Heteryl- and Arylaminomethylenebisphosphonates: Synthesis and Biologic Activity

Article abstract of DOI:10.1023/A:1024775501781

A series of substituted aminomethylenebisphosphonates is prepared by reaction of dialkyl hydrogen phosphites with triethyl orthoformate and corresponding heteryl- or arylamine. The determining factor in the formation of the target products is appearance of an intermediate associate between the ortho ester and amine. The rate and selectivity of the process can be increased by using a nonpolar solvent to stabilize the transition state of the reaction. Crown ether catalysis increases the selectivity of the process by 10-20%. The synthesized aminomethylenebisphosphonates whose structure was established by NMR spectroscopy exhibit a moderate antibacterial and anticholinesterase activity.

Full text of DOI:10.1023/A:1024775501781

Russian Journal of General Chemistry, Vol. 73, No. 2, 2003, pp. 187 191. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 2, 2003,
pp. 205 210.
Original Russian Text Copyright
2003 by Krutikov, Erkin, Pautov, Zolotukhina.
Heteryl- and Arylaminomethylenebisphosphonates: Synthesis
and Biologic Activity
V. I. Krutikov, A. V. Erkin, P. A. Pautov, and M. M. Zolotukhina
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received November 20, 2001
Abstract A series of substituted aminomethylenebisphosphonates is prepared by reaction of dialkyl hydro-
gen phosphites with triethyl orthoformate and corresponding heteryl- or arylamine. The determining factor
in the formation of the target products is appearance of an intermediate associate between the ortho ester and
amine. The rate and selectivity of the process can be increased by using a nonpolar solvent to stabilize the
transition state of the reaction. Crown ether catalysis increases the selectivity of the process by 10 20%. The
synthesized aminomethylenebisphosphonates whose structure was established by NMR spectroscopy exhibit a
moderate antibacterial and anticholinesterase activity.
Bisphosphonates are the most widely investigated
bisphosphoryl compounds. This group of compounds
includes hundreds of names. Their biologic activity is
in the first turn provided by a favorable combination
of two acid groups with highly basic fragments:
hydroxy, amino, and alkylamino groups. Evidently,
the strength of the biologic effect correlates with the
lipophilicity and basicity of such fragments, but no
systematic studies on this correlation have not yet
been performed. Recently [1, 2] we described the pre-
paration of compounds containing two phosphorus
atoms whose properties were first of all determined
by the character of bonds between the phosphorus
atoms and their substituents. Geminal dichlorome-
thylenebisphosphonates [1] exhibit a high antimi-
crobial activity due to enhanced hydrophobicity. In
the present work we report the synthesis of sym-
metrical bisphosphoryl compounds. The products
contain two geminal phosphono groups with similar
surroundings of the phosphorus atoms and a highly
lipophilic and potentially biologically active fragment.
Such fragments are nitrogen-containing cyclic and
substituted aryl structures. Analysis of patents, too,
gives evidence for the conclusion that methylenebis-
phosphonates comprising a nitrogenous heterocyclic
fragment may well exhibit a high biologic activity.
The lability of the N C bonds stipulates the facility
of their nonenzymatic hydrolysis in the physiologic
medium and cleavage of the A NH2 fragment. There-
fore, pharmacophores, such as radicals A, may
impart some physiologic activity to the molecule.
Specifically, such compounds were found to exhibit
antitumor and bactericide activity [3]. The biologic
activity of compounds I is undoubtedly much contri-
buted by the bisphosphonate fragment.
P(O)(OH)2
A NH CH
P(O)(OH)2
I
The role of phosphoryl groups attached to a prochi-
ral center in the biologic activity of such compounds
probably consists in that they provide specific ori-
entation of the molecules on the surface of a biologic
object via hydrogen-bond formation. This assumption
is based on our data on the antimicrobial activity of
dichloromethylenebisphosphonates containing hydr-
oxy groups [2].
The choice of potential pharmacophores used in the
present work is motivated by the following factors.
Biologically active compounds are frequently obtained
from halogenated six-membered cyclic structures
containing 1 3 nitrogen atoms in the ring. Disposition
of chlorine in different positions of the pyrimidine
ring and fluorine substitution at C5 should change the
electron density distribution in and reactivity of hetero-
cyclic fragments in biochemical processes. Our recent
studies on the biologic activity of 4-R-2-chloro-5-
fluoropyrimidines showed that they possess pro-
nounced fungicidal properties in relation to some
yeasty and filamentous fungi [4]. Halogen-substituted
aryl radicals in the molecules of organic compounds
are also considered evidence for a possible anti-
microbial activity [5].
The synthesis of heterocyclic and aromatic amino-
1070-3632/03/7302-0187$25.00 2002 MAIK Nauka/Interperiodica

188
KRUTIKOV et al.
Yields, melting points, and elemental analyses of substituted heteryl and arylaminomethylenebisphosphonates II
mp, C, or
bp,
(p, mm)
Found, %
N
Calculated, %
Comp.
no.
Yield,
%
R
R
C
Formula
C
P
C
N
30.8 10.8
26.8 17.8
P
IIa
CH3 4-Chloro-6-methoxy- 62
pyrimidin-2-yl
129
30.9 10.7
15.8 C10H18ClN3O7P2
19.6 C7H16N4O6P2
15.9
IIb
IIc
IId
CH3 1,2,4-Triazol-4-yl
77
Oil
127
111
26.2 17.6
19.7
CH3 3-Phenylthiazol-2-yl 64
41.0
6.75 15.4 C14H20N2O6P2S
41.4
6.90 15.2
CH3 2-Chloro-5-fluoro-
pyrimidin-4-yl
CH3 4-(2-Chloro-5-fluo-
ropyrimidin-4-yl)-
piperazin-1-yl
C2H5 1,2,4-Triazol-4-yl
C2H5 4-(2-Pyridyl)
C2H5 2-Thiazolyl
C2H5 2-ClC6H4
72
65
28.1 11.6
16.2 C9H15ClFN3O6P2 28.6 11.1
16.4
IIe
259
Oil
34.4 12.7
13.7 C13H22ClFN4O6P2 35.0 12.5
13.9
IIf
IIg
IIh
IIi
IIj
IIk
IIl
35
28
31
20
20
28
26
35
34
15
32
35.2 15.6
16.8 C11H24N4O6P2
35.7 15.3
16.7
44.1
37.9
43.0
43.6
45.3
40.1
36.0
39.2
57.1
56.9
7.30 16.2 C14H26N2O6P2
6.99 15.9 C12H24N2O6P2S
3.37 14.9 C15H26ClNO6P2
3.47 15.1 C15H26ClNO6P2
3.50 15.5 C15H26FNO6P2
3.11 13.7 C15H25Cl2NO6P2
2.68 12.5 C15H26INO6P2
3.07 13.7 C15H26BrNO6P2
2.59 12.1 C24H45NO6P2
2.62 12.4 C24H45NO6P2
44.2
37.3
43.5
43.5
45.4
40.2
35.7
39.3
57.0
57.0
7.36 16.3
7.25 16.0
3.39 15.0
3.39 15.0
3.53 15.6
3.12 13.8
2.77 12.3
3.06 13.5
2.77 12.2
2.77 12.2
C2H5 3-ClC6H4
C2H5 4-FC6H4
C2H5 2,5-Cl2C6H3
93
61
Oil
127
Oil
190
Oil
IIm C2H5 4-IC6H4
IIn
IIo
IIp
C2H5 3-BrC6H4
C4H9 C6H5CH2
C4H9 2-CH3C6H4
methylenebisphosphonates II was based on reac-
tion between dialkyl hydrogen phosphites, triethyl
orthoformate, and corresponding amines.
of dialkyl hydrogen phosphite with orthoformate
to form ethanol is also probable.
OC2H5
(RO)2P(O)H + H5C2O
2(RO)2P(O)H + R NH2 + (C2H5O)3CH
OC2H5
(RO)2(O)P P(O)(OR)2 + 3C2H5OH
(1)
OR
OC2H5
(2)
NHR
H P
O
+ C2H5OH
OC2H5
II
An analogous reaction yielding a functionally
substituted phosphonous acid have been observed
with metaphosphorous acid [7].
R = alkyl, R = aryl or heteryl (for radicals, see table).
The reaction was carried out under the conditions
described in [6]: The reagents were mixed in a stoi-
chiometric ratio, and the reaction mixture was heated
with simultaneous distillation of the ethanol formed.
The temperature of the reaction mixture gradually rose
from the boiling point of ethanol to the boiling point
of triethyl orthoformate (80 150 C). The reaction
progress was controlled by TLC, and its completion
was established by the amount of the ethanol distilled.
Regretfully, performing the reaction under the tradi-
tional conditions did not always lead to satisfactory
yields of the target products. The amount of the
ethanol formed, while convenient to determine, cannot
be a strict criterion of the purposeful synthesis of
aminomethylenebisphosphonates, because the reaction
Orthoesters are highly reactive compounds which
are widely used in synthetic practice. Therewith, the
structure of final compounds frequently depends on
whether reaction conditions are strictly followed.
Reactions of ortho esters with amines may give
imidoesters III or formamidines IV [8], and each of
these reactions involves ethanol formation.
Hence, the liberation of a stoichiometric amount of
ethanol indicates nothing more than complete con-
sumption of triethyl orthoformate, and the low yields
of the target aminomethylenebisphosphonates are
explained by the diversity of reactions the starting
compounds may be involved.
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HETERYL- AND ARYLAMINOMETHYLENEBISPHOSPHONATES: SYNTHESIS
189
R
OC2H5
NH2 + H5C2O
2000
1800
OC2H5
R
OC2H5
N
+ 2C2H5OH,
1600
1400
1200
1000
800
III
R
R
R
+
OC2H5
2
1
3
NH2
N
N
600
400
+ C2H5OH.
R
260
270
280
290
300 , nm
NH
IV
Long-wave absorption band in the UV spectra of aniline
triethyl orthoformate mixtures in cyclohexane at varied
reagent ratio. (1) 1:0.15, (2) 1:0.5, and (3) 1:1.
With the purpose of optimizing conditions for
preparing substituted aminomethylenebisphosphonates,
we considered probable mechanisms of their forma-
tion. Neither intermediate III nor intermediate IV can
be precursors of the target aminomethylenebisphos-
phonate. Indeed, in view of the fact that formamidines
undergo further transformations similar to those of
azomethines, the formation of bis(arylamino)methyl-
phosphonates is readily explained by phosphorylation
of the methine carbon atom of formamidine IV. At the
same time, the formation of the second phosphono
group can hardly be explained.
transformation of associate V to the corresponding
geminal dialkoxymethylaniline and subsequent
phosphorylation of the latter.
To gain insight into the character of reaction
between aniline and orthoformate, we recorded the
UV spectra of their mixtures at varied reagent ratio.
Note that the spectra vary in a complicated fashion.
The effect of different molecular interactions on elec-
tronic absorption spectra is most often characterized
in terms of shifts of absorption maxima. However,
this characteristic can be considered correct only for
bands having a well-defined vibrational structure [10].
As hydrogen-bond formation is a weak perturbation
compared to the energy of an electronic transition, it
was reasonable to suggest that the spectrum of a
hydrogen-bonded associate looks like a slightly per-
turbed spectrum of the free molecule. The spectrum
of aniline in a nonpolar solvent has a broad long-wave
absorption band with an ill-defined fine structure. On
addition of a little triethyl orthoformate (see figure),
Reactions in a ternary system comprising an amine,
an ortho ester, and a compound with an active me-
thylene group have been described [9]. Since the re-
activity of hydrophosphoryl compounds is mostly
contributed by the lability of proton, we can assume
that the final bisphosphonates are formed via phos-
phorylation of some intermediate species.
Triethyl orthoformate can react with various re-
agents, and its reactivity is determined by the presence
of unshared electron pairs on the oxygen atoms. This
explains the enhanced nucleophilicity of its methine
carbon atom, as well as the ability of triethyl ortho-
formate to hydrogen-bond formation [9]. We sug-
gested that the reaction system under study can in-
volve associates whose structure is based on hydrogen
bonds between orthoformate oxygen atoms and pri-
mary amino groups of substituted anilines or hetero-
cyclic amines.
the structureless absorption band at
288 nm
acquires the vibrational structure charmaacxteristic of
hydrogen-bonded associates. Further increase of the
concentration of triethyl orthoformate (1:0.5 reagent
ratio) causes appearance of a third absorption band
(
317 nm, log 2.3) not belonging to any of the
max
starting compounds. At a 1:1 (and higher) ester ani-
line molar ratio, the absorption band gets similar to
the band of aniline but less intense. The complicated
character of variation of electronic transitions in the
aniline triethyl orthoformate mixture can be explained
by formation of hydrogen-bonded associates of dif-
ferent composition. Evidence for this assumption is
provided by the observation of isobestic points at 248
and 300 nm at low concentrations of the ortho ester.
H
C2H5
N
H5C2
O
O H
R
O
C2H5
V
The final bisphosphonates are evidently formed via
If the formation of associates via proton transfer
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 2 2003

190
KRUTIKOV et al.
between the ortho ester and aniline can be considered
established, the question as to whether these as-
sociates take part in the formation of target com-
ounds is still open.
on dialkyl hydrogen phosphites we noted in [12].
While studying the mechanism of the Kabachnik
Fields reaction, we observed an analogous role of
crown ethers in the selective formation of amino-
methylphosphonates. The choice of a crown ether
catalyst can be substantiated by two reasons. First,
such catalyst improves the solubility of the starting
amines in the chosen solvents and is readily soluble
in them itself. This permits to prepare aryldiphos-
phonomethanes under mild conditions in a homo-
geneous medium inspite of the different aggregative
states of the starting reagents. Second, crown ether
oxygen atoms form, by their unshared electron pairs,
hydrogen bonds with hydrogen atoms of the starting
reagents, specifically, phosphite, and convert them
into a more reactive form.
Note that the yields of target compounds derived
from substituted anilines are much lower than the
yields of heterylaminomethylenebisphosphonates.
Considering that substituted anilines are more basic
than heterocyclic amines, which largely determines
the higher probability of formation of imidoesters III
and formamidines IV, the higher selectivity of forma-
tion of products IIa IIe as compared to bisphos-
phonates IIf IIp can be explained by the purposeful
phosphorylation of the dialkoxymethylaniline formed
from associate V. This assumption is confirmed by
some specific features of reaction progress. The TLC
control of the reaction progress in the systems
3-chloroaniline + orthoformate (A) and 3-chloroani-
line + orthoformate + diethyl hydrogen phosphite (B)
showed appearance of a spot belonging neither to
the starting aniline nor to the target bisphosphonate.
After ca. 2 h, this spot disappeared and only the target
product was detected.
Using dibenzo-24-crown-8 in the synthesis of aryl-
diphosphonomethane from 3-chloro- and 4-fluoro-
aniline increases the reaction rate and product yields
by about 10 20%.
The reaction products are colorless crystalline or
oily substances soluble in DMSO, DMF, and acetic
acid. Crystalline products were purified by recrystal-
lization, and oils, by treatment with charcoal. The
physicochemical parameters of the obtained com-
pounds are listed in the table.
The 31P NMR spectra of compounds IIa IIh
contain a doublet at 13.3 16.2 ppm. The spectra of
aryl-substituted bisphosphonates IIIi IIIp display
doublets at 16 18 ppm, confirmatory of their bisphos-
phonate structure.
It is known that nonpolar solvents considerably
affect the structure of associates with a strong hydro-
gen bond and determine the rate of the limiting stage
of the process. To study solvent effects in the reaction
in question, we carried out 3 parallel experiments in
aprotic solvents with different dielectric constants
(heptane, dioxane, and nitromethane). The synthesis
of bis(diethoxyphosphoryl)(3-chlorophenylamino)me-
thynebis(diethyl phosphonate) (IIj) was chosen as the
test experiment. It was established that the effect of
polar solvents is negative. The rate of formation of
the target bisphosphonates in nitromethane is half that
in heptane (reaction completion was detected by the
disappearance of the spot of the starting aniline). This
result can be explained by the fact that the transition
state of the reaction is less polar than the starting
reagents (due to delocalization of unshared electron
density in structure V). In a medium with a high di-
electric constant, the nonpolar associate located on
the reaction coordinate aquires the character of the
reagent-like early transition state and destabilizes not
passing into the target products. The reaction rate in
dioxane is approximately the same as in heptane,
which points to stabilization of the product-like late
transition state and shift of the equilibrium to the
target products. Moreover, the yield of compound IIj
in dioxane is 88%. This result, unexpected from the
first glance, can be explained not only by nonspecific
solvation of the transition state, but also by the con-
version of diethyl hydrogen phosphite into a more
reactive symmetrical form. Such an effect of ethers
1
The H NMR spectra of the obtained compounds
contain a multiplet of the methine proton (3.10 3.50
ppm for IIa IIh and 4.50 4.65 ppm for IIi IIp) and
a doublet of the proton of the secondary amino group
(3.7 4.2 ppm for IIa IIh and 5.61 6.02 ppm for IIi
IIp).
1
In the range 1 4 ppm, all the H NMR spectra
show signals characteristic of alkyl and alkoxy
protons of the phosphoester part of the molecule. The
proton signals of the heterocyclic and aromatic frag-
ments of compounds IIa IIp appear at 6.3 8.4 ppm.
The UV spectra of aminomethylenebisphospho-
nates lack the long-wave absorption band characte-
ristic of related heterocycles and substituted anilines.
Substituted aminomethylenebisphosphonates
exhibit a moderate antimicobacterial activity (100%
growth inhibition of pathogenic micobacteria
at the concentration 300 g/ml) and a weak antiviral
effect in relation to the herpes virus of the I type
(VPG-I/Leningrad/248/88). At the same time, amino-
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HETERYL- AND ARYLAMINOMETHYLENEBISPHOSPHONATES: SYNTHESIS
methylenebisphosphonates are characterized by a
191
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