N-alkylated sulfamic acid derivatives as organocatalyst in multicomponent synthesis of fatty dihydropyrimidinones

Article abstract of DOI:10.21577/0103-5053.20180112

In this work, N-alkylated sulfamic acid derivatives are introduced as promising acidic organocatalysts with convenient acidity and easy synthesis. The new organocatalysts derived from different nitrogenated compounds (amines, chitosan, urea and thiourea) were applied in multicomponent reactions to synthesize several dihydropyrimidinones (DHPMs). All tested organocatalysts resulted in good DHPM yields, using classic 1,3-dicarbonyl compounds and long-chain 1,3-dicarbonyl derivatives, demonstrating catalytic efficiency. N-Alkylated sulfamic acid derived from benzylamine showed good results (ca. 80percent yields). In addition, excellent results were obtained with organocatalysts based on sulfamic acid and thiourea (ca. 80-97percent yields), demonstrating the catalytic efficiency of new derivatives of thiourea organosulfamic catalysts.

Full text of DOI:10.21577/0103-5053.20180112

http://dx.doi.org/10.21577/0103-5053.20180112
J. Braz. Chem. Soc., Vol. 29, No. 11, 2342-2349, 2018
Printed in Brazil - ©2018 Sociedade Brasileira de Química
Article
N-Alkylated Sulfamic Acid Derivatives as Organocatalyst in Multicomponent
Synthesis of Fatty Dihydropyrimidinones
Carolina R. L. Hack,a Larissa Porciuncula,a Andressa C. H. Weber,a
Caroline R. M. D’Oca,a Dennis Russowsky,b Jaqueline M. Moura,c
Luiz A. A. Pintoc and Marcelo G. M. D’Oca*,a
aLaboratório Kolbe de Síntese Orgânica, Universidade Federal do Rio Grande,
Av. Itália, km 08 s/n, 96203-900 Rio Grande-RS, Brazil
bLaboratório de Síntese Orgânica, Universidade Federal do Rio Grande do Sul,
Av. Bento Gonçalves 9500, 91501-970 Porto Alegre-RS, Brazil
cLaboratório de Operações Unitárias, Universidade Federal do Rio Grande,
Av. Itália, km 08 s/n, 96203-900 Rio Grande-RS, Brazil
In this work, N-alkylated sulfamic acid derivatives are introduced as promising acidic
organocatalysts with convenient acidity and easy synthesis. The new organocatalysts derived
from different nitrogenated compounds (amines, chitosan, urea and thiourea) were applied in
multicomponent reactions to synthesize several dihydropyrimidinones (DHPMs). All tested
organocatalysts resulted in good DHPM yields, using classic 1,3-dicarbonyl compounds and
long-chain 1,3-dicarbonyl derivatives, demonstrating catalytic efficiency. N-Alkylated sulfamic
acid derived from benzylamine showed good results (ca. 80% yields). In addition, excellent results
were obtained with organocatalysts based on sulfamic acid and thiourea (ca. 80-97% yields),
demonstrating the catalytic efficiency of new derivatives of thiourea organosulfamic catalysts.
Keywords: sulfamic acid, organocatalysts, Biginelli reaction, multicomponent reaction, fatty
derivatives
Introduction
synthesis.5 It is a relatively stable, white crystalline and
odorless solid, non-volatile, non-hygroscopic, non-corrosive,
and inexpensive.6 In addition, it is a heterogeneous catalyst,
and can be recovered by simple filtration and is considered
an efficient green catalyst.7,8 It has been used in acid
catalyzed reactions, for functional group protections8 and
deprotections,9 and some important organic transformations,
such as the Beckmann rearrangement,10 Michael addition,11
imino Diels-Alder,12 Pechmann reaction,13 esterifications,14
transesterification,15 Hantzsch reaction16 and Biginelli
condensations.17 According to a previous study,18 the pKa
of SA in water is 1.19. As would be expected, the aliphatic
derivatives of SA are weaker acids than SA itself. For
example: the pKa value for cyclohexylsulfamic acid was
found to be 1.90.19 Dupont et al.20 synthesized recently
different N-alkylated sulfamic acid (NSA) as acidic metal
extractants (pKa ca. 2). Other species such as N-alkylated
sulfamic acid ionic liquids ([R2NH–SO3H][Tf2N]) were
presented as a new and safe to handle class of super acids
(pKa value < –7).20 The N-alkylated sulfamic acid derivatives
Organocatalysis is a field of organic chemistry in
constant growing and several organocatalysts have
been developed for different organic reactions. The first
mention date to back the second decade of 20th century
from the Bredig and Fiske’s report1 using some alkaloids
as catalysts in the addition reaction of hydrocyanic acid to
benzaldehyde. Organocatalytic methods has been currently
widely applied in the synthesis of various bioactive
compounds including large-scale intermediates in the
pharmaceutical industry.2 The main advantage is due to
avoiding contamination risk by metals.3 In addition, the
organocatalysts are generally cheap, stable in atmospheric
conditions, allow reproducible results and require simple
reaction conditions.4
Sulfamic acid (SA, H2NSO3H) has emerged as a substitute
for conventional Bronsted and Lewis acid catalysts in organic
*e-mail: dqmdoca@furg.br

Vol. 29, No. 11, 2018
Hack et al.
2343
showed a good miscibility in various organic solvents such
as methanol, ethanol and acetone. This fact turns its use
in organic media compatible and interesting to apply as a
promoter in organic reactions.
In the present work, we describe the synthesis of new
N-alkylated sulfamic acid derivatives and their uses as
organocatalysts in multicomponent Biginelli reactions
to achieve the synthesis of dihidropirimidin-2-ones and
their respective fatty derivatives following up the research
previously developed in our research group.21
of 10 mol% of SA as pattern, under reflux of methanol.23
The reaction was monitored by thin-layer chromatography
(TLC) and the aldehyde consumption was observed after
4 h. In this case, the dihydropyrimidinone 1 was formed in
84% yield. The result is shown in Table 1 (entry 2).
Next, we examined the ability of NSA’s as
organocatalysts to promote the Biginelli reaction. The
loading of 10 or 20 mol% of NSA 01-06 were investigated
under the same experimental conditions (Scheme 2). The
dihydropyrimidinone 1 was formed in good yields in all
examined cases (Table 1, entries 3-14). The best catalytic
behavior was observed when 20 mol% of organocatalyst
NSA 04 was employed (Table 1, entry 10).
Results and Discussion
A series of N-alkylated sulfamic acid derivatives
NSA 01-06 was synthesized using a set of primary
amines and chlorosulfonic acid, according to a procedure
described in the literature (Scheme 1).22 The NSA 01-06
organocatalysts were characterized by melting point, and
infrared (IR), nuclear magnetic resonance (NMR), and high
resolution mass spectrometries (HRMS).
As a part of our ongoing efforts to synthesize new fatty
hybrid molecules, we applied this protocol to the synthesis
of hybrid fatty dihydropyrimidinones (fatty-DHPMs).16,21,24
Thus, the multicomponent Biginelli reaction was performed
in the presence of long-chain octadecyl acetoacetate 2c,24
benzaldehyde, urea and 20 mol% of NSA 01-06 under
reflux of methanol. In these cases, 24 h were necessary to
complete the reactions (Scheme 3). The crude product was
purified by column chromatography and characterized by
Initially, the Biginelli reaction was carried out reacting
methyl acetoacetate, benzaldehyde and urea in the presence
Scheme 1. Synthesis of N-alkylated sulfamic acid organocatalysts NSA 01-06.

2344
N-Alkylated Sulfamic Acid Derivatives as Organocatalyst in Multicomponent Synthesis of Fatty Dihydropyrimidinones J. Braz. Chem. Soc.
Table 1. Multicomponent synthesis of DHPM 1 using SA and
organocatalysts NSA 01-06
Table 2. Synthesis of fatty DHPM 1c using SA and organocatalysts
NSA 01-06
entry
1
Catalyst
–
Catalyst / mol%
1, Yield / %
entry
Catalyst
NH2SO3H
NSA 01
NSA 02
NSA 03
NSA 04
NSA 05
NSA 06
Catalyst / mol%
1c, Yield / %
–
9
1
2
3
4
5
6
7
20
20
20
20
20
20
20
74
79
56
53
80
72
53
2
NH2SO3H
NSA 01
NSA 01
NSA 02
NSA 02
NSA 03
NSA 03
NSA 04
NSA 04
NSA 05
NSA 05
NSA 06
NSA 06
10
10
20
10
20
10
20
10
20
10
20
10
20
84
57
75
49
68
54
67
63
79
45
65
59
67
3
4
5
6
7
8
9
several aromatic aldehydes.After reactions using 20 mol%
of catalyst NSA 04, the fatty dihydropyrimidinones 1-6a-d
were obtained (Scheme 4). All tested examples resulted in
good fatty dihydropyrimidinones yields, demonstrating
the catalytic efficiency of the new aminosulfamic
organocatalysts NSA 04 derived from benzylamine in the
multicomponent Biginelli reaction (Table 3).
In recent decades, interest has arisen in the development
of organocatalysts based on mono or bifunctional urea or
thiourea capable of double hydrogen bonding. A broad
variety of monofunctional and bifunctional achiral double
hydrogen-bonding thiourea organocatalysts have been
developed to accelerate various synthetically useful organic
transformations employing H-bond-accepting substrates
(Figure 1).26
10
11
12
13
14
1
IR, H and 13C NMR spectroscopies.23,24 The results are
showed in Table 2. The fatty-DHPM 1c was formed in good
yields, with NSA 04 and NSA 01 catalysts showing the
most relevant results (Table 2, entries 2 and 5, respectively).
Although the yield of reactions involving the NSA 06
catalyst, derived from chitosan, were modest (Table 1,
entries 13-14 and Table 2, entry 7), they could be considered
relevant, since the chitosan is easily obtained from natural
resources and is a low-cost material.25
A recent report of Puripat et al.27 based on computational
calculations, suggested that urea would be a good catalyst
for the synthesis of 3,4-dihydropyrimidinones via Biginelli
reaction. Taking this into account, we decided to investigate
To increase the scope of products obtained through this
process, we utilized long-chain alkyl acetoacetates derived
from fatty alcohols, combined with urea or thiourea and
Scheme 2. Synthesis of DHPM 1 using aminosulfamic organocatalysts NSA 01-06.
Scheme 3. Synthesis of fatty DHPM 1c under catalysis with NSA 01-06.

Vol. 29, No. 11, 2018
Hack et al.
2345
Scheme 4. Synthesis of long chain DHPMs 1-6a-d under catalysis with NSA 04.
Figure 1. Bifunctional organocatalysts based on thiourea.
Table 3. Synthesis of fatty DHPMs 1-6a-d using organocatalyst NSA 04
N-substituted sulfamic acid derivatives based on the
structure of sulfamic acid and urea (NSA 07), as well as
thiourea (NSA 08) as organocatalysts. The procedure used
to obtain compounds NSA 07 and NSA 08 (Scheme 5) was
the same to NSA 01-06.
entry
1
R1
R2
H
X
O
O
O
O
O
O
O
O
O
O
O
O
S
Yield / %
1a, 89
2a, 86
3a, 80
1b, 70
2b, 87
3b, 82
1c, 80
2c, 68
3c, 81
1d, 87
2d, 84
3d, 83
4a, 82
5a, 84
6a, 76
4b, 87
5b, 65
6b, 62
4c, 81
5c, 61
6c, 65
4d, 70
5d, 76
6d, 66
2
OH
OMe
H
3
4
5
OH
OMe
H
6
7
8
OH
OMe
H
9
Scheme 5. Synthesis of organosulfamic catalysts NSA 07 and NSA 08.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
OH
OMe
H
NSA 07 and NSA 08 were obtained at satisfactory yields
(82 and 81%, respectively), after removing hydrochloric
acid. Then these catalysts were submitted to a Biginelli
reaction, using them individually with benzaldehyde, urea
and acetoacetates (Scheme 6). Different times and two
concentrations of NSA 07 and NSA 08 (10 and 20 mol%)
in the presence of methanol were evaluated in this reaction.
The results are shown in Table 4.
In general, both NSA 07 and NSA 08 were able to
catalyze the multicomponent reaction in reasonable to
good yields, with the latter showing superior catalytic
ability when used at 20 mol%, leading to excellent yields
of 81-97%. In addition, higher yields were obtained
with NSA 08 (Table 4, entries 11-20) compared to that
OH
OMe
H
S
S
S
OH
OMe
H
S
S
S
OH
OMe
H
S
S
S
OH
OMe
S
S

2346
N-Alkylated Sulfamic Acid Derivatives as Organocatalyst in Multicomponent Synthesis of Fatty Dihydropyrimidinones J. Braz. Chem. Soc.
Scheme 6. Organocatalytic properties of NSA 07 and NSA 08 in Biginelli reaction.
Table 4. Synthesis of DHPM 1, 1a-d under catalysis of NSA 07 and NSA 08
entry
R1
Catalyst
Catalyst / mol%
time / h
Yield / %
1, 47
1
2
3
4
5
6
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
10
20
10
20
10
20
5
5
6
6
8
8
1, 54
1, 52
1, 65
1, 57
1, 67
7
20
20
20
20
24
24
24
24
1a, 51
1b, 57
1c, 61
1d, 65
8
9
10
11
12
13
14
15
16
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
10
20
10
20
10
20
5
5
6
6
8
8
1, 74
1, 87
1, 80
1, 81
1, 84
1, 88
17
18
19
20
20
20
20
20
24
24
24
24
1a, 97
1b, 91
1c, 82
1d, 93
with organocatalysts NSA 07 and NSA 01-06 (Table 2,
The literature suggests that the catalytic behavior of
sulfamic acid and derivatives is associated to the presence
entries 2-7).
1
1
Figure 2 shows the H NMR spectra of new sulfamic
of zwitterion.20,28 Thus, according to H NMR spectra
organocatalysts NSA 08 and thiourea precursor. The
spectrum of NSA 08 indicates the disappearance of singlet
in 7.05 ppm from NH2 of thiourea and the appearance of
a singlet in 1.78 ppm, attributed to OH and NH hydrogens
present in the catalyst structure. In addition, two broad
singlets are observed, referent to hydrogens NH2 and
NH2+ from neutral and zwitterionic forms, with d 9.07 and
5.92 ppm, respectively (Figure 2b).
(Figure 2b), the results obtained with NSA 08 could be
related to the presence of zwitterion in approximately 70%,
showing superior catalytic activity.
Conclusions
In this work, N-alkylated sulfamic acid (NSA) derivatives
are introduced as promising acidic organocatalysts with

Vol. 29, No. 11, 2018
Hack et al.
2347
Figure 2. 1H NMR (400 MHz, DMSO-d6) spectra of thiourea (a) and NSA 08 (b).
convenient acidity and easy synthesis. These organocatalysts
could successfully be applied in multicomponent Biginelli
reaction, and good yields were observed in the synthesis
of DHPMs using classic 1,3-dicarbonyl compounds and
long-chain 1,3-dicarbonyl derivatives.
All tested examples resulted in good to reasonable DHPM
yields, demonstrating catalytic efficiency. The N-alkylated
aminosulfamic acid catalysts NSA 04 and NSA 01 derived
from benzylamine and butylamine, respectively, showed
the most relevant results. In addition, excellent results were
obtained with organocatalysts NSA 08 based on the structure
of sulfamic acid and thiourea with good yields (80-97%),
demonstrating, for the first time, its catalytic efficiency in
multicomponent Biginelli reaction.
Co. and used without further purification. Column
chromatography was performed using a silica gel 60A
(ACROS Organics, 0.035-0.070 mesh). The reactions
were monitored using thin-layer chromatography (TLC)
performed with plates containing silica gel (Merck,
60GF245), gradients of hexane:ethyl acetate as eluents,
and the spots were visualized using iodine. The melting
points were obtained on a Fisatom 430D apparatus and
are uncorrected. Infrared (IR) spectra were measured on
a Shimadzu PRESTIGIE-21 FT-IR spectrophotometer.
1
Nuclear magnetic resonance experiments for H, 13C
and 15N nuclei were conducted using a Bruker Ascend
400 MHz spectrometer, equipped with BBO probe
with z-axis gradients in CDCl3, DMSO-d6 or CD3CN.
Chemical shifts are reported in d (ppm) downfield from
the tetramethylsilane (TMS) internal standard or residual
Experimental
1
solvent. 15N data were acquired from H-15N HMBC
Apparatus and chemistry
(heteronuclear multiple bond correlation) experiments
at room temperature. Coupling constants (J) are reported
in Hz and refer to apparent peak multiplicities.
The reagents were purchased from Aldrich Chemical

2348
N-Alkylated Sulfamic Acid Derivatives as Organocatalyst in Multicomponent Synthesis of Fatty Dihydropyrimidinones J. Braz. Chem. Soc.
General procedure for synthesis of catalysts NSA 01-05,
NSA 07 and NSA 08
d 24.2, 25.0, 30.7, 49.8; HRMS calcd. to C6H7NO3S [M–]
178.0538; found 178.0543.
To a round-bottom flask containing nitrogenated
compound (1 mmol, amine, urea or thiourea) in dry
acetonitrile (3 mL), it was added chlorosulfonic acid
(1 mmol) dropwise over a period of 20 min at room
temperature. The reactional mixture was stirred for 2 h,
and the acetonitrile was removed under vacuum to give
the catalysts.
Carbamoylsulfamic acid (NSA 07)
139.99 g mol-1; white solid; mp 58-60 °C; FTIR
(KBr) ν / cm-1 1282, 1354, 1550.7, 1728, 3327; 1H NMR
(400 MHz, DMSO-d6) d 1.80 (s), 6.01 (s), 6.96 (s);
15N NMR (40 MHz, DMSO-d6) d 22.4, 112.1; CHN calcd.
to CH4N2O3S: C, 8.57; H, 2.88; N, 19.99%; found: C, 8.65;
H, 2.74; N, 19.87%.
Butylsulfamic acid (NSA 01)
Carbamothioylsulfamic acid (NSA 08)
153.04 g mol-1; colorless oil; FTIR (KBr) ν / cm-1
3516, 2962, 1597, 1496, 1465, 1220, 1058, 869; 1H NMR
(400 MHz, DMSO-d6) d 7.69 (sl, 2H), 2.77 (m, 2H), 1.51
(q, J 8 Hz, 2H), 1.32 (q, J 8 Hz, 2 H), 0.89 (t, J 4 Hz,
8H); 13C NMR (100 MHz, DMSO-d6) d 13.9, 19.5, 29.5,
39.1; HRMS calcd. to C4H11NO3S [M–] 152.0381; found
152.0381.
155.97 g mol-1; white solid; mp 137-140 °C; FTIR
(KBr) ν / cm-1 1217, 1357, 1531.4, 1712.7, 3280; 1H NMR
(400 MHz, DMSO-d6) d 1.78 (s), 5.91 (s), 9.08 (s);
15N NMR (40 MHz, DMSO-d6) d 63.3, 86.6; CHN calcd.
to CH4N2O3S2: C, 7.69; H, 2.58; N, 17.94%; found: C, 7.96;
H, 2.59; N, 17.62%.
Chitosan production
(R)-1-Phenylethylsulfamic acid (NSA 02)
201.04 g mol-1; yellow viscous liquid; FTIR (KBr)
ν / cm-1 3460, 2868, 1764, 1517, 1452, 1087, 887; 1H NMR
(400 MHz, DMSO-d6) d 1.48 (d, J 8 Hz, 3H), 4.41 (sl, 9H),
7.45 (m, 5H), 8.30 (sl, 2H); 13C NMR (100 MHz, DMSO-d6)
d 21.2, 50.5, 127.2 (2C), 128.9, 129.2 (2C), 139.7; HRMS
calcd. to C8H11NO3S [M–] 200.0381; found 200.0372.
Chitin was extracted from pink shrimp wastes
(Farfantepenaeus brasiliensis) through chemical treatments,
demineralization, deproteination, deodorization and
depigmentation. Deacetylation of chitin was carried out with
150 g of chitin and 3 L of concentrated sodium hydroxide
solution (45% m/v) at 130 1 °C, under constant agitation
of 50 rpm.25
1-Phenylethylsulfamic acid (NSA 03)
201.04 g mol-1; yellow viscous liquid; FTIR (KBr)
Preparation of chitosan sulfonic acid NSA 06
1
ν / cm-1 3523, 3062, 1598, 1510, 1292, 887; H NMR
(400 MHz, DMSO-d6) d 1.50 (d, J 8 Hz, 3H), 4.26 (m,
11H), 7.48 (m, 5H), 8.26 (sl, 3H); 13C NMR (100 MHz,
DMSO-d6) d 21.2, 50.5, 127.2 (2C), 129.0, 129.2 (2C),
139.6; HRMS calcd. to C8H11NO3S [M–] 200.0381; found
200.0372.
To a round-bottom flask containing a mixture of 5.0 g
of chitosan and 20 mL of hexane was added dropwise 1.0 g
of chlorosulfonic acid (9 mmol), at room temperature and
magnetic stirring. After the addition was complete, the
mixture was stirred for 1 h. Then the mixture was filtered
and washed with 30 mL of acetonitrile, and the solvent was
removed under vacuum to afford chitosan sulfonic acid as
a pale yellow powder.
Benzylsulfamic acid (NSA 04)
187.03 g mol-1; white solid; mp 69-70 °C; FTIR (KBr)
ν / cm-1 3600, 3018, 1591, 1473, 1215, 1050, 887; 1H NMR
(400 MHz, CD3CN) d 7.39 (m, 7H), 5.42 (sl, 2H), 4.07 (m,
2H); 13C NMR (100 MHz, CD3CN) d 134.0, 130.2 (2C),
129.8 (2C), 118.3, 44.5; HRMS calcd. to C7H9NO3S [M–]
186.0225; found 186.0224.
Chitosan sulfonic acid (NSA 06)
257.0205 g mol-1; pale yellow power; mp > 250 °C;
FTIR (KBr) ν / cm-1 3577, 3352, 1637, 1517, 1207, 1182,
1014, 889; HRMS calcd. to C6H11NO8S [M–] 256.0491;
found 255.2351.
Cyclohexylmethylsulfamic acid (NSA 05)
173.01 g mol-1; white solid; mp 163-165 °C; FTIR (KBr)
ν / cm-1 3616, 2935, 1597, 1496, 1226, 1066, 887; 1H NMR
(400 MHz, DMSO-d6) d 7.75 (sl, 3H), 4.61 (sl, 7H), 2.93
(m, 1H), 1.88 (m, 2H), 1.72 (m, 2 H), 1.56 (m, 1H), 1.24
(m, 4H), 1.08 (m, 1H); 13C NMR (100 MHz, DMSO-d6)
General procedure for synthesis of fatty 3,4-dihydropyrimidin-
2(1H)-one/thiones (1-6a-d)
A mixture of acetoacetate (1 mmol), aldehyde (1 mmol),
urea or thiourea (1 mmol) in 3 mL methanol was refluxed

Vol. 29, No. 11, 2018
Hack et al.
2349
for 24 h in the presence of catalyst (0.2 mmol). The progress
of the reaction was monitored by TLC (hexane/ethyl acetate
8:2). After completion of the reaction, the mixture was
evaporated under reduced pressure and the crude product
was recrystallized from ethyl acetate to obtain pure fatty
3,4-dihydropyrimidin-2(1H)-one/thiones according to
literature.23,24
Morón-Villarreyes, J. A.; D’Oca, M. G. M.; J. Am. Oil Chem.
Soc. 2016, 93, 1393.
15. Weber, A. C. H.; Batista, T. C.; Gonçalves, B.; Hack, C. R.
L.; Porciuncula, L. M.; Treptow, T. G. M.; D’Oca, C. R. M.;
Russowsky, D.; D’Oca, M. G. M.; J. Am. Oil Chem. Soc. 2016,
93, 1399.
16. Fontecha-Terazona, H. D.; Brinkerhoff, R. C.; Oliveira, P. M.;
Rosa, S. B.; Flores, D. C.; D’Oca, C. R. M.; Russowsky, D.;
D’Oca, M. G. M.; RSC Adv. 2015, 5, 59638; Cabrera, D. C.;
Rosa, S. B.; de Oliveira, F. S.; Marinho, M. A. G.; D’Oca, C.
R. M.; Russowsky, D.; Horn, A. P.; D’Oca, M. G. M.; Med.
Chem. Commun. 2016, 7, 2167.
Supplementary Information
Supplementary data are available free of charge at
http://jbcs.sbq.org.br as PDF file.
17. Wang, B.; Synlett 2005, 2005, 1342; Li, J.-T.; Han, J.-F.;Yang,
J.-H.; Li, T.-S.; Ultrason. Sonochem. 2003, 10, 119; Chen, W.
Y.; Qin, S. D.; Jin, J. R.; Synth. Commun. 2007, 37, 47.
18. Benson, G. A.; Spillane, W. J.; Chem. Rev. 1980, 80, 151.
19. Kojima, S.; Ichibagase, H.; Iguchi, S.; Chem. Pharm. Bull.
1966, 14, 965.
Acknowledgments
The authors would like to thank Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
and Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq) for financial support and fellowships.
20. Dupont, D.; Renders, E.; Raiguel, S.; Binnemans, K.; Chem.
Commun. 2016, 52, 7032.
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Submitted: March 21, 2018
Published online: June 11, 2018
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