A one-pot catalyst/external oxidant/solvent-free cascade approach to pyrimidines: Via a 1,5-hydride transfer

Article abstract of DOI:10.1039/c8gc03507e

A cascade 3-component reaction of 6-aminouracil, aldehyde and tetrahydroisoquinolines under heating in the absence of catalyst, external oxidant and solvent is performed. The reaction constructs a new pyrimidine ring by the functionalization of the C-H bond adjacent to nitrogen through a 1,5-hydride shift. No chromatographic techniques were required to purify the products.

Full text of DOI:10.1039/c8gc03507e

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A one-pot catalyst/external oxidant/solvent-free
cascade approach to pyrimidines via a 1,5-hydride
transfer†
Cite this: Green Chem., 2019, 21, 69
Received 9th November 2018,
Accepted 20th November 2018
Mohit L. Deb, * Paran J. Borpatra
and Pranjal K. Baruah
*
DOI: 10.1039/c8gc03507e
rsc.li/greenchem
A cascade 3-component reaction of 6-aminouracil, aldehyde and catalyst, in which various chiral nitrogen heterocycles have
tetrahydroisoquinolines under heating in the absence of catalyst, been synthesized.8a,c,f–j
external oxidant and solvent is performed. The reaction constructs
Inspired by our recently published work on the synthesis of
a new pyrimidine ring by the functionalization of the C–H bond 1,3-oxazines (Scheme 1a), we planned to construct a pyrim-
adjacent to nitrogen through a 1,5-hydride shift. No chromato- idine nucleus under identical reaction conditions to our pre-
graphic techniques were required to purify the products.
vious report, but failed. Pyrimidines and pyrimido[4,5-d]pyr-
imidines are highly useful bio-active ingredients.9 Many pro-
cedures are available in the literature for their synthesis,
however long synthetic steps, toxic catalysts/solvents and low
yields make them less attractive.10 Multi-component one-pot
processes have gained much attention in synthetic chemistry
due to their advantages of reduced reaction steps/waste/cost,
inherent atom economy, simpler procedures, and energy
savings.11 Many of the solvents used in the laboratory and for
industrial purposes are volatile organic compounds (VOCs),
which are unsafe for human beings and the surroundings.12
Therefore, a solvent-free synthesis is now in high demand.
Moreover, the use of external oxidants sometimes generates
unwanted oxidation products, which can be avoided by using
aerial oxidation.13 Nowadays, catalyst-free synthetic methods
have attracted much attention because of their lower costs,
mild conditions, reduced pollution and ease of purification.14
On top of that, avoiding all types of chromatographic purifi-
cation adds additional advantages in terms of cost and time
savings.15 Keeping these advantages in mind, here we disclose
C–H functionalization is a growing field that has continued to
draw the attention of enthusiastic organic chemists for the last
two decades due to it not having the prefunctionalization
requirements of substrates, being step economical, being
directive, and having environment-friendly features.1 The
functionalization of the α-sp3 C–H bonds of tertiary amines is
very common and much more studied because of its massive
importance. A lot of progress has been made by various
research groups to extend the scope and to develop new syn-
thetic strategies for this particular reaction.2 Salts of many
transition metals such as Cu, Fe, Ru, Rh, V, and Co are usually
used as catalysts along with various oxidants for this purpose.3
Molecular iodine or its salts has also been used to catalyze the
reaction.4 Brønsted acids were recently reported to catalyze the
reaction in the presence of an oxidant.5 Very recently, our
group reported one such C–H functionalization in the
presence of only molecular oxygen in H2O.6
Redox-neutral processes which operate via hydride transfer
mechanisms have recently attracted the attention of research-
ers due to their simple mode of operation, low cost, and step
and atom economy.7 This technique was also utilized for the
functionalization of the α-sp3 C–H bonds of tertiary amines.8
However, most of these synthetic methods require pre-
structurally arranged amine substrates in the presence of a
Department of Applied Sciences, GUIST, Gauhati University, Guwahati-781014,
Assam, India. E-mail: mohitdd.deb@gmail.com, baruah.pranjal@gmail.com;
Fax: +91-8876998905; Tel: +91-8876998905
†Electronic supplementary information (ESI) available. CCDC 1850632, 1850625
and 1850631. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8gc03507e
Scheme 1 Intramolecular α-C–H cyclization of tert-amine.
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a one-pot cascade reaction of 6-aminouracil, aldehyde and considered as the optimal conditions for the reaction
tetrahydroisoquinolines (THIQs). (entry 11, Table 1). At room temperature the reaction only
The reaction constructs a new pyrimidine ring in the formed the intermediate 4′a (27%) and we recovered the
absence of any catalyst, external oxidant and solvent under unreacted 6-aminouracil 1a (entry 13, Table 1).
heating which proceeds through the cyclization of intermedi-
To examine the substrate scope we used a variety of alde-
ate 4′ by a 1,5-hydride shift (Scheme 1b). The cyclization of hydes in the reaction. We observed that aromatic aldehydes
this type of intermediate usually requires a catalyst and/or and aromatic heterocyclic aldehydes both gave excellent yields
oxidant and therefore, this method has advantages over of the product. However, aliphatic aldehydes such as isobutyr-
previously known ones.16 The products of the reactions were aldehyde and formaldehyde gave a slightly lower yield (4o–p,
isolated in pure form only by trituration with ethanol and no Scheme 2). We did not notice much difference in the yield of 4,
chromatographic techniques were required. To the best of our obtained by electron-donating and -withdrawing aldehydes.
knowledge, this is the first example of C–H functionalization When we used -o, -m and -p substituted aldehydes for the reac-
adjacent to a tertiary nitrogen atom in which no external tion, an excellent yield of 4 was obtained in each case (4b–d,
agents are required.
Scheme 2). We then treated various N-substituted/unsubsti-
We started our study with the 3-component reaction of tuted 6-aminouracils and observed relatively lower yields in the
1,3-dimethyl-6-aminouracil (1a), benzaldehyde (2a) and THIQ case of 1,3-dipropyl-6-aminouracil (4q–t, Scheme 2). On the
(3) in the presence of an oxygen balloon in water under reflux other hand, 1-methyl-6-aminouracils and unsubstituted-
conditions (entry 1, Table 1). However, the reaction completely 6-aminouracils produced a higher yield of the products (4u–z,
failed to form the product. We then performed the reaction in Scheme 2). When we replaced THIQ with tryptoline, an indole
different solvents like ethanol, toluene, p-xylene, and DMF in fused derivative of THIQ, we obtained a comparatively lower
the presence of molecular oxygen. However, only DMF pro- yield of 4 (4z1 & 4z2, Scheme 2).
duced an average yield after 4 h of heating at 120 °C (entry 6,
In our previous report, we isolated both the regioisomers of
Table 1). Very interestingly, when we performed the reaction in the intermediate Mannich product,6 which we did not observe
open air in DMF, we obtained the same yield as in the pres- here and therefore, the reaction was clean. The structures of
ence of molecular oxygen (entry 7, Table 1). This result tells us all of the products were established through spectroscopic ana-
that external oxygen is not required for the reaction. We lysis as well as single crystal X-ray studies of a few compounds
further attempted the reaction in DMSO and acetonitrile in (Fig. 1). To scale-up the reaction process, we carried out the
open air under heating, but afforded a poor yield (entries 8–9, first reaction in a 10 g scale under the optimized conditions
Table 1). However, the reaction under neat conditions gave us and obtained a 97% yield of 4a only after trituration with
an excellent yield only after 2 h of stirring at 130 °C, which we ethanol.
To extend the scope of the reaction further, we performed a
reaction using 1,3-dimethylbarbituric acid assuming that it
would produce compounds like pyrimido-1,3-oxazines.
Table 1 Optimization of the reaction conditionsa
However, we didn’t obtain the expected one and instead, the
reduced compound of the Knoevenagel product was isolated
(Scheme 3). THIQ reduced the unsaturated double bond to a
single bond and consequently isolated DHIQ (8) from the reac-
tion mixture. The reducing properties of THIQ were reported
earlier by Seidel et al., in which they observed the reduction of
iminium ions by THIQ.7d One of the possible explanations
Entry
Oxidant
Solvent
Temp. (°C)
Time (h)
Yield (%)
they proposed was an intermolecular hydride transfer from
THIQ to the iminium ions with the concurrent oxidation of
the former to DHIQ. Here, we for the first time observed that
THIQ can even reduce the activated CvC double bond. To
understand the route of the reaction, we first synthesized
compound 6 17 which was then reacted with THIQ under the
same reaction conditions. In both of the reactions, we
obtained the product 7.
To study the mechanism of the formation of 4, we per-
formed some control experiments (Scheme 4). We isolated
compound 4a′ along with 4a when the reaction was stopped
after 1 h (Scheme 4a). When 4a′ was heated at 130 °C, it pro-
duced 4a in excellent yield (Scheme 4b). Therefore, we believe
that the reaction proceeded through 4′ which was formed from
the 3-component Mannich reaction. Since, in many instances,
the α-C–H functionalization/cyclization of tertiary nitrogen
1
2
3
4
5
6
7
8
O2 (1 atm)
O2 (1 atm)
O2 (1 atm)
O2 (1 atm)
O2 (1 atm)
O2 (1 atm)
Open air
Open air
Open air
Open air
Open air
Open air
Open air
H2O
100 °C
90 °C
12 h
12 h
12 h
12 h
12 h
4 h
4 h
4 h
4 h
4 h
—
10
—
EtOH
Toluene
p-Xylene
DMF
DMF
DMF
DMSO
CH3CN
Neat
Neat
Neat
110 °C
130 °C
120 °C
120 °C
120 °C
120 °C
90 °C
120 °C
130 °C
140 °C
RT
—
58
65
65
15
Trace
80
92
84
—
9
10
11
12
13b
2 h
2 h
12 h
Neat
a Unless otherwise mentioned, all of the reactions were performed
using 1a (1 mmol, 155 mg), 2a (1 mmol, 106 mg) and 3a (1 mmol,
133 mg) under neat conditions under heating. The products were
purified by trituration with ethanol and the yields are for the isolated
products. b At room temperature the reaction gave 27% of the
intermediate 4′.
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Scheme 2 Synthesis of pyrimido[4,5-d]pyrimidines. The products were purified by trituration with ethanol except 4q–t in which column chromato-
graphy was required for purification.
proceeds via a free radical route,18 we carried out the reaction tion gave a trace amount of 4a (Scheme 4d). This reaction
by adding radical scavengers. In the presence of BHT and inferred that oxygen is essential for the formation of the
TEMPO the reaction gave 90% and 87% of 4a, respectively, product 4, however an external supply of oxygen is not
under the optimized reaction conditions (Scheme 4c). These required. Next, we attempted a reaction under dark conditions
reactions ruled out the possibility of a radical pathway for the bearing in mind that light may promote the reaction, but did
cyclization step. As our reactions were performed in open air, not observe any decrease in the yield of 4a (Scheme 4e).
we thought aerial oxygen might play some crucial role in the Therefore, based on our control experiments and relevant
cyclization. When the reaction was carried out under the reports8 a tentative mechanism for the formation of 4 is
careful exclusion of air/oxygen (in a N2 atmosphere), the reac- proposed (Scheme 5).
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Fig. 1 X-ray structures of 4a, 4m and 4q. CCDC no. for 4a is 1850632, 4m is 1850625 and 4q is 1850631.†
Scheme 3 Reaction of barbituric acid, aldehyde and THIQ.
Scheme 5 Tentative mechanism for the reaction.
Conclusions
In conclusion, here we have developed a 3-component one-pot
coupling of 6-aminouracil, aldehyde and tetrahydroisoquino-
lines in which a new pyrimidine ring is constructed. The reac-
tion proceeds through the cyclization of the Mannich inter-
mediate. The reaction does not necessitate any catalyst, exter-
nal oxidant and solvent. No inert atmosphere is required for
the reaction and it could be scaled up with an excellent yield.
Furthermore, the reaction avoids chromatographic techniques
for the purification of the products. Further studies to evaluate
the bioactivity of the synthesized compounds are in progress
in our collaborating laboratory.
Conflicts of interest
Scheme 4 Control experiments to establish the reaction mechanism.
There are no conflicts to declare.
The 3-component reaction of 6-aminouracil, aldehyde and
THIQ gave the Mannich product 4′ via an iminium ion [A].
Under the reaction conditions, 4′ may tautomerise to 4′x,
Acknowledgements
which undergoes a 1,5-hydride shift to form a zwitterion 4′y. MLD is thankful to the Science and Engineering Research
This molecule then cyclizes to the intermediate Z′ which Board (SERB), India, (Grant No. SB/FT/CS-073/2014) for the
finally oxidises to the desired product 4. The final oxidation financial support. PKB is also thankful to SERB, India (Grant
probably takes place by aerial oxygen. Further work is under No. SB/FT/CS-100/2012) for the financial support. PJB acknowl-
way to develop a better understanding of the mechanism.
edges MHRD, Govt of India, for the research fellowship under
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the TEQIP-III Project. We acknowledge the Sophisticated
Analytical Instrumentation Facility (SAIF), GU, for the use of
the single-crystal X-ray diffractometer. The authors acknowl-
edge Dr Ranjit Thakuria, Dept. of Chemistry, Gauhati
University for the X-ray structure analysis.
C. Xia and F. Li, Chem. Commun., 2014, 50, 4751;
(e) I. Andivelu and S. Gandhesiri, J. Org. Chem., 2014, 79,
4984; (f) L.-T. Li, H.-Y. Li, L.-J. Xing, L.-J. Wen, P. Wang and
B. Wang, Org. Biomol. Chem., 2012, 10, 9519;
(g) Z. Ningning, C. Ran, Z.-N. Daisy, D. Yunfei and Z. Kang,
J. Org. Chem., 2014, 79, 10581; (h) L.-T. Li, J. Huang,
H.-Y. Li, L.-J. Wen, P. Wang and B. Wang, Chem. Commun.,
2012, 48, 5187; (i) L. Le, D. Liang, Z.-N. Daisy and
D. Yunfei, RSC Adv., 2015, 5, 29774; ( j) T. Froehr,
C. P. Sindlinger, U. Kloeckner, P. Finkbeiner and
B. J. Nachtsheim, Org. Lett., 2011, 13, 3754; (k) M. L. Deb,
C. D. Pegu, P. J. Borpatra and P. K. Baruah, Tetrahedron
Lett., 2016, 57, 5479.
5 (a) B. Schweitzer-Chaput, J. Demaerel, H. Engler and
M. Klussmann, Angew. Chem., Int. Ed., 2014, 53, 8737;
(b) A. Pinter, A. Sud, D. Sureshkumar and M. Klussmann,
Angew. Chem., Int. Ed., 2010, 49, 5004; (c) W. Chen and
D. Seidel, Org. Lett., 2016, 18, 1024; (d) S. Lai, X. Ren,
J. Zhao, Z. Tang and G. Li, Tetrahedron Lett., 2016, 57, 2957;
(e) B. Schweitzer-Chaput and M. Klussmann, Eur. J. Org.
Chem., 2013, 666.
Notes and references
1 (a) M. Klussmann and D. Sureshkumar, Synthesis, 2011,
353; (b) W.-J. Yoo and C.-J. Li, Top. Curr. Chem., 2010, 292,
281; (c) C. S. Yeung and V. M. Dong, Chem. Rev., 2011, 111,
1215; (d) P. Anastas and N. Eghbali, Chem. Soc. Rev., 2010,
39, 301; (e) C.-J. Li, Acc. Chem. Res., 2009, 42, 335; (f) Z. Shi
and F. Glorius, Angew. Chem., Int. Ed., 2012, 124, 9354;
(g) C. Zhang, C. Tang and N. Jiao, Chem. Soc. Rev., 2012, 41,
3464; (h) Z. Li, D. S. Bohle and C.-J. Li, Proc. Natl. Acad.
Sci. U. S. A., 2006, 103, 8928.
2 (a) K. Alagiri and K. R. Prabhu, Org. Biomol. Chem., 2012,
10, 835; (b) K. R. Campos, Chem. Soc. Rev., 2007, 36, 1069;
(c) X. Z. Shu, X. F. Xia, Y. F. Yang, K. G. Ji, X. Y. Liu and
Y. M. Liang, J. Org. Chem., 2009, 74, 7464; (d) G. Jiang,
J. Chen, J. S. Huang and C. M. Che, Org. Lett., 2009, 11,
4568; (e) K. M. Jones and M. Klussmann, Synlett, 2012, 23,
159; (f) F. Kienzle, Tetrahedron Lett., 1983, 24, 2213;
(g) C. L. Mathis, B. M. Gist, C. K. Frederickson,
K. M. Midkiff and C. C. Marvin, Tetrahedron Lett., 2013, 54,
2101; (h) C.-K. Chen, A. G. Hortmann and M. R. Marzabadi,
J. Am. Chem. Soc., 1988, 110, 4829; (i) J. Xuan, Z.-J. Feng,
S.-W. Duan and W.-J. Xiao, RSC Adv., 2012, 2, 4065;
( j) G. Pandey, G. Kumaraswamy and P. Y. Reddy,
Tetrahedron, 1992, 48, 8295; (k) M. Okimoto, K. Ohashi,
H. Yamamori, S. Nishikawa, M. Hoshi and T. Yoshida,
Synthesis, 2012, 44, 1315; (l) M. L. Deb, C. D. Pegu,
P. J. Borpatra and P. K. Baruah, RSC Adv., 2016, 6, 40552.
3 (a) O. Basle, N. Borduas, P. Dubois, J. M. Chapuzet,
T.-H. Chan, J. Lessard and C.-J. Li, Chem. – Eur. J., 2010, 16,
8162; (b) W. Han and A. R. Ofial, Chem. Commun., 2009, 45,
5024; (c) W. Han, P. Mayer and A. R. Ofial, Adv. Synth.
Catal., 2010, 352, 1667; (d) S.-I. Murahashi and D. Zhang,
Chem. Soc. Rev., 2008, 37, 1490; (e) M. Rueping, C. Vila,
R. M. Koenigs, K. Poscharny and D. C. Fabry, Chem.
Commun., 2011, 47, 2360; (f) A. J. Catino, J. M. Nichols,
B. J. Nettles and M. P. Doyle, J. Am. Chem. Soc., 2006, 128,
5648; (g) S. Singhal, S. L. Jain and B. Sain, Chem. Commun.,
2009, 45, 2371; (h) M. Ghobrial, K. Harhammer,
M. D. Mihovilovic and M. Schnurch, Chem. Commun., 2010,
46, 8836; (i) A. Sud, D. Sureshkumar and M. Klussmann,
Chem. Commun., 2009, 45, 3169; ( j) Z. Li and C.-J. Li, J. Am.
Chem. Soc., 2005, 127, 3672; (k) Z. Li and C.-J. Li, J. Am.
Chem. Soc., 2005, 127, 6968.
6 M. L. Deb, C. D. Pegu, P. J. Borpatra, P. J. Saikia and
P. K. Baruah, Green Chem., 2017, 19, 4036.
7 (a) S. Mahato, M. A. Haque, S. Dwaria and C. K. Jana, RSC
Adv., 2014, 4, 46214; (b) B. M. Trost and R. J. Kulaweic,
J. Am. Chem. Soc., 1993, 115, 2027; (c) M. T. Richers,
M. Breugst, A. Y. Platonova, A. Ullrich, A. Dieckmann,
K. N. Houk and D. Seidel, J. Am. Chem. Soc., 2014, 136,
6123; (d) D. A. Evans and A. H. Hoveyda, J. Am. Chem. Soc.,
1990, 112, 6447; (e) L. Ma, A. Paul, M. Breugst and
D. Seidel, Chem. – Eur. J., 2016, 22, 18179.
8 (a) Y. K. Kang, S. M. Kim and D. Y. Kim, J. Am. Chem. Soc.,
2010, 132, 11847; (b) A. Dieckmann, M. T. Richers,
A. Y. Platonova, C. Zhang, D. Seidel and K. N. Houk, J. Org.
Chem., 2013, 78, 4132; (c) Y.-P. He, Y.-L. Du, S.-W. Luo and
L.-Z. Gong, Tetrahedron Lett., 2011, 52, 7064; (d) C. Zhang,
C. K. De, R. Mal and D. Seidel, J. Am. Chem. Soc., 2008, 130,
416; (e) C. Zhang, S. Murarka and D. Seidel, J. Org. Chem.,
2009, 74, 419; (f) Y.-Y. Han, W.-Y. Han, X. Hou, X.-M. Zhang
and W.-C. Yuan, Org. Lett., 2012, 14, 4054; (g) K. Mori,
K. Ehara, K. Kurihara and T. Akiyama, J. Am. Chem. Soc.,
2011, 133, 6166; (h) W. Cao, X. Liu, W. Wang, L. Lin and
X. Feng, Org. Lett., 2011, 13, 600; (i) L. Chen, L. Zhang,
J. Lv, J.-P. Cheng and S. Luo, Chem. – Eur. J., 2012, 18, 8891;
( j) S. Murarka, I. Deb, C. Zhang and D. Seidel,
J. Am. Chem. Soc., 2009, 131, 13226; (k) M. Asikainen,
O. Jauhiainen, O. Aaltonen and A. Harlin, Green Chem.,
2013, 15, 3230.
9 (a) A. S. Jones, J. R. Sayers, R. T. Walker and E. D. Clercq,
J. Med. Chem., 1988, 31, 268; (b) H. Mitsuya, R. Yarchoan
and S. Broder, Science, 1990, 249, 1533; (c) R. Pontikis and
C. Monneret, Tetrahedron Lett., 1994, 35, 4351;
(d) N. J. Curtin, H. C. Barlow, K. J. Bowman, A. H. Calvert,
R. Davison, B. T. Golding, B. Huang, P. J. Loughlin,
D. R. Newell, P. G. Smith and R. J. Griffin, J. Med. Chem.,
2004, 47, 4905.
4 (a)
U.
Kloeckner,
N.
M.
Weckenmann
and
B. J. Nachtsheim, Synlett, 2012, 97; (b) Z.-Q. Lao,
W.-H. Zhong, Q.-H. Lou, Z.-J. Li and X.-B. Meng, Org.
Biomol. Chem., 2012, 10, 7869; (c) Y. Yan, Y. Zhang, Z. Zha
and Z. Wang, Org. Lett., 2013, 15, 2274; (d) J. Zhao, P. Li,
This journal is © The Royal Society of Chemistry 2019
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View Article Online
Communication
Green Chemistry
10 (a) S. Badvel, R. R. Gopireddy, T. B. Shaik, S. Hasti,
V. R. Tummaluru and N. R. Chamarthi, Chem. Heterocycl.
54, 5043; (c) M. L. Deb, B.-S. Saikia, G. K. Rastogi and
P. K. Baruah, ChemistrySelect, 2018, 3, 1693.
Compd., 2015, 51, 749; (b) F. M. Moghaddam, 14 (a) J. Zhang, Z. Cui, F. Wang, Y. Wang, Z. Miao and
M. R. Khodabakhshi and A. Mariyeh, Tetrahedron Lett.,
2014, 55, 4720; (c) H. Wang, C. Wang and T. D. Bannister,
Tetrahedron Lett., 2015, 56, 1949; (d) L. Suresh,
R. Chen, Green Chem., 2007, 9, 1341; (b) G. Choudhary and
R. K. Peddinti, Green Chem., 2011, 13, 276; (c) J. Liu, M. Lei
and L. Hu, Green Chem., 2012, 14, 2534.
P. S. V. Kumar, Y. Poornachandra, C. G. Kumar and 15 (a) A. M. Wawro, T. Muraoka, M. Kato and K. Kinbara, Org.
G. V. P. Chandramouli, Bioorg. Med. Chem. Lett., 2017, 27,
1451; (e) M. Kidwai, K. Singhal and S. Kukreja, Z.
Naturforsch., B: J. Chem. Sci., 2007, 62, 732;
(f) W. S. Hamama, M. A. Ismail, H. A. Al-Saman and
H. H. Zoorob, J. Adv. Res., 2013, 4, 115.
Chem. Front., 2016, 3, 1524; (b) X. Lei, A. Jalla,
M. A. A. Shama, J. M. Stafford and B. Cao, Synthesis, 2015,
47, 2578; (c) P. A. Byrne, K. V. Rajendran, J. Muldoon and
D. G. Gilheany, Org. Biomol. Chem., 2012, 10, 3531.
16 (a) K. S. V. Gupta, D. V. Ramana, B. Vinayak, B. Sridhar and
M. Chandrasekharam, New J. Chem., 2016, 40, 6389;
(b) S. Mahato, S. Haldar and C. K. Jana, Chem. Commun.,
2014, 50, 332; (c) X.-J. Shang and Z.-Q. Liu, Tetrahedron
Lett., 2015, 56, 482; (d) V. Satheesh, M. Sengoden and
T. Punniyamurthy, J. Org. Chem., 2016, 81, 9792.
11 (a) A. Dömling and I. Ugi, Angew. Chem., Int. Ed., 2000, 39,
3168; (b) M. Shiri, Chem. Rev., 2012, 112, 3508;
(c) A. Dömling, Chem. Rev., 2006, 106, 17; (d) J.-C. Xiang,
Z.-X. Wang, Y. Cheng, S.-Q. Xia, M. Wang, B.-C. Tang,
Y.-D. Wu and A.-X. Wu, J. Org. Chem., 2017, 82, 9210;
(e) H. Wang, Q. Xu, S. Shen and S. Yu, J. Org. Chem., 2017, 17 M. L. Deb and P. J. Bhuyan, Tetrahedron Lett., 2005, 46,
82, 770; (f) L.-J. Cheng and N. P. Mankad, J. Am. Chem. 6453.
Soc., 2017, 139, 10200; (g) M. L. Deb, P. J. Borpatra, 18 (a) M. L. Deb, S. S. Dey, I. Bento, M. T. Barros and
P. J. Saikia and P. K. Baruah, Synthesis, 2017, 49, 1401.
12 (a) J.-Y. Chin, C. Godwin, E. Parker, T. Robins, T. Lewis,
P. Harbin and S. Batterman, Indoor Air, 2014, 24, 403;
(b) W.-T. Tsai, Environments, 2016, 3, 23.
13 (a) A. N. Campbell and S. S. Stahl, Acc. Chem. Res., 2012,
45, 851; (b) D. Munz and T. Strassner, Inorg. Chem., 2015,
C. D. Maycock, Angew. Chem., Int. Ed., 2013, 52, 9791;
(b) P. J. Borpatra, M. L. Deb and P. K. Baruah, Synlett, 2018,
29, 1171; (c) K. Chen, B. Gao, Y. Shang, J. Du, Q. Gu and
J. Wang, Org. Biomol. Chem., 2017, 15, 8770;
(d) A. V. A. Gholap, S. Maity, C. Schulzke, D. Maiti and
A. R. Kapdi, Org. Biomol. Chem., 2017, 15, 7140.
74 | Green Chem., 2019, 21, 69–74
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