A practical synthesis for the key intermediate (G) of Apixaban

doi:10.21203/rs.3.rs-3287464/v1

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A practical synthesis for the key intermediate (G) of Apixaban

https://doi.org/10.21203/rs.3.rs-3287464/v1

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published 13 Dec, 2023

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Apixaban is a highly potent, selective, and efficacious inhibitor of blood coagulation factor Xa. A practical and efficient process has been developed for the preparation of the key intermediate (G) of Apixaban. Starting from inexpensive 4-chloronitrobenzene and piperidine, an eight-step procedure for G has been developed. In this case, sodium chlorite is used twice to oxidize the piperidine cycle to the corresponding lactam under a CO₂ atmosphere, resulting in the construction of two lactams. Furthermore, most of these reactions are highly efficient and practical as they occur under mild conditions. Most of the intermediates can be obtained through simple slurry or recrystallization, and column chromatography purification is not necessary.

Apixaban intermediate

Synthesis

Amine oxidation

Sodium chlorite

Apixaban, marketed by Bristol-Myers Squibb under the trade name Eliquis, is a highly potent, selective, and efficacious inhibitor for blood coagulation factor Xa [1–3]. This oral anticoagulant is an immediate-release form of a peroral drug with quick dissolution, linear pharmacokinetics, good bioavailability, and a rapid onset and offset of action [4–5]. It has been approved for the clinical treatment of various thromboembolic disorders, including the reduction of stroke risk in nonvalvular atrial fibrillation, thromboprophylaxis after hip or knee replacement, treatment of deep vein thrombosis or pulmonary embolism, and prevention of recurrent deep vein thrombosis and pulmonary embolism [6–9]. Currently, it is one of the most widely used oral anticoagulant drugs in the world. Since 2018, Apixaban has become the top-selling small molecule drug among the Top 200 Brand Name Drugs [10–11]. As shown in Scheme 1, Apixaban contains two different six-ring lactam (I and II) structures, making the synthesis of Apixaban reliant on the construction of these six-ring lactams. So far, several processes have been developed for this purpose.

In 2007, Donald J. P. Pinto et. al. developed the first-generation process for Apixaban [12]. In this case, two six-ring lactams was introduced subsequently using piperidin-2-one as a key synthon, and a copper(I) catalyzed Goldberger coupling reaction was applied. (Scheme 2)

Later, a similar strategy was adopted for Apixaban starting from 2-piperidone [13–14], a three-step procedure affords the key intermediate 5, which is condensed with 1 to give 6. Finally, another lactam ring is introduced by a Goldberger coupling reaction between 6 and 1-(4-iodophenyl) piperidin-2-one 7. (Scheme 3)

Both processes have some disadvantages, such as the difficulty in preparing intermediates iodide 2 and 5, as well as 7, and the high cost of 2-piperidone used as the main material. In addition, transition metal is used in the second step to last, which may result in the presence of heavy metal residues.

In 2013, several synthesis processes starting from 4-nitroaniline for Apixaban were reported [15–17]. Both six-ring lactams were constructed by condensation of a primary aromatic amine with 5-bromopentanoyl chloride (or 5-chloropentanoyl chloride). This protocol for six-ring lactam formation is usually insufficient, especially for the second lactam formation, due to the structural complexity as well as the instability of intermediate 9 under harsh reaction conditions. (Scheme 4). Furthermore, 5-bromopentanoyl chloride and 5-chloropentanoyl chloride are expensive, leading to a high cost of Apixaban.

Recently, a facile and efficient oxidation of tertiary amines to lactams was developed using inexpensive sodium chlorite as an oxidant [18–19]. Based on this finding, a novel process was designed for constructing lactams in Apixaban. As shown in Scheme 5, 4-chloronitrobenzene is condensed with piperidine to afford 1-(4-nitrophenyl) piperidine A, which is then oxidized by sodium chlorite under a CO₂ atmosphere to achieve 1-(4-nitrophenyl) piperidin-2-one B. Then, with FeCl₃/charcoal as catalyst, the nitro group is reduced by aqueous hydrazine to form the corresponding aniline C. In the presence of K₂CO₃, cyclization between C and 1,5-dibromo pentane affords 1-(4-(piperidin-1-yl) phenyl) piperidin-2-one D, which is chlorinated with PCl₅ to produce E. Using the same oxidation system, E is then transformed into the key intermediate F, but in this case, a side product F' is also formed with a ratio of 1:1. In the presence of lithium carbonate, the mixture of F and F' is eliminated to yield G and G' respectively, and G' can be transferred to G using Zinc-acetic acid as reductant. (Scheme 5).

Reaction conditions: a) piperidine (1 mol), 4-chloro nitrobenzene (0.5 mol), Na₂CO₃ (1 mol), 100°C, 3 h; b) NaClO₂ (3 eq), CO₂ balloon; c) FeCl₃/Charcoal (15 mol %), aq. hydrazine (5 eq), reflux; d) 1,5-dibromopentane (1.2 eq), K₂CO₃ powder (1.5 eq); e) PCl₅ (3 eq); f) NaClO₂ (3 eq), CO₂ balloon; g) Li₂CO₃/LiCl

Our investigation was initiated by condensing 4-chloronitrobenzene and piperidine in the presence of sodium carbonate. After the reaction, we obtained 1-(4-nitrophenyl) piperidine A with a 98% yield through simple slurring with water. A was of sufficient purity and could be used in the next step without further purification. We then smoothly oxidized A to lactam B using sodium chlorite as an oxidant and CO₂ for precise pH adjustment, resulting in a 97% yield. By utilizing a catalytic amount of FeCl₃/Charcoal as a catalyst and aqueous hydrazine, we were able to reduce the nitro group in B with a 95% yield. In the presence of K₂CO₃, the cycloaddition of C with 1,5-dibromo pentane provided D with a 90% yield. Thus far, four subsequent steps allowed for the easy production of D with a high total yield (81%), and no special workup was necessary (Scheme 5).

However, when attempting to convert D to E using PCl₅ as a chlorinating reagent, the reaction mixture became very messy, and no major products were detected. This may be attributed to the destructive nature of the amine under the reaction conditions. Thus, in order to protect the amino group in D, we attempted to use HCl, resulting in the formation of a D-HCl salt which was then converted to E with a 90% yield using PCl₅ as the chlorinating reagent. The oxidation of E was carried out using a procedure similar to that used for the oxidation of A. However, an impurity F' was formed alongside the target compound, and the ratio of F to F' was found to be close to 1:1, resulting in a total yield of over 90%. The structure of F' was confirmed to be 3,3-dichloro-1-(4-(3-chloro-2-oxopiperidin-1-yl) phenyl) piperidin-2-one. In the presence of Li₂CO₃/LiCl, the mixture of F and F' was eliminated, yielding G and G' with a total yield of 90%. and G' is easily converted quantitatively to G using Zn/acetic acid as reductant. (Scheme 6)

With the key intermediate G at hand, Apixaban can be easily obtained by cyclization G with ethyl (Z)-2-chloro-2-(2-(4-methoxyphenyl)hydrazono) acetate, subsequently amidation gives Apixaban [18–19].

In conclusion, a novel and practical process for the key intermediate (G) of Apixaban has been developed. In this case, a transient metal-free oxidation of tertiary amine to corresponding lactam is achieved using cheap and innocuous sodium chlorite as an oxidant, and CO₂ is used for precise pH adjustment. All materials involved in this process are low-cost and easily available. Furthermore, working up the reaction in the whole process is quite convenient, requiring no column chromatography. The product and intermediates can be obtained via a simple slurry and filtration, making it suitable for industrial production.

Commercial reagents were acquired from Macklin, Adamas-beta, Aladdin, Bidepharm or, and used as received. All commercially available reagents were used without further purification unless otherwise stated. All solvents were dried according to established procedures. Reactions were monitored by thin layer chromatography (TLC), the potassium K₂CO₃ powder was prepared by dissolving recrystallization.

¹H NMR and ¹³C NMR were recorded on a Bruker AV400 spectrometer at room temperature. Proton chemical shifts are reported in ppm downfield from tetramethylsilane or from the residual solvent as internal standard in CDCl₃ (δ 7.26 ppm) and in (CD₃)₂SO (2.50 ppm). Carbon chemical shifts were internally referenced to the deuterated solvent signals in CDCl₃ (δ 77.16ppm) and in (CD₃)₂SO (39.52 ppm).

1-(4-nitrophenyl) piperidine (A, C₁₁H₁₄N₂O₂).

A mixture of p-nitrochlorobenzene (78.8 g, 0.5 mol), Na₂CO₃ (106.1 g, 1 mol) and piperidine (85.1 g, 1 mol) is heated to reflux under nitrogen for 3 h. After the reaction, it is cooled to room temperature, and 100 mL of water is added. The solid is collected by filtration and slurred again in water to afford 1-(4-nitrophenyl) piperidine A as an orange-yellow solid, yield of 98%. ¹H NMR (400 MHz, CDCl₃) δ: 1.69 (m, 6H), 3.50 (m, 4H), 6.77–6.80 (d, J = 12.0 Hz, 2H), 8.08–8.11 (d, J = 12.0 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 154.90, 137.41, 126.15, 112.48, 112.30, 48.38, 25.19, 24.24.

1-(4-nitrophenyl) piperidin-2-one (B, C₁₁H₁₂N₂O₃).

A solution of A (10.3 g, 50 mmol) in acetonitrile is stirred at 50 ℃ with a CO₂ balloon, then aqueous NaClO₂ solution (24%, 75.0 g, 200 mmol) is added drop wisely. After all of A is consumed completely in three hours (detected by TLC), the reaction is quenched with aqueous saturated Na₂SO₃ solution. Then acetonitrile is removed by vacuum distillation, and a large amount of solid is precipitated, which is collected by filtration and the filter cake is washed with water, and dried to achieve a light-yellow solid in 97% yield. ¹H NMR (400 MHz, CDCl₃) δ: 2.04–1.94 (m, 4H), 2.61 (t, J = 6.0 Hz, 2H), 3.73 (t, J = 6.0 Hz, 2H), 7.49 (d, J = 8.0 Hz, 2H), 8.24 (d, J = 8.0 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 170.29, 148.92, 145.23, 125.87, 124.36, 50.91, 33.10, 23.42, 21.25.

1-(4-aminophenyl) piperidin-2-one (C, C₁₁H₁₄N₂O).

To a solution of B (11.0 g, 0.05 mol) in 110 mL ethanol, anhydrous ferric chloride (1.2 g, 7.5 mmol) and activated carbon (4.2 g) are added subsequently. Then the reaction is heated to reflux, 14.7 g of hydrazine hydrate (85% aqueous solution, 0.25 mol) is added dropwise in 1 h. TLC and HPLC detection showed that this reaction could be completed in 3 h. Charcoal was removed by filtration, and the solution was concentrated on a rotatory evaporator to leave a residue, which was recrystallized from acetonitrile to afford pure C as a white solid in 95% yield; ¹H NMR (400 MHz, CDCl₃) δ: 1.91–1.89 (m, 4 H), 2.52 (t, J = 4.0 Hz, 2 H), 3.56 (t, J = 4.0 Hz, 2 H), 3.69 (b, 2H), 6.65 (d, J = 8.0Hz, 2H), 6.99 (d, J = 8.0 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 170.15, 145.23, 134.42, 127.22, 115.59, 52.10, 32.85, 23.63, 21.55.

1-(4-(piperidin-1-yl) phenyl) piperidin-2-one (D, C₁₆H₂₂N₂O).

A mixture of compound C (9.5 g, 50 mmol), 1,5-dibromopentane (13.8 g, 60 mmol) and K₂CO₃ powder (10.4 g, 75 mmol) in xylene (20 mL) is heated at 120 ℃ under nitrogen atmosphere for 10 h. After reaction, the mixture is cooled to room temperature, and 50 mL water is added and the mixture is stirred for 0.5 h. Solid is collected by filtration and dried under vacuum to produce D as a white solid in 90%yield. ¹H NMR (400 MHz, CDCl₃) δ: 1.56 (m, 2H), 1.69 (m, 4H), 1.91 (m, 4H), 2.53 (m, 2H), 3.14 (m, 4H), 3.58 (s, 2H), 6.92 (d, 2H, J = 8.0 Hz), 7.09 (d, 2H, J = 8.0 Hz); ¹³C NMR (100 MHz, CDCl₃) δ: 170.14, 150.86, 134.73, 126.76, 116.93, 52.00, 50.64, 32.88, 25.84, 24.26, 23.64, 21.56.

3,3-dichloro-1-(4-(piperidin-1-yl) phenyl) piperidin-2-one (E, C₁₆H₂₀Cl₂N₂O).

D (12.9 g, 50 mmol) is dissolved in 50 mL of hydrochloric acid methanol solution (2 M). After removal of all volatiles yields hydrochloride salt of D, which is dissolved in anhydrous dichloromethane (150 mL). To this solution, PCl₅ (31.2 g, 150 mmol) is added in one portion to form a mixture, which is refluxed to form a clear solution. The reaction is completed in 5 h, and it is poured into ice-water, and neutralized with NaOH(2 N), then it is extracted with DCM (100 mL × 3) and dried with anhydrous sodium sulfate. The solution is passed through a pad of silica gel for decolorization. Removal of all solvent leaves a light-yellow solid E, yield is 90%. ¹H NMR (400 MHz, CDCl₃) δ: 1.56–1.59 (m, 4H), 1.71 (m, 4H), 2.18–2.24 (m, 2H), 2.88–2.91(m, 2H), 3.15–3.18 (m, 4H), 3.70 (t, 2H, J = 6.4 Hz), 6.91 (d, 2H, J = 8.8 Hz), 7.12 (d, 2H, J = 8.8 Hz); ¹³C NMR (100 MHz, CDCl₃) δ: 163.52, 151.16, 133.38, 126.29, 116.58, 83.40, 52.03, 50.40, 44.02, 25.71, 24.23, 20.68; HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₆H₂₁Cl₂N₂O: 327.1031; found: 327.1030.

Oxidation of E with sodium chlorite:

3,3-dichloro-1-(4-(2-oxopiperidin-1-yl)phenyl)piperidin-2-one (F, C₁₆H₁₈Cl₂N₂O₂).

3,3-dichloro-1-(4-(3-chloro-2-oxopiperidin-1-yl)phenyl)piperidin-2-one (F', C₁₆H₁₇Cl₃N₂O₂).

A solution of E (16.4 g, 50 mmol) in acetonitrile (240 mL) is heated at 50 ℃ with a CO₂ balloon, and aqueous NaClO₂ (13.6 g in 42 mL water, 150 mmol) is added drop wisely. Then the mixture is stirred at 50 ℃ for 3 h. After the reaction is completed, an aqueous saturated Na₂SO₃ solution is added and stirred at room temperature for 1 h. Removal of most of acetonitrile leaves a suspension, and the solid materials is collected by filtration, and washed with water dried. The product is proved to be the mixture of F and F' in total 90% yield with a ratio of 1:1. And F and F' are separated with silica gel column, and their structure are confirmed by NMR and MS spectrums.

F: ¹H NMR (400 MHz, CDCl₃) δ: 1.91–1.99 (m, 4H), 2.21–2.27 (m, 2H), 2.55–2.58 (m, 2H, J = 6.1 Hz), 2.90–2.93 (m, 2H), 3.65 (t, 2H, J = 5.8 Hz), 3.75 (t, 2H, J = 6.2 Hz), 7.30 (s, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 170.15, 163.61, 142.33, 140.29, 127.09, 126.62, 83.09, 51.79, 51.56, 43.94, 32.89, 23.52, 21.42, 20.66; MS (ESI): m/z = 340.97 {[M + H]⁺}.

F': ¹H NMR (400 MHz, CDCl₃) δ: 1.92–2.02 (m, 1H), 2.22 ~ 2.28 (m, 2H), 2.30–2.46 (m, 3H), 2.90–2.93 (m, 2H), 3.71 (t, 2H, J = 5.4 Hz), 3.76 (t, 2H, J = 6.3 Hz), 4.58 (t, 1H, J = 4.5 Hz), 7.31 (s, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 166.21, 163.64, 141.45, 140.77, 126.83, 126.72, 83.04, 55.33, 51.77, 51.27, 43.92, 31.15, 20.65, 19.06; MS (ESI): m/z = 374.97 {[M + H]⁺}.

Elimination of F and F':

3-chloro-1-(4-(2-oxopiperidin-1-yl)phenyl)-5,6-dihydropyridin-2(1H)-one (G, C₁₆H₁₇ClN₂O₂).

3-chloro-1-(4-(3-chloro-2-oxopiperidin-1-yl)phenyl)-5,6-dihydropyridin-2(1H)-one (G', C₁₆H₁₆Cl₂N₂O₂).

To a mixture of F and F' (17.9 g, 50 mmol) in 35 mL DMF, 1.06 g LiCl (25 mmol) is added. Then it is stirred at 105 ℃, and 4.43 g Li₂CO₃ (60 mmol) is added in several portions with 5–10 minutes interval. After addition, the reaction continues stirring at 105 ℃ for 2 h. After F and F' are consumed, most of DMF is removed by vacuum distillation, and water (60 mL) is added, and extracted with DCM (80 mL × 3). The combined organic layer is dried with sodium sulfate. Removal of all solvent leaves a residue, which is slurred with 20 mL mixture of ethyl acetate/ petroleum ether (10:1, v/v) to afford solid. Structure conformation shows that elimination products G and G' are obtained in ratio of 1:1, Total yield is 90%.

G: ¹H NMR (400 MHz, CDCl₃) δ: 1.91–1.98 (m, 4H), 2.54–2.64 (m, 4H), 3.63–3.66 (m, 2H), 3.90 (t, 2H, J = 6.9 Hz), 6.85 (t, 1H, J = 4.6 Hz), 7.27 (d, 2H, J = 8.7 Hz), 7.33 (d, 2H, J = 8.7 Hz); ¹³C NMR (100 MHz, CDCl₃) δ: 170.17, 159.94, 141.34, 140.53, 136.17, 128.58, 126.79, 125.81, 51.62, 48.76, 32.88, 25.20, 23.53, 21.44; MS (ESI): m/z = 304.95 {[M + H]⁺}.

G': ¹H NMR (400 MHz, CDCl₃) δ: 1.91–1.99 (m, 1H), 2.30–2.46 (m, 3H), 2.59–2.64 (m, 2H), 3.70–3.74 (m, 2H), 3.90 (t, 2H, J = 6.9 Hz), 4.58 (t, 1H, J = 4.3 Hz), 6.86 (t, 1H, J = 4.6 Hz), 7.28 (d, 2H, J = 8.9 Hz), 7.35 (d, 2H, J = 8.8 Hz); ¹³C NMR (100 MHz, CDCl₃) δ: 166.20, 159.94, 140.96, 140.44, 136.29, 128.51, 126.48, 125.84, 55.36, 51.31, 48.72, 31.16, 25.19, 19.05; MS (ESI): m/z = 338.95 {[M + H]⁺}.

Reduction G' to G with zinc/acetic acid:

To a solution of G and G' (1 g) in 5 mL acetic acid, 2 g zinc powder is added, which is heated at 100 ^oC for 5–6 h. LCMS shows that all G' is converted to G. All acetic acid is removed by evaporation, and 20 mL CH₂Cl₂ is added. After unreacted zinc is removed by filtration, the organic layer is concentrated, and recrystallization from ethyl acetate gives target compound G.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21372081, 21172072).

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Schemes 1 and 6 are available in the Supplementary Files section.

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You are reading this latest preprint version

A practical synthesis for the key intermediate (G) of Apixaban

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Journal Publication

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Abstract

Introduction

Results and Discussion

Conclusion

Experimental

Oxidation of E with sodium chlorite:

Elimination of F and F':

Reduction G' to G with zinc/acetic acid:

Declarations

References

Schemes

Supplementary Files

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