Dimethylurea/L-tartaric acid as deep eutectic solvent for one-pot synthesis of 2-(methylamino)-3-nitrospiro-[chromene] and N-methyl-3-nitro-4H chromen-2-amines

Article abstract of DOI:10.1080/00397911.2021.1878225

An application of dimethyl urea and L-tartaric acid as deep eutectic solvent (DES) is demonstrated for the synthesis of 2-(methylamino)-3-nitrospiro[chromene] and N-methyl-3-nitro-4H-chromen-2-amines by a reaction of substituted isatins/pyrazole aldehydes

Full text of DOI:10.1080/00397911.2021.1878225

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Dimethylurea/L-tartaric acid as deep
eutectic solvent for one-pot synthesis of 2-
(methylamino)-3-nitrospiro-[chromene] and N-
methyl-3-nitro-4H chromen-2-amines
Kota Sathish, Sakkani Nagaraju & Dhurke Kashinath
To cite this article: Kota Sathish, Sakkani Nagaraju & Dhurke Kashinath (2021) Dimethylurea/
L-tartaric acid as deep eutectic solvent for one-pot synthesis of 2-(methylamino)-3-nitrospiro-
[chromene] and N-methyl-3-nitro-4H chromen-2-amines, Synthetic Communications, 51:8,
1242-1251, DOI: 10.1080/00397911.2021.1878225
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2021, VOL. 51, NO. 8, 1242–1251
https://doi.org/10.1080/00397911.2021.1878225
Dimethylurea/L-tartaric acid as deep eutectic solvent for
one-pot synthesis of 2-(methylamino)-3-nitrospiro-
[chromene] and N-methyl-3-nitro-4H chromen-2-amines
Kota Sathish, Sakkani Nagaraju, and Dhurke Kashinath
Department of Chemistry, National Institute of Technology, Warangal, India
ABSTRACT
ARTICLE HISTORY
Received 1 December 2020
An application of dimethyl urea and L-tartaric acid as deep eutectic
solvent (DES) is demonstrated for the synthesis of 2-(methylamino)-
3-nitrospiro[chromene] and N-methyl-3-nitro-4H-chromen-2-amines
by a reaction of substituted isatins/pyrazole aldehydes, nucleophiles
and (E)-N-methyl-1-(methylthio)-2-nitroethenamine. Systematic stud-
ies proved that the dimethyl urea and L-tartaric acid in 2:1 ratio at
80 ꢀC is suitable to give the desired products in good yields in
shorter period of reaction time (45 min). This method was extended
for water as reaction medium and good yields of the products was
obtained in 1 h.
KEYWORDS
Chromenes; catalyst-free;
deep eutectic solvent (DES);
spirooxindoles; water
GRAPHICAL ABSTRACT
Introduction
The development of environmentally benign methods is the need of the hour. Over the
years, many non-conventional reaction media like the use of water,[1a] glycerol,[1b] ionic
liquids,[1c] perfluorinated,[1d] supercritical fluids[1e] and deep eutectic solvents (DES)
have been developed. Among these, DES (popularly known as twenty-first century sol-
vents) are gaining attention recently in biocatalysis,[2a] separation technology,[2b,c] elec-
trochemistry[2d,e] and organic synthesis (cross-coupling,[2f] multicomponent,
condensation and Michael addition reactions).[2g,h,i,j] DES are proven to be relatively
cheaper, nontoxic and biodegradable[2a,b] compared to ionic liquids. DES are obtained
by mixing two or three safe hydrogen bond donors and hydrogen-bond acceptors as
biodegradable components (from the natural source) in different ratios at room tem-
perature or by heating (below 100 ꢀC).[2]
Though many organic compounds are insoluble in water, the natural abundance,
nontoxic, and non-flammable properties make water as a preferred reaction
CONTACT Dhurke Kashinath
kashinath@nitw.ac.inkashinath.dhurke@gmail.com
Department of Chemistry,
National Institute of Technology, Warangal, 506 004, India.
ß 2021 Taylor & Francis Group, LLC

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Figure 1. Biologically active chromene and spirochromene heterocyclic compounds.
medium.[3–6], it is the first priority solvent because of the development of “on-water”
and “in-water” concepts in which the formation of aggregates via hydrophobic interac-
tions with organic molecules is the key for the formation of product in organic
transformation.[5,7]
The chromenes and spirochromenes are well-known scaffolds in medicinal chemistry.
Chromene derivatives (A and B) show anticancer properties[8,9] and C–F are reported
for their antimicrobial,[10] anti-tubercular, antimalarial,[11] antibacterial,[12] anti-apop-
totic (Bcl-2 proteins),[13] anti-fungal,[14] anti-rheumatic,[15] anti-hyperglycemic and
a-glucosidase inhibitory activities.[16] Some of these compounds play a key role in the
central nervous system, possess excellent binding capacity toward many receptors in the
biological systems and are used as cosmetics, pigments and biodegradable agrochemi-
cals[17a] (Fig. 1). The chromene and spirochromenes are synthesized in many conditions
including magnetic graphitic carbon nitride as heterogeneous catalyst,[17b] urea and cho-
line chloride as deep eutectic solvent.[17c]
Nitroolefin N,S acetals (NMSM)[18a] have been found as interesting building blocks
used for the synthesis of five- and six-membered heterocyclic compounds. NMSM
possesses unique structural features of having electrophilic and nucleophilic vicinal
carbons because of the presence of alkylamino (electron donating) and nitro group
(electron withdrawing) on an olefinic system.[18b] There have been reports on the
synthesis of chromenes using NMSM with aldehydes and different nucleophiles. The
methods are known so far include solvent and catalyst-free conditions at 110 ꢀC,[18c]
6,60-thiobis(-methylene)-b-cyclodextrin dimer,[18d] and poly(-oligo ethylene glycol
methacrylate)-g-supported coated double hydroxides (LDHs-g-POEGMA)[18e] as
reusable promoters, silica-supported tungstic acid (STA) as a heterogeneous cata-
lyst,[18f] microwave irradiation in the presence of NH4OAc,[18g] ZnCl2, and InCl3 as
acid catalysts,[18h–18i] H2O and triethylamine in ethanol,[18j–18k] However, only two
methods have been reported for the synthesis of spirochromenes so far. One of
them is InCl3-mediated reaction of NMSM, isatin and pyrazolone as nucleophile by
Perumal and coworkers[19a] and the second report is by the Saigal group in EtOH
reflux conditions (i.e., the reaction of cyclic-1,3-dinucleophiles, isatin and
NMSM)[19b] (Fig. 2). Though these reports are good methods for the generation of
complex spiro compounds from NMSM, the reaction times are relatively high.
Considering this, and in continuation with our efforts of developing synthetic meth-
ods under homogeneous conditions,[20a,b] we applied our reported condition (L-
Proline and Melamine; 3:1 in DMSO, RT) for this reaction but the attempts were
futile even at 80 ꢀC (Scheme 1). In addition, as mentioned in the introduction part,

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K. SATHISH ET AL.
Figure 2. Closely related literature methods for the synthesis of chromene and spirochromenes and
present study.[19]
Scheme 1. One-pot three-component synthesis of chromene and spirochromenes.
the DES are finding a lot of applications in organic synthesis and have been used
for multicomponent reactions. However, to the best of our knowledge, there is no
report so far on the use of DES for the generation of spiro compounds using
NMSM as starting material. In this context, herein we wish to report a new method
for the construction of chromene and spirochromene/spirooxindole derivatives using
deep eutectic solvent (DES)/water as green reaction medium in short reaction times
(both methods) with excellent yields.

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Table 1. Optimization of the reaction conditions for the synthesis of compound 4b’.
Entry
Solvent
Temp (oC)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
DMSO [L-proline and melamine (3:1)]
DMSO [L-proline and melamine (3:1)]
EtOH
MeOH
EtOH
rt
24
24
6
6
6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.5
0.5
0.5
0.5
0.5
0.5
1
ND
ND
50
Trace
70
ND
ND
trace
20
55
40
70
60
50
50
80
70
80
25
50
30
70
50
60
55
75
85
80
80
90
85
80
50
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
Urea þ ChCl (1:1)
Urea þ ChCl (2:1)
Urea þ ZnCl2 (2:1)
9
DMU þ ZnCl2 (1:1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
DMU þ ZnCl2 (2:1)
DMU þ ZnCl2 (3:1)
DMU þ ChCl (1:1)
DMU þ ChCl (2:1)
DMU þ ChCl (3:1)
SnCl2 þ ChCl (1:1)
SnCl2 þ ChCl (2:1)
SnCl2 þ ChCl (3:1)
Ethylene glycol þ ChCl (2:1)
ZnCl2 þ ChCl (2:1)
L-tartaric acid þ ChCl (2:1)
Proline þ Oxalic acid (1:1)
Proline þ L-tartaric acid (1:1)
DMU þ Oxalic acid (1:1)
DMU þ Oxalic acid (2:1)
DMU þ Oxalic acid (3:1)
DMU þ Citric acid (1:1)
DMU þ Citric acid (2:1)
DMU þ Citric acid (3:1)
DMU þ L-tartaric acid (1:1)
DMU þ L-tartaric acid (2:1)
DMU þ L-tartaric acid (3:1)
H2O (3 mL; 0.158 M)
95
All the reactions were performed with 0.475 mmol of isatin.
aIsolated yields.
Results and discussion
Owing to the biological importance of chromene and spirochromenes and in our con-
tinuous interest in sustainable organic transformations, we planned for the synthesis of
chromene and spirochromenes using (E)-N-methyl-1-(methylthio)-2-nitroethenamine.
Initially, a model reaction of aldehyde (1a’), pyrazole (2) and (E)-N-methyl-1-(methyl-
thio)-2-nitroethenamine (3) in presence of L-proline and melamine (3:1 ratio) as cata-
lyst in DMSO at room temperature[20b] could not afford the final product but the same
reaction under heating conditions (80 ꢀC) gave 4a’ in 60% yield in 2 h. After confirm-
ation of the product, similar conditions were applied for isatin (1b’) but the desired
product (4b’) was not formed even after prolonged reaction time (24 h) (Scheme 1).
Then the above reaction was carried out in Ethanol under catalyst-free condition (50 ꢀC

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K. SATHISH ET AL.
Figure 3. Various spirochromene derivatives (4–8) under optimized reaction conditions.
in 6 h)[19b] leading to the formation of the product 4b’ in 50% yield (Table–1; entry-3).
1
The formation of the desired product was confirmed by H, 13C NMR and mass spec-
tral data.
After confirmation of the product, the next task was to optimize the reaction condi-
tions. Thus, the reaction was carried out in methanol and ethanol at 80 ꢀC without cata-
lyst (Table–1, entries 4–5) in which EtOH could give the desired product 4b’ in 70%
yield in 6 h. Toward finding the best reaction conditions (time and yield), we choose
deep eutectic solvent as a reaction medium. Accordingly, the above reaction was per-
formed using urea-choline chloride and urea-ZnCl2 as deep eutectic solvent (Table–1,
entries 6–8). However, both the conditions fail to give the desired product (4b’). Then,
efforts were continued using different combinations of DES [DMU þ ZnCl2,
DMU þ ChCl, SnCl2 þ ChCl, Ethylene glycol þ ChCl, ZnCl2 þ ChCl, L-tartaric
acid þ ChCl, Proline þ Oxalic acid and Proline þ L-tartaric acid] in different ratio
(Table–1, entries 9–22), which afford desired product in 20-80% yields in 60 min at

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Figure 4. Synthesis of chromene derivatives (10a–l) under optimized condition.
80 ꢀC. Following these results, changing the reaction medium to dimethyl urea þ oxalic
acid, dimethyl urea þ citric acid and dimethyl urea þ L-tartaric acid in different combi-
nations gave product 4b’ in good yields in 30 min at 80 ꢀC (Table–1, entries 23–31).
Among the screened combinations, dimethyl urea þ L-tartaric acid (2:1) was found to
be superior in terms of product yield (90%) and reaction time (30 min) at 80 ꢀC. To
explore further to find a green method, the same experiment was performed using water
as solvent at 80 ꢀC. To our delight, the product 4b’ was obtained in 95% in 1 h
(Table–1; entry-32).
With the optimization condition in hand, protocol was explored for structurally dif-
ferent isatins (substitution on the aromatic ring and nitrogen), nucleophiles (pyrazole,
dimedone, 1,3-cyclohexanedione, 4-hydroxy coumarin and N,N0-dimethyl barbituric
acid) and (E)-N-methyl-1-(methylthio)-2-nitroethenamine under both conditions (both
in DES and water) to a library of spirochromene hybrids with good yields as shown in
Figure 3. The structures of all the obtained products were confirmed by NMR and mass
spectra (for detail See ESI). Keeping in view of the biological activity of pyrazole-chro-
mene hybrids C, D (in Fig. 1), we extended this method for the synthesis of different
chromene hybrids with pyrazole moiety. Thus, the reaction of pyrazole aldehyde (9a;
prepared via literature methods) dimedone (2), and (E)-N-methyl-1-(methylthio)-2-
nitroethenamine (3) under the optimized reaction conditions in DES and water (as
reaction media) gave the products with excellent yields up to 92% (Figure 4) in shorter
reaction times. Later, various nucleophiles (dimedone, 1,3-cyclohexanedione and

1248
K. SATHISH ET AL.
(A)
(B)
Figure 5. Plausible Mechanism for the reaction, (A) for DES and (B) for Water.
4-hydroxy coumarin) were treated with pyrazole aldehyde and (E)-N-methyl-1-(methyl-
thio)-2-nitroethenamine under optimized conditions to generate functionalized pyra-
zole-chromene hybrids with excellent yields both in DES and water (catalyst-free
conditions). All the products obtained were characterized using spectroscopic data.
Based on the experimental observations, we propose plausible mechanisms as shown
in Figure 5A and B. The DES induces the keto-enol tautomerism in dimedone. The
activated dimedone (Enol form) undergo Knoevenagel condensation with isatin (The
both carbonyl groups are associated with DES via intermolecular hydrogen bonding)

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afforded intermediate (I), which act as Michael acceptor, the adduct (I) immediately
undergoes Michael type addition with (E)-N-methyl-1-(methylthio)-2-nitroethenamine
to generate intermediate (II). Subsequent imine-enamine tautomerism of (II) afforded
intermediate (III) followed by intramolecular O-cyclization to give the product (4a).
The water-mediated reaction follow the on-water concept.[21] The water induces enoli-
zation in dimedone. Simultaneously, the isatin is activated and reacts with enolic form
of dimedone to give the Knoevenagel product followed by the Michael addition, imine-
enamine tautomerism and O-cyclization, as shown in Figure 5B.
Conclusion
In summary, we have developed a new efficient protocol for the synthesis of chromene
and spirochromene derivatives using dimethylurea/L-tartaric acid as a deep eutectic
solvent. The reaction proceeds via Knoevenagel condensation, Michael addition, imine-
enamine tautomerism followed by O-cyclization. This procedure offers notable advan-
tages such as catalyst-free, environmentally benign reaction conditions and good yields
of the products.
Experimental section
General procedure for the one-pot synthesis of chromene and spirochromenes
Method-A
A mixture of isatin/pyrazole aldehyde (0.475 mmol, 1 eq), Nucleophile (0.475 mmol, 1
eq) and (E)-N-methyl-1-(methylthio)-2-nitroethenamine (0.475 mmol, 1 eq) was stirred
at 80 ꢀC in DMU þ L-tartaric acid (2:1) (DES, 1 mL). The progress of the reaction was
monitored by TLC. After this period, the mixture was treated with water and extracted
with EtOAc (2 ꢁ 10 mL). The collected organic phases were dried over anhydrous
Na2SO4 and the solvent was removed under reduced pressure. Evaporation of the solv-
ent gave the crude product, which was further recrystallized from ethanol to afford the
pure product.
Method-B
A mixture of isatin/pyrazole aldehyde (0.475 mmol, 1 eq), Nucleophile (0.475 mmol, 1
eq) and (E)-N-methyl-1-(methylthio)-2-nitroethenamine (0.475 mmol, 1 eq) was stirred
at 80 ꢀC in water. The progress of the reaction was monitored by TLC. Upon comple-
tion, the reaction mixture was cooled and filtered. Then, the precipitate was washed
with cold water (2 ꢁ 5 mL) and the crude product was recrystallized from ethanol to
afford the pure product.
7,7-Dimethyl-2-(methylamino)-3-nitro-7,8-dihydrospiro[chromene-4,3’-indoline]-2’,5(6H)-
1
dione (4a):. Yield ¼ A: 90%; B: 95% (Light yellow solid); m.p.: 302–304 ꢀC, H NMR
(400 MHz, DMSO-d6): d 10.47 (s, 1H), 10.44 (s, 1H), 7.09 (t, J ¼ 7.6 Hz, 1H), 7.00 (d,
J ¼ 7.2 Hz, 1H), 6.80 (t, J ¼ 7.6 Hz, 1H), 6.72 (d, J ¼ 7.6 Hz, 1H), 3.11 (d, J ¼ 4.4 Hz, 3H),
2.65 (q, J ¼ 48.8 Hz, 2H), 2.13 (q, J ¼ 53.6, 2H), 1.02 (s, 3H), 0.95 (s, 3H). 13C NMR

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K. SATHISH ET AL.
(101 MHz, DMSO-d6): d 196.02, 178.26, 163.06, 157.59, 144.24, 130.99, 128.84, 122.86,
122.11, 113.07, 109.40, 107.88, 50.81, 49.18, 39.95, 31.85, 28.74, 27.92, 26.87. Mass (ESI-
MS): m/z Calculated: 369.13; Observed: 395.1140 (M þ Na).
Funding
KS thanks DST for the INSPIRE fellowship. SN thanks, UGC-New Delhi for the fellowship. DK
thanks, DST (SERB), New Delhi for the financial support [SB/FT/CS-136/2012 and SB/EMEQ-
103/2014].
References
[1] (a) Cheng, T. Y.; Zhang, D. C.; Li, H. X.; Liu, G. H. Green Chem. 2014, 16, 3401–3427.
DOI: 10.1039/C4GC00458B. (b) Garcia, J. I.; Garcia-Marin, H.; Pires, E. Green Chem.
2014, 16, 1007–1033. DOI: 10.1039/C3GC41857J. (c) Petkovic, M.; Seddon, K. R.; Rebelo,
L. P. N.; Pereira, C. S. Chem. Soc. Rev. 2011, 40, 1383–1403. DOI: 10.1039/c004968a. (d)
Zhang, W. Green Chem. 2009, 11, 911–902. DOI: 10.1039/b820740b. (e) Medina-Gonzalez,
Y.; Camy, S.; Condoret, J. S. ACS Sustainable Chem. Eng. 2014, 2, 2623–2636. DOI: 10.
1021/sc5004314.
[2] (a) Xu, P.; Zheng, G. W.; Zong, M. H.; Li, N.; Lou, W. Y. Bioresour. Bioprocess 2017, 4,
34. DOI: 10.1186/s40643-017-0165-5. (b) Tang, S.; Baker, G. A.; Zhao, H. Chem. Soc. Rev.
2012, 41, 4030–4066. DOI: 10.1039/c2cs15362a. (c) Atanassova, M.; Kurteva, V.; Lubenov,
L.; Billard, I. RSC Adv. 2014, 4, 38820–38829. DOI: 10.1039/C4RA04302B. (d) Hillman,
A. R.; Ryder, K. S.; Zaleski, C. J.; Ferreira, V.; Beasley, C. A.; Vieil, E. Electrochim. Acta
2014, 135, 42–51. DOI: 10.1016/j.electacta.2014.04.062. (e) Sebastian, P.; Valles, E.; Gomez,
E. Electrochim. Acta 2014, 123, 285–295. DOI: 10.1016/j.electacta.2014.01.062. (f) Vidal, C.
; Garcia-Alvarez, J.; Hernan-Gomez, A.; Kennedy, A. R.; Hevia, E. Angew. Chem. Int. Ed.
2014, 53, 5969–5973. DOI: 10.1002/anie.201400889. (g) Liu, P.; Hao, J.-W.; Mo, L.-P.;
Zhang, Z.-H. RSC Adv. 2015, 5, 48675–48704. DOI: 10.1039/C5RA05746A. (h) Alonso,
ꢀ
D. A.; Baeza, A.; Chinchilla, R.; Guillena, G.; Pastor, I. M.; Ramon, D. J. Eur. J. Org.
Chem. 2016, 612–632. DOI: 10.1002/ejoc.201501197. (i) Jr Longo, L. S.; Craveirob, M. V.
J. Braz. Chem. Soc. 2018, 29, 1999–2025. (j) Krishnakumar, V.; Vindhya, N. G.; Mandal,
B. K.; Khan, F.-R. N. Ind. Eng. Chem. Res. 2014, 53, 10814–10819. DOI: 10.1021/
ie501320a.
[3] Li, C.-J.; Chen, L. Chem. Soc. Rev. 2006, 35, 68–82. DOI: 10.1039/b507207g.
[4] Li, C.-J.; Chan, T.-H. Comprehensive Organic Reactions in Aqueous Media; John Wiley &
Sons, Inc.: Hoboken, NJ, 2007.
ꢀ
[5] Gawande, M. B.; Bonifacio, V. D. B.; Luque, R.; Branco, P. S.; Varma, R. S. Chem. Soc.
Rev. 2013, 42, 5522–5551. DOI: 10.1039/c3cs60025d.
[6] Horv’ath, I. T. Green Chem. 2008, 10, 1024–1028. DOI: 10.1039/b812804a.
[7] Michael, C. P. Chem. Eur. J. 2006, 12, 1312–1317.
[8] Cai, S. X.; Drewe, J.; Kemnitzer, W. Anticancer Agents Med. Chem. 2009, 9, 437–456.
DOI: 10.2174/1871520610909040437.
[9] Fujiwara, A.; Mori, T.; Lida, A.; Ueda, S.; Hano, Y.; Nomura, T.; Tokuda, H.; Nishino, H.
J. Nat. Prod. 1998, 61, 629–632. DOI: 10.1021/np9800147.
[10] Khafagy, M. M.; Abd El-Wahab, A. H. F.; Eid, F. A.; El-Agrody, A. M. Il Farmaco. 2002,
57, 715–722. DOI: 10.1016/S0014-827X(02)01263-6.
[11] Kalaria, P. N.; Satasia, S. P.; Raval, D. K. New J. Chem. 2014, 38, 1512–1521. DOI: 10.
1039/c3nj01327h.
[12] Zhang, G.; Zhang, Y.; Yan, J.; Chen, R.; Wang, S.; Ma, Y.; Wang, R. J. Org. Chem. 2012,
77, 878–888. DOI: 10.1021/jo202020m.

R
SYNTHETIC COMMUNICATIONSV
1251
[13] Tian, D.; Das, S. G.; Doshi, J. M.; Peng, J.; Lin, J.; Xing, C. Cancer Lett. 2008, 259,
198–208. DOI: 10.1016/j.canlet.2007.10.012.
[14] Pradeep, K. P.; Rao, J. S.; Shubha, J. Med. Chem. Res. 2013, 22, 2984–2990.
[15] Smith, C. W.; Bailey, J. M.; Billingham, M. E. J.; Chandrasekhar, S.; Dell, C. P.; Harvey,
A. K.; Hicks, C. A.; Kingston, A. E.; Wishart, G. N. Bioorg. Med. Chem. 1995, 5,
2783–2788. DOI: 10.1016/0960-894X(95)00487-E.
[16] Raju, B. C.; Tiwari, A. K.; Kumar, J. A.; Ali, A. Z.; Agawane, S. B.; Saidachary, G.;
Madhusudana, K. Bioorg. Med. Chem. 2010, 18, 358–365. DOI: 10.1016/j.bmc.2009.10.047.
[17] (a) Ellis, G. P. Chromenes, Chromanones, and Chromones. In The Chemistry of
Heterocyclic Compounds Chromenes, Weissberger. A., Taylor, E. C., Eds.; John Wiley: New
York, NY, USA, 1977; chapter 2, pp. 11–139. (b) Azizi, N.; Ahooie, T. S.; Hashemi,
M. M.; Yavari, I. Synlett. 2018, 29, 645–649. DOI: 10.1055/s-0036-1589145. (c) Azizi, N.;
Dezfooli, S.; Khajeh, M.; Hashemi, M. M. J. Mol. Liq. 2013, 186, 76–80. DOI: 10.1016/j.
molliq.2013.05.011.
[18] (a) Saigal; Khan S.; Rahman, H.; Shafiullah, Khan; M. M. RSC Adv. 2019, 9, 14477–14502.
DOI: 10.1039/C9RA00630C. (b) Surya Prakash Rao, H.; Parthiban, A. Org. Biomol. Chem.
2014, 12, 6223–6238. DOI: 10.1039/C4OB00628C. (c) Khan, M. M.; Saigal, B.; Shareef, S.;
Khan, S.; Sahoo, S. C. Synth. Commun. 2018, 48, 2683–2694. DOI: 10.1080/00397911.2018.
1517218. (d) Shinde, V. V.; Jeong, D.; Cho, E.; Jung, S. Tetrahedron 2018, 74, 194–198.
DOI: 10.1016/j.tet.2017.11.065. (e) Krishnammagari, S. K.; Lee, S. M.; Jeong, Y. T. Res.
Chem. Intermed. 2018, 44, 517–533. DOI: 10.1007/s11164-017-3117-y. (f) Jadhav, A. M.;
Krishnammagari, S. K.; Kim, J. T.; Jeong, Y. T. Tetrahedron 2017, 73, 5163–5169. DOI:
10.1016/j.tet.2017.07.008. (g) Mao, J.; Wang, J.; Zhang, W.; Li, Z.; Zhu, J.; Guo, C.
ARKIVOC 2016, iii, 171–186. DOI: 10.3998/ark.5550190.p009.398. (h) Gunasekaran, P.;
Prasanna, P.; Perumal, S.; Almansour, A. I. Tetrahedron Lett. 2013, 54, 3248–3252. DOI:
10.1016/j.tetlet.2013.04.022. (i) Survase, D. N.; Chavan, H. V.; Dongare, S. B.; Ganapure
S. D.; Helavi, V. B. Iran. Chem. Commun. 2017, 5, 105–114. (j) Reddy, M. V.; Reddy, G.
D.; Kim, J. T.; Jeong, Y. T. Tetrahedron 2016, 72, 6484–6491. DOI: 10.1016/j.tet.2016.08.
059. (k) Sivakumar, S.; Kanchithalaivan, S.; Kumar, R. R. RSC Adv. 2013, 3, 13357–13364.
DOI: 10.1039/c3ra41510d.
[19] (a) Poomathi, N.; Kamalraja, J.; Mayakrishnan, S.; Muralidharan, D.; Perumal, P. T.
Synlett. 2014, 25, 0708–0712. (b) Saigal Irfan, M.; Khan, P.; Abid, M., Khan; M. M. ACS
Omega. 2019, 4, 16794–16807. DOI: 10.1021/acsomega.9b01571. (c) Ghadiri, S.; Bayat, M.;
Hosseini, F. S. J. Heterocycl. Chem. 2018, 55, 2693–2697. DOI: 10.1002/jhet.3326.
[20] (a) Nagaraju, S.; Sathish, K.; Paplal, B.; Kashinath, D. Tetrahedron Lett. 2017, 58,
2865–2871. DOI: 10.1016/j.tetlet.2017.06.029. (b) Nagaraju, S.; Paplal, B.; Sathish, K.; Giri,
S.; Kashinath, D. Tetrahedron Lett. 2017, 58, 4200–4204. DOI: 10.1016/j.tetlet.2017.09.060.
[21] (a) Jung, Y.; Marcus, R. A. J. Am. Chem. Soc. 2007, 129, 5492–5502. DOI: 10.1021/
ja068120f. (b) Butler, R. N.; Coyne, A. G. Org. Biomol. Chem. 2016, 14, 9945–9960. DOI:
10.1039/c6ob01724j.