WO2008105593A1

WO2008105593A1 - METHOD FOR STEREOSELECTIVE PREPARATION AND SEPARATION OF TRI-O-ACETYL-5-DEOXY-ß-D-RIBOFURANOSE

Info

Publication number: WO2008105593A1
Application number: PCT/KR2008/000969
Authority: WO
Inventors: Jaeheon Lee; Gha Seung Park; Weon Ki Yang; Jin Hee Kim; Chul Hyun Park; Yong-Hoon An; Chang-Ju Choi; Young-Kil Chang; Gwan Sun Lee
Original assignee: Hanmi Pharmaceutical Co Ltd; Hanmi Pharmaceutical Industries Co Ltd
Current assignee: Hanmi Pharmaceutical Co Ltd; Hanmi Pharmaceutical Industries Co Ltd
Priority date: 2007-02-28
Filing date: 2008-02-19
Publication date: 2008-09-04
Anticipated expiration: 2009-08-28
Also published as: TW200900417A; KR20080079833A; KR100908363B1; AR065305A1

Abstract

The present invention discloses a method for preparing highly pure tri-O-acetyl-5-deoxy-β-D-ribofuranose which comprises a highly stereoselective acetylation step of 1-methylacetonide, and the pure β-anomer thus obtained can be advantageous used for preparing capecitabine.

Description

METHOD FOR STEREOSELECTIVE PREPARATION AND SEPARATION OF TRI-O- ACETYL-S-DEOXY-β-D-RIBOFURANOSE

Field of the Invention

The present invention relates to a method for the stereoselective preparation and separation of tri-O-acetyl-5-deoxy-β-D-ribofuranose, which is useful as an intermediate in preparing capecitabine.

Background of the Invention

Capecitabine, a nucleoside having a ribofuranose backbone, is an anti-tumor agent orally administered it to treat metastatic breast cancer and rectal cancer, and has the stereochemical structure shown below wherein the

5-fiuorocytosine moiety attached at position 1 of the ribofuranose has β- orientation.

As described in US Patent No. 5,453,497, the capecitabine can be prepared by glycosylating tri-O-acetyl-5-deoxy-β-D-ribofuranose with fluorocytosine, and then subjecting the resulting product to carbamoylation and hydrolysis as shown in Reaction Scheme (A). Reaction Scheme (A)

The starting material used above, tri-O-acetyl-5-deoxy-β-D- ribofuranose of formula (I), is prepared according to the method described in

US Patent No. 4,340,729 as shown in Reaction Scheme (B). Specifically,

(i) the compound of formula (IV) is hydrolyzed to obtain a triol compound of formula (III); (ii) the triol compound of formula (III) is acetylated using acetic anhydride in pyridine to obtain tri-O-acetyl-5-deoxy-D-ribofuranose of formula (II) which is a β-/α-anomer mixture with respect with the acetyl at position 1; (iii) the reaction mixture is subjected to vacuum distillation to purify the β-/α-anomer mixture; and (iv) β-anomer of formula (I) is separated from the mixture.

Reaction Scheme (B)

(IV) (III) (H) (I)

Further, the acetylation of the triol compound with the acetic anhydride in pyridine to obtain tri-O-acetyl-5-deoxy-D-ribofuranose has also been described in other reports [J. Med. Chem., 2000, vol. 43, pp 2566-

2574]; [Carbohydrate Research, 2003, vol. 338, pp 303-306]; [Nuclear Medicine and Biology, 2004, vol. 31, pp 1033-1041]; and [ J. Am. Chem. Soc, 1957, vol. 79, pp 5534-5540].

However, the above-mentioned method of preparing tri-O-acetyl-5- deoxy-D-ribofiiranose is hampered by various problems due to the use of pyridine as the solvent: the work up volume after the reaction is very large and requirs a complicated, multi-step work up procedure; the selectivity of the β-anomer formation is low, the β-/α-anomer ratio of the tri-O-acetyl-5- deoxy-D-ribofuranose product being no more than 3:1 to 3.5:1; and the isolation of the desired β-anomer of formula (I) from the β-/α-anomer mixture by recrystallization is difficult due to the presence of a relatively large amount of the α-anomer, and gives yield of only about 35 to 40 %.

Therefore, the present inventors have attempted to develop an improved method of preparing tri-O-acetyl-5-deoxy-β-D-ribofuranose, and have found that the β-/α-anomer ratio can be markedly improved by using alkylamine or cyclic amine bases instead of pyridine during the acetylation.

Summary of the Invention

Accordingly, it is an object of the present invention to provide a method for preparing tri-O-acetyl-5-deoxy-β-D-ribofuranose in a high β- anomer ratio. It is another object of the present invention to provide a simple and easy method for separating pure β-anomer of tri-O-acetyl-5- deoxy-D-ribofuranose from a β-/α-anomer mixture thereof. In accordance with one aspect of the present invention, there is provided a method for preparing tri-O-acetyl-5-deoxy-β-D-ribofuranose of formula (I), which comprises the steps of:

(i) conducting the raction of the compound of formula (III) with acetic anhydride in an organic solvent in the presence of an alkylamine or a cyclic amine to obtain the compound of formula (II); and

(ii) isolating the compound of formula (I) from the compound of formula (II) by recrystallization using a solvent and an anti-solvent:

QAc

(I)

AcO QAc

Detailed Description of the Invention

The method for the stereoselective acetylation and separation of the β-anomer according to the present invention is shown in Scheme (C).

Scheme (C)

Specifically, the methylacetonide compound of formula (IV) is deprotected by the conventional method described in US Patent No.

4,340,729 to obtain the triol compound of formula (III) in the form of an α- /β-anomer mixture having an α- to β- ratio of about 1 :1 to 2:1. The anomers may be optionally separated by column chromatography for use in the next step. The methylacetonide compound of formula (IV) can be easily prepared by the method described in US Patent No. 4,340,729.

In the step of acetylating the triol compound of formula (III), instead of using neat pyridine as the solvent as is taught by the prior art, an alkylamine or a cyclic amine may be used in an amount of 3 mole equivalents or more based on the compound of formula (HI) to achieve a highly stereoselective reaction of the triol compound with the acetic anhydride: The product, tri-O-acetyl-5-deoxy-D-ribofuranose of formula (II) contains a marked high β-anomer content, the α- to β-anomer ratio being

1:10 or higher. This β-anomer-rich product can be subjected to recrystallization using a solvent or an anti-solvent to obtain highly pure β- anomer of formula (I) in a high yield of 99.5 % and more.

As described above, unlike the conventional methods using an excessive amount of neat pyridine as a solvent, the present inventive method gives tri-O-acetyl-5-deoxy-D-ribofuranose having a high β-anomer content, which is carried out using an equivalent amount of an alkylamine or a cyclic amine, together with the organic solvent. Therefore, the work up process after the reaction is greatly simplified. That is, pure tri-O-acetyl-5-deoxy-β- D-ribofuranose can be recovered from the acetylation reaction mixture, and can be easily obtained in a high yield of 80 % and more by simple recrystallization using a solvent and an anti-solvent, instead of conducting high vacuum distillation employed in the conventional method.

The organic solvent used in the acetylation according to the present invention may be selected from the group consisting of tetrahydrofuran, acetonitrile, methylene chloride, chloroform, dibromoethane, dichloroethane, ethylacetate, toluene and a mixture thereof, preferably methylene chloride.

The alkylamine or cyclic amine used in the present invention may be selected from the group consisting of triethylamine, n-tributylamine, dicyclohexylamine, tetramethylethylenediamine, diisopropylethylamine, N- methylmorphorine and a mixture thereof, preferably triethylamine.

According to the kind of the alkylamine or cyclic amine used, the selectivity of the β-anomer over the α-anomer increases from at least 10:1 to 15:1, in which the use of triethylamine is preferred, while piperidine is ineffective.

In the present invention, the amount of the alkylamine or cyclic amine may be 3 to 12 mole equivalents, preferably 4 to 5 mole equivalents based on the compound of formula (HI). The acetylation according to the present invention may be carried out at 10 to 50 ⁰C, preferably 5 to 25 ^°C for 6 to 24 hours.

After the acetylation, the tri-O-acetyl-5-deoxy-D-ribofuranose product of formula (II) is obtained as a solid due to its high β-anomer content, unlike the product obtained by the conventional method using pyridine is an oil or semi-solid.

Tri-O-acetyl-5-deoxy-D-ribofuranose of formula (II) obtained by the inventive acetylation method is composed mainly of the desired β-anomer, the α-anomer content being only 6 to 9 %. To obtain the pure β-anomer therefrom, the α-anomer can be removed by recrystallization in accordance with any of the conventional methods. For example, pure β-anomer may be easily separated by adding an anti-solvent to a solution obtained by dissolving the anomer mixture in an appropriate solvent, filtering to remove the filtrate containing the α-anomer, and collecting pure β-anomer as a solid. The appropriate solvent used in the recrystallization may be a polar organic solvent, which is selected from the group consisting of methanol, ethanol, propanol, isopropanol, n-butanol, acetone, acetonitrile, tetrahydrofiiran, chloroform, methylene chloride, ethylacetate, diethyl ether and a mixture thereof, preferably isopropanol. The anti-solvent used in the recrystallization may be a nonpolar organic solvent, which is selected from the group consisting of n-hexane, heptane, octane, nonane, petroleum ether, isopropyl ether and a mixture thereof, or water, preferably n-hexane or water.

The recrystallization may be carried out at - 60 to 30 ^°C, preferably - 10 to 5 °C, and the amounts of the solvent and anti-solvent may be 2 to 5 times (v/wt) and 5 to 40 times (v/wt) based on tri-O-acetyl-5-deoxy-D- ribofuranose of formula (II), respectively.

The β-anomer obtained after the recrystallization has a purity of at least 99.5 %, the α-anomer content being less than 0.5 %.

As described above, the inventive method for the stereoselective preparation of tri-O-acetyl-5-deoxy-β-D-ribofuranose of formula (I) shows markedly improved stereoselectivity as compared with the conventional method, and gives a high yield of at least 75 %, as compared with less than

35 to 40 % yield obtained by the conventional method.

The following Examples are intended to further illustrate the present invention without limiting its scope.

In the following Examples, in case of the compound of formula (I), GC (gas chromatography) was operated by using cyanopropylphenyl polysiloxane column (inner diameter: 0.32 mm, length: 60 m, thickness: 1.8 μm) (e.g.: DB 624), and a test solution was prepared by dissolving 100 mg of the compound of formula (I) in dimethylsulfoxide to make 25 ml of the solution.

Example 1: Stereoselective preparation of tri-O-acetyI-5-deoxy-D- ribofuranose of formula (II)

(IV) (III) (II)

(1-1) Procedure when triethylamine is used as a base

100 g of methyl-2,3-O-isopropylidene-5-deoxy-β-D-ribofuranose was added to 500 ml of 0.02 M sulfuric acid, and stirred at 80 to 85 ^°C for 2 hours. The temperature was lowered to 45 to 50 ^°C, and water was removed therefrom under a reduced pressure until the total volume was reduced by 2/3 to 1/2. 500 ml of 0.02 M sulfuric acid was added to the resulting solution, and stirred at 80 to 85 ^"C for 1 hour. The temperature was lowered to room temperature, the resulting solution was neutralized to pH 5.3 to 5.5 with 0.2 M sodium carbonate, and concentrated under a reduced pressure. The resulting residue was suspended in 1,000 ml of acetonitrile, 50 g of anhydrous sodium sulfate was added thereto, and stirred for 1 hour. The resulting mixture was filtered through a cellite column, which was then washed with 100 ml of acetonitrile, the filtrate and the wash were combined, and concentrated under a reduced pressure to remove the solvent. 1,000 ml of methylene chloride and 370 ml of triethylamine were added to the resulting residue, and while keeping the temperature at 5 ^"C, 235.5 ml of acetic anhydride was added thereto, and stirred for 20 hours. After completion of the reaction, 1,000 ml of water was added to the resulting reaction mixture, stirred, and the organic layer was separated. The organic layer was then washed successively with 1,000 ml portions of IN HCl, saturated sodium bicarbonate, and brine, and then, dried over anhydrous sodium sulfate. The resulting organic layer was concentrated under a reduced pressure to obtain the title compound (140.8 g). The product thus obtained was analyzed by GC, which showed that the product contained the α- and β-anomers in a ratio of 1 : 13.8.

¹H NMR (300MHz, CDCl₃): (β-anomer) δ 6.09(s, IH), 5.30 5.32(dd, IH), 5.06 5.10(dd, IH), 4.26(q, IH), 2.10(s, 3H), 2.07(s, 3H), 2.05(s, 3H), 1.35(d, 3H). 1H NMR (300MHz, CDCl₃): (α-anomer) δ 6.2 l(d, IH), 5.11(dd,

IH), 4.8(dd, IH), 4.15 4.20(m, IH), 1.97(s, IH), 1.92(s, 3H), 1.89(s, 3H), 1.21(d, 3H).

(1-2) Procedure when tri-n-butylamine is used as a base The initial procedure of (1-1) was repeated except for using 5 g of methyl-2,3-O-isopropylidene-5-deoxy-β-D-ribofuranose to obtain a triol compound. Then, 50 ml of methylene chloride and 31.6 ml of tri-n- butylamine were added to the resulting residue, 11.8 ml of acetic anhydride was added slowly thereto, and stirred at room temperature for 12 hours. After completion of the reaction, 50 ml of water was added to the reaction mixture, stirred, and the organic layer was separated. The organic layer was then washed successively with 50 ml portions of IN HCl, saturated sodium bicarbonate, and brine, dried over anhydrous sodium sulfate, and filtered. The resulting filtrate was concentrated under a reduced pressure to obtain the title compound (7 g). The product thus obtained was analyzed by GC, which showed that the product contained the α- and β-anomers in a ratio of 1 :11.2.

(1-3) Procedure when tetramethylethylenediamine is used as a base The initial procedure of (1-1) was repeated except for using 5 g of methyl-2,3-O-isopropylidene-5-deoxy-β-D-ribofuranose to obtain a triol compound. Then, 50 ml of methylene chloride and 20 ml of tetramethylethylenediamine were added to the resulting residue, 11.8 ml of acetic anhydride was added slowly thereto, and stirred at room temperature for 12 hours. After completion of the reaction, 50 ml of water was added to the reaction mixture, stirred, and the organic layer was separated. The organic layer was then washed successively with 50 ml portions of IN HCl, saturated sodium bicarbonate, and brine, dried over anhydrous sodium sulfate, and filtered. The resulting filtrate was concentrated under a reduced pressure to obtain the title compound (7 g). The product thus obtained was analyzed by GC, which showed that the product contained the α- and β-anomers in a ratio of 1 : 10.3.

Comparative Example 1: Procedure when pyridine is used as an organic base

5 g of methyl-2,3-O-isopropylidene-5-deoxy-β-D-ribofuranose was added to 25 ml of 0.02 M sulfuric acid, and stirred at 80 to 85 ^°C for 2 hours. The temperature was lowered to 45 to 50 ^°C, and water was removed therefrom under a reduced pressure until the total volume was reduced by 2/3 to 1/2. 25 ml of 0.02 M sulfuric acid was added to the resulting solution, and stirred at 80 to 85 ^°C for 1 hour. The temperature was lowered to room temperature, the resulting solution was neutralized to pH 5.3 to 5.5 with 0.2 M sodium carbonate, and concentrated under a reduced pressure. The resulting residue was suspended in 50 ml of acetonitrile, 5 g of anhydrous sodium sulfate was added thereto, and stirred for 1 hour. The resulting mixture was filtered through a cellite column, which was then washed with 5 ml of acetonitrile, the filtrate and wash were combined, and concentrated under a reduced pressure. 11.8 ml of acetic anhydride and 25 ml of pyridine were added to the resulting residue, and stirred for 12 hours. After completion of the reaction, 100 ml of water was added thereto, stirred for 1 hour, and the resulting solution was extracted with 100 ml of methylene chloride. The organic layer was washed three times with 100 ml portions of IN HCl, and then washed successively with 100 ml portions of saturated sodium bicarbonate and brine. The organic layer was dried over anhydrous sodium sulfate and filtered. The resulting filtrate was concentrated under a reduced pressure to obtain the title compound (6.8 g). The oily product thus obtained was analyzed by GC, which showed that the product contained the α- and β-anomers in a ratio of 1:3.4.

Example 2: Selective crystallization of β-anomer (tri-O-acetyl-5-deoxy- β-D-ribofuranose of formula (I)

(II) (I)

140 g of the anomer mixture (α-anomer:β-anomer=l :13.8) obtained in Example 1-1 was divided in 5 portions, and each portion was subjected to recrystallization as described follow.

(2-1)

28 g of the α-/β-anomer mixture was dissolved in 56 ml of isopropanol, and 168 ml of hexane was added dropwise thereto. The resulting solution was aged at -10 ^°C for 3 hours, and filtered to obtain the title compound as a white solid (21.9 g; total yield: 79.5 %).

m.p: 65-66 ^"C β-anomer content: 99.6 % (α-anomer content: 0.1 %)

¹H NMR (300MHz, CDCl₃): (β-anomer) δ 6.09(s, IH), 5.30 5.32(dd, IH), 5.06 5.10(dd, IH), 4.26(q, IH), 2.10(s, 3H), 2.07(s, 3H), 2.05(s, 3H), 1.35(d, 3H).

(2-2) 28 g of the α-/β-anomer mixture was dissolved in 56 ml of isopropanol, and 168 ml of hexane was added dropwise thereto. The resulting solution was aged at 0 to 5 ^°C for 3 hours, and filtered to obtain the title compound as a white solid (21.0 g; total yield: 76.4 %).

β-anomer content: 99.7 % (α-anomer content: 0.05 %)

The m.p and ¹H-NMR data were identical with those obtained in Example 2-1.

(2-3) 28 g of the α-/β-anomer mixture was dissolved in 56 ml of isopropanol, and 168 ml of distilled water was added dropwise thereto. The resulting solution was aged at 5 ^°C for 7 hours, and filtered to obtain the title compound as a white solid (21.3 g; total yield: 77.4 %).

β-anomer content: 99.5 % (α-anomer content: 0.1 %)

The m.p and ¹H-NMR data were identical with those obtained in Example 2-1.

(2-4) 28 g of the α-/β-anomer mixture was dissolved in 56 ml of isopropanol, and 112 ml of distilled water was added dropwise thereto. The resulting solution was aged at 5 ^°C for 7 hours, and filtered to obtain the title compound as a white solid (20.7 g; total yield: 75.4 %). β-anomer content: 99.6 % (α-anomer content: 0.1 %) The m.p and ¹H-NMR data were identical with those obtained in Example 2-1.

(2-5)

28 g of the α-/β-anomer mixture was dissolved in 56 ml of methylene chloride, and 168 ml of hexane was added dropwise thereto. The resulting solution was aged at 10 ^°C for 3 hours, and filtered to obtain the title compound as a white solid (21.1 g; total yield: 76.5 %).

β-anomer content: 99.5 % (α-anomer content: 0.1 %)

The m.p and ¹H-NMR data were identical with those obtained in

Example 2-1.

Comparative Example 2

6.8 g of the anomer mixture (α-anomer :β-anomer= 1 :3.4) of

Comparative Example 1 obtained by using pyridine as an organic base was dissolved in 13.6 ml of isopropanol, and 40.8 ml of hexane was added dropwise thereto. The resulting solution was aged at 0 to 5 ^"C for 3 hours, and filtered to obtain the title compound as a cream white solid (2.5 g; total yield: 36.2 %).

m.p: 63-65 ^°C β-anomer content: 99.0 % (α-anomer content: 0.6 %)

Claims

What is claimed is:

1. A method for preparing tri-O-acetyl-5-deoxy-β-D-ribofuranose of formula (I), which comprises the steps of: (i) conducting the raction of the compound of formula (III) with acetic anhydride in an organic solvent in the presence of an alkylamine or a cyclic amine to obtain the compound of formula (II); and

Vv* "cOAc

(ID

Ac- O OAc

2. The method of claim 1, wherein the alkylamine or cyclic amine base used in step (i) is selected from the group consisting of triethylamine, n- tributylamine, dicyclohexylamine, tetramethylethylenediamine, diisopropylethylamine, N-methylmorphorine, and a mixture thereof.

3. The method of claim 2, wherein the amount of the alkylamine or cyclic amine ranges from 3 to 12 mole equivalents based on the compound of formula (TH).

4. The method of claim 1, wherein the solvent used in step (ii) is selected from the group consisting of methanol, ethanol, propanol, isopropanol, n- butanol, acetone, acetonitrile, tetrahydrofuran, chloroform, methylene chloride, ethylacetate, diethylether, and a mixture thereof.

5. The method of claim 1, wherein the anti-solvent used in step (ii) is selected from the group consisting of n-hexane, heptane, octane, nonane, petroleum ether, isopropyl ether, water, and a mixture thereof.

6. The method of claim 4 or 5, wherein the solvent is isopropanol, and the anti-solvent is n-hexane or water.