The Pudovik reaction: the synthesis of bioactive α-aminophosphonates with long alkyl chains

Article abstract of DOI:10.1080/10426507.2018.1539848

In this research we investigated the reactions of substituted naphthaldehyde with amines. The condensation of do-, tetra-, hexa-, octadecan-1-amines with 2-hydroxy-1-naphthaldehyde yielded a series of azomethines in good yields. Subsequent reaction of these compounds with didodecylphosphine oxide yielded α-aminophosphonates. The in vitro microbiological activity of the synthesized phosphorus compounds against gram-positive, gram-negative bacteria and the yeast Candida albicans was determined in comparison to standard agents. The synthesized compounds showed a high antibacterial and antimycotic activity against human and animal pathogenic microflora. Every newly synthesized compound was characterized by elemental analyses, IR, ¹H NMR, ³¹P NMR spectral studies. The thermal stability was studied by synchronous thermogravimetry and differential scanning calorimetry (TG/DSC).

Full text of DOI:10.1080/10426507.2018.1539848

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: https://www.tandfonline.com/loi/gpss20
The Pudovik reaction: the synthesis of bioactive α-
aminophosphonates with long alkyl chains
Irina V. Galkina, Khasan R. Khayarov, Rustam R. Davletshin, Aynaz Z.
Gaynullin, Alexander V. Gerasimov, Marina P. Shulaeva, Oskar K. Pozdeev,
Svetlana N. Egorova, Luiza M. Usupova & Vladimir I. Galkin
To cite this article: Irina V. Galkina, Khasan R. Khayarov, Rustam R. Davletshin, Aynaz Z.
Gaynullin, Alexander V. Gerasimov, Marina P. Shulaeva, Oskar K. Pozdeev, Svetlana N. Egorova,
Luiza M. Usupova & Vladimir I. Galkin (2019): The Pudovik reaction: the synthesis of bioactive
α-aminophosphonates with long alkyl chains, Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2018.1539848
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2018.1539848
The Pudovik reaction: the synthesis of bioactive a-aminophosphonates with long
alkyl chains
Irina V. Galkinaa, Khasan R. Khayarovb, Rustam R. Davletshina, Aynaz Z. Gaynullina, Alexander V. Gerasimova
Marina P. Shulaevab, Oskar K. Pozdeevb, Svetlana N. Egorovac, Luiza M. Usupovad, and Vladimir I. Galkina
,
b
c
aKazan Federal University, Kazan, Russia; Kazan State Medical Academy, Kazan, Russia; Kazan State Medical University, Kazan, Russia;
dKazan National Research Technological University, Kazan, Russia
ABSTRACT
ARTICLE HISTORY
Received 27 September 2018
Accepted 15 October 2018
In this research we investigated the reactions of substituted naphthaldehyde with amines. The
condensation of do-, tetra-, hexa-, octadecan-1-amines with 2-hydroxy-1-naphthaldehyde yielded a
series of azomethines in good yields. Subsequent reaction of these compounds with didodecyl-
phosphine oxide yielded a-aminophosphonates. The in vitro microbiological activity of the synthe-
sized phosphorus compounds against gram-positive, gram-negative bacteria and the yeast
Candida albicans was determined in comparison to standard agents. The synthesized compounds
showed a high antibacterial and antimycotic activity against human and animal pathogenic micro-
flora. Every newly synthesized compound was characterized by elemental analyses, IR, 1H NMR,
31P NMR spectral studies. The thermal stability was studied by synchronous thermogravimetry and
differential scanning calorimetry (TG/DSC).
KEYWORDS
a-aminophosphonates; long
alkyl chains;
antimicrobial activity
GRAPHICAL ABSTRACT
Introduction
synthesized and their biological activities have not
been tested.
In organophosphorus chemistry, the Pudovik reaction is a
method for preparing of the various aminophosphonates
and phosphorylated imines. The amination of 2-hydroxy-
1-naphthaldehyde with long-chain aliphatic amines gave
products of condensation (1–5). The phosphorylation of
azomethines lead to the formation of a-aminophospho-
nates (6–10). All new obtained compounds with aliphatic
long-chain, due to their unique reactivity, are important in
the construction of various multifunctional compounds
with a broad spectrum of biological activity.[1–6] The
search for novel agents to combat resistant bacteria has
become one of the most important areas of antibacterial
research today.[7,8] Pharmaceutical and organic chemists
are trying to synthesize new drugs with better pharmaco-
kinetic and dynamic properties. We have earlier described
the methods of preparation various azomethines and us
a-aminophosphonates reported on their structure and
reactivity.[9] However, literature survey has indicated that
Results and discussion
Chemistry
In this paper we present the synthesis and antimicrobial
activity of a series of 1-(alkylamino)methyl)naphthalene-2-
oles (1–5) on the basis of 2-hydroxy-1-naphthaldehyde and
aliphatic alkyl amines and corresponding us a-amino-
phosphonates bases (6–10) by the reaction of azomethines
(1–5) with didodecylphosphine oxide. All the synthesized
1
compounds were characterized by elemental analysis, IR, H
NMR, and 31P NMR spectroscopy.
We have found that the amination of 2-hydroxy-1-naph-
thaldehyde with long-chain aliphatic amines ie decan-,
dodecan-, tetradecan-, hexadecan and octadecan-1-amines in
rations in ethanol at room temperature under vigorous stir-
ring gave products of condensation – azomethines (1–5).
Heating in ethanol these compounds with didodecylphos-
all obtained by us a-aminophosphonates have not been phine oxide leads to the formation of (6–10).
CONTACT Vladimir I. Galkin
vig54@mail.ru
ß 2019 Taylor & Francis Group, LLC

2
I. V. GALKINA ET AL.
Table 1. Physical data for the azomethines 1–5 and us a-aminophosphonates 6–10.
Found (calc) (%)
H
Comp. No.
Mw (g/mol)
Yield (%)
Empirical formula
C
N
P
m.p. (ꢀC)
1
2
3
4
5
6
7
8
9
311
339
367
395
423
697
725
753
781
809
90.7
85.2
90.8
92.4
89.4
65.7
71.4
82.3
57.4
77.5
C21H29NO
C23H33NO
C25H37NO
C27H41NO
C29H45NO
81.03 (81.37)
81.42 (81.13)
81.74 (81.38)
82.03 (82.21)
82.27 (82.50)
77.47 (77.71)
77.79 (77.52)
78.09 (78.38)
78.36 (78.02)
78.62 (78.99)
9.32 (9.51)
9.73 (9.28)
4.50 (4.47)
4.13 (3.95)
3.81 (4.17)
3.54 (3.43)
3.31 (3.79)
2.01 (2.11)
1.93 (2.17)
1.86 (2.12)
1.73 (2.06)
1.73 (1.57)
—
—
—
—
—
67.7
74.9
76.3
84.7
89.7
oil
oil
oil
oil
103
10.08 (10.31)
10.38 (9.94)
10.64 (10.78)
11.48 (11.26)
11.59 (11.73)
11.69 (11.55)
11.78 (12.01)
11.87 (11.33)
C
45H80NO2P
4.45 (4.37)
4.28 (4.09)
4.12 (3.93)
3.97 (4.23)
3.83 (3.66)
C47H84NO2P
C49H88NO2P
C51H92NO2P
10
C
53H96NO2P
Table 2. Antimicrobial activity (growth inhibition zone, mm) of compounds (1–10) (c ¼ 50 lg/0.1 mL).
Compound
Staphylococcus aureus Escherichia coli Bacillus cereus Pseudomonas aeruginosa Candida albicans
1
2
3
4
5
6
7
8
23 0.5
25 0.3
25 0.4
25 0.3
37 0.2
15 0.5
12 0.4
13 0.35
14 0.4
17 0.3
16 0.5
0
—
17 0.6
36 0.7
30 0.5
26 0.4
36 0.3
11 0.5
17 0.25
13 0.3
47 0.5
28 0.7
20 0.35
40 0.8
15 0.2
12 0.3
10 0.25
14 0.3
14 0.4
21 0.75
30 0.5
31 0.7
29 0.25
30 0.5
33 0.5
26 0.7
18 0.3
28 0.15
—
12 0.2
11 0.2
—
9
8
0.3
0.2
7
0.2
15 0.5
17 0.3
18 0.2
17 0.3
18 0.2
9
10
11 0.5
15 0.5
8 0.1
15 0.2
Chlorohexidine
Griseofulvin
9
0.2
0
7
0.1
0
0
Azomethines and a-aminophosphonates were synthesized carried out by using cup-plate method using Sabouraud’s
in good yield and were fully characterized by physical data agar medium. The standard drug used was Griseofulvin
in Table 1.
(50 mg/0.1 mL) and the test compounds at concentration of
50 mg/0.1 mL by using of the mixture of solvents – ethanol-
water at different ratios (1:10). After incubation the average
of inhibition was recorded in mm.
Biological evaluations
All tests were performed in triplicate. Zone of inhibition
22 to 33: highly significant, between 13 to 21 mm: less sig-
nificant, below 12 mm: poor active.
The antibacterial and antifungal activity of a series of azo-
methines (1–5) and us a-aminophosphonates (6–10) were
investigated in vitro against several pathogenic representative
Gram-negative bacteria (Pseudomonas aeruginoza ATCC
27853 and Escherichia coli ATCC 25922), Gram-positive
bacteria (Staphylococcus aureus ATCC 29213 and Bacillus
cereus 11778), and pathogenic fungi Candida albicans ATCC
885-653. The results were summarized in Table 2.
Conclusions
In conclusion, new azomethines and us a-aminophospho-
nates were synthesized and their structures were determined
by NMR, TG-DSC and elemental analyses. All obtained
derivatives with long alkyl aliphatic chains of various lengths
were synthesized in good yield, characterized by different
spectral studies, and their antimicrobial activity has been
Cup-plate Agar method was used for evaluation of anti-
bacterial activity. The nutrient agar medium is used. The
medium with bacteria was poured into sterilized Petri dishes
under
aseptic
conditions.
Standard
drugs
were
Chlorhexidine (50 mg/0.1 mL) and test compounds at con-
centration of 50 mg/0.1 mL. Solvent used was mixture of evaluated. So, it may be concluded from our results that the
water and isopropanol at different rations (1:10). Plates were synthesized compounds (2–5) are potent antimicrobial
incubated at 37 ꢀC for 24 hours. The antifungal activity was agents against pathogenic bacteria and fungi.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
1-((octadecylimino)methyl)naphthalen-2-ol 5. Mw.: 423,7.
Yield: 89%. M.p.: 86.7 ꢀC. FTIR (ATR, cmꢁ1): 3111 (OH),
2817, 2852 (aromatic C-H), 1661 (C ¼ N). 1H-NMR
(400 MHz, CDCl3, d, ppm): 0.93 (3H, t, CH3), 1.26 (28H, m,
CH2), 1.45 (2H, p, CH2), 1.81 (2H, p, CH2), 3,67 (2H, t, CH2),
6.91 (1H, d, naph-H), 7.28 (1H, t, naph-H), 7.41 (1H, t, naph-
H), 7.69 (1H, d, naph-H), 7.61 (1H, d, naph-H), 7.85 (1H, d,
naph-H), 8.68 (1H, d, CH), 14.47 (1H, s, OH).
Experimental
Chemistry
All chemicals purchased from Sigma-Aldrich were reagent
grade and used without purification. IR spectra were taken
on a spectrophotometer Spectrum Two PERKIN ELMER in
the range 400–3700 cmꢁ1 1H NMR (D2O) and 31P NMR
.
(DMSO-d6) spectra were determined on a Bruker Avance
digital spectrometer 400.13 MHz. Chemical shifts were deter-
mined with respect to external reference, 85% H3PO4. The
purity and thermal stability of crustal compounds were
determined by simultaneous TG-DSC analysis on
NETZSCH STA 449C instrument (temperature range
40–400 ꢀC, heating rate 10 degrees/min, argon atmosphere).
General procedure for the synthesis of 1-((dialkylphos-
phoryl)alkylamino)naphthalen-2-ol 6-10
a
A mixture of equimolar quantities of azomethines 1-5
(0.01 mol) and appropriate didodecylphosphine oxide
(0.01 mol) was refluxed in dry ethanol (100 ml) for 25 h. The
reaction mixture was left overnight at room temperature,
wherein the oil of the product was obtained. Excess of solv-
ent was removed under reduced pressure. The resulting
compounds were obtained as yellow oils and purified by
ethanol and diethyl ether from starting reagents.
Materials and methods
General procedure for the synthesis of 1-((alkylamino)me-
thyl)naphthalen-2-ol and its various derivatives 1–5
((Decylimino)(2-hydroxynaphthalen-1-y)methyl)didode-
cylphosphine oxide 6. Mw.: 697. Yield: 62%. Yellow oil.
FTIR (ATR, cmꢁ1): 3400-3300 (NH), 1126 (P ¼ O), 776 (P-
A
mixture of equimolar quantities of alkyl-1-amine
(0.01 mol) and appropriate 2-hydroxy-1-naphthaldehyde
(0.01 mol) was refluxed in dry ethanol (50 ml) for 1 h.
Excess of solvent was removed under reduced pressure. The
resulting azomethines were obtained as yellow crystals and
purified by diethyl ether from starting reagents.
1
C). H-NMR (400 MHz, DMSO-d6, d, ppm): 0.87 (9H, m,
CH3), 1.04 (6H, t, CH2), 1.22 (52H, m, CH2), 1.63 (2H, s,
CH2), 4,44 (2H, m, CH2), 7.50 (2H, dd, naph-H) [7.69 (1H,
d, naph-H), 8.45 (1H, d, naph-H)], 7.63 (1H, t, naph-H),
7.72 (1H, d, naph-H), 7.81 (1H, d, naph-H), 8.16 (1H, t,
naph-H), 8.3 (1H, s, OH), 8.50 (1H, s, CH). 31P-NMR
(400 MHz, CDCl3, d, ppm): 55.36.
((Dodecylimino)(2-hydroxynaphthalen-1-y)methyl)dido-
decylphosphine oxide 7. Mw.: 725. Yield: 65%. Yellow oil.
FTIR (ATR, cmꢁ1): 3400-3300 (NH), 1129 (P ¼ O), 775 (P-
C). 1H-NMR (400 MHz, DMSO-d6, d, ppm): 1H-NMR
(400 MHz, DMSO-d6, d, ppm): 0.84 (9H, m, CH3), 1.07 (6H,
t, CH2), 1.28 (56H, m, CH2), 1.65 (2H, s, CH2), 4,43 (2H,
m, CH2), 7.53 (2H, dd, naph-H) [7.69 (1H, d, naph-H), 8.45
(1H, d, naph-H)], 7.63 (1H, t, naph-H), 7.72 (1H, d, naph-
H), 7.81 (1H, d, naph-H), 8.16 (1H, t, naph-H), 8.3 (1H, s,
OH), 8.50 (1H, s, CH).31P-NMR (400 MHz, CDCl3, d,
ppm): 57.13.
1-((decylimino)methyl)naphthalen-2-ol 1. Mw.: 311.5.
Yield: 82%. M.p.: 75.0 ꢀC. FTIR (ATR, cmꢁ1): 3110 (OH),
2817, 2849 (aromatic C-H), 1643 (C ¼ N). 1H-NMR
(400 MHz, CDCl3, d, ppm): 0.90 (3H, t, CH3), 1.27 (12H, m,
CH2), 1.48 (2H, p, CH2), 1.79 (2H, p, CH2), 3,60 (2H, t,
CH2), 6.97 (1H, d, naph-H), 7.23 (1H, t, naph-H), 7.51 (1H,
t, naph-H), 7.63 (1H, d, naph-H), 7.69 (1H, d, naph-H),
7.81 (1H, d, naph-H), 8.65 (1H, d, CH), 14.43 (1H, s, OH).
1-((dodecylimino)methyl)naphthalen-2-ol 2. Mw.: 339.
Yield: 85%. M.p.: 79.5 ꢀC. FTIR (ATR, cmꢁ1): 3100 (OH),
2815, 2850 (aromatic C-H), 1647 (C ¼ N). 1H-NMR
(400 MHz, CDCl3, d, ppm): 0.93 (3H, t, CH3), 1.23 (16H, m,
CH2), 1.49 (2H, p, CH2), 1.77 (2H, p, CH2), 3,62 (2H, t,
CH2), 6.90 (1H, d, naph-H), 7.21 (1H, t, naph-H), 7.54 (1H,
t, naph-H), 7.60 (1H, d, naph-H), 7.70 (1H, d, naph-H),
7.84 (1H, d, naph-H), 8.63 (1H, d, CH), 14.41 (1H, s, OH).
1-((tetradecylimino)methyl)naphthalen-2-ol 3. Mw.: 367.
Yield: 91%. M.p.: 76.3 ꢀC. FTIR (ATR, cmꢁ1): 3087 (OH),
2811, 2849 (aromatic C-H), 1649 (C ¼ N). 1H-NMR
(400 MHz, CDCl3, d, ppm): 0.97 (3H, t, CH3), 1.28 (20H, m,
CH2), 1.45 (2H, p, CH2), 1.76 (2H, p, CH2), 3,66 (2H, t,
CH2), 6.93 (1H, d, naph-H), 7.27 (1H, t, naph-H), 7.49 (1H,
t, naph-H), 7.57 (1H, d, naph-H), 7.63 (1H, d, naph-H),
7.87 (1H, d, naph-H), 8.62 (1H, d, CH), 14.57 (1H, s, OH).
1-((hexadecylimino)methyl)naphthalen-2-ol 4. Mw.:
395,62. Yield: 90%. M.p.: 87.2 ꢀC. FTIR (ATR, cmꢁ1): 3091
(OH), 2813, 2855 (aromatic C-H), 1655 (C ¼ N). 1H-NMR
(400 MHz, CDCl3, d, ppm): 0.91 (3H, t, CH3), 1.25 (24H, m,
CH2), 1.43 (2H, p, CH2), 1.77 (2H, p, CH2), 3,62 (2H, t,
CH2), 6.90 (1H, d, naph-H), 7.33 (1H, t, naph-H), 7.44 (1H,
t, naph-H), 7.61 (1H, d, naph-H), 7.67 (1H, d, naph-H),
7.92 (1H, d, naph-H), 8.67 (1H, d, CH), 14.39 (1H, s, OH).
((Tetradecylimino)(2-hydroxynaphthalen-1-y)methyl)di-
dodecylphosphine oxide 8. Mw.: 753. Yield: 73%. Yellow oil.
FTIR (ATR, cmꢁ1): 3410-3300 (NH), 1130 (P ¼ O), 777 (P-
1
C). H-NMR (400 MHz, DMSO-d6, d, ppm): 0.90 (9H, m,
CH3), 1.10 (6H, t, CH2), 1.32 (60H, m, CH2), 1.69 (2H, s,
CH2), 4,41 (2H, m, CH2), 7.55 (2H, dd, naph-H) [7.69 (1H,
d, naph-H), 8.45 (1H, d, naph-H)], 7.63 (1H, t, naph-H),
7.72 (1H, d, naph-H), 7.81 (1H, d, naph-H), 8.16 (1H, t,
naph-H), 8.3 (1H, s, OH), 8.50 (1H, s, CH). 31P-NMR
(400 MHz, CDCl3, d, ppm): 53.66.
((Hexadecylimino)(2-hydroxynaphthalen-1-y)methyl)di-
dodecylphosphine oxide 9. Mw.: 781. Yield: 52%. Yellow oil.
FTIR (ATR, cmꢁ1): 3400-3300 (NH), 1129 (P ¼ O), 774 (P-
1
C). H-NMR (400 MHz, DMSO-d6, d, ppm): 0.86 (9H, m,
CH3), 1.09 (6H, t, CH2), 1.29 (64H, m, CH2), 1.6371 (2H, s,
CH2), 4,40 (2H, m, CH2), 7.57 (2H, dd, naph-H) [7.69 (1H,
d, naph-H), 8.45 (1H, d, naph-H)], 7.63 (1H, t, naph-H),
7.72 (1H, d, naph-H), 7.81 (1H, d, naph-H), 8.16 (1H, t,

4
I. V. GALKINA ET AL.
naph-H), 8.3 (1H, s, OH), 8.50 (1H, s, CH). 31P-NMR
(400 MHz, CDCl3, d, ppm): 56.90.
[2] Thavaudurai, K.; Antonysamy, K.; Ponnusamy, T. S.;
Characterization, S. Electrochemical and Antimicrobial Studies of
Copper(II), Nickel(II), Cobalt(II) and Zinc(II) Complexes Derived
((Octadecylimino)(2-hydroxynaphthalen-1-y)methyl)di-
dodecylphosphine oxide 10. Mw.: 809. Yield: 56%. Yellow
oil. FTIR (ATR, cmꢁ1): 3400-3300 (NH), 1127 (P ¼ O), 777
(P-C). 1H-NMR (400 MHz, DMSO-d6, d, ppm): 0.87 (9H,
m, CH3), 1.00 (6H, t, CH2), 1.27 (68H, m, CH2), 1.68 (2H,
s, CH2), 4,47 (2H, m, CH2), 7.60 (2H, dd, naph-H) [7.69
(1H, d, naph-H), 8.45 (1H, d, naph-H)], 7.63 (1H, t, naph-
H), 7.72 (1H, d, naph-H), 7.81 (1H, d, naph-H), 8.16 (1H, t,
naph-H), 8.3 (1H, s, OH), 8.50 (1H, s, CH). 31P-NMR
(400 MHz, CDCl3, d, ppm): 57.37.
from
1-Phenyl-2,3-Dimethyl-4(2-Iminomethylbenzylidene)-
Pyrozol-5-(a-Imino)-Indole-3-Propionic Acid. Chem. Sci. Trans.
2013, 2, 25–32. doi:10.7598/cst2013.3.
[3] Karthikeyan, M. S.; Prasad, D. J.; Poojary, B.; Bhat, K. S.; Holla,
B. S.; Kumari, N. S. Synthesis and Biological Activity of Schiff
and Mannich Bases Bearing 2,4-Dichloro-5-Fluorophenyl
Moiety. Bioorg. Med. Chem. 2006, 14, 7482–7489. doi:10.1016/
j.bmc.2006.07.015.
[4] Pareek, S.; Vyas, S.; Seth, G.; Vyas, P. C. Synthesis,
Characterization, and Biological Activity of Phosphorylated/
Thiophosphorylated
Compounds
of
2-Substituted
Benzimidazoles. Heteroatom Chem. 2008, 19, 154–157. doi:
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Disclosure statement
[5] Kasthuraiah, M.; Kumar, K. A.; Reddy, C. S.; Reddy, C. D.
Syntheses, Spectral Property, and Antimicrobial Activities of 6-
a-Amino Dibenzo [d,f][1,3,2]Dioxaphosphepin 6-Oxides.
Heteroatom Chem. 2007, 18, 2–8. doi:10.1002/hc.20226.
[6] Srinivasulu, D.; Reddy, C. D.; Reddy, B. S. Synthesis and
Antimicrobialactivity of 2- Alkylcarbamato-2,3-Dihydro-5-
Propylthio-1H1,3,2-Benzodiazaphosphole 2-Oxides. Heteroatom
Chem. 2000, 11, 336–340. doi:10.1002/1098-1071(2000)11:
5 < 336::AID-HC4 > 3.0.CO;2-0.
No potential conflict of interest was reported by the authors.
Funding
This work was funded by the subsidy allocated to Kazan Federal
University for the state assignment in the sphere of scientific activities
(No. 4.5888.2017/8.9).
[7] Aysel, G.; Taylan, I.; Nalan, T.; Gulten, O. Synthesis and Antimicrobial
Evaluation of Some Novel Imidazolylmercaptoacetylthiosemicarbazide
and 4-Thiazolidinone Analogs. Turk. J. Pharm. Sci. 2005, 2, 1–10.
[8] Costerton, J. W.; Cheng, K.-J. The Role of the Bacterial Cell
Envelope in Antibiotic Resistance. J. Antimicrob. Chemother.
1975, 1, 363–377. doi:10.1093/jac/1.4.363.
ORCID
Alexander V. Gerasimov
http://orcid.org/0000-0003-4213-9724
[9] Galkina, I. V.; Kiyamova, E. R.; Gaynullin, A. Z.; Bakhtiyarov,
D. I.; Shulaeva, M. P.; Pozdeev, O. K.; Egorova, S. N.; Ivshin,
K. A.; Kataeva, O. N.; Bakhtiyarova, Y. V.; et al. Synthesis and
Structure of Novel Phosphorylated Azomethines. Phosphorus,
Sulfur Silicon Relat. Elem. 2016, 191, 1679–1681. doi:10.1080/
10426507.2016.1227822.
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