Highly efficient one-pot multi-component synthesis of α- aminophosphonates and bis-α-aminophosphonates catalyzed by heterogeneous reusable silica supported dodecatungstophosphoric acid (DTP/SiO2) at ambient temperature and their antitubercular evaluation against Mycobactrium Tuberculosis

Article abstract of DOI:10.1039/c3ra45853a

A highly efficient one-pot multi-component reaction (MCR) protocol over DTP/SiO₂ has been developed for the synthesis of α- aminophosphonate derivatives (4a-x) in excellent yields. The α-aminophosphonate derivatives were for the first time evaluated for their antitubercular activity against the M. tuberculosis H37Ra (MTB) strain. An evaluation of the data on the cytotoxicity and antimicrobial activity shows that 4n and 4v are promising antitubercular agents.

Full text of DOI:10.1039/c3ra45853a

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Highly efficient one-pot multi-component
synthesis of a-aminophosphonates and bis-a-
aminophosphonates catalyzed by heterogeneous
reusable silica supported
Cite this: RSC Adv., 2014, 4, 7666
dodecatungstophosphoric acid (DTP/SiO2) at
ambient temperature and their antitubercular
evaluation against Mycobactrium Tuberculosis†
Shafeek A. R. Mulla, a Mohsinkhan Y. Pathan,a Santosh S. Chavan,a Suwara P. Gampleb
*
and Dhiman Sarkarb
A highly efficient one-pot multi-component reaction (MCR) protocol over DTP/SiO2 has been developed for
the synthesis of a-aminophosphonate derivatives (4a–x) in excellent yields. The a-aminophosphonate
derivatives were for the first time evaluated for their antitubercular activity against the M. tuberculosis
H37Ra (MTB) strain. An evaluation of the data on the cytotoxicity and antimicrobial activity shows that 4n
and 4v are promising antitubercular agents.
Received 15th October 2013
Accepted 25th November 2013
DOI: 10.1039/c3ra45853a
www.rsc.org/advances
Clay-MW,11 TiO2,12 scandium (tris-dodecyl sulfate),13 I2,14 Nano
Fe3O4,15 EAN,16 H-beta zeolite,17 Nano ZnO,18 AIKIT-5,19
ZrOCl2$8H2O,20 SbCl3/Al2O3,21 and Amberlyst-15,22 which have
Introduction
The synthesis of a-aminophosphonates has become the center of
interest among world researchers due to their wide spectrum of
biological and/or medicinal activities and structural analogy to
natural a-amino acids and a-aminophosphoric acids. They have
achieved a signicant importance as a key moieties having wide
applications not only in agriculture but also in biological/
medicinal chemistry, as anti-cancer agents,1a–c inhibitors of
synthase,1d HIV protease,1e antibiotics,1f enzyme inhibitors,1g
anti-thrombotic agents,1h cytotoxicity,1c antibacterial activity,1i
antifungal activity,1j antiproliferative activity,1j inhibitors of
protein tyrosine phosphatases,1k herbicides, fungicides,1l insec-
ticides,1m plant growth regulators,1n and as substrates in the
synthesis of phosphonopeptides. Owing to the potential impor-
tance of a-aminophosphonate derivatives as a key moieties in life
sciences, pharmaceuticals, agriculture, world-wide efforts have
made in the last few decades by researchers and various proto-
cols have been developed for their synthesis using various cata-
lysts such as SnCl4,2 SnCl2,3 ZnCl2,4 BF3$OEt2,5 InCl3,6
Mg(ClO4)2,7 M(OTf)3,8 AlCl3,9 CF3CO2H,10 Montmorillonite
been reported in the literature. The acid2–10,20 catalyzed synthesis
of a-aminophosphonates by the nucleophilic addition reaction
of phosphite to amines is convenient and the most preferred
method, however, its practical approach is limited since the
water formed during the course of the reaction either deactivates
or decomposes the acid catalyst. Even though signicant
improvements and/or developments using either homogeneous
or heterogeneous catalysts have been achieved, almost all of
the methods reported so far lack general applicability and
commercial scale implementation, and suffer from one or more
limitations such as the use of excess or stoichiometric quantity,
moisture sensitive, toxic, corrosive, expensive catalysts, which are
non-recoverable and/or recoverable with tedious separation
procedures involving lots of toxic waste generation besides a long
reaction time, high temperature and low yield for the desired
product.
As a-aminophosphonate derivatives are the key constituent
and structural backbone of many pharmaceutical and agricul-
tural compounds, the development of more general and cost
effective, one-pot multi-component protocols for their synthesis
under much milder, more efficient, environment friendly
conditions using recyclable, eco friendly catalysts is still a
possibility to explore. As part of our continuous efforts to
develop a green, ecofriendly, general and cost effective protocol
for the organic transformation using DTP/SiO223a and recyclable
catalysts,23b–f we report herein a highly efficient, cost effective,
aChemical Engineering
& Process Development Division, National Chemical
Laboratory, Dr Homi Bhabha Road, Pune-411008, Maharashtra, India. E-mail: sa.
mulla@ncl.res.in; Fax: +91-20-25902676; Tel: +91-20-25902316
bCombi. Chem. Bio. Resource Center, Organic Chemistry Division, National Chemical
Laboratory, Pashan Road, Pune 411 008, India
† Electronic supplementary information (ESI) available: experimental details and
spectral data of all the new compounds. See DOI: 10.1039/c3ra45853a
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Table 1 Optimizations of the reaction conditions for the synthesis of
a-aminophosphonatea
general, and much milder one-pot multi-component protocol
for the synthesis of a-aminophosphonate and bis-a-amino-
phosphonate derivatives in excellent yields via a one-pot three
component condensation of various aldehydes, amines and di
or tri alkyl phosphites using a heterogeneous reusable silica
supported dodecatungstophosphoric acid catalyst at ambient
temperature in a short reaction time (Scheme 1).
TB is a chronic infectious disease24 and a serious threat to
the public health worldwide. The world health organization
(WHO) declared25 TB as an international public health crisis
and appealed to develop the anti-TB drug or vaccine, which
could be licensed by 2020. Even though a-aminophosphonate
derivatives, which are constituents of various potent drugs,1a–k
and their bioevaluation for anticancer activity are well repor-
ted,1a–c surprisingly its antitubercular (TB) activity has not been
reported so far. Therefore, herein we report for the rst time the
preliminary results on the antitubercular activity of a-amino-
phosphonate derivatives against the Mycobactrium Tuberculosis
H37Ra (MTB) strain.
To develop a one-pot multi-component protocol for synthesis
of a-aminophosphonate derivatives, benzaldehyde (10 mmol),
aniline (10 mmol), and dimethylphosphite (10 mmol), catalyzed
by 50 mg (0.35 mol%) 20% DTP/SiO2 catalyst in a 5 ml solvent at
ambient temperature for 1 h was selected as a model reaction to
optimize the reaction conditions. Initially, the screening of
different solvents such as methanol, ethanol, dichloromethane,
Entry
Solvent
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
8
Methanol
Ethanol
DTP/SiO2
DTP/SiO2
DTP/SiO2
DTP/SiO2
DTP/SiO2
DTP/SiO2
—
DTP/SiO2 (25 mg)
DTP/SiO2 (100 mg)
10% DTP/SiO2
30% DTP/SiO2
74
78
40
98
N.R.
N.R.
N.R.
84
98
76
Dichloromethane
Acetonitrile
Dimethylformamide
Water
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
9
10c
11d
98
a
Reaction conditions: benzaldehyde (10 mmol), aniline (10 mmol),
dimethylphosphite (10 mmol), catalyst 20% DTP/SiO2: 25–100 mg
(0.18–0.7 mol% of DTP) in 5 ml solvent, room temperature 1 h.
b
d
c
Isolated yields aer column chromatography. 50 mg ¼ 0.17 mol%.
50 mg ¼ 0.52 mol%.
acetonitrile, dimethylformamide and water were performed. reaction conditions. To establish the general applicability, a
However, the acetonitrile solvent gives the desired a-amino- variety of substituted aldehydes, substituted amines and di/tri
phosphonate product in a 98% yield (Table 1, entry 4) whereas alkyl phosphites were subjected for
a three-component
methanol, ethanol and dichloromethane give 74%, 78%, and condensation (Kabachnik-Fields) reaction. Interestingly, a wide
40% yields, respectively (Table 1, entries 1–3). The formation of range of aryl/heteroaromatics aldehydes. and amines possess-
the desired product was not observed using dimethylformamide ing various electron donating and electron withdrawing func-
or water as the solvent (Table 1, entry 5, 6) and also in the tional groups reacted smoothly with di or tri alkyl phosphites
absence of catalyst in an acetonitrile solvent (Table 1, entry 7). over a DTP/SiO2 catalyst at ambient temperature for 1 h to give
The promising results using acetonitrile as a solvent over a DTP/ the desired products in excellent yields (Table 2, entries 4a–x).
SiO2 catalyst allowed us to further optimize the DTP loading and The aromatic/heteroaromatics aldehydes such as benzaldehyde,
catalyst loading, and the results in Table 1 (entry 8–11) reveal 4-methoxy benzaldehyde, 4-chloro benzaldehyde, 4-methyl
that the catalyst with 20% DTP loading and 50 mg catalyst shows benzaldehyde, 2,5-dimethoxy benzaldehyde and furfural
excellent catalytic activity (Table 1, entry 4).
reacted well with aniline/3-chloroaniline/2,4,6-trimethylaniline/
The excellent yield, using 20% DTP/SiO2 in an acetonitrile 1-naphthylamine/4-nitroaniline/4-methoxyaniline and dime-
solvent, motivated us to investigate the scope of the one-pot thylphosphite to produce the corresponding a-aminophosphate
multi-component protocol for the synthesis of the a-amino- in excellent yields (Table 2, entries 4a–o). To further elaborate
phosphonate and bis-a-aminophosphonate derivatives from the scope of a one-pot multi-component protocol over a DTP/
various substituted aldehydes, amines and di or tri alkyl phos- SiO2 catalyst, the substituted aromatic amines such as aniline,
phites in the presence of a DTP/SiO2 catalyst at optimized 3-chroloaniline and 1-naphthylamine reacted smoothly with
benzaldehyde/3,4,5-trimethoxybenzaldehyde/4-chloro benzal-
dehyde/4-methylbenzaldehyde/4-methoxy benzaldehyde, and
dibenzylphosphite/triethylphosphite to obtain the correspond-
ing a-aminophosphonate in an excellent yield (Table 2, entries
4p–u). The excellent performance of the DTP/SiO2 catalyst for
the synthesis of a-aminophosphonate derivatives from the
various combinations of aryl/heteroaromatics aldehydes/
substituted aldehydes, amines/substituted amines and di or tri
alkyl phosphites made us excited to investigate the dimer
formation of a-aminophosphonates. Amazingly, benzaldehyde/
4-methoxyaldehyde (20 mmol) reacted very well with dimethyl
phosphite/triethylphosphite (20 mmol) and 4-amino aniline
Scheme 1 The synthesis of a-aminophosphonate derivatives via a
one-pot three-component condensation reaction.
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Table 2 The substrate scope for the one-pot three-component condensation reaction for the synthesis of a-aminophosphonate derivativesa,b
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Table 2 (Contd.)
a
Reaction conditions: aldehyde (10 mmol), amine (10 mmol), phosphite (10 mmol), DTP/SiO2 (50 mg) in 5 ml CH3CN, room temperature 1 h.
Isolated yields aer column chromatography. IC90 for stander drugs. IC50 against M. tuberculosis H37Ra for antitubercular activity. IC50
b
c
d
e
against THP 1 cell line for cytotoxicity.
(10 mmol) to provide the corresponding bis-a-amino- aldehyde by DTP/SiO2 (I) followed by the nucleophilic addition
phosphonate as a dimer in a high yield (Table 2 entries 4v–x). of amine to afford the imine (II) by the removal of water. The
The results in Table 2 clearly reveal that the one-pot three- subsequent activation of imine (II) by DTP/SiO2 facilitated the
component condensation reactions over the DTP/SiO2 catalyst addition of phosphite to give an activated phosphonium inter-
show a remarkable and excellent performance irrespective to mediate (III), which then gave the desired product IV (Scheme 2).
the presence of an electron donating/electron withdrawing
The recyclability and recovery of the DTP/SiO2 catalyst was
groups on the aromatic/heterocyclic aldehydes and/or amines investigated for the synthesis of a-aminophosphonates by a
and hence the one-pot three-component protocol is highly one-pot three-component condensation of benzaldehyde and
effective, promising, and general for the synthesis of a-amino- aniline with dimethyl phosphite as a model substrate in an
phosphonate and bis-a-aminophosphonate derivatives.
acetonitrile solvent at room temperature for 1 h, and the results
As per earlier literature,5,23a–c a probable mechanism is shown are provided in Table 3. The DTP/SiO2 catalyst was recovered
in Scheme 2 for the synthesis of a-aminophosphonate. The quantitatively from reaction mixture by ltration and reused
mechanism involves the activation of the carbonyl group of several times without the loss of catalytic activity (Table 3,
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a
Table 3 The recyclability of DTP/SiO2
Cycles
Yieldb (%)
1
2
3
4
5
6
98
98
97
98
96
97
a
Reaction conditions: Benzaldehyde (10 mmol), aniline (10 mmol),
dimethylphosphite (10 mmol), 20% DTP/SiO2 in 5 ml CH3CN, room
b
temp. 1 h. Isolated yields aer column chromatography.
the a-aminophosphonate derivatives (4a–x) were evaluated for
their cytotoxicity using a THP 1 (Human acute monocytic
leukemia cell line) in vitro MTT assay. Surprisingly, the 4n and
4v compounds were found to be non active towards cytotoxicity.
However, a few compounds, 4b, 4c, 4p, 4q, 4r and 4s (Table 2),
showed a good cytotoxicity.
The selectiveness of the 4n and 4v compounds towards
antitubercular activity made us enthusiastic to evaluate these
compounds for their antimicrobial activity. 4n and 4v were
evaluated for their antimicrobial activity against gram-negative
(Escherichia coli) and gram-positive (Staphylococcus aureus and
Bacillus) bacteria (Table 4). Miraculously, 4n and 4v show no
antibacterial activity towards gram-negative and gram-positive
bacteria. The evaluation data on cytotoxicity using the THP 1
(Human acute monocytic leukemia) cell line (Table 2), antimi-
crobial activity against gram-positive and gram-negative
Scheme 2 A possible mechanism for the synthesis of a-amino-
phosphonates over a DTP/SiO2 catalyst.
entries 2–6). The isolated yield obtained for the product at the
end of the 5th recycle (Table 3, entry 6) is very much consistent
with the fresh DTP/SiO2 catalyst (Table 3, entry 1). The consis-
tent catalytic activity of the recovered and reused DTP/SiO2
catalyst indicates that the reused catalyst shows an excellent
performance for the synthesis of a-aminophosphonates.
All of the synthesized a-aminophosphonate and bis-a-ami-
nophosphonate derivatives (4a–x) were screened using 100 mg
mlꢀ1 concentrations for their in vitro antitubercular activity
against the M. tuberculosis H37Ra (ATCC 25177) strain by XTT
reduction menadione assay. As shown in Fig. 1, the 4e, 4f, 4g, 4i,
4j, 4n, 4o and 4v a-aminophosphonate derivatives exhibited
inhibition. However, only the 4n and 4v derivatives exhibited
more than 90% inhibition, and they were further screened
using various concentrations for their in vitro antitubercular
activity to achieve IC50 which is compared with standard drugs
such as isoniazid, ethambutol and pyrazinamide (Table 2, entry
4y–aa).
Fig. 1 The analysis of the antitubercular activity of compounds using
an XTT Reduction menadione assay. 100 mg mlꢀ1 of the compounds
were added to 2 M tuberculosis culture at 0 days after inoculation. The
cell growth was estimated by monitoring the extent of the XTT
reduction after 8 days of incubation with respect to the DMSO vehicle
control and media as a blank. The percent inhibition of the compounds
is as shown in a graph. Further details are provided in the ESI.† The
results are the average of three identical experiments ꢁ the standard
The 4n and 4v a-aminophosphonate derivatives exhibited
half maximal concentration (IC50) values of 8.62 and 7.51 mg
mlꢀ1 (Table 2 entry 4n and 4v), respectively, which indicate that
the compounds are promising antitubercular agents. These
ndings inspired us to evaluate their cytotoxicity. Hence, all of deviation.
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Table 4 The antibacterial screening of 4n and 4v against gram-
positive and gram-negative bacteria
General experimental procedure for the 20% DTP/SiO2
mediated synthesis of a-aminophosphonates
The reaction mixture of aldehyde (10 mmol), amine (10 mmol)
and di/tri alkyl phosphite (10 mmol) was stirred in a 10 ml
round bottom ask containing 5 ml acetonitrile solvent in the
presence of 50 mg DTP/SiO2 catalyst at room temperature for
1 h. The completion of the reaction was monitored by TLC. Aer
the completion of the reaction, the reaction mixture was diluted
with ethyl acetate (10 ml) and the catalyst was recovered by
ltration. The ltrate was washed with aqueous NaHCO3 and
then with water followed by the separation of an aqueous layer
and organic layer. The organic layer is dried over anhydrous
Na2SO4 and concentrated in a vacuum to give the crude product.
The crude product was puried by silica gel column chroma-
tography using a 70 : 30 ratio of pet ether : ethyl acetate to
afford the pure a-amino phosphonate. The products obtained
were characterized by NMR.
% Inhibition
Concentration
Compounds
(mg mlꢀ1
)
E. coli
S. aureus
Bacillus
4n
4v
100
100
0.8
35
87.3
2.5
10.1
ꢀ10.3
bacteria (Table 4) clearly shows that 4n and 4v are highly
selective towards antitubercular activity against the M. tuber-
culosis H37Ra (MTB) strain and were found to be promising
antitubercular agents for further drug discoveries.
Conclusions
In conclusion, a novel, environment friendly, highly efficient,
cost effective, one-pot multi-component protocol has been
developed for the efficient synthesis of a-aminophosphonate
derivatives in excellent yields via a one-pot three-component
condensation of various substituted aldehydes, substituted
amines and di or tri alkyl phosphites using an ecofriendly,
heterogeneous, reusable silica supported dodecatungstophos-
phoric acid (DTP/SiO2) catalyst at ambient temperature in a
short reaction time. The one-pot multi-component condensa-
tion reactions (MCR) over the DTP/SiO2 catalyst show a
remarkable and excellent performance irrespective of the
presence of electron donating/electron withdrawing groups on
the aromatic/heterocyclic aldehydes and/or amines and hence
the one-pot three-component protocol is highly effective,
promising and general for the synthesis of a-amino-
phosphonate and bis-a-aminophosphonate derivatives.
The catalyst was recycled several times without the loss of
catalytic activity. These a-aminophosphonate derivatives were
evaluated for the rst time for the antitubercular activity
against the M. tuberculosis H37Ra (MTB) strain by using an
XTT reduction menadione assay (XRMA) protocol. However,
the 4n and 4v a-aminophosphonate derivatives exhibited half
maximal concentration (IC50) values of 8.62 and 7.51 mg mlꢀ1
(Table 2 entry 4n and 4v), respectively. An evaluation of the
data on the cytotoxicity and antimicrobial activity shows that
4n and 4v are highly selective towards antitubercular activity
against the M. tuberculosis H37Ra (MTB) strain and were found
to be promising antitubercular agents for further drug
discoveries.
Acknowledgements
MYP and SSC are thankful to CSIR New Delhi for a SRF. The
authors also thank Dr V. V. Ranade, Chair of CE-PD division for
helpful discussion, encouragement and support.
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