New class of diethyl substituted phosphoramidimidates and phosphonimidates: synthesis, spectral characterization and antimicrobial activity

Article abstract of DOI:10.1080/10426507.2018.1513934

A series of new class of diethyl N-2-hydroxyethyl-N'-substituted phosphoramidimidates 6(a–e) and diethyl P-morpholino-N-substituted phosphonimidates 6(f–j) was synthesized. The precursor intermediates, diethyl substituted phosphoramidites 3(a–b) were prepared initially by a reaction of various amines 1(a–b) and diethyl phosphorochloridite (2) and then they were treated by in situ with aromatic/alkyl azides through Staudinger reaction to accomplish title products. Structures of all the synthesized compounds were characterized by spectroscopic data such as IR, NMR (¹H, 13C, ³¹P), mass, and elemental analyses. The synthesized compounds were screened for their in vitro antimicrobial activity to understand their biological potency. The biological screening results disclosed that compounds 6b, 6c, 6e, 6g, 6h and 6j having potent antimicrobial activity against all the tested pathogens.

Full text of DOI:10.1080/10426507.2018.1513934

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
New class of diethyl substituted
phosphoramidimidates and phosphonimidates:
synthesis, spectral characterization and
antimicrobial activity
Mavallur Varalakshmi, Chamarthi Nagaraju & Palaa Krishna
To cite this article: Mavallur Varalakshmi, Chamarthi Nagaraju & Palaa Krishna (2018): New
class of diethyl substituted phosphoramidimidates and phosphonimidates: synthesis, spectral
characterization and antimicrobial activity, Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2018.1513934
To link to this article: https://doi.org/10.1080/10426507.2018.1513934
Published online: 10 Oct 2018.
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2018.1513934
New class of diethyl substituted phosphoramidimidates and phosphonimidates:
synthesis, spectral characterization and antimicrobial activity
a
b
c
Mavallur Varalakshmi , Chamarthi Nagaraju , and Palaa Krishna
a
Basic Sciences & Humanities, Centre for Applied Sciences, Sree Vidyanikethan Engineering College, Tirupati, Andhra Pradesh, India;
Department of Chemistry, Sri Venkateswara University, Tirupat, Andhra Pradesh, India; Department of Chemistry, Geethanjali Institute of
b
c
Science and Technology, Gangavaram, Nellore, India
ABSTRACT
ARTICLE HISTORY
A series of new class of diethyl N-2-hydroxyethyl-N’-substituted phosphoramidimidates 6(a–e) and
diethyl P-morpholino-N-substituted phosphonimidates 6(f–j) was synthesized. The precursor inter-
mediates, diethyl substituted phosphoramidites 3(a–b) were prepared initially by a reaction of
various amines 1(a–b) and diethyl phosphorochloridite (2) and then they were treated by in situ
with aromatic/alkyl azides through Staudinger reaction to accomplish title products. Structures
of all the synthesized compounds were characterized by spectroscopic data such as IR, NMR
Received 13 March 2018
Accepted 15 August 2018
KEYWORDS
Iminophosphoranes; diethyl
phosphorochloridite;
Staudinger reaction;
antimicrobial activity
1
31
(
H, 13C, P), mass, and elemental analyses. The synthesized compounds were screened for their
in vitro antimicrobial activity to understand their biological potency. The biological screening
results disclosed that compounds 6b, 6c, 6e, 6g, 6h and 6j having potent antimicrobial activity
against all the tested pathogens.
GRAPHICAL ABSTRACT
Introduction
phosphoranimines, or phosphine imides, where phosphorus
atom is linked to a nitrogen atom by a double bond and to
Phosphorus compounds includes trivalent and pentavalent
states bonded with nitrogen species have become most
attractive molecules in diverse fields. They have been used
[8]
three other atoms or groups by single bonds.
These
derivatives are useful in many applications like building
[1]
[9,10]
blocks of polymers,
sis,
key components in organic synthe-
[2]
as catalysts in numerous organic transformations
and
[11,12]
super-bases (e.g. Schwesinger base) with nonme-
tallic, nonionic and low nucleophilic characteristics
and serves as key intermediates in the synthesis of various
ligands in asymmetric synthesis to achieve high enantiose-
[13,14]
[
3–5]
lectvity
as well as inhibitors for the treatment of various
[
6]
diseases. Particularly, phosphoramidites belong to a family
of amides of trivalent phosphorous acid, distinct from other
trivalent phosphorous compounds, as they possess one P-N
and two P-O bonds. These phosphoramidites are important
intermediates in the synthesis of numerous libraries of
[15]
[16]
biologically active compounds
aplysinopsin,
leucett-
[
17]
[18]
amine B,
Fascaplysin.
Staudinger reaction is the most
widely employed protocol for the synthesis of iminophos-
phoranes. This reaction involves the right components of
Staudinger reaction such as trivalent phosphorus and
organic molecules and can be acted as ligands in asymmetric organic azide species leading to the formation of P¼N
synthesis because of their easy accessibility and rapid tuning linkages and it can be used to enables the straightforward
[
19]
of properties with the variation of substituents at the nitro- synthesis of various phosphorous nitrogen compounds.
[
7]
gen atoms.
Iminophosphoranes (R P¼NR) are
To the best of literature knowledge, there is no reports on
class of the investigation of biological activity of phosphoramimi-
a
3
organic compounds, also known as phosphazenes, date derivatives.
CONTACT Mavallur Varalakshmi
varalakshmi.mavallur@gmail.com
Supplemental data for this article can be accessed on the publisher’s website at https://doi.org/10.1080/10426507.2018.1513934.
ß 2018 Taylor & Francis Group, LLC

2
M. VARALAKSHMI ET AL.
Scheme 1. Synthesis of diethyl substituted phosphoramidimidates and phosphonimidates 6(a–j).
1
Considering the overview facts, we herein report the syn- -NH and P ¼ N, respectively. In H NMR spectra, the chem-
thesis of new class of diethyl substituted phosphoramidimi- ical shifts in the regions of d 1.39–1.42 ppm as a triplet and
date/phosphonimidate derivatives through Staudinger d 4.13–4.18 ppm as a quartet confirmed CH3 (H-16 and
reaction using substituted phosphoramidites and heterocyc- H-18) and –OCH2 (H-15 and H-17) protons present in
lic/alkyl azides. The antimicrobial activity was evaluated to ethoxy groups, respectively. The –OH (H-1) and –NH (H-4)
understand their biological potency.
protons resonated as singlets at d 4.76 and d 3.73 ppm
respectively. The remaining aliphatic protons appeared in
the regions of d 2.05–3.17 ppm (H-2 and H-3). In 13C NMR
spectra, the chemical shift values at d 65.5 and d 154.5 ppm
Results and discussion
The synthesis of new iminophosphorane derivatives, diethyl are corresponding to CH
N-2-hydroxyethyl-N’-substituted
phosphoramidimidates (C ) carbons, respectively. The remaining aliphatic carbons
(a–e) and diethyl P-morpholino-N-substituted phosphoni- were observed in the range of d 19.3–51.2 ppm (C , C
OH (C15 and C17) and –P¼N–C
2
9
6
2
3
, C16
31
midates 6(f–j) was depicted in Scheme 1. All the title com- and C18). The P NMR signal of compound 6c was
pounds were purified by column chromatography and observed as a singlet at d 12.6 ppm.
stored in nitrogen atmospheric conditions.
The structures of all the newly synthesized compounds
(a–j) were characterized by IR, NMR ( H, 13C & P),
Biological activity
1
31
6
mass spectroscopy and CHN analyses and the details In vitro antimicrobial activity of the newly synthesized
provided in the experimental section and supporting infor- phosphoramidimidate/phosphonimidate derivatives 6(a–j)
mation. In IR spectrum of compound 6c, the absorption was screened to understand their biological potency. The
bands appeared in the regions of 3472, 3267 and bacterial strains such as Staphylococcus aureus (S. aureus),
ꢀ
1
1
342 cm corresponding to the functional groups, –OH, Lactobacillus acidophillus (L. acidophillus) (Gram positive)

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
and Escherichia coli (E. coli) and Vibrio cholerae (V. Procedure for the synthesis of 2-azidopyrazine 5c
cholerae) (Gram negative), and four fungal strains like
The mixture of 2-chloropyrazine (4c) (235.42 mg, 1.5 mmol)
Trichoderma
longibrachiatum
(T.
longibrachiatum),
and sodium azide (9 mg, 1.5 mmol) in tetrahydrofuran
Clostridium tetani (C. tetani), Aspergillus niger (A. niger)
and Aspergillus fumigates (A. fumigates) were selected to
investigate the activity. The agar diffusion method and poi-
son plate technique were used to screen the antibacterial
and antifungal activities, respectively. Tetracycline in anti-
bacterial activity and Fluconazole in antifungal activity were
used as the standards. The bacterial/fungal growths of zone
of inhibition exhibited by the title compounds at the con-
centration of 200 mg/mL were measured in millimeters. The
experiments were conducted in duplicate and average value
was taken as final result. The results of antibacterial and
ꢁ
(
THF) (10 mL) was stirred for 2 h at 40–45 C. After com-
pletion of the reaction as checked by TLC, the reaction mass
was filter at ambient temperature to remove sodium chloride
as residue and then the filtrate was concentrated under vac-
ꢁ
uum at 45 C to afford 2-azidopyrazine (5c). It was used dir-
ectly without purification in the Staudinger reaction. The
same procedure was adopted to prepare remaining organic
azide derivatives used in the present study.
Procedure for the synthesis of compound 6c
antifungal activities were summarized in Table S1 and Diethyl phosphorochloridite (2) (0.21 mL, 1.5 mmol) solution in
Table S2, respectively (Supplemental Materials).
THF (5 mL) was added slowly in drop wise to the cold solution
As can be seen in Table S1 and Table S2, all the com- of ethanolamine (1a) (0.09 mL, 1.5 mmol) and triethylamine
ꢁ
pounds showed moderate to potential antimicrobial activity (TEA) (1.5 mmol) in THF (10 mL) at 0–5 C for 45min under
at the concentration of 200 mg/mL except compound 6f and nitrogen atmospheric conditions. The organics were stirred
ꢁ
6
i against fungal strains. Whereas, 2-hydroxy ethylamine for 2.0 h at 15–20 C to give precursor intermediate, diethyl
substituted phosphoramidimidate derivatives 6b linked with 2-hydroxyethylphosphoramidite (3a). The reaction completion
4
6
-bromo-2-pyridine, 6c bonded with cyclopropyl methyl and was judged by TLC. The solution of 2-azidopyrazine (5c)
e bearing thiazol-2-yl, and morpholine substituted phos- (1.5mmol) in THF (5mL) was added to the above reaction mass
ꢁ
phonimidate derivatives 6g linked with 4-bromo-2-pyridine, at 15–20 C for 30 min. The resulting organics were agitated for
6
ꢁ
h bonded with cyclopropyl methyl and 6j bearing thiazol- 5.0 h at 60-65 C and TLC confirmed the completion of reaction.
2
-yl showed potential activity which are almost closer activ- The reaction mass was filtered under nitrogen atmospheric condi-
ity to the standards. The substituents, 4-bromo-2-pyridine, tions to remove the salt, triethyl ammonium hydrochloride. The
cyclopropyl methyl and thiazol-2-yl on the title compounds filtrate was concentrated under vacuum to obtain the crude com-
could be reason to exhibited potential activity. Future pound. It was subjected to column chromatography using ethyl
research by the synthesis of library compounds through acetate and n-hexane (3:7) as a mobile phase to obtain diethyl N-
altering different substituted amines and organic azides fol- 2-hydroxyethyl-N’-(pyrazin-2-yl)phosphoramidimidate (6c). The
lowed by screening of biological activity is required to same experimental procedure was followed for the synthesis of
understand clearly the structural-activity relationship.
remaining title compounds 6a, 6b & 6(d–j).
Diethyl N-2-hydroxyethyl-N’-(pyrazin-2-
yl)phosphoramidimidate (6c)
Experimental
The analytical grade chemicals and GC-grade solvents used
in the reactions were purchased from Sigma Aldrich and S.
D. fine chem (India) and used without further purification.
The progress of reactions was monitored by TLC on Merck
silica plates. All the reactions were carried out under nitro-
gen atmospheric conditions. Neutral alumina was used as a
solid phase and moisture free solvents ethyl acetate and
n-hexane was used as eluents in the column chromatog-
raphy to purify the title compounds. Melting points were
determined in open capillaries on Guna melting point
apparatus and are uncorrected. IR spectra were recorded on
JASCO FT-IR 5300 using KBr discs. Nuclear magnetic res-
onance (NMR) spectroscopic data were recorded on Bruker
AV-400 spectrometer. Tetramethylsilane (TMS) and 85%
H PO were used as an internal and external standards in
ꢀ1
Semi-solid, Yield:78%; IR (KBr, cm ): 1342 (-P¼N, str),
1
3
267 (-P-NH, str), 3472 (-OH, str); H NMR (CDCl ,
3
4
(
(
00 MHz): d 1.39 (t, J ¼ 7.2 Hz, 6H, H-16, H-18), 2.05-2.27
q, 2H, H-3), 3.15-3.17 (q, 2H, H-2), 3.73 (s, 1H, H-4), 4.76
s, 1H, H-1), 4.13-4.18 (q, 4H, H-15, H-17), 7.79 (s, 1H, H-
1
0), 8.10 (d, J ¼ 8.8 Hz, 1H, H-12), 8.47 (d, J ¼ 5.2 Hz, 1H,
H-13); 13C NMR (CDCl3 100.6 MHz): d 19.3 (C , C ),
,
16
18
3
1
1
4
9.9 (C ), 51.2 (C , C ), 65.5 (C ), 140.8 (C ), 147.2 (C ),
3 15 17 2 12 13
31
50.5 (C ), 154.5 (C ). P NMR (CDCl , 161.9 MHz): d
1
0
9
3
þ
2.6. MS (m/z): 273 (M-H ). Anal.Calcd.forC H N O P: C
3.79, H 6.98, N, 20.43; Found: C 43.82, H 6.87, N 20.45.
10
19 4 3
4
H
6
N
2
14
5
N
O
9
1
3
HO
8
P
1
3
7
3
4
O
N
1
31
10
12
the recording of H NMR, 13C NMR and P NMR spectra,
respectively. CHN analysis was recorded on Thermo
Finnigan Flash 1112 instrument. Results are presented as,
chemical shift d in ppm, multiplicity, J values in Hertz (Hz).
The Supplemental Materials contains complete characteriza-
17
15
1
1
1
8
16
Biological activity
1
13
31
tion data for products 6, and select H, C and P NMR The antibacterial activity of the newly synthesized
spectra for the products (Figures S1–S10). compounds 6(a–j) was carried out using agar diffusion

4
M. VARALAKSHMI ET AL.
20]
[
method
against the bacterial strains, Staphylococcus
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Escherichia coli and Vibrio cholerae (Gram negative). The
results are presented in Table S1. The antifungal activity
was screened using poison plate technique against fungal
strains such as Trichoderma longibrachiatum, Clostridium
tetani, Aspergillus niger and Aspergillus fumigates and the
results summarized in Table S2. The detailed procedure has
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The author (M. Varalakshmi) is grateful to acknowledge University
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0.25135/.acg.oc.35.17.12.055.