Synthesis and biological activity of polyfluorinated p-aminosalicylic acids and their amides

Article abstract of DOI:10.1016/j.mencom.2020.09.028

Polyfluorinated analogues of salicylamide and p-aminosalicylic acid have been synthesized based on methyl polyfluorosalicylates. Polyfluorosalicylamides were obtained by the reaction with aqueous ammonia, while 4-aminopolyfluorosalicylic acids were prepar

Full text of DOI:10.1016/j.mencom.2020.09.028

Mendeleev
Communications
Mendeleev Commun., 2020, 30, 636–638
Synthesis and biological activity
of polyfluorinated p-aminosalicylic acids and their amides
Yanina V. Burgart,a,b Irina V. Shchur,a Evgeny V. Shchegolkova,b and Victor I. Saloutin*a,b
a I. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
620108 Ekaterinburg, Russian Federation. Fax: +7 343 374 5954; e-mail: saloutin@ios.uran.ru
b Institute of Chemical Engineering, Ural Federal University, 620002 Ekaterinburg, Russian Federation
DOI: 10.1016/j.mencom.2020.09.028
R
F
Polyfluorinated analogues of salicylamide and p-amino-
salicylic acid have been synthesized based on methyl
polyfluorosalicylates. Polyfluorosalicylamides were obtained
by the reaction with aqueous ammonia, while 4-amino-
polyfluorosalicylic acids were prepared in two steps via
regio-oriented nucleophilic replacement of para-positioned
fluorine atom with azido group followed by its reduction.
3,4,5-Trifluorosalicylamide showed pronounced analgesic
in vivo activity in hot plate test while 4-amino-3,5-
difluorosalicylic acid revealed high tuberculostatic activity.
F
F
C(O)NH2
OH
Amidation
R
F
F
C(O)OMe
OH
R
F
Nucleophilic
substitution
F
C(O)OH
OH
F
H2N
R = H, F
Keywords: salicylamide, p-aminosalicylic acid, polyfluoroarenes, organofluorine compounds, aromatic nucleophilic substitution,
tuberculostatic activity, analgesic activity, acute toxicity.
Modification of natural compounds is a fruitful approach to new
pharmaceutical agents. For example, salicylic acid initially
isolated from willow bark still serves as a basis for creating a
wide range of non-steroidal anti-inflammatory drugs of the
salicylate series.1–6 Among them, 4-aminosalicylic acid (PASA)1
is used as a specific anti-tuberculosis drug. On the other hand,
organofluorine compounds have great potential for the creation
of new drugs, since the presence of electron-withdrawing
fluorine atoms affects their metabolic stability, bioavailability,
lipophilicity, acid–base properties, intermolecular interactions,
etc.7–12 Mono-3-, 4-, 5-, 6-fluoro- and 4,5-difluorosalicylamides
are known. 4-Fluorosalicylamide was obtained by the reaction
of methyl 4-fluorosalicylate with concentrated aqueous
ammonia,13 while 6-fluorosalicylamide was prepared by
treatment of 2,6-difluorobenzamide with benzyl alcohol
followed by removal of benzyl protection.14 Plant protection
agents were found among these amides.15 Based on mono- and
difluorosalicylamides, alkoxy derivatives were synthesized and
patented as poly(ADP-ribose) polymerase inhibitors16,17 and
bactericidal agents.18 Data on the synthesis of fluorine-
containing 4-aminosalicylic acids as PASA analogues are
limited to two examples. 5-Amino-4-fluorosalicylic acid was
obtained by reduction of 4-fluoro-5-(phenylazo)salicylic acid
with Na2S2O4.13 Isomeric 4-amino-5-fluoro-2-hydroxybenzoic
acid was synthesized by selective fluorination of methyl
4-acetylsalicylamide in the presence of Selectfluor™ followed
by alkaline hydrolysis.19,20 The MIC value of 5-fluoro-
substituted PASA toward M. tuberculosis H37Rv was determined
to be 6.5–12.5 µg ml–1.
analogues of salicylamide and PASA, and also estimated the
prospects of their biological action.
Attempts to obtain polyfluoro-containing salicylamides or
PASA analogues via direct amidation of tri- and
tetrafluorosalicylic acids with aqueous ammonia by reflux in
alcohols or heating at 80 °C in polar DMSO or MeCN failed.
Therefore, we turned to the corresponding methyl
polyfluorosalicylates 1a,b synthesized from the corresponding
acids.23 Refluxing esters 1a,b with aqueous ammonia in
methanol results in replacement of the methoxy group with the
amino one to give 2,3,4,5-tetrafluoro- and 3,4,5-trifluoro-6-
hydroxybenzamides 2a,b in high yields (Scheme 1). We have
noticed that if the final neutralization in this procedure is
performed with HCl of higher concentration or if overheating is
admitted, the amido group in compounds 2a,b is hydrolyzed to
liberate polyfluorosalicylic acids 3a,b.
We implemented a synthesis of 4-aminopolyfluorosalicylic
acids by preliminary preparation of 4-azido derivatives.
Replacement of the fluorine atom with the azido group at
activated position 4 of compounds 1a,b was performed by
refluxing them with sodium azide in aqueous acetone as
described for methyl pentafluorobenzoate (see Scheme 1).24
However, complete conversion of tetrafluorosalicylate 1a took
twice as long (16 h), while the similar process for
trifluorosalicylate 1b took even 40 h (19F NMR monitoring).
Anyway, yields of 4-azido benzoates 4a,b were good enough.
It should be noted that prolonged processing of tetrafluoro-
substituted ester 1a favored the nucleophilic substitution not
only at position 4 but also at position 2 (the appearance of extra
two fluorine signals corresponding to ~17% of 2,4-diazido
product in the 19F NMR spectrum of the reaction mixture after
40 h processing). It should also be noted that acids 3a,b did not
undergo noticeable changes upon refluxing with sodium azide in
Recently, we have developed a versatile synthesis of
polyfluorosalicylic acids21,22 that was used to obtain a series of
analogues of known clinically used salicylates.23 In this work,
we suggest a convenient synthesis of polyfluoro-containing
© 2020 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2020, 30, 636–638
R
R
F
F
F
F
CO2H
OH
C(O)NH2
OH
ii
i
82–85%
81–83%
R
F
2a,b
F
3a,b
F
F
CO2Me
OH
R
R
F
CO2Me
OH
F
CO2X
OH
F
iv
iii
1a,b
70–72%
85–87%
N3
H2N
a R = F
b R = H
F
F
4a,b
5a,b X = Me
v
6a,b X = H (65–67%)
Scheme 1 Reagents and conditions: i, NH4OH, MeOH, D, then 10% HCl, room temperature; ii, 10–15% HCl, 50–60 °C; iii, NaN3, acetone–H2O, D, 16 h
(for 1a) or 40 h (for 1b); iv, Zn, NH4Cl, MeOH, D; v, morpholine, MeCN, DIPEA, D, then 10% HCl, room temperature.
aqueous acetone. The azido group in compounds 4a,b was
reduced with zinc metal in the presence of NH4Cl upon refluxing
in methanol to afford the target methyl 4-amino-2,3,5-trifluoro-
and 3,5-difluoro-2-hydroxybenzoates 5a,b (see Scheme 1).
Esters 5a,b were found to be resistant to classical alkaline and
acid hydrolyses. However, we have previously found that the
reaction of esters 1a,b with morpholine involves, in addition to
the substitution of the fluorine atom at position 4, intramolecular
acid hydrolysis of the amido group due to catalysis by the
adjacent hydroxy group to give 4-morpholino-3,5,6-trifluoro-
and 3,5-difluorosalicylic acids.25 Taking these results into
account, we treated esters 4a,b with morpholine upon refluxing
in MeCN in the presence of DIPEA for 2 days followed by
hydrolysis in 10% HCl to obtain the desired 4-amino-2,3,5-
trifluoro- and 4-amino-3,5-difluoro-2-hydroxybenzoic acids
6a,b (see Scheme 1).
We also tried to obtain amino acid 6b from amino ester 5b by
alternative protocol based on our finding that amides 1a,b were
readily hydrolyzed into acids 3a,b with HCl of higher
concentration or upon heating (see Scheme 1). However,
treatment of ester 5b with ammonia in methanol followed by
hydrolysis with 10% HCl at 60–70 °C (Scheme 2) afforded a
mixture of amidation product 7, target compound 6b and
decarboxylated derivative 8 in 9:5:86 ratio (1H and 19F NMR
data). Recrystallization of this crude mixture from chloroform
gave pure 3-amino-2,5-difluorophenol 8. This may be an
alternative synthesis of compound 8 previously obtained from
2,6-difluoro-3-triisopropylsilyloxybenzoyl chloride by the
Curtius reaction26 or by dealkylation of 2,6-difluoro-3-
methoxyaniline.27,28 When the final hydrolysis with HCl was
performed at room temperature (see Scheme 2, conditions ii), a
~3:1 mixture of compounds 7 and 6b was formed. We failed to
isolate amino amide 7 in a pure form.
Note that the resulting compounds 2a,b, 6a,b are moderately
soluble in water on heating.
Earlier, we found that tetra- and trifluorosalicylic acids 3a,b
demonstrated an analgesic effect, which exceeded the activity of
diclofenac, at a dose of 50 mg kg–1.23 However, acids 3a,b were
more toxic than aspirin and diclofenac when administered
intraperitoneally. Further, we found a way to reduce the toxicity by
incorporating a cyclic amine residue into polyfluorosalicylic
acids,25 though this caused some loss of activity. Here we estimated
the analgesic effect of amide 2b in the in vivo hot plate test in
rats29,30 at a dose of 15 mg kg–1. This compound had a pronounced
analgesic effect by the 1st hour of the test, which exceeded
threefold the effects of aspirin at a dose of 25 mg kg–1 and
diclofenac at a dose of 10 mg kg–1, while by the 2nd hour of the
estimation, the effect remained at the level of diclofenac. Table 1
summarizes the data on the analgesic activity and acute toxicity of
trifluorsalicylic acid 3b, amide 2b and derivatives 9, 10 bearing
cyclic amine moieties. Analysis of these data showed that amide
2b was the most active among these compounds. Moreover, its
expected acute toxicity was the lowest among these derivatives.
F
CO2H
OH
F
CO2H
OH
N
N
F
F
O
N
Me
10
9
It is known that PASA metabolically competes with
p-aminobenzoic acid required to tuberculous mycobacteria,
which explains the anti-tuberculosis effect of the former.33
Previously,
a
higher tuberculostatic activity (MIC of
0.7–1.5 µg ml–1) was found for 3,5- and 4,5-difluorosalicylic acids
compared to the 3,4,5-trifluoro-substituted analogue 3b (MIC of
6.5 µg ml–1).22 Therefore, in this work we screened 4-amino-3,5-
difluorosalicylic acid 6b and its previously synthesized analogues
9, 1025 for activity against Mycobacterium tuberculosis H37Rv
strains. It was found that compounds 9, 10 showed a tuberculostatic
effect at the level of the acid 3b (MIC of 6.5 µg ml–1), while its
4-amino derivative 6b inhibited this strain with a MIC of
1.5 µg ml–1, which is slightly inferior to the efficiency of PASA
(Table 2). Extensive testing of acid 6b revealed its inhibitory
effect against M. avium and M. terrae strains and against MDR
(multidrug resistance) at a MIC of 1.5 µg ml–1, which makes its
undoubtedly superior to PASA that has low efficiency against
M. avium and is inactive against M. terrae and MDR.
F
CO2Me
OH
i or ii
H2N
F
5b
F
F
F
CO2H
OH
C(O)NH2
OH
+
+
H2N
OH
H2N
H2N
F
F
F
7
6b
8
for i: 7/6b/8 = 9:5:86
for ii: 7/6b/8 = 77:23:0
In summary, we have synthesized new polyfluorine-
containing analogues of known pharmaceuticals, viz.,
salicylamide and PASA. A preliminary estimation of the
Scheme 2 Reagents and conditions: i, NH4OH, MeOH, D, then 10% HCl,
80 °C; ii, NH4OH, MeOH, D, then 10% HCl, room temperature.
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Mendeleev Commun., 2020, 30, 636–638
Table 1 Analgesic activity and acute toxicity of acid 3b and its analogues.
Analgesic activity: latent period increase (%)
2 h
Compound
Dose/mg kg–1
Acute toxicity: dose/mg kg–1 [survival (%)]a Reference
1 h
3b
50
15
25
25
25
178.1b
150.4b
31.4b
60.6b
54.7 ± 15.7c
not tested
80.1
300 (66)
23
2b
300 (100)
this work
25
9
66.2
300 (66)
10
not tested
300 (66)
25
Aspirin
300 (33)
23, 31
LD50 179.6 (109.1–295.5)
LD50 74d
Diclofenac
10
56.0 ± 10.3
83.4 ± 18d
23, 32
a Mice, intraperitoneal administration; b p < 0.01; c average value from five different independent experiments ± standard deviation; d average value of a for six
different independent experiments ± standard deviation.
Table 2 Tuberculostatic activity of acid 3b and its analogues.
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Activity against Mycobacterium tuberculosis (MIC/µg ml–1
)
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Compound
H37Rv
M. avium
M. terrae
MDR
3b
6.25
1.5
not tested
1.5
not tested
1.5
not tested
1.5
6b
9
6.25
6.25
0.3–134
not tested
not tested
32–12834
not tested
not tested
not active35
not tested
not tested
not active
10
PASA
biological effects showed that 3,4,5-trifluorosalicylamide had a
high analgesic activity exceeding the action of aspirin and
diclofenac, while 4-amino-3,5-difluorosalicylic acid exhibited a
high capability to inhibit four strains of Mycobacterium
tuberculosis, including MDR. These data indicate the prospects
of a further deeper biological study of the compounds obtained.
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23 E. V. Shchegol’kov, I. V. Shchur, Y. V. Burgart, V. I. Saloutin,
A. N. Trefilova, G. A. Ljushina, S. Y. Solodnikov, L. N. Markova,
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This study was financially supported by the Russian Science
Foundation (grant no. 16-13-10255). Equipment of the
Spectroscopy andAnalysis of Organic Compounds Center for Joint
Use at the Postovsky Institute of Organic Synthesis of the Russian
Academy of Sciences (Ural Branch) was used in this work. Access
to literature sources and databases was provided by the Ural Federal
University. Experiments on the analgesic activity and acute toxicity
of the compounds synthesized were carried out at Perm National
Research Polytechnic University, Perm (O. P. Krasnykh, Ph.D. in
Chemistry). Tuberculostatic activity assays were performed at the
Ural Scientific Research Institute of Phthisiopulmonology,
Ekaterinburg (M. A. Kravchenko, Ph.D. in Biology).
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Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.09.028.
28 J. Zhang, Y. Gao, X. Kang, Z. Zhu, Z. Wang, Z. Xi and L. Yi,
Org. Biomol. Chem., 2017, 15, 4212.
29 Drug Discovery and Evaluation: Pharmacological Assays,
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Received: 23rd April 2020; Com. 20/6205
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