A method for synthesizing emtricitabine by Mitsunobu condensation method

Abstract

The invention discloses a method for synthesizing emtricitabine by Mitsunobu condensation, which belongs to the field of pharmaceutical chemistry technology. The key synthesis step is: (2R, 5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) and N4-protected 5-fluorocytosine (II) are condensed by Mitusnobu reaction, followed by sodium borohydride reduction, and finally the protecting group is removed under acidic conditions to obtain emtricitabine. The method avoids the generation and separation of isomers, has short steps and high yield. The emtricitabine obtained by adopting the method has low cost, is easy to operate, and has mild reaction conditions, which shows the superiority and simplicity of the synthetic method.

Description

Technical Field

[0001] The present invention belongs to the technical field of medicinal chemistry, and particularly relates to a method for synthesizing emtricitabine through a Mitsunobu condensation method. Background Art

[0002] Emtricitabine (FTC), chemically known as 4-amino-5-fluoro-1-[(2R,5S)-2-hydroxymethyl-1,3-oxathiolan-5-yl]-2(1H)-pyrimidinone, is a novel nucleoside reverse transcriptase inhibitor developed by Gilead Sciences in the United States. It is an antiviral drug. It was approved by the US FDA in 2003 for the treatment of AIDS caused by human immunodeficiency virus (HIV) and hepatitis B caused by hepatitis B virus (HBV). Since its launch, FTC has been clinically proven to have a low antiviral failure rate and excellent therapeutic efficacy, making it one of the most successful drugs for the treatment of AIDS and hepatitis B.

[0003]

[0004] Emtricitabine is a l,3-oxathioheterocyclic nucleoside analogue with two chiral centers in its molecular structure and four optical isomers, but emtricitabine is a drug with a single (2R,5S) optical structure.

[0005] There are currently three basic types of methods for synthesizing emtricitabine at home and abroad:

[0006] (1) Emtricitabine was obtained by splitting optical isomers using a chiral column. In 1992, Choi synthesized a 1,3-oxosulfur heterocyclic compound of flucytosine and obtained emtricitabine by splitting it using a chiral column. However, the optical process for splitting emtricitabine was complex and the scale of splitting was limited, which seriously restricted its development and utilization.

[0007] (2) Emtricitabine is obtained by enzymatic resolution of optical isomers. In 1996, Dionne et al. synthesized a 1,3-oxosulfur heterocyclic compound of flucytosine, formed a dephosphate salt, and then selectively dephosphorylated it with 5-nuclease to obtain emtricitabine. Enzymatic resolution can obtain high-purity emtricitabine and can be produced on a large scale. However, the disadvantage of this method is that the preparation of 5-nuclease is cumbersome, resulting in increased costs.

[0008] (3) By first synthesizing a chiral intermediate and then inducing the synthesis of emtricitabine. In 2002, Gong Ping used hydrated glyoxylic acid as a raw material, reacted it with thiazane and 1-menthol to form a chiral intermediate, which was then selectively coupled with a base. After reduction, emtricitabine was obtained by separation on a silica gel column. This method has few synthetic steps and simple conditions, but the overall yield is not high. Summary of the Invention

[0009] To address the deficiencies of the prior art, the present invention provides a method for synthesizing emtricitabine via the Mitsunobu condensation method. The key synthetic steps are: condensation of (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) and N4-protected 5-fluorocytosine (II) via a Mitsunobu reaction, followed by sodium borohydride reduction, and finally removal of the protecting group under acidic conditions to obtain emtricitabine. The chirality of the secondary alcohol in the condensation step is reversed, resulting in a stereoselective reaction that avoids the generation and separation of isomers, resulting in a simple process and high yield. The emtricitabine obtained by this method is low-cost, easy to operate, and operates under mild reaction conditions, demonstrating the superiority and simplicity of this synthetic method.

[0010] The technical solution of the present invention is: a method for synthesizing emtricitabine by Mitsunobu condensation method, and the reaction equation is expressed as follows:

[0011]

[0012] Wherein: Tr is trityl; Boc is tert-butyloxycarbonyl.

[0013] The following steps are involved:

[0014] Step 1: (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) and N4-protected 5-fluorocytosine (II) are condensed in an organic solvent using a Mitsunobu reaction to obtain an intermediate (III);

[0015] Step 2: Intermediate (III) reacts in an alcohol solvent in the presence of a reducing agent to obtain intermediate (IV);

[0016] Step 3: Under acidic conditions, the intermediate (IV) is deprotected to obtain emtricitabine.

[0017] Furthermore, in the above technical solution, the molar ratio of compound I to compound II is 1:1-0.5.

[0018] Furthermore, in the above technical solution, the Mitsunobu reaction uses a Ph3P / DIAD or Ph3P / DEAD combination reagent; wherein the molar ratio of Ph3P to DIAD / DEAD is 1:1.

[0019] Furthermore, in the above technical solution, the molar ratio of compound II to Mitsunobu reagent is 1:1.2-1.5.

[0020] Furthermore, in the above technical solution, the organic solvent is selected from one or more of tetrahydrofuran, acetonitrile, chloroform, dimethyl sulfoxide, ethyl acetate, and dichloromethane.

[0021] Furthermore, in the above technical solution, the reducing agent is selected from sodium borohydride, lithium borohydride or potassium borohydride.

[0022] Furthermore, in the above technical solution, the alcohol solvent is selected from methanol or ethanol.

[0023] Furthermore, in the above technical solution, the third deprotection step adopts hydrogen chloride / alcohol solution, acetyl chloride / alcohol or trifluoroacetic acid.

[0024] Emtricitabine is obtained by adopting this method, which has low cost, simple operation and mild reaction conditions, indicating the superiority and simplicity of this synthesis method. DETAILED DESCRIPTION

[0025] The present invention is described in detail below with reference to the embodiments.

[0026] Example 1

[0027] Add (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) (3.00 g, 10.4 mmol), N4-trityl-5-fluorocytosine (IIa, 3.86 g, 10.4 mmol), and triphenylphosphine (3.27 g, 12.5 mmol) to tetrahydrofuran (200 mL). Add DEAD (1.96 mL, 12.5 mmol) dropwise while controlling the temperature at 0-10°C. After the addition is complete, warm the mixture to room temperature and stir for 12 hours. Then, warm the mixture to 40°C and stir for an additional 3 hours.

[0028] The reaction temperature was lowered to room temperature and diluted with tetrahydrofuran (100 mL). The organic phase was washed with water (2 × 100 mL), and the aqueous layer was again extracted with tetrahydrofuran (100 mL). Drying was performed over sodium sulfate, filtering, and the solvent and low-boiling-point compounds were removed using a rotary evaporator and a vacuum pump. The resulting crude product was suspended in tetrahydrofuran and allowed to stand, yielding a white solid. This was filtered under reduced pressure and washed with ether and n-hexane in a 1:1 volume ratio. The filtrate was concentrated on a rotary evaporator. The resulting yellow oil was dissolved in dichloromethane (10 mL) and purified by column chromatography to yield 5.66 g of intermediate IIIa in an 85% yield. 1 H NMR(500MHz,DMSO-d6)δ8.02(d,J=6.80Hz,1H),7.74(brs,1H),7.25-7.33(m,15H),6.24(t,J=4.60Hz,1H),5.46(s,1H) ,3.52(dd,J=4.42Hz,1H),3.12(dd,J=4.46Hz,1H),1.02-2.05(m,10H),0.93(dd,J=6.82Hz,6H),0.78(d,J=6.82Hz,3H).13 C NMR (125MHz, DMSO-d6) δ169.6,158.0,150.1,129.2,126.1,126.3,127.1,128.6,129. 2,89.8,80.2,78.6,76.0,46.3,35.8,34.3,31.2,26.6,23.0,22.2,21.6,20.4,16.3.

[0029] Example 2

[0030] In a 250 mL round-bottom flask, intermediate IIIa (2.0 g, 3.12 mmol) and 15 mL of anhydrous ethanol were added, and a 25% sodium hydroxide (0.04 mL) / sodium borohydride (0.353 g, 9.36 mmol) aqueous solution (4 mL) was added dropwise. The mixture was stirred at room temperature for 4 h, allowed to stand and separate, and the pH of the upper layer was adjusted to 4-5 with hydrochloric acid. The mixture was neutralized with sodium hydroxide, and organic matter was removed by extraction with toluene three times. The remaining solvent was evaporated to dryness, and anhydrous ethanol was added to the residue. The mixture was heated to 60-70°C, filtered while hot to remove inorganic salts, and then concentrated under reduced pressure and recrystallized from ethyl acetate to obtain 1.23 g of intermediate IVa in a yield of 81%. 1 H NMR(500MHz,DMSO-d6)δ8.04(d,J=6.80Hz,1H),7.71(brs,1H),7.22-7.30(m,15H),6.26(t,J=4.60Hz,1 H),5.82(brs,1H),5.42(t,J=5.11Hz,1H),4.74-4.81(dd,J=4.38Hz,2H),4.16-4.18(dd,J=4.65Hz,2H). 13 CNMR(125MHz,DMSO-d6)δ169.2,158.1,150.3,129.4,129.2,128.2,127.2,126.3,126.1,87.3,62.8,36.5.

[0031] Example 3

[0032] Intermediate IVa (2.0 g, 4.09 mmol) was added to methanol (50 mL), and TFA (0.61 mL, 8.18 mmol) was added dropwise. The mixture was heated to 60°C for 4 h, neutralized with saturated sodium bicarbonate solution, concentrated under reduced pressure, and recrystallized from ethyl acetate to obtain 0.93 g of emtricitabine in a yield of 92%. 1H NMR(500MHz,DMSO-d6)δ8.20(d,J=6.80Hz,1H),7.72(brs,1H),7.45(brs,1H),6.14(t,J=4.60Hz,1H),5.16(m,1H) ,4.74-4.81(m,1H),4.16-4.18(dd,J=4.65Hz,1H),3.76(dt,1H),3.42(dd,J=4.46Hz,1H),3.13(dd,J=4.46Hz,1H). 13 C NMR (125MHz, DMSO-d6) δ158.3,153.2,137.3,135.4,126.3,87.3,62.8,36.9.

[0033] Example 4

[0034] (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) (3.00 g, 10.4 mmol), N4-Boc-5-fluorocytosine IIb (2.38 g, 10.4 mmol), and triphenylphosphine (3.27 g, 12.5 mmol) were added to tetrahydrofuran (200 mL). DEAD (1.96 mL, 12.5 mmol) was added dropwise while controlling the temperature between 0 and 10°C. After the addition was complete, the mixture was warmed to room temperature and stirred for 12 hours; then the temperature was raised to 40°C and stirred for an additional 3 hours. The reaction temperature was lowered to room temperature and diluted with tetrahydrofuran (100 mL). The organic phase was washed with water (2 × 100 mL), and the aqueous layer was back-extracted with tetrahydrofuran (100 mL). The mixture was dried over sodium sulfate, filtered, and the solvent and low-boiling-point compounds were removed using a rotary evaporator and vacuum pump. The crude product was suspended in tetrahydrofuran and allowed to stand to produce a white solid, which was filtered under reduced pressure and washed with ether and n-hexane in a volume ratio of 1:1. The filtrate was concentrated on a rotary evaporator, and the residual yellow oil was dissolved in dichloromethane (10 mL). After purification by column chromatography, a white crystalline solid was obtained (yield 4.57 g, yield 88%). 1 H NMR(500MHz,DMSO-d6)δ8.05(d,J=6.80Hz,1H),7.78(brs,1H),6.26(t,J=4.58Hz,1H),5.42(s,1H),4.13- 4.18(dd,J=4.65Hz,2H),1.38(s,9H),1.02-2.05(m,10H),0.89(dd,J=6.81Hz,6H),0.81(d,J=6.80Hz,3H). 13C NMR (125MHz, DMSO-d6) δ171.0,161.2,158.0,156.3,153.2,121.0,102.5,98.2,89.8,78.6,47.2,40.9,34.3,31.2,26.0,23.7,22.2,21.0,20.7.

[0035] Example 5

[0036] In a 250 mL round-bottom flask, intermediate IIIb (2.0 g, 4 mmol) and 15 mL of anhydrous ethanol were added, and a 25% sodium hydroxide (0.03 mL) / sodium borohydride (0.275 g, 7.30 mmol) aqueous solution (4 mL) was added dropwise. The mixture was stirred at room temperature for 4 h, allowed to stand for separation, and the pH of the supernatant was adjusted to 4-5 with hydrochloric acid. The mixture was neutralized with sodium hydroxide, and organic matter was removed by extraction with toluene three times. The remaining solvent was evaporated to dryness, and anhydrous ethanol was added to the residue. The mixture was heated to 60-70°C, filtered while hot to remove inorganic salts, concentrated under reduced pressure, and recrystallized from ethyl acetate to obtain 1.08 g of intermediate IVb in a yield of 78%. 1 H NMR(500MHz,DMSO-d6)δ8.03(d,J=6.80Hz,1H),7.65(brs,1H),6.24(t,J=4.60Hz,1H),5.81(brs,1H ), 5.39 (t, J = 5.10Hz, 1H), 4.71-4.78 (dd, J = 4.35Hz, 2H), 4.12-4.15 (dd, J = 4.65Hz, 2H), 1.38 (s, 9H). 13 C NMR (125MHz, DMSO-d6) δ162.1,157.5,152.1,121.2,109.7,98.2,87.6,84.1,66.3,29.8,27.4.

[0037] Example 6:

[0038] Intermediate IVb (2.0 g, 5.76 mmol) was added to methanol (50 mL), and TFA (0.85 mL, 11.5 mmol) was added dropwise. The mixture was heated to 60°C for 2 h, neutralized with saturated sodium bicarbonate solution, concentrated under reduced pressure, and recrystallized from ethyl acetate to obtain 1.27 g of emtricitabine in a yield of 90%.

[0039] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing emtricitabine by Mitsunobu condensation method, characterized in that: The synthesis is carried out according to the following reaction equation: The following steps are involved: Step 1: (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid 1-menthyl ester (I) and N4-protected 5-fluorocytosine (II) are condensed in an organic solvent using a Mitsunobu reaction to obtain an intermediate (III); Step 2: Intermediate (III) reacts in an alcohol solvent in the presence of a reducing agent to obtain intermediate (IV); Step 3: The intermediate (IV) is deprotected under acidic conditions to obtain emtricitabine.

2. The method for synthesizing emtricitabine by the Mitsunobu condensation method according to claim 1, characterized in that: The molar ratio of compound I to compound II is 1:1-0.

5.

3. The method for synthesizing emtricitabine by the Mitsunobu condensation method according to claim 1, characterized in that: The Mitsunobu reaction uses a Ph3P / DIAD or Ph3P / DEAD combination reagent; wherein the molar ratio of Ph3P to DIAD / DEAD is 1:

1.

4. The method for synthesizing emtricitabine by the Mitsunobu condensation method according to claim 3, characterized in that: The molar ratio of compound II to Mitsunobu reagent is 1:1.2-1.

5.

5. The method for synthesizing emtricitabine by Mitsunobu condensation according to claim 1, characterized in that: The organic solvent is selected from one or more of tetrahydrofuran, acetonitrile, chloroform, dimethyl sulfoxide, ethyl acetate, and dichloromethane.

6. The method for synthesizing emtricitabine by Mitsunobu condensation according to claim 1, characterized in that: The reducing agent is selected from sodium borohydride, lithium borohydride or potassium borohydride.

7. The method for synthesizing emtricitabine by Mitsunobu condensation according to claim 1, characterized in that: The alcohol solvent is selected from methanol or ethanol.

8. The method for synthesizing emtricitabine by the Mitsunobu condensation method according to claim 1, characterized in that: The third step of deprotection uses hydrogen chloride / alcohol solution, acetyl chloride / alcohol or trifluoroacetic acid.

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