Synthesis of 2-amino-4H-chromen-4-ylphosphonates and β-phosphonomalonates via tandem Knoevenagel–Phospha-Michael reaction and antimicrobial evaluation of newly synthesized β-phosphonomalonates

Article abstract of DOI:10.1007/s11164-017-3077-2

In this manuscript we have demonstrated a new approach for the synthesis of 2-amino-4H-chromen-4-ylphosphonates and β-phosphonomalonates linked 2-chloroquinoline-3-carbaldehyde by modified one-pot three-component tandem Knoevenagel–Phospha-Michael reaction of salicylaldehyde/aryl aldehyde/2-chloroquinoline-3-carbaldehyde, malononitrile/ethylcyanoacetate, and phosphite ester using triethylamine (1–10?mol%) in ethanol under reflux conditions. The desired products were obtained in 86–97% yield in 8–35?h. The advantages of this protocol are its operational simplicity, low catalytic loading, no side product formation, and high yield of product. The newly synthesized β-phosphonomalonates, diethyl-(2-chloroquinolin-3-yl)-2,2-dicyanoethyl)-phosphonates (4a–i) have been tested on two fungal strains (C. albicans and A. fumigatus) and two bacterial strains (S. aureus and E. coli) and their minimum inhibitory concentration was also determined by microbroth dilution method.

Full text of DOI:10.1007/s11164-017-3077-2

Res Chem Intermed
DOI 10.1007/s11164-017-3077-2
Synthesis of 2-amino-4H-chromen-4-ylphosphonates
and b-phosphonomalonates via tandem Knoevenagel–
Phospha-Michael reaction and antimicrobial evaluation
of newly synthesized b-phosphonomalonates
Parteek Kour1 Anil Kumar1
Rashmi Sharma2
•
•
•
Reena Chib2 Inshad Ali Khan2 Vijai K. Rai3
•
•
Received: 1 February 2017 / Accepted: 12 July 2017
Ó Springer Science+Business Media B.V. 2017
Abstract In this manuscript we have demonstrated a new approach for the syn-
thesis of 2-amino-4H-chromen-4-ylphosphonates and b-phosphonomalonates linked
2-chloroquinoline-3-carbaldehyde by modified one-pot three-component tandem
Knoevenagel–Phospha-Michael reaction of salicylaldehyde/aryl aldehyde/2-
chloroquinoline-3-carbaldehyde, malononitrile/ethylcyanoacetate, and phosphite
ester using triethylamine (1–10 mol%) in ethanol under reflux conditions. The
desired products were obtained in 86–97% yield in 8–35 h. The advantages of this
protocol are its operational simplicity, low catalytic loading, no side product for-
mation, and high yield of product. The newly synthesized b-phosphonomalonates,
diethyl-(2-chloroquinolin-3-yl)-2,2-dicyanoethyl)-phosphonates (4a–i) have been
tested on two fungal strains (C. albicans and A. fumigatus) and two bacterial strains
(S. aureus and E. coli) and their minimum inhibitory concentration was also
determined by microbroth dilution method.
Keywords Triethylamine Á Multi-component reaction Á Tandem reaction Á b-
Phosphonomalonates Á 2-Amino-4H-chromen-4-ylphosphonates Á Antimicrobial
activity
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3077-2)
contains supplementary material, which is available to authorized users.
&
Anil Kumar
anilsharmachemistry@gmail.com
1
2
3
Synthetic Organic Chemistry Laboratory, Faculty of Sciences, Shri Mata Vaishno Devi
University, Katra, Jammu and Kashmir 182 320, India
Clinical Microbiology Division, Indian Institute of Integrative Medicine, Canal Road,
Jammu 180001, India
Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh 495009,
India
123

P. Kour et al.
Introduction
Multi-component (MCRs) and tandem reactions have played a pivotal role in
organic synthesis. MCR provides a powerful platform to quickly generate novel and
complex molecules including bioactive heterocyclic scaffolds in a minimum
number of steps employing diversity-oriented synthesis [1–5].
In spite of the investments in antimicrobial drug discovery, the difficulty to treat
infections due to antibiotic resistance poses a significant threat to global health
[6–10]. Consequently, tackling antimicrobial resistance and search for new
antimicrobial agents will always remain a significant job for medicinal chemists.
Quinolines are important structural building block of several natural products and
also considered one of the most commonly encountered heterocycles in medicinal
chemistry. Literature survey revealed that compounds containing quinoline moiety
possess diverse chemotherapeutic activities, which includes anti-malarial [11], anti-
cancer [12], anti-microbial [13], anti-bacterial [14], anti-fungal [15], and anti-
inflammatory [16] activities. Ciprofloxacin, sparfloxacin, and mefloquine are well
known anti-microbial quinoline drugs (Fig. 1) [17, 18]. Further, 2-chloroquinoline-
3-carbaldehyde is a versatile intermediate for the synthesis of potential bioactive
heterocycles and for functional group transformation [19, 20].
Likewise, phosphonates have been used as enzyme inhibitors [21], metabolic
probes [22], peptide mimetics [23], as well as intermediates in organic synthesis
[24]. In addition, 2-amino-4H-chromen-4-ylphosphonates exhibits wide variety of
biological activities such as anti-cancer [25], anti-inflammatory [26], anti-malarial
[27] and anti-microbial activity [28].
Numerous reports are accessible in the literature for the synthesis 2-amino-4H-
chromen-4-ylphosphonates [29–40] and b-phosphonomalonates [37–55]. However,
domino Knoevenagel-phospha-Michael strategy, which involves the reaction
between aromatic aldehyde, active methylene compound, and phosphite ester is
the most reliable route used for synthesizing 2-amino-4H-chromen-4-ylphospho-
nates and b-phosphonomalonates.
Despite the promising bioactivity of quinoline and phosphonate moieties, there is
no report for the synthesis of b-phosphonomalonates containing pharmaceutically
and chemically important quinoline moiety, which appear to be attractive scaffolds
to screen for lead candidates. Thus, keeping these facts in mind and our interest in
developing new methodologies [57–59] especially C–P bond formation [40, 60], we
envisioned to employ the tandem Knoevenagel–Phospha-Michael approach for the
NH2
O
N
H
O
O
N
F
H
COO
N
H
F
H
COO
N
N
HN
F
N
H
C
3
HN
F
C
3
Sparfloxacin
Mefloquine
Ciprofloxacin
Fig. 1 Structures of representative quinoline based antimicrobial drugs
123

Synthesis of 2-amino-4H-chromen-4-ylphosphonates and…
synthesis of b-phosphonomalonates utilizing 2-chloroquinoline-3-carbaldehyde as
aldehyde partner.
Result and discussion
The remarkable biological activity of quinoline and phosphonate moieties has
encouraged us to synthesize a series of hybrid molecules containing both quinoline
and phosphonate moiety as building blocks to evaluate anti-microbial activity and
enrich the quinoline and phosphorous chemistry. In this context, we intended to
synthesize a series of new b-phosphonomalonates linked with 2-chloroquinoline-3-
carbaldehyde moiety using our previously reported protocols, but did not achieve
good yield of products [40]. This is probably due to (1) the low solubility of
2-chloroquinoline-3-carbaldehydes in aqueous condition and (2) possible side
reactions. Therefore, we thought it worth developing a convenient and high yielding
synthetic route for b-phosphonomalonates possessing quinoline moiety.
In this context, we began by evaluating the reaction of 2-chloroquinoline-3-
carbaldehyde (1a), malononitrile (2a), and triethylphosphite (3a) in the presence of
triethylamine (10 mol%) under reflux in EtOH (Table 1). Gratifyingly, reaction
proceeded smoothly to deliver the desired product, 2-chloroquinolinephosphonates
in 97% yield in 8 h (Table 1, entry 1). The product was isolated and characterized
1
by spectroscopic and elemental analysis. The H NMR spectrum of product 4a
exhibited peaks at d 1.17 and 1.41 for two –CH3 groups and doublet at d 3.90–4.00
and 4.07 for two –CH (CH–P and CH(CN)2 , respectively) protons. In 13C spectra
the C–P peak at d 38.88 was also observed. Encouraged by these results, we next
Table 1 Optimization of reaction conditions for the synthesis of 4a
Entry
Catalyst (mol%)
Solvent
Temperature
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
Triethylamine (10)
Triethylamine (5)
Triethylamine (2)
Triethylamine (1)
Triethylamine (2)
Triethylamine (2)
DABCO (2)
EtOH
EtOH
EtOH
EtOH
CHCl3
CH3CN
EtOH
EtOH
EtOH
EtOH
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
8
16
22
30
27
27
27
27
27
24
97
96
95
88
84
82
70
50
30
32
DBU (2)
p-TsOH (2)
Bi(NO3)3Á5H2O (2)
Reaction was performed using 2-chloroquinoline-3-carbaldehyde (1 mmol), malononitrile (1 mmol),
triethylphosphite (1 mmol)
a
Isolated yields
123

P. Kour et al.
Table 2 Synthesis of b-phosphonomalonates
Time (h)
Entry Aldehydes Product
1(a–p) 4(a–p)
Yield (%)a
M.P (°C)
2 mol% of 10 mol% 2 mol% of 10 mol% of
catalyst
of catalyst
8
catalyst
catalyst
97
1
2
3
22
95
148–149
153–155
151–152
24
25
9
9
92
92
94
93
4
25
9
93
94
144–146
5
6
23
24
8
9
93
92
95
94
153–154
127–128
7
8
27
30
10
10
94
90
95
92
130–131
165–166
9
30
10
90
91
176–177
10
11
26
28
9
94
93
95
95
55
(53–57)
[46]
10
94
(93–95)
[46]
123

Synthesis of 2-amino-4H-chromen-4-ylphosphonates and…
Table 2 continued
Time (h)
Entry Aldehydes Product
1(a–p) 4(a–p)
Yield (%)a
M.P (°C)
2 mol% of 10 mol% 2 mol% of 10 mol% of
catalyst
of catalyst
catalyst
catalyst
59
12
13
22
8
96
97
(60–62)
[46]
94
(93–95)
[46]
29
9
91
92
76
14
15
30
30
10
10
90
89
93
90
(75–77)
[46]
72
(73) [55]
16
35
12
86
86
Oil
(Oil) [51]
Reaction conditions: 1 (1 mmol), 2 (1 mmol), 3 (1 mmol), triethylamine (2/10 mol%) and EtOH (4 mL)
a
Isolated yield
examined the amount of catalyst by using varied catalytic concentrations such as 5,
2, and 1 mol% (Table 1, entries 2–4). The results indicated that 2 mol% of catalyst
was enough to trigger the reaction to afford 2-chloroquinolinephosphonates in
excellent yield (Table 1, entry 3). However, further decrease in catalytic loading
results in a decrease in yield of product (Table 1, entry 4). Next, optimization of
solvent was carried out, and ethanol was the best solvent for this conversion
(Table 1, entries 5 and 6). Subsequently, several basic and acidic catalysts such as
DABCO, DBU, p-TsOH, and Bi(NO3)3Á5H2O were tested for comparative purpose,
but the yield decreased to 30–70% (Table 1, entries 7–10), hence triethylamine
(2 mol%) was found to be effective catalyst for this conversion and hence selected
for subsequent reactions.
Subsequently, the substrate scope was investigated and the results are
summarised in (Table 2, entries 1–9). Delightfully, differently substituted 2-chloro-
quinoline-3-carbaldehyde derivatives (1a–i) with either electron withdrawing or
donating group were well tolerated to generate the desired product in excellent
yields. Further, differently substituted benzaldehydes (aryl aldehydes) also reacted
efficiently to afford the anticipated product in excellent yields (Table 2, entries
10–16).
Further, to study the effect of catalytic loading in detail we extended the substrate
scope with 10 mol% of triethylamine and the results showed that reaction
123

P. Kour et al.
Table 3 Comparative data of triethylamine with previously reported catalysts for the synthesis of 4j
Entry
Catalyst (mol%)
Method
Time
Yield (%)
1
2
Triethylamine (10)
Triethylamine (2)
Conventional
Conventional
Conventional
Conventional
Conventional
Conventional
Conventional
Conventional
Conventional
Conventional
9 h
95a
94a
26 h
12 h
1 h
3
DMAP (20)
92 [40]
90 [50]
92 [49]
85 [47]
93 [41]
92 [42]
80 [39]
85 [38]
4
Py-GO (3)
5
PS-DABCO (3)
1 h
6
Iodine (10)
2 h
7
HClO4–SiO2 (3)
1 h
8
3-Aminopropylated silica gel (30)
Silica-bonded 2-HEAA (1)
Ethylenediamine diacetate (20)
3 h
9
15 min
3 h
10
Reactions were carried out with 1 mmol each of benzaldehyde, malononitrile, triethyl phosphate in EtOH
(4 mL) under reflux
a
Yields are for isolated products
completed in around 8–12 h with slight improvement in the yield of product, but
with 2 mol% reactions took 22–35 h for completion (Table 2, entires 1–16).
The work-up procedure is very simple, and product was mostly purified by
simple crystallization from ethanol. The structure of new compounds (4a–i) were
characterized by 1H NMR, 13C NMR, 31P NMR spectra and HRESIMS data;
however, the structure of the known compounds (4j–p) were confirmed and
authenticated by comparison of its physical and spectral data with that of reported
in the literature.
In order to quantitatively compare the efficiency of different catalysts, for 4j our
catalyst gave better yield to that of reported in the literature (Table 3, entries 1–10)
despite using low catalytic loading (Table 3, entries 1–10).
In order to broaden further the synthetic utility of current methodology, we
extended this standardized protocol to explore the tandem Knoevenagel–Phospha-
Michael reaction of substituted salicylaldehyde, malononitrile/ethylcyanoacetate,
and phosphite ester. To our delight, the reaction worked well and series of
2-amino-4H-chromen-4-ylphosphonates were obtained in very good to excellent
yields (Table 4, entries 1–8) akin to that of our earlier reported protocol [40].
Further, it is pertinent to mention that these 2-amino-4H-chromen-4-ylphospho-
nates have been reported to possess the anti-microbial activity [32].
A plausible mechanism for the Knoevenagel–Phospha-Michael reaction for the
formation of 2-chloroquinolinephosphonates is depicted in Scheme 1. The first step
involves Knoevenagel condensation of 2-chloroquinoline-3-carbaldehyde (A) and
malononitrile in the presence of triethylamine to afford Knoevenagel product (B)
followed by nucleophilic attack of triethylphosphite to deliver the desired product,
2-chloroquinolinephosphates (C).
123

Synthesis of 2-amino-4H-chromen-4-ylphosphonates and…
Table 4 Synthesis of 2-amino-4H-chromen-4-ylphosphonates
Entry Aldehydes
Product
6(a–h)
Time (h)
2 mol% of 10 mol% of 2 mol% of 10 mol% of
Yield (%)a
M.P (°C)
5(a–h)
catalyst
catalyst
catalyst
catalyst
1
2
24
8
95
96
139–141
(140–142) [34]
29
10
92
94
151
(156–158) [34]
3
4
28
31
10
12
95
93
96
94
177–178
(177–178) [34]
159–161
(160–161) [34]
5
6
28
30
9
94
92
94
93
150–151
(148–150) [34]
11
167–169
(166–168) [34]
7
8
33
34
12
12
89
88
90
90
Oil
(Oil) [32]
Oil
(Oil) [32]
Reaction conditions: 5 (1 mmol), 2 (1 mmol), 3 (1 mmol), triethylamine (2/10 mol%), EtOH (4 mL)
a
Isolated yield
Evaluation of antimicrobial activity
The antimicrobial activity of the newly synthesized compounds (4a–i) were
evaluated against Gram positive (S. aureus) and Gram negative bacteria (E. coli)
and fungus (C. albicans and A. fumigatus) using microdilution method [61–63].
123

P. Kour et al.
Scheme 1 Proposed mechanism for the synthesis of 4a
Table 5 MIC determination of
Serial no
Tested samples
In lg/mL MIC
S.aureus ATCC 29213
the active sample
1
2
3
4
5
6
4a
128
128
64
4b
4g
4h
64
4i
64
Ciprofloxacin
0.125
Antibacterial testing was performed in Muller Hinton Broth (Becton-Dickenson,
Cockeysville, MD, USA), whereas for antifungal testing, RPMI 1640 with L-
glutamine (Sigma-Aldrich, St. Louis, MO, USA) buffered to pH 7.0 supplemented
with 0.165 M 3-(N-morpholino) propanesulfonic acid (MOPS) (Sigma-Aldrich)
was used. The stock solution of the compounds was prepared in DMSO. The MIC
(minimum inhibitory concentration) of the compounds was determined by serial
twofold diluting of the compounds in the prepared media in 100 lL volume in a
96-well, U bottom microtitre plate. The final concentrations of compound ranged
from 128 to 0.25 lg/mL. Amphotericin B and ciprofloxacin (16–0.03 lg/mL)
(both from Sigma-Aldrich) were used as standard antifungal and antibacterial
agents respectively. The bacterial and fungal suspension of the overnight grown
bacterial and fungal was prepared in sterile normal saline and the density was
adjusted to 0.5 Mcfarland. The bacterial cultures were further diluted and added in
100 lL volume at a final inoculum of 1 9 105 CFU/mL. For fungal cultures 1 9
103 CFU/mL was used. The plates were incubated at 37 °C for 24 h for bacterial
cultures and 48 h for fungal cultures. The plates were read visually and the
minimum concentration of the compound showing no turbidity was recorded as
MIC.
Five compounds (4a, 4b, 4g, 4h, 4i) were found active against S. aureus
(ATCC 29213). Further, the minimum inhibitory concentration (MIC) of these
active samples was determined (Table 5). The comparison of the MICs of the
compounds with standard drug (ciprofloxacin) revealed that five compounds (4a,
4b, 4g, 4h, 4i) exhibited moderate antibacterial activity against Gram positive
bacteria (S. aureus).
123

Synthesis of 2-amino-4H-chromen-4-ylphosphonates and…
Conclusion
In conclusion, we have devised a new strategy for the synthesis of 2-amino-4H-
chromen-4-ylphosphonates and b-phosphonomalonate derivatives by modified
domino Knoevenagel–Phospha-Michael reaction using triethylamine. The present
methodology is very simple, efficient, and selective and thus will be a valuable entry
to a general method for preparing 2-amino-4H-chromen-4-ylphosphonates and b-
phosphonomalonates. The newly synthesized b-phosphonomalonate derivatives
(4a–i) have been evaluated for their antimicrobial activity. The screening results
revealed that five compounds (4a, 4b, 4g, 4h, 4i) exhibited moderate antibacterial
activity against Gram positive bacteria (S. aureus).
Experimental
All the reactants and reagents were used as received from commercial suppliers
without further purification. 2-Chloroquinoline-3-carbaldehyde derivatives were
synthesized according to literature method [56]. Progress of reaction was monitored
by thin layer chromatography (TLC) technique using alumina silica gel plates (60
F
254). Column chromatography was performed over silica gel (60–120 mesh) using
ethyl acetate/n-hexane. All the products were identified by their physical and
spectral data. Melting points were determined on analab melting point apparatus.
1H, 13C, and 31P NMR spectra were recorded on a Bruker 400, 100, and 162 MHz
spectrometer, respectively, in CDCl3 solvent using tetramethylsilane as internal
standard. Chemical shifts (d) are reported in parts per million. The following
abbreviations are used to explain the multiplicities: s = singlet, d = doublet, t =
triplet, and m = multiplet. HRESIMS data were obtained from Agilent 6540 UHD
Q-ToF Mass Spectrometer coupled LC system. The following conditions were used:
capillary voltage 3500 V, capillary temperature 350 °C, auxiliary gas flow rate 7.0
L/min, spray voltage 4.5 kV, mass range 100–1000 amu (maximum resolution
30,000).
General procedure for the synthesis of b-phosphonomalonates
Triethylamine (2 mol%) was added to a mixture of substituted aldehydes 1
(1 mmol), malononitrile/ethyl cyanoacetate 2 (1 mmol), and phosphite ester 3
(1 mmol) in EtOH (4 mL). The contents of the reaction mixture were heated at
80 °C for an appropriate time (Table 2). After completion of the reaction as
monitored by TLC, the crude product was isolated and purified by crystallization
with ethanol. In some cases (Table 2, entries 7, 9, and 16) the crude product was
purified by column chromatography using ethyl acetate and n-hexane (8:2 to 6:4).
The structure of the product, 4a–p, was confirmed by their spectral data (see the
Supporting Information).
123

P. Kour et al.
General procedure for the synthesis of 2-amino-4H-chromen-4-ylphosphonates
Triethylamine (2 mol%) was added to a mixture of substituted salicylaldehyde 5
(1 mmol), malononitrile/ethyl cyanoacetate 2 (1 mmol), and phosphite ester 3
(1 mmol) in EtOH (4 mL). The contents of the reaction mixture were heated at
80°C for an appropriate time as shown in (Table 4). After completion of the reaction
as monitored by TLC, the crude product was isolated and purified by crystallization
with ethanol. In some cases (Table 3, entries 7 and 8) the crude product was purified
by column chromatography using ethyl acetate and n-hexane (8:2–6:4). The
structure of the product, 6a–h, was confirmed by spectral data, which is consistent
with that reported in the literature (see the Supporting Information).
Acknowledgements We are thankful to SAIF Chandigarh for providing spectral data. P. K. thanks to
UGC, Govt. of India for BSR fellowship.
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