A scalable one-pot process for the synthesis of florfenicol phosphodiester

Article abstract of DOI:10.1021/op500038s

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.

Full text of DOI:10.1021/op500038s

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.
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 purifica-
tion. 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
INTRODUCTION
■
Florfenicol (1, Figure 1), a broad-spectrum antibiotic with
excellent activities against both Gram-negative and Gram-
positive 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 Schering-
Plough 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.
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-
Received: February 4, 2014
Published: March 20, 2014
© 2014 American Chemical Society
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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
a
Table 1. Base and solvent screen for the preparation of 3
purity of 3
yield of 3
content of the
maximum impurity
(%)
b
entry
organic base
solvent
(%)
molar ratio
yield of purity of
b
entry (1:POCl3:pyridine)
3 (%)
3 (%)
1
Et3N
CH3CN (110 mL)
CH3CN (110 mL)
acetone (110 mL)
CH3CN (110 mL)
THF (110 mL)
8.32
9.58
1
1:0.5:1
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
2
(n-Bu)3N
2
1:1:1
3
pyridine
21.61
39.83
17.48
10.44
16.68
24.42
42.66
42.54
41.83
3
1:1.5:1.5
1:2:2
4
pyridine
4
5
pyridine
5
1:2.5:2.5
1:1.5:1
6
N,N-dimethylaniline
4-methylmorpholine
CH3CN (110 mL)
CH3CN (110 mL)
6
7
7
1:1.5:2
8
4-dimethylaminopyridine CH3CN (110 mL)
8
1:1.5:2.5
1:1.5:3
9
pyridine
pyridine
pyridine
CH3CN (80 + 30 mL)
CH3CN (90 + 20 mL)
CH3CN (100 + 10 mL)
9
10
11
10
11
12
13
14
15
16
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
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).
a
All experiments in this table were carried out on 30-g scale according
to Scheme 3. The purity of 3 and the content of the maximum
b
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.
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Organic Process Research & Development
Technical Note
(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.
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
1
(CDCl3) −2.40; H 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;
CONCLUSION
■
In conclusion, we have developed a concise and scalable one-
pot 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.
MS (ESI− ) 777 [M
C24H28Cl4F2N2O10PS2 [M + H] 778.9616, found 778.8802.
− H]; HRMS calcd for
ASSOCIATED CONTENT
* Supporting Information
Spectra for florfenicol phosphodiester (3). This material is
■
S
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86 27 87195671. Fax: +86 27 87194465. E-mail:
xiulianju2008@aliyun.com.
■
EXPERIMENTAL SECTION
■
General. Solvents and reagents were obtained from
commercial sources and used without further purification. H
1
Notes
NMR spectra were recorded in DMSO-d6 on a Varian Mercury
spectrometer plus 400 MHz using TMS as an internal standard.
31P 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
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.
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