Practical preparation of trimethoprim: A classical antibacterial agent

Article abstract of DOI:10.1080/00397911.2011.644380

An efficient, simple, and mild preparation of the classical antibacterial agent trimethoprim (1) was achieved in 85% overall yield from 3,4,5-trimethoxybenzaldehyde (2). First, the addition of propenenitrile (3) with dimethylamine almost quantitatively afforded 3-dimethylaminopropanenitrile (7). Then, by condensation of 7 with 2 as well as the continuous replacement of 3-dimethylamino group with aniline in situ, the key intermediate 3-anilino-2-(3,4,5-trimethoxybenzyl)propenenitrile (9) was obtained in an excellent yield of 91% with a one-pot procedure. Finally, the cyclization of 9 with guanidine nitrate furnished 1 in yields as good as 95% in the presence of the excessive sodium methoxide. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications1 to view the free supplemental file. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2011.644380

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Practical Preparation of Trimethoprim: A
Classical Antibacterial Agent
Ya-Fei Ji a , Jian-An Jiang a , Hong-Wei Liu a , Dao-Hua Liao a & Xian-
Yong Wei b
a School of Pharmacy, East China University of Science and
Technology, Shanghai, China
b School of Chemical Engineering, China University of Mining and
Technology, Xuzhou, China
Accepted author version posted online: 05 Jun 2012.Version of
record first published: 06 Mar 2013.
To cite this article: Ya-Fei Ji , Jian-An Jiang , Hong-Wei Liu , Dao-Hua Liao & Xian-Yong Wei (2013):
Practical Preparation of Trimethoprim: A Classical Antibacterial Agent, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:11, 1517-1522
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Synthetic Communications1, 43: 1517–1522, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.644380
PRACTICAL PREPARATION OF TRIMETHOPRIM:
A CLASSICAL ANTIBACTERIAL AGENT
Ya-Fei Ji,1 Jian-An Jiang,1 Hong-Wei Liu,1 Dao-Hua Liao,1
and Xian-Yong Wei2
1School of Pharmacy, East China University of Science and Technology,
Shanghai, China
2School of Chemical Engineering, China University of Mining and
Technology, Xuzhou, China
GRAPHICAL ABSTRACT
Abstract An efficient, simple, and mild preparation of the classical antibacterial agent
trimethoprim (1) was achieved in 85% overall yield from 3,4,5-trimethoxybenzaldehyde
(2). First, the addition of propenenitrile (3) with dimethylamine almost quantitatively
afforded 3-dimethylaminopropanenitrile (7). Then, by condensation of 7 with 2 as well as
the continuous replacement of 3-dimethylamino group with aniline in situ, the key intermedi-
ate 3-anilino-2-(3,4,5-trimethoxybenzyl)propenenitrile (9) was obtained in an excellent
yield of 91% with a one-pot procedure. Finally, the cyclization of 9 with guanidine nitrate
furnished 1 in yields as good as 95% in the presence of the excessive sodium methoxide.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Antibacterial agent; highly effective preparation; trimethoprim; 3,4,5-
trimethoxybenzaldehyde
INTRODUCTION
Trimethoprim (1) is a classical antibacterial agent that serves as a potent and
selective inhibitor of bacterial dihydrofolate reductase. It is used solely, or acts as
a potentiator in combination with sylfamethoxazole, to treat a wide range of
bacterial infections in humans and poultry.[1] In recent decades, antibacterial agent
Received September 18, 2011.
Address correspondence to Ya-Fei Ji, or to Xian-Yong Wei, School of Chemical Engineering,
China University of Mining and Technology, Xuzhou 221008, China. E-mail: ji_yafei@yahoo.com.cn;
wei_xianyong@163.com
1517

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Y.-F. JI ET AL.
Scheme 1. Route to 1.
1 has been embodied in the pharmacopoeias of many countries in the world.
In China, more than 3,000 tons of 1 were output in 2010 for the global market,
which shows an increasing demand because of 1’s antimicrobial and antimalarial
applications. So far, antibacterial agent 1 has been regarded as the most mature
dihydrofolate reductase inhibitor in clinical application.
Based on the previously known synthesis involving a base-catalyzed 1,3-
prototropic rearrangement to form enol ether=acetal intermediates (5=6) along the
route shown in Scheme 1,[1–3] an improved manufacturing process of 1 was carried
out with an unsatisfactory yield of 65% or less from 3,4,5-trimethoxybenzaldehyde
(2), a relatively expensive material.[4] Because of the limitation of the reactivity of
5 and 6 in the manufacturing process, it is very difficult to further increase the yield
of 1. The previous patents, which we named the ‘‘Cresswell approach,’’[5,6] displayed
a superior synthesis of 1 using a powerful cyclization of 3-anilino-2-(3,4,5-
trimethoxybenzyl)-propenenitrile (9) and guanidine.
In pursuit of greater yield and lower cost, we re-examined the Cresswell
approach to develop a practical preparation of 1 (Scheme 2). In this report, we
describe a highly efficient preparation of 1, taking advantage of the crucial cycliza-
tion of 9 and guanidine. By means of dimethylamine and aniline as assistant
reagents, the key intermediate 9 was effectively prepared via the condensation of
3-dimethylaminopropanenitrile (7) and 2, followed by substituting 3-dimethylamino
group with aniline in situ with a convenient one-pot procedure. Besides, the addition
reaction of propenenitrile (3) and dimethylamine was achieved to afford 7 in nearly
quantitative yield.
Scheme 2. Practical route to preparing 1.

PREPARATION OF TRIMETHOPRIM
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RESULTS AND DISCUSSION
In comparison with the excellent cyclization capability of 9 with guanidine,[5,6]
we noticed that the analog 3-dimethylamino-2-(3,4,5-trimethoxy-benzyl)propen-
enitrile (8) does not possess a corresponding cyclization capability because of its
aliphatic amino-substituted pattern. It has been proved that the condensation of 8
with guanidine was extremely sluggish to give 1.[1,6] Therefore, we had to exclude
8 as a proper cyclization substrate to directly prepare 1. Further, direct formation
of 9 from 3-anilinopropanenitrile and 2 requires stoichiometric potassium tert-
butoxide as a condensing agent,[5,6] leading to an unsatisfactory yield and a relatively
high cost. In addition, the reported preparation of 3-anilinopropanenitrile was
considered unpractical and uneconomical owing to the relatively poor yield[7] or
the use of expensive organometallic catalyst.[8] Thus, the Cresswell approach
prompted us to investigate its improvement.
The Cresswell approach[5,6] has clearly revealed that 3-morpholinopropaneni-
trile is absolutely qualified for the condensation with the aldehyde 2 in excellent
yield. We reasonably presumed that aliphatic amino-3-substituted propanenitrile
7, as the most inexpensive alternative to 3-morpholinopropanenitrile, should
similarly possess a prominent capability to form 8 in the condensation of 7 with 2.
Moreover, we wished to efficiently perform an acid-mediated substitution of aniline
for the dimethylamino group of 8 in situ to achieve the key intermediate 9. Thus,
it would lead to accomplishing a one-pot procedure for synthesis of 9 using quite
inexpensive dimethylamine as an assistant reagent.
Compared with Scheme 1, antibacterial agent 1 was prepared in an overall
yield as high as 85% from 2 with the improved synthesis (Scheme 2).Because of
the intrinsically high reactivity for the addition reaction of 3 and dimethylamine,[9]
we could readily perform this Michael addition, preparing 7 in aqueous media with
nearly quantitative yield. At the reaction end, sodium hydroxide should be added
to further promote its thorough completion and simultaneously to partition the
solution into two phases. Practically, it is unnecessary to employ N-donor ligand[10]
[11]
or InCl3 as catalyst in the addition reaction.
In the presence of the catalytic amount of potassium hydroxide and
methanol, the aldehyde 2 reacted entirely with an excess of 7 via a base-catalyzed
1,3-prototropic isomerization to form 8 in the solvent dimethylsulfoxide (DMSO).
Subsequently, after adjusting pH to 3.0–3.5 with inorganic acid for the reaction
solution, an acid-mediated substitution of aniline for the dimethylamino group
in situ was smoothly administrated to afford 9 in an isolated yield of 91% with
a one-pot procedure. The use of the catalytic amount of potassium hydroxide would
profitably lead to an economical consumption of inorganic acid. Here, an intrinsic
hydrolysis–condensation sequence (Scheme 3) was implicated in the substitution.
In the course of the substitution, the hydrolysis of enamine 8 brought about
the steady dimethylammonium ion and the corresponding aldehyde under acidic
conditions. Meantime, the aldehyde underwent a condensation with aniline ulti-
mately to result in the insoluble and more stable enamine 9, which comprises
a steadily p–p–p–P conjugated structure.
An Indian patent reported that enamine 9 could be synthesized in 91%
yield from 2 in the presence of the catalysts potassium methoxide and PEG-600 in

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Y.-F. JI ET AL.
Scheme 3. Intrinsic hydrolysis–condensation sequence leading to 9.
methanol.[12] We attempted to reproduce the process to prepare 9 with a one-pot
procedure, but 9 was obtained only in ca. 80–83% yields. Actually, we found that
DMSO acts as an important factor to efficiently facilitate the formation of 8.
Finally, in the presence of the excessive sodium methoxide, the treatment of 9
with guanidine nitrate gave 1 in 95% yield under reflux for 10 h and synchronously
released the assistant reagent aniline. Understandably, the solvents and the assistant
reagents would be recovered and reused through appropriate separation engineering
during large-scale production.
In conclusion, we have accomplished an improved and practical preparation of
1 using dimethylamine and aniline as assistant reagents. The preparation, which fea-
tures good yield, cost efficiency, convenient manipulation, and better feasibility, will
meet the demands in industrial manufacture of this classic drug.
EXPERIMENTAL
Melting points were determined by capillary method without correction.
NMR spectra were recorded on Bruker AV400 in CDCl3 or DMSO-d6 as indicated
in parentheses and chemical shifts were expressed in d ppm relative to tetrame-
thylsilane (TMS). Mass spectrometry (MS) and high-resolution mass spectrometry
(HRMS) were carried out on a QSTAR Pulsar I LC=TOF MS mass spectrometer.
Thin-layer chromatography (TLC) was performed on silica-gel plates (HF254), and
TLC visualizations were performed with I2 vapor and ultraviolet (UV) light. All
solvents and reagents were obtained from commercial sources and used without
further purification.
Preparation of 3-Dimethylaminopropanenitrile (7)[10]
Propenenitrile 3 (53.1 g, 1.0 mol) was added dropwise under vigorous stirring
to a 40 mass % aqueous solution of dimethylamine (118.3 g, 1.05 mol) for 5 h to con-
trol the temperature within the range of 18–20 ꢀC with an external ice bath, and then
the solution was stirred further for 0.5 h. To this solution ca. 30 g solid sodium
hydroxide to dissolve the mixture was added for demixing. The top organic layer
was separated and distilled to provide 7 as colorless oil. Yield: 96.1 g, 98%; bp
110–113 ꢀC = 29–31 mmHg; 1H NMR (400 MHz, CDCl3, ppm) d: 2.58 (t, J ¼ 7.2 Hz,
Hz, 2H, CH2), 2.45 (t, J ¼ 7.2 Hz, 2H, CH2), 2.24 (s, 6H, CH3). 13C NMR (100 MHz,
CDCl3, ppm) d: 118.7, 54.5, 44.9, 16.2; ESI-MS (m=z): 99.1 (M þHþ).

PREPARATION OF TRIMETHOPRIM
1521
Preparation of 3-Anilino-2-(3,4,5-trimethoxybenzyl)propenenitrile (9)
A mixture of DMSO (400 mL), methanol (40 mL), solid potassium hydroxide
(8.3 g, 95% purity, 0.14 mol), and 7 (88.3 g, 0.90mol) was heated to 35ꢀC. The solution
of 2 (117.7 g, 0.60 mol) in DMSO (100 mL) was slowly added to the mixture, and then
the reaction solution was heated to 40–45ꢀC. After vigorous stirring for 5 h at the same
temperature, the solution was cooled to 30ꢀC and aniline (56.8 g, 0.61 mol) was added.
A dilute hydrochloric acid (2 N) was added slowly to the mixture, which was vigorously
stirred for 1 h under reflux and properly adjusted to maintain pH 3.0–3.5. Finally, the
resulting mixture was cooled to ambient temperature. After the addition of water
(200 mL), the mixture was cooled once again to 5 ꢀC. The product was collected by
filtration and dried in vacuo to give 9 as a yellowy solid. Yield: 177.1 g, 91%; mp
1
138–142ꢀC (lit.[5] mp 132–133ꢀC). H NMR (400 MHz, DMSO-d6, ppm) d: 7.63 (s,
1H, ¼CH), 7.27 (t, J ¼ 8.0Hz, 2H, Ar), 7.19 (d, J ¼ 8.0 Hz, 2H, Ar), 6.92 (t, J ¼ 8.0 Hz,
Hz, 1H, Ar), 6.58 (s, 2H, Ar), 3.77 (s, 6H, OCH3), 3.65 (s, 3H, OCH3), 3.44 (s, 2H, CH2);
13C NMR (100 MHz, DMSO-d6, ppm), d: 153.3, 142.9, 142.0, 136.6, 136.0, 129.7, 121.8,
119.9, 115.6, 106.0, 80.8, 60.5, 56.3, 37.1; ESI-MS (m=z): 325.2 (M þHþ), 347.1 (M
þNaþ), 363.1 (M þKþ). HRMS (ESI): calculated for (C19H20N2O3 þHþ), 325.1552;
m=z found, 325.1557; calculated for (C19H20N2O3 þNaþ), 347.1372; m=z found,
347.1382; calculated for (C19H20N2O3 þKþ), 363.1111; m=z found, 363.1122.
Preparation of Trimethoprim (1)[1]
Guanidine nitrate (97.7 g, 0.80mol) was added to a solution of sodium methoxide
freshly prepared from clean sodium (24.1 g, 1.05mol) and methanol (500mL). The
solution was heated to reflux for 0.5 h to free guanidine and then cooled to 30ꢀC. Sub-
sequently, compound 9 (162.2 g, 0.50mol) was added to the solution and the mixture
was stirred under reflux for 10h. The solvent methanol (ca. 250 mL) was boiled off
and the mixture was cooled to 5 ꢀC. The resulting crystal was collected by filtration,
and further concentration of the filtrate gave the second crop of the crystal. The com-
bined product was washed with water (2ꢁ 300 mL) and cold acetone (2ꢁ 150 mL) in
1
turn to give 1 as a off-white solid. Yield: 137.8 g, 95%; mp 200–203 ꢀC; H NMR
(400MHz, DMSO-d6, ppm), d: 7.53 (s, 1H, pyrimidine), 6.55 (s, 2H, Ar), 6.10 (s,
2H, NH2), 5.70 (s, 2H, NH2), 3.73 (s, 6H, OCH3), 3.62 (s, 3H, OCH3), 3.54 (s, 2H,
CH2); 13C NMR (100 MHz, DMSO-d6, ppm), d: 162.63, 162.59, 156.1, 153.2,
136.289, 136.280, 106.323, 106.248, 60.4, 56.3, 33.4; ESI-MS (m=z): 291.1 (M þHþ).
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China (Project No.
21176074) and Shandong Haoyuan Co. Ltd. for financial support.
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