-421777.

The preparation of a nucleoside or derivative thereof often results in the production of a mixture of the α- and ß-anomers. It is often difficult to obtain just the desired anomer, which is usually the ß-anomer. For example, in EP-A-409575 referred to above, the ß-anomer of the compound of formula (A) is preferred but often a lower yield of the ß-anomer is obtained compared with the yield of the corresponding α-anomer.

Accordingly, this invention provides a process for the production of the α- or ß-anomer of a nucleoside, said process comprising anomerisation of the ß- or α-anomer of a nucleoside using an anhydride and a strong acid.

The anomerisation reaction is believed to be catalysed by an "intermediate" which is a reaction product of the strong acid and anhydride. In order to increase the rate of reaction it is preferred that the process of the invention is carried out in two steps of (i) reacting together the anhydride and strong acid, and (ii) mixing the reaction product thereby obtained with the starting nucleoside. Step (i) may be effected by adding the anhydride to a solution of the acid, by mixing the acid and the anhydride with the solvent or,

preferably, by adding the acid to a solution of the anhydride in an inert solvent (e.g. acetonitrile or dichloromethane). Step (i) is suitably carried out at ambient temperature (i.e. 20°C). Heating the reaction mixture would tend to destroy the intermediate produced by reaction of the strong acid and anhydride. The time duration of step (i) may be, for example, about 5 minutes.

However, the intermediate may also be generated in situ and the nucleoside, anhydride and acid may be added to the reaction system simultaneously or separately in any order. For example, the acid may be added last to a solution already containing the nucleoside and anhydride. In this case, when operating at laboratory scale with sulphuric acid as the acid it is preferred that the acid is dropped from above into the solution using a syringe, for example, a 22 guage syringe and a height of 2 to 3 cm. Dropping the acid from this height is believed to allow the acid drop to "stay together" rather than disperse when it enters the solution, which is believed to generate localised conditions favourable to the production of the intermediate.

The chemical nature of the intermediate is not known, but is postulated to be a mixed anhydride between the acid and the anhydride of a type referred to in Bourne et al, Journal of the Chemical Society, March 1951, (155), pp 718-720. Any combination of an acid and an anhydride capable of

producing the reactive intermediate may be used in the process of the invention.

The mechanism of the reaction is unknown, but a postulated mechanism is shown below. "CH₃CO⁺" represents the intermediate. The sugar moiety is shown with acetyl protecting groups. Postulated Mechanism

Mechanism (A) gives the desired anomerisation. Mechanism (B) gives an open chain form of the sugar which is a "dead end" and the reaction conditions are therefore generally chosen to avoid mechanism (B).

The starting nucleoside which is anomerised may, for example, be of the formula (i)

wherein

B is a purine or pyrimidine base,

Y is oxygen or sulphur,

Z¹ is hydroxy or OR¹ wherein R¹ is a hydroxy protecting group, Z² is hydrogen, hydroxy, fluoro, azido or OR² wherein R² is hydroxy protecting group, and

Z³ is hydrogen, hydroxy or OR³ wherein R³ is a hydroxy

protecting group.

Preferably, Z¹ is hydroxy or OR¹ wherein R¹ is a hydroxy protecting group, Z² is hydrogen, hydroxy, or OR² wherein R² is a hydroxy protecting group, and Z³ is hydrogen.

It is generally preferred that the nucleoside produced is the ß-anomer. Nucleosides of formula (i) wherein Y is sulphur are also preferred. More preferably the process is used to produce ß-anomers of nucleosides of formula (I) wherein

Y is sulphur. The nucleoside may be either a D- or L- isomer.

The group B in the nucleoside of formula (I) should generally be one which is stable under the conditions required for anomerisation. The group may be a pyrimidine base. The pyrimidine base is preferably of the formula (II)

wherein A is hydroxy, amino, monoalkylamino or dialkylamino, and X is hydrogen, halo, alkoxy, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, amino, monoalkylamino, dialkylamino, cyano or nitro.

In formula (II), an alkyl moiety of a

monoalkylamino, dialkylamino, alkoxy, alkyl or haloalkyl group may be linear, branched or cyclic, but linear alkyl moieties are preferred. An alkyl moiety is suitably methyl or C_2-8 alkyl, preferably methyl or C_2-4 alkyl, for example methyl, ethyl, propyl or isopropyl. A haloalkyl group may be a mono-, di- or tri- halo-substituted alkyl group wherein halo is fluoro, chloro, bromo or iodo. A haloalkenyl group is suitably C_2-4 bromoalkenyl.

Particularly preferred pyrimidine bases of formula (II) are those wherein X is chloro, bromo, iodo, C_2-4 alkyl, C_2-4 alkenyl, C_2-4 alkynyl, C_2-4 haloalkyl or C_2-4 haloalkenyl.

The group B may also be a purine base, for example adenine, guanine, 2,6-diaminopurine and 1-6-dihydro-6- oxopurine, or an analogue thereof, for example, 3-deazapurine, 6-amino-3-deazapurine, 3-deaza-6-oxopurine, 7-deazapurine, 6- amino-7-deazapurine, 7-deaza-6-oxopurine, 8-azapurine, 6-amino- 8-azapurine, 8-aza-6-oxopurine, 2-azapurine, 6-amino-2- azapurine, 2-aza-6-oxopurine, or a derivative thereof.

Derivatives include 2-halo and 2-amino substituted derivatives of the purine bases and analogues listed above.

Examples of nucleosides which may be anomerised are:

5-(2-chloroethyl)-2'-deoxy-4'-thiouridine;

5-nitro-2'-deoxy-4'-thiouridine; 5-amino-2'-deoxy-4'-thiouridine;

5-methylamino-2'-deoxy-4'-thiouridine;

5-iodo-2'-deoxy-4'-thiouridine;

5-ethyl-2'-deoxy-4'-thiouridine;

5-bromo-2'-deoxy-4'-thiouridine;

5-chloro-2'-deoxy-4'-thiouridine;

5-trifluoromethyl-2'-deoxy-4'-thiouridine;

5-propyl-2'-deoxy-4'-thiouridine;

E-5-(2-bromovinyl)-2'-deoxy-4'-thiouridine;

2'-deoxythymidine;

3'-deoxy-3'-fluorothymidine;

2',3'-dideoxy-3'-fluorouridine;

5-chloro-2'3'-dideoxy-3' -fluorouridine;

2'-deoxy-3'-azido-thymidine;

cis-1-(2-(hydroxymethyl)-1,3-oxathiolan-5-yl)- cytosine;

2' ,3'-dideoxy-5-ethynyl-3'-fluorouridine;

1-(ß-D-arabinofuranosyl)-5-propynyluracil; and hydroxy protected and physiologically acceptable derivatives thereof.

The groups Z¹, Z² and Z³ of the nucleoside of formula (I) may be -OR¹, -OR² and -OR³ respectively wherein the groups R¹, R² and R³ are hydroxy protecting groups.

Conventional protecting groups may be used for R¹, R² and R³; examples include acyl groups, for example C_1-6 alkanoyl (e.g. acetyl) or aroyl (e.g. benzoyl or toluoyl); other groups such as silyl groups, for instance tri-C_1-6 alkylsilyl (e.g.

trimethylsilyl) or tert-butyl diphenylsilyl; or arylmethyl groups such as benzyl or triphenylmethyl. Acetyl groups are preferred from the viewpoint of the solubility of the

nucleoside. Primary benzyl groups (i.e. R¹ = benzoyl) should generally be avoided because they exchange and use up reagent, although other acyl groups do not exchange. Also,

triphenylmethyl and silyl ethers tend to be unstable. With addition of sufficient anthydride, unprotected nucleosides can be acylated and anomerised simultaneously.

Physiologically acceptable derivatives of compounds of the invention include pharmaceutically acceptable salts; esters and salts of esters, or any other compound which, upon administration to human subject, is capable of providing

(directly or indirectly) the active metabolite or residue thereof.

Preferred mono- and di-esters according to the invention include carboxylic acid esters in which the non-carbonyl moiety of the ester grouping is selected from straight or branched chain alkyl, (e.g. tertiary butyl); cyclic alkyl (e.g. cyclohexyl); alkoxyalkyl (e.g. methoxymethyl),

carboxyalkyl (e.g. carboxyethyl), aralkyl (e.g. benzyl), aryloxyalkyl (e.g. phenoxymethyl), aryl (e.g. phenyl optionally substituted by halogen, C_1-4 alkyl or C_1-4 alkoxy); sulphonate esters such as alkyl- or aralkyl-sulphonyl (e.g.

methanesulphonyl); mono-, di- or tri- phosphate esters which may or not be blocked, amino acids esters and nitrate esters. With regard to the above-described esters, unless otherwise specified, any alkyl moieties present in such esters

advantageously contain 1 to 18 carbon atoms, particularly 1 to 4 carbon atoms, in the case of straight chain alkyl groups, or 3 to 7 carbon atoms in the case of branched or cyclic alkyl groups. Any aryl moiety present in such esters advantageously comprises a phenyl group. Any reference to any of the above compounds also includes a reference to a physiologically acceptable salt thereof.

Salts according to the invention which may be conveniently used in therapy include physiologically acceptable base salts, e.g. derived from an appropriate base, such as alkali metal (e.g. sodium), alkaline earth metal (e.g.

magnesium) salts, ammonium and NR₄ (wherein R₄ is C_1-4 alkyl) salts. When A represents an amino group, salts include

physiologically acceptable acid addition salts, including the hydrochloride and acetate salts.

The derivatives of the compounds of formula (i) may be prepared in conventional manner. For example, esters may be prepared by treating a compound of formula (I) with an

appropriate esterifying agent, for example, an acyl halide or anhydride. Salts may be prepared by treating a compound of formula (I) with an appropriate base, for example an alkali metal, alkaline earth metal or ammonium hydroxide, or where necessary, an appropriate acid, such as hydrochloric acid or an acetate, e.g. sodium acetate.

The reagents used for the anomerisation of the ß- or α-anomer of the nucleoside of formula (I) are an anhydride and a strong acid.

Anhydrides which may be used include those of the formula R⁴COOCOR⁵ wherein R⁴ and R⁵ are each independently alkyl, haloalkyl, aryl or aralkyl. An alkyl group may be branched, linear or cyclic, and is suitably C_1-8 alkyl,

preferably C_1-4 alkyl, for example methyl, ethyl or propyl. Most preferably, when R⁴ and R⁵ are alkyl, R⁴ and R⁵ are methyl and the anhydride is thus acetic anhydride. A haloalkyl group is suitably C_1-8 haloalkyl, preferably C_1-4 haloalkyl, and the halogen may be fluoro, chloro or bromo. Most preferably, when R⁴ and R⁵ are haloalkyl, R⁴ and R⁵ are CF₃ and the anhydride is thus trifluoracetic anhydride. An aryl group is suitably C₆ or C₁₀ aryl. An aralkyl group is suitably C₇ or C₈ aralkyl, for example benzyl.

The strong acid preferably has a negative pK_a value, for example a pK_a value of from 0 to -10. The acid may be inorganic or organic. The process is preferably carried out under anhydrous conditions and the acid is therefore preferably anhydrous. Examples of acids are hydrochloric acid, sulphuric acid, nitric acid, chlorosulphonic acid, trifluoroacetic acid and superacid resins (e.g. Nafion, Trade Name of Aldrich

Chemicals). Sulphuric acid is preferred.

The process of the invention is generally carried out in the presence of a solvent, preferably an inert solvent. The solvent is one which is compatible with the solubility of the reagents. The solvent is preferably anhydrous. Acetic acid has been used but favours formation of the open-chain by- product and is preferably avoided as a solvent for the

reaction. Examples of suitable inert solvents include

dichloromethane, tetrahydrofuran, dimethylformamide,

acetonitrile and dimethylsulphoxide. A preferred solvent is acetonitrile because it stabilizes the carbocation intermediate and hence speeds up the reaction. However, especially if one or more of the reagents is liquid, it is not essential to use any solvent. When the process is carried out in one step, the following amounts and concentrations of the reagents are suitably used:

The anhydride may be used in a large molar excess relative to the nucleoside and may be used as a solvent. For example, up to a 1000 fold molar excess may be used. However, the rate of reaction tends to increase as the amount of

anhydride is reduced from a 1000 fold excess, and as little as 0.1 or 1 mole equivalent can be used. Thus, the anhydride may be used in an amount of from 0.1 to 30, for example 0.1 to 20, 0.1 to 10 or 1 to 10 mole equivalents relative to the

nucleoside. When the process is performed on nucleosides having free OH groups, the OH groups are acylated and a larger amount of anhydride is therefore required: this may be avoided by use of conventional protecting groups which do not exchange (as discussed above) and removal thereof after the anomerisation reaction.

The strong acid is suitably used in an amount of less than 10, for example from 0.01 to 6 mole equivalents relative to the nucleoside; within this range use of minimum quantities of acid favours the anomerisation process over the formation of open chain by-product. For oxygen-containing nucleosides, the acid is generally used in an amount of below 6 equivalents, preferably about 0.3 equivalent. For 4'- thionucleosides, more than 0.01 equivalent is generally used and about 1.4 equivalents is preferred. The concentration of acid is suitably 0.002 M to 0.2 M. For oxygen-containing nucleosides, a concentration of below 0.2 M is generally used, and about 0.02 M is preferred. For 4'-thionucleosides, a concentration of above 0.002 M is generally used and about 0.2 M is preferred.

When the process is conducted in two steps

corresponding amounts and concentrations of the reagents are used.

There are no particular limitations on the

temperature at which the process of the invention is carried out and reduced, ambient or elevated temperatures may be used. Temperatures from the freezing point to the boiling point of the reaction mixture will be convenient and temperatures of 0 to 60ºC are suitable, preferably 5 to 40ºC. From the viewpoint of convenience, ambient temperature (i.e. about 20ºC) is preferred. A higher temperature tends to give a faster reaction, and affects the position of the equilibrium which exists between the α- and ß-anomers.

The length of time for which the reaction is carried out is suitably from 5 min to 240 h, e.g. 15 min to 24 h, 30 min to 18 h or 1 h to 16 h.

It is possible to conduct the reaction in the presence of a carboxylic acid in addition to the specified reagents. However, for oxygen-containing nucleosides this tends to cause accumulation of the acyclic sugar. It is therefore preferred that the reaction is conducted in the absence of any carboxylic acid. More preferably the reaction mixture consists only of the starting anomer (or mixture of anomers), the anhydride, the strong acid (or the reaction product of the anhydride and the acid) and any solvent.

The anomerisation reaction is believed to be reversible and, if allowed to go to completion, will establish an equilibrium between the α- and ß-anomers. Clearly, it is advantageous to adjust the conditions of the reaction to increase the equilibrium proportion of the desired anomer.

Many factors may influence the final equilibrium position, including the choice of the anhydride, the groups Z¹, Z² and Z³, the inert solvent (if any) and the temperature. It is believed that a large group Z² and a small group Z¹ may tend to favour the ß-anomer.

The reaction conditions should generally be chosen to reduce the amount of open chain sugar formed. Larger amounts of acid tend to result in more open chain sugar.

However, in the case of 4'-thionucleosides, open chain sugars have not been observed (presumably because sulphur is a better nucleophile than oxygen and closes the ring) and a larger amount of acid is used than in the case of oxygen-containing nucleosides. A large amount of acid should also be avoided in the case of purine nucleosides and analogues thereof to prevent loss of the base.

In order to produce an α- or ß-anomer of a

nucleoside containing a group which is unstable under the anomerisation conditions, a stable precursor of the nucleoside may be subjected to anomerisation and the resulting anomer converted to the desired nucleoside. The precursor may be a nucleoside in which the unstable group is protected, or a nucleoside not containing the unstable group to which the group can be added after anomerisation. The unstable group may be one which is acid labile.

The process of the invention may be used to convert pure α- or ß-anomer into the other anomer or to convert a single anomer or a mixture of α- and ß-anomers into a mixture containing a higher concentration of the desired anomer. If necessary, the process may be repeated more than once on a given sample, with the desired anomer being separated out after each repeat and the process then being repeated on the

remaining unwanted anomer. In a case where the desired anomer precipitates out or is otherwise removed, the process may be conducted continuously. The desired anomer is usually the β-anomer and is also the anomer which tends to precipitate out.

Preferably the process comprises the further step of recovering the desired α- or ß-anomer and, optionally, purifying the desired anomer. Recovery and purification may be effected by well known methods.

The starting ß- or α-anomer of the nucleoside of formula (I) (or mixture of ß- and α-anomers of the nucleoside of formula (I)) is obtained by known processes. For example, pyrimidine 4'-thionucleosides may be obtained as described in EP-A-421777 and EP-A-409575. Purine 4 '-thionucleosides may be obtained as described in WO-A-91/04033. Pyrimidine nucleosides and purine nucleosides are commercially available (see also Hubbard, Jones and Walker, Nucleic Acids Res. 1984, 12.6827- 6837). A synthesis for each of the following compounds is described in the publication in parentheses: 3'-deoxy-3'- fluorothymidine and 2 ',3'-dideoxy-3'-fluorouridine (Journal F. prackt Chemie vol 315, 895-900, 1973), 5-chloro-2'3'-dideoxy- 3'-fluorouridine (EP-A-0305117), 2'-deoxy-3'azido-thymidine (EP-A-0198185), cis-1-(2-(hydroxymethyl)-1,3-oxathiolan-5-yl)- cytosine (BCH-189) (EP-A-0382526), 2 ',3^,-dideoxy-5-ethynyl-3'- fluorouridine (EP-A-0356166), and 1-(ß-D-arabinofuranosyl)-5- propynyluracil (EP-A-0272065). L-isomers of nucleosides which may be anomerised in the invention are disclosed in GB Patent Application Nos 9218812.7 and 9218810.1 filed on the same day as this Application.

The process of the invention may further comprise one or more of the following steps in any order:

a) removing one or more hydroxy protecting

groups, for example R¹, R² and/or R³ when Z¹, Z² and/or Z³ are OR¹, OR² and/or OR³,

b) converting the nucleoside produced or a

protected form thereof into a further

nucleoside or a protected form thereof, c) converting the nucleoside produced or a

protected form thereof into a physiologically acceptable derivative of the said nucleoside or a protected form thereof,

d) converting a physiologically acceptable

derivative of the nucleoside produced or a protected form thereof into another

physiologically acceptable derivative of the said nucleoside or a protected form thereof, e) when 4'-sulphone or sulphoxide sugar moiety derivatives are required, partially or completely oxidising the sulphur of the 4'- thio sugar moiety of the nucleoside produced

(e.g. with a peracid), and

f) where necessary, further purifying the α- or ß-anomer of the nucleoside produced or a protected form thereof or a physiologically acceptable derivative thereof.

Each of steps a) to d) and f) may be effected in a manner described in EP-A-409575. Step e) may be effected in a manner described in EP-A-421777.

The process of the invention may further comprise formulating the nucleoside produced or a physiologically acceptable derivative thereof into a pharmaceutically

acceptable composition with a carrier or diluent. EP-A-409575 describes suitable carriers and diluents and give Examples of suitable pharmaceutical formulations.

Examples 1 to 6

Table 1 shows the reaction conditions and results, and Table 2 shows the 2'-deoxynucleoside substrate in each of Examples 1 to 6.

General Procedure A

The 2'-deoxynucleoside substrate was dissolved in dichloromethane (DCM) and acetic anhydride, and stirred at ambient temperature (Examples 1-3, 5 and 6) or at 40ºC (Example 4). Concentrated sulphuric acid was then added dropwise using a syringe. The reaction was allowed to continue at ambient temperature for the required length of time, whereupon the reaction mixture was diluted with dichloromethane and

neutralised with saturated aqueous sodium bicarbonate. The organic fraction was separated, dried with magnesium sulphate, filtered and the solvent removed in vacuo. The residue was co- evaporated with toluene then ethanol to remove any residual acetic anhydride. The anomeric mixture of products was isolated as a foam and did not require further purification. The anomeric composition of the products was calculated from the ratio of the H-1' resonances in the ¹H-NMR spectrum and the identity of the products was established by comparison with the ¹H-NMR spectra of authentic samples.

Example 7

Sulphuric acid (98%, 0.3 molar equivalents relative to the nucleoside) was added to DCM (9ml per 200 mg of

nucleoside), followed by 10 equivalents of acetic anhydride.

This solution was boiled under reflux (with precautions to keep the mixture dry) for 5 min and allowed to cool.

Diacetylthymidine (ß-anomer) was then added and the

anomerisation allowed to take place for 18 h. An α:ß ratio of

65:35 was obtained.

Examples 8 to 23

The following general procedure B was used in Examples 8 to 23. General Procedure B

The reagents used to anomerise the nucleoside were as follows: sulphuric acid (>98% purity, supplied by Fisons);

acetic anhydride (Fisons) which had been distilled and stored over a type 4A molecular sieve; acetonitrile (BDH) which had been dried by refluxing over calcium hydride for at least 2 h and distilling off dry acetonitrile, and stored over a type 4A molecular sieve.

The reaction was carried out as follows:

To acetonitrile (400 μl) was added acetic anhydride and sulphuric acid to form the "catalyst". The catalyst was added, with stirring, to β-diacetyl-2'-deoxythymidine in acetonitrile (400 μl) at room temperature.

The mixture was poured into a separating funnel and quenched with saturated sodium bicarbonate solution.

The products were extracted with dichloromethane,

separated, and washed with water until the washings were neutral. The organic layer was dried with MgSO₄ , filtered, and the solvent removed in vacuo. The residue was co- evaporated with toluene/ethanol to leave the product as a white solid.

The reaction time, the amounts and concentrations of the reagents, and the results of each of Examples 8 to 23 are shown in Table 3. TABLE 3

Example Time % α-anomer % open chain acetic Sulphuric acid rnucleoside anhydride/mol /mol eqv. rngμl^-1 of eqv.relative to relative acetonitrile to nucleoside to nucleoside

8 5h 30min 66 40 10 0.3 0.25

9 2h 66 <5 10 0.3 0.25

10 15min 66 0 10 0.3 0.25

11 15min 61 0 10 0.3 0.025

12 45min 66 <5 10 0.3 0.025

13 5h 66 33 10 0.3 0.025

14 40min 66 0 1 0.3 0.25

15 5h 66 0 1 0.3 0.25

16 23h 0 0 0.1 0.3 0.25

17 23h 0 0 0.3 0.3 0.25

18 2h 66 <5 10 0.1 0.25

19 18h 30min 66 30 10 0.1 0.25

20 25min 7 0 10 0.03 0.25

21 2h 38 0 10 0.03 0.25

22 17h 60 0 10 0.03 0.25

23 42h 63 11 10 0.03 0.25

Example 24

General procedure B of Examples 8 to 23 was carried out, except that β-diacetyl-4'-thio-5-ethyl-2'-deoxyuridine (200 mg, 0.56 mmol) was used instead of β-diacetyl-2'-deoxy-thymidine. The catalyst was made using acetonitrile (400 μl), acetic anhydride (1040 μl, 11.2 mol, 20 eqv), and sulphuric acid (43 μl, 1.4 eqv). The anomerisation reaction mixture was stirred at room temperature for 18 h.

Example 25

To anomerise β-3',5'-diacetyl-2'-deoxyuridine harsher conditions are preferably used than for the corresponding thymidine nucleoside (β-3',5'-diacetyl-2'-deoxythymidine).

Acetic anhydride (28 μl, 3 eqv) and concentrated sulphuric acid (7.5 μl, 1.4 eqv) were added to 60 μl of dry acetonitrile. The so formed catalyst was added to a solution of β-3 ',5'-diacetyl-2'-deoxyuridine (31.2 mg, 0.1 mmol) in acetonitrile (60 μl). On addition the solution turned yellow.

After 30 min, the solution was poured into a separating funnel containing sodium hydrogen carbonate. After

neutralization, the aqueous layer was extracted with

dichloromethane. The combined extracts were dried (magnesium sulphate), filtered and the solvent removed in vacuo. The residue was co-evaporated with toluene/ethanol to give the product. Examination of the ¹H-NMR spectrum revealed an α : β ratio of 2:1.

Claims

1. A process for the production of the α- or ß-anomer of a nucleoside, said process comprising anomerisation of the ß- or α-anomer of a nucleoside using an anhydride and a strong acid.

2. A process according to claim 1 wherein (i) the anhydride and strong acid are first reacted together, and (ii) the reaction product thereby obtained is then mixed with the starting nucleoside.

3. A process according to claim 1 wherein the anhydride and strong acid react together in situ to produce a reaction product which reacts with the starting nucleoside.

4. A process for the production of the α- or ß-anomer of a nucleoside, said process comprising reacting the ß- or α-anomer of a nucleoside with a reaction product obtainable by reacting an anhydride and a strong acid.

5. A process according to any one of the

preceding claims wherein the starting nucleoside is of formula (I)

wherein

B is a purine or pyrimidine base,

Y is oxygen or sulphur,

Z¹ is hydroxy or OR¹ wherein R¹ is a hydroxy protecting group, Z² is hydrogen, hydroxy, fluoro, azido or OR² wherein R² is a hydroxy protecting group, and

Z³ is hydrogen, hydroxy or OR³ wherein R³ is a hydroxy

protecting group.

6. A process according to claim 5 wherein Z¹ is hydroxy or OR¹ wherein R¹ is a hydroxy protecting group,

Z² is hydrogen, hydroxy or OR² wherein R² is a hydroxy

protecting group, and Z³ is hydrogen.

7. A process according to claim 6 wherein Z¹ is -OR¹ and Z² is -OR² wherein R¹ and R² are both either benzyl or acetyl, and Z³ is hydrogen.

8. A process according to any one of the

preceding claims wherein the nucleoside produced is the ß-anomer.

9. A process according to any one of claims 5 to 8 wherein Y is sulphur.

10. A process according to any one of claims 5 to 9 wherein B is a pyrimidine base.

11. A process according to claim 10 in which B is of the formula (II)

12. A process according to claim 11 wherein X is chloro, bromo, iodo, C_2-4 alkyl, C_2-4 alkenyl, C_2-4 alkynyl, C_2-4 haloalkyl or C_2-4 haloalkenyl.

13. A process according to any one of claims 1 to 4 in which the starting nucleoside is selected from:

5-(2-chloroethyl)-2'-deoxy-4'-thiouridine;

5-nitro-2'-deoxy-4'-thiouridine;

5-amino-2'-deoxy-4'-thiouridine;

5-methylamino-2'-deoxy-4'-thiouridine;

5-iodo-2'-deoxy-4'-thiouridine;

5-ethyl-2'-deoxy-4'-thiouridine;

5-bromo-2'-deoxy-4'-thiouridine;

5-chloro-2'-deoxy-4'-thiouridine;

5-trifluoromethyl-2'-deoxy-4'-thiouridine;

5-propyl-2'-deoxy-4'-thiouridine;

E-5-(2-bromovinyl)-2'-deoxy-4'-thiouridine;

2'-deoxythymidine;

3'-deoxy-3'-fluorothymidine;

2',3'-dideoxy-3'-fluorouridine;

5-chloro-2'3'-dideoxy-3'-fluorouridine;

2'-deoxy-3'-azido-thymidine;

cis-1-(2-(hydroxymethyl)-1,3-oxathiolan-5-yl)- cytosine;

2',3'-dideoxy-5-ethynyl-3'-fluorouridine;

14. A process according to any one of the

preceding claims wherein the anhydride is acetic anhydride.

15. A process according to any one of the

preceding claims wherein the strong acid is sulphuric acid.

16. A process according to any one of the

preceding claims which is carried out in the presence of an inert solvent.

17. A process according to claim 16 wherein the inert solvent is acetonitrile.

18. A process according to any one of the

preceding claims which further includes one or more of the following steps in any order:

a) removing one or more hydroxy protecting

groups,

b) converting the nucleoside produced or a

protected form thereof into a further

d) converting a physiologically acceptable

derivative of the nucleoside produced or a protected form thereof into another

physiologically acceptable derivative of the said nucleoside or a protected form thereof, e) when 4'-sulphone or sulphoxide sugar moiety derivatives are required, partially or

completely oxidising the sulphur of the 4'- thio sugar moiety of the nucleoside produced, and

19. A process according to any one of the

preceding claims which further comprises formulating the nucleoside produced or a physiologically acceptable derivative thereof into a pharmaceutically acceptable composition with a carrier or diluent.