Technical Note pubs.acs.org/OPRD
A Scalable One-Pot Process for the Synthesis of Florfenicol Phosphodiester Shuang-Xi Gu,† Jia-Wen Du,† Xiu-Lian Ju,*,† and Qing-Ping Chen‡ †
Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, P.R. China ‡ Hubei Longxiang Pharmaceutical Co., Ltd., Wuxue 435402, P.R. China S Supporting Information *
ABSTRACT: A practical and scalable one-pot process for the preparation of florfenicol phosphodiester (3), a new water-soluble prodrug of florfenicol (1), has been developed by adopting the phosphorylating system of POCl3/pyridine/CH3CN. The yield of 3 was 80.72%, and its HPLC purity reached as high as 99.20%, while the content of the maximum impurity was reduced to 0.28% under the optimum conditions. The present process has proven to be reliable on 500-g and 4-kg scale in the pilot plant.
■
INTRODUCTION Florfenicol (1, Figure 1), a broad-spectrum antibiotic with excellent activities against both Gram-negative and Grampositive bacteria, has been applied in the prevention and treatment of bacterial infections in fishes,1 birds,2 and mammals3 for decades. It is most often administered either orally in solid dosage forms or parenterally in soluble dosage forms. Due to its poor water solubility (approximately 1 mg/ mL), organic solvents such as N-methylpyrrolidinone and propylene glycol as well as polyethylene glycol are used to improve the solubility of florfenicol to above 300 mg/mL, which is often desired in commercial formulations.4 However, the organic solvents often cause significant localized irritation. Therefore, some water-soluble prodrugs of florfenicol have emerged. Among the prodrugs, florfenicol phosphomonoester and its salts (2a and 2b, Figure 1), developed by ScheringPlough Ltd., were rather attractive for their good solubility and pharmacokinetic properties. Unfortunately, the preparation of 2a and 2b (Scheme 1) involved extremely low temperature (−78 °C) and an expensive reagent, which resulted in a high production cost.4 Thus, 2a and 2b have not yet been brought into market for wide applications. In recent years, our group has aimed to develop an inexpensive and water-soluble prodrug of florfenicol. To our delight, florfenicol phosphodiester (3, Figure 1) as a new prodrug of 1 was found to possess good water solubility (at least 700 mg/mL) and excellent pharmacokinetic properties.5 Herein, 3 was prepared from cheap reagents by a simple phosphorylating system of POCl3/pyridine/CH3CN.
phate, was only suitable for the preparation of 2a. Moreover, the required protection and deprotection of hydroxyl resulted in too high of a cost. Interestingly, the most common phosphorylating reagent, POCl3, was found to be efficient to prepare 3, and the ratio of the byproduct 2a could be reduced by control of reaction conditions. In the following experiments, POCl3 was adopted as the phosphorylating reagent, and the organic bases and solvents were screened according to the procedure depicted in Scheme 2. The results are shown in Table 1. The heterocyclic bases (entries 4, 7, and 8) were superior to the tertiary amines with no heterocyclic structure (entries 1, 2, and 6), and CH3CN (entry 4) as solvent was more favorable than the other two polar aprotic solvents acetone and tetrahydrofuran (entries 3 and 5). Obviously, POCl3/pyridine/CH3CN was the most attractive phosphorylating system to prepare 3. Moreover, it is more favorable for the yield of 3 to reserve 20−30 mL of CH3CN for diluting POCl3 (entries 9−10). To facilitate the large-scale production, an improved and practical procedure to prepare 3 (Scheme 3) was developed to avoid the above-mentioned column chromatographic purification. A typical procedure is as follows: After thin layer chromatography (TLC) shows the complete consumption of 1, the theoretical amount of H2O was added to the reaction mixture to quench the P−Cl bond in both the exess POCl3 and the generating phosphodiester chloride. The pH was adjusted to 6−7, followed by removal of most of the solvent. The residue was readjusted to pH 11 with 10% NaOH. The mixture was extracted with dichloromethane (DCM) to remove pyridine and other lipophilic impurities. Next, the alkaline aqueous layer was acidified to pH 2 with 20% aqueous HCl to precipitate 3 as a gel or oil and was immediately followed by extraction with ethyl acetate (EA). The combined organic layers were washed with brine, decolorized with active carbon, dried with anhydrous sodium sulfate, and concentrated under reduced pressure to obtain crude product 3, which was further
■
RESULTS AND DISCUSSION Generally, phosphoesters are prepared by reaction of compounds featuring a hydroxyl group and phosphorylating reagents such as POCl3,6 P2O5,7 protected chlorophosphate,4,8 etc. In our preliminary experiments, the following results were found. The reaction of florfenicol and P2O5 was prone to form the mixture of florfenicol phosphomonoester 2a and phosphodiester 3, and it is difficult to control the ratio of 2a and 3. Dibenzylphosphoryl chloride, a protected chlorophos© 2014 American Chemical Society
Received: February 4, 2014 Published: March 20, 2014 552
dx.doi.org/10.1021/op500038s | Org. Process Res. Dev. 2014, 18, 552−554
Organic Process Research & Development
Technical Note
Figure 1. Structures of florfenicol and its phosphoesters.
Scheme 1. Reported synthesis of 2a and 2b4
According to the improved procedure (Scheme 3), many experiments were carried out to optimize the reaction conditions, and it was found that the molar ratio of 1, POCl3, and pyridine plays an important role in the yield and purity of 3 (Table 2). Although theoretically 1 and POCl3 react
Scheme 2. Synthesis and column chromatographic purification of 3
Table 2. Molar ratio of 1, POCl3 and base on the yield and purity of 3a
Table 1. Base and solvent screen for the preparation of 3 entry
organic base
solvent
yield of 3 (%)
1 2 3 4 5 6 7 8 9 10 11
Et3N (n-Bu)3N pyridine pyridine pyridine N,N-dimethylaniline 4-methylmorpholine 4-dimethylaminopyridine pyridine pyridine pyridine
CH3CN (110 mL) CH3CN (110 mL) acetone (110 mL) CH3CN (110 mL) THF (110 mL) CH3CN (110 mL) CH3CN (110 mL) CH3CN (110 mL) CH3CN (80 + 30 mL) CH3CN (90 + 20 mL) CH3CN (100 + 10 mL)
8.32 9.58 21.61 39.83 17.48 10.44 16.68 24.42 42.66 42.54 41.83
molar ratio entry (1:POCl3:pyridine) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Reaction conditions: 1 (55.83 mmol), molar ratio (1:POCl3:base) 1:1.5:1.5, 110 mL of solvent, 0 °C for 14 h, the yield of 3 refers to the isolated yield by column chromatography. In entries 1−8, 110 mL of solvent was added into the reaction mixture of 1 and organic base in one batch before the addition of POCl3. In entries 9−11, 80/90/100 mL of CH3CN was added into the reaction mixture of 1 and pyridine, followed by adding dropwise of the solution of POCl3 in CH3CN (30/ 20/10 mL).
1:0.5:1 1:1:1 1:1.5:1.5 1:2:2 1:2.5:2.5 1:1.5:1 1:1.5:2 1:1.5:2.5 1:1.5:3 1:1.4:2.5 1:1.3:2.5 1:1.2:2.5 1:1.2:1.9 1:1.3:2.3 1:1.3:2.1 1:1.3:1.9
yield of 3 (%)
purity of 3b (%)
content of the maximum impurityb (%)
15.93 27.38 42.71 44.19 45.82 28.07 51.41 68.28 68.56 71.69 77.23 70.08 78.68 81.14 80.72 77.77
83.03 88.76 96.57 95.70 96.36 95.45 96.35 97.83 97.88 98.12 97.17 96.82 96.58 98.87 99.20 98.86
11.35 7.68 0.60 1.31 1.26 3.50 1.08 1.06 0.55 0.73 1.35 2.03 2.56 0.62 0.28 0.56
a
All experiments in this table were carried out on 30-g scale according to Scheme 3. bThe purity of 3 and the content of the maximum impurity are measured by HPLC.
Scheme 3. Improved and scalable one-pot synthesis and purification of 3
on a molar ratio of 1:0.5 to prepare 3 and 1 equiv of pyridine should be enough, the entry 1 afforded product 3 in a yield of only 15.93%, and simutaneously increasing the amount of POCl3 and pyridine seems to be favorable in a range from 0.5 to 2.5 equiv (entries 2−5). In the presence of 1.5 equiv of POCl3, an increase in the amount of pyridine from 1 to 3 equiv resulted in an increase in yield of 3 (entries 3, 6−9). Typically, in entries 8−9, the yields of 3 reached 68.28% and 68.56% respectively, the purity of 3 both reached above 97.80%, and the contents of the maximum impurity were 1.06% and 0.55%, respectively. To our excitement, the optimum molar ratio
purified by trituration with the mixed solvent of acetone and petroleum ether (3:5, v/v) by stirring and filtering to afford 3 with higher purity. 553
dx.doi.org/10.1021/op500038s | Org. Process Res. Dev. 2014, 18, 552−554
Organic Process Research & Development
Technical Note
for 3 h at this temperature. The mixture was filtered and dried under vacuum at 50 °C for 3 h to obtain a white solid 3. Yield 17.54 g (80.72%) with 99.20% HPLC purity; 31P NMR: δ (CDCl3) −2.40; 1H NMR (DMSO-d6): δ 3.17 (s, 6H, CH3), 4.21−4.33 (m, 4H, CH2F), 4.55−4.68 (m, 2H, CHN), 5.44 (d, 2H, J = 9.2, CHO−P), 6.41 (s, 2H, CHCl2), 7.43 (d, 4H, J = 8.4, Ar-H), 7.74 (d, 4H, J = 8.4, Ar-H), 8.88 (d, 2H, J = 8.0, NHCO); IR (KBr, cm−1): 3408, 3017, 2929, 1694, 1542, 1468 1411, 1300, 1151, 1063, 961, 887, 815, 772, 735, 678, 628, 551; MS (ESI−) 777 [M − H]; HRMS calcd for C24H28Cl4F2N2O10PS2 [M + H] 778.9616, found 778.8802.
(1:1.3:2.1) was screened out in further optimization. Under the optimal molar ratio, the yield of 3 is 80.72%, the purity of 3 reached as high as 99.20%, and the content of the maximum impurity was reduced to 0.28%. The present process has proven to be reliable on 500-g and 4-kg scale in the pilot plant.
■
CONCLUSION In conclusion, we have developed a concise and scalable onepot process for the synthesis of florfenicol phosphodiester (3). The phosphorylating system of POCl3/pyridine/CH3CN proved to be efficient to prepare 3. Under the optimum conditions, the yield of 3 was up to 80.72%, the purity of 3 reached as high as 99.20%, and the content of the maximum impurity was reduced to as low as 0.28%. The present process has been proven to be feasible on 500-g and 4-kg scale in the pilot plant.
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
S Supporting Information *
Spectra for florfenicol phosphodiester (3). This material is available free of charge via the Internet at http://pubs.acs.org.
■
Corresponding Author
EXPERIMENTAL SECTION General. Solvents and reagents were obtained from commercial sources and used without further purification. 1H NMR spectra were recorded in DMSO-d6 on a Varian Mercury spectrometer plus 400 MHz using TMS as an internal standard. 31 P NMR spectra were obtained in DMSO-d6 on a Varian Mercury spectrometer plus 162 MHz. IR spectra were recorded in the solid state as KBr dispersion using a Perkin-Elmer 1650 FT IR spectrometer. Mass spectra (MS) were obtained on a Waters Quattro Micromass instrument using electrospray ionization (ESI) techniques. High -resolution mass spectra (HRMS) were performed on a Waters SYNAPT G1 HDMS instrument. All the reactions were monitored by thin layer chromatography (TLC) on precoated silica gel G plates at 254 nm under a UV lamp. Column chromatography separations were obtained on silica gel (300−400 mesh). HPLC analysis was performed on a Agilent 1100 HPLC system with a UV detector (λ) at 28 °C, using an Intersil column (250 mm × 4.6 mm) which is eluted with a solution of CH3CN (10−40% volume) in 0.01 M Na2HPO4 at a speed of 1.0 mL/min. Bis((1R,2S)-2-(2,2-Dichloroacetamido)-3-fluoro-1-(4(methylsulfonyl)phenyl)propyl) Hydrogen Phosphate (3). A mixture of 1 (20.00 g, 55.83 mmol), pyridine (9.27 g, 117.25 mmol), and acetonitrile (80 mL) was stirred for 10 min at room temperature (23−25 °C) and another 20 min at 0 °C. Then a solution of POCl3 (11.13 g, 72.58 mmol) in acetonitrile (30 mL) was added dropwise in 1 h. The reaction mixture was stirred for 1 h at 0 °C and 10 h at 23−25 °C. Upon completion of the reaction (monitored by TLC), 3 mL H2O was added dropwise in 10 min at 0−5 °C, and the resulting solution was stirred for 8 h at 23−25 °C. Next, the solution was adjusted to pH 6−7 with 10% NaOH at 5−10 °C, and most of solvent was recovered under reduced pressure, followed by basification to pH 11 with 10% NaOH at 5−10 °C. Afterwards, the solution was washed with DCM (4 × 200 mL), and the aqueous phase was acified to pH 2 with 20% aqueous HCl at 5−10 °C, immediately followed by extraction with EA (4 × 250 mL). The combined organic layers were washed with saturated brine (1 × 50 mL), decolorized with active carbon (20 g) at 23−25 °C, dried with anhydrous Na2SO4, filtered, concentrated under reduced pressure, and dried under vacuum at 50 °C for 3 h to obtain an off-white crude product 3 (19.06 g). The crude product was added into the chilled mixed solvent of acetone (60 mL) and petroleum ether (100 mL) at −5 °C, and stirred
*Telephone: +86 27 87195671. Fax: +86 27 87194465. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We are grateful to Hubei Novel Reactor and Green Chemical Technology Key Laboratory Open Fund (No. RGCT201201), Wuhan Institute of Technology Scientific Research Fund (No. 10128301) for the financial support. We thank Mr. Chun-Xi Cheng, Mr. Hua-Wei Liu, and Mr. Ying-Jie Liu in Haiso Technology Co., Ltd. for their support for HPLC analysis of the product.
■
REFERENCES
(1) Horsberg, T. E.; Hoff, K. A.; Nordmo, R. J. Aquat. Anim. Health 1996, 8, 292. (2) Shen, J. Z.; Hu, D. F.; Wu, X. A..; Coats, J. J. Vet. Pharmacol. Ther. 2003, 26, 337. (3) Ueda, Y.; Ohtsuki, S.; Narukawa, N. J. Vet. Med. Sci. 1995, 57, 261. (4) Hecker, S.; Pansare, S. WO Pat. 2005/063257, 2005. (5) Ju, X. L.; Gu, S. X.; Zhao, Y.; Chen, Q. P.; Du, J. W. CN Pat. 103,242,363, 2013. (6) (a) Imai, K.; Fujii, S.; Takanohashi, K.; Furukawa, Y.; Masuda, T.; Honjo, M. J. Org. Chem. 1969, 34, 1547. (b) Mahmoodi, N. O. Phosphorus, Sulfur Silicon Relat. Elem. 2002, 177, 2887. (c) Sun, Y.; Feng, Y.; Dong, H.; Chen, Z.; Han, L. Cent. Eur. J. Chem. 2007, 5, 620. (7) Wang, X. C.; Zhang, M. R.; Shi, B. Fine Chem. 2002, 19, 658. (8) Kurtz, R. R. U.S. Pat. 4,531,004, 1985.
554
dx.doi.org/10.1021/op500038s | Org. Process Res. Dev. 2014, 18, 552−554
