Microwave-assisted synthesis of n-heterocycles and their evaluation using an acetylcholinesterase immobilized capillary reactor

Article abstract of DOI:10.5935/0103-5053.20140056

Polarized ketene dithioketals have been recognized as useful building blocks in many synthetic operations. In this work, a transition-metal-free annulations of 1,1-bis(thiomethyl)-2-nitroethylene with hydroxylalkylamines or alkyldiamines have been reported. This methodology provides a directed approach to N-heterocycles, e.g., imidazolidines, oxazolidines and benzoxazoles under microwave conditions. These compounds were evaluated as acetylcholinesterase inhibitors by using an enzyme immobilized capillary reactor-tandem mass spectrometry.

Full text of DOI:10.5935/0103-5053.20140056

http://dx.doi.org/10.5935/0103-5053.20140056
J. Braz. Chem. Soc., Vol. 25, No. 5, 887-889, 2014.
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0103 - 5053 $6.00+0.00
Article
A
Microwave-Assisted Synthesis of N-Heterocycles and Their Evaluation Using an
Acetylcholinesterase Immobilized Capillary Reactor
Diego P. Sangi,a,b Julia L. Monteiro,a Kenia L. Vanzolini,a Quezia B. Cass,a
Marcio W. Paixãoa and Arlene G. Corrêa*,a
aDepartamento de Química, Universidade Federal de São Carlos, 13565-905 São Carlos-SP, Brazil
bInstituto de Ciências Exatas, Universidade Federal Fluminense, 27213-145 Volta Redonda-RJ, Brazil
Os etenoditioacetais polarizados são conhecidos como importantes intermediários em
muitos processos de síntese. Neste trabalho, reporta-se a anelação livre de metal de transição
de 1,1-bis(tiometil)-2-nitroetileno com hidroxilalquilaminas e alquildiaminas. Esta metodologia
permite a síntese direta de N-heterociclos tais como imidazolidinas, oxazolidinas e benzoxazóis sob
irradiação de micro-ondas. Estes compostos foram avaliados como inibidores da acetilcolinesterase
usando um reator capilar contendo a enzima imobilizada acoplado a um espectrômetro de massas.
Polarized ketene dithioketals have been recognized as useful building blocks in many synthetic
operations. In this work, a transition-metal-free annulations of 1,1-bis(thiomethyl)-2-nitroethylene
with hydroxylalkylamines or alkyldiamines have been reported. This methodology provides a
directed approach to N-heterocycles, e.g., imidazolidines, oxazolidines and benzoxazoles under
microwave conditions. These compounds were evaluated as acetylcholinesterase inhibitors by
using an enzyme immobilized capillary reactor-tandem mass spectrometry.
Keywords: dithioketals, oxazolidines, imidazolidines, benzoxazoles, microwaves, green
technology
Introduction
N-heterocycles, which present different biological
properties, such as enzyme inhibition,3,4 insecticides,5 and
tuberculostatic activity.6 More recently, the bis-vinylic
substitution of 1,1-bis(methylthio)-2-nitroethylene with
diamines or hidroxylamines has been reported for the
synthesis of 5-member or 6-member nitrovinyl derivatives.7
In general, the reaction conditions employed for the
bis-vinylic substitution include reflux in ethanol for several
hours,4,8 in presence of catalysts such as p-toluenesulfonic
acid (PTSA).3,5 As part of our studies on ketene dithioacetal
for the synthesis of heterocycles, we have found that
microwave irradiation accelerates the vinylic substitution
of nitroketene S,S-acetals with anilines and amines
using ethanol as solvent.9 Employing this method, we
have synthesized a new class of hydrogen bond donor
organocatalysts which was applied to the activation of
trans-β-nitrostyrenes toward reactions with a range of
carbon-based nucleophiles, affording the corresponding
adducts in excellent yields.10
Association of biological activity with heterocyclic
motifs is well known and is an important strategy in
drug discovery programs. In this way, the development
of new, easy and fast environmental benign strategies
to the formation of these scaffolds have been receiving
great attention.1 Over the past several years, polarized
ketene dithioketals 1 (Figure 1) have been recognized as
useful building blocks in many synthetic transformations.
They can be converted into the corresponding S,N- and
N,N-ketals, 2 and 3, respectively, making them important
as precursors for a large variety of functionalized ketals.2
Furthermore, the ketene dithioketals have also been
employed as intermediates for the synthesis of several
Figure 1. Ketene dithioketals 1, S,N- and N,N-ketals 2 and 3, respectively.
In this work, we report the microwave-assisted metal-
free-annulation reaction of hydroxylakylamines and
alkyldiamines to the formation of several heterocycles, such
*e-mail: agcorrea@ufscar.br

888
Microwave-Assisted Synthesis of N-Heterocycles and Their Evaluation
J. Braz. Chem. Soc.
as imidazolidines, oxazolidines and benzoxazoles, and their
evaluation as acetylcholinesterase inhibitors.
Results and Discussion
We have started our studies with the reaction of
1,1-bis(thiomethyl)-2-nitroethylene (4) with a range of
alkyldiamines and hidroxylalkylamines (Scheme 1).
Irradiation of a suspension formed by nitroethylene 4
with the corresponding amines in ethanol afforded five
and six membered N-heterocycles 5a-f in good yields.11
However, when 4- or 5-aminopentanol were employed,
only mono-substituted acyclic products 6a-b were obtained.
Furthermore, the same reactions were performed under
solvent free conditions and comparable yields could be
obtained in a shorter reaction time. Only ten minutes were
able to finish the monosubstitution, however maintaining
the irradiation for more time, double substitution products
were not produced.
Scheme 2.
the anti-cholinesterase activity of 5-amino-4-cyano-1,3-
oxazoles. Recently, Zhu et al.14 reported the synthesis
of 2-aminobenzimidazoles and their activity as novel
inhibitors of AChE.
The enzyme AChE acts in the central nervous system
and rapidly hydrolyzes the active neurotransmitter
acetylcholine into the inactive compounds choline and
acetic acid.15 Low levels of acetylcholine in the synaptic
cleft are associated with a decrease in cholinergic function
characterizing Alzheimer’s disease, which is the most
common cause of dementia among the elderly. As AChE
inhibitors are currently one of the few therapies approved
for the treatment of Alzheimer’s disease, the identification
of novel ligands that could modulate AChE activity is of
great therapeutic importance.16
Recently, we have reported the screening of a small
library of coumarins using acetylcholinesterase from
Electrophorus electricus immobilized onto a fused silica
capillary enzyme reactor (eelAChE-ICER) in a liquid
chromatograph-tandem ion-trap mass spectrometer
(LC-IT-MS/MS).17 Employing this method, we were able
to identify a new competitive inhibitor of AChE, as potent
as tacrine, with an IC50 = 0.356 µM.
In order to contribute with the structure-activity
relationship study, we evaluated the N-heterocycles 5a-f and
7a-e using eelAChE-ICER in the LC-IT-MS/MS system
and tacrine was employed as a standard inhibitor. Among
the tested compounds, the benzoxazoles showed the best
inhibition profile, being 7c and 7d the most potent with 53.5
and 70.0% inhibition at 200 µM, respectively (Table 1).
Scheme 1.
Benzoxazoles are an important class of heterocyclic
compound that can be applied to diverse therapeutic
areas.12 Therefore, to our delight, by applying the same
methodology, a series of benzoxazoles 7a-e could be easily
synthesized using 1,1-bis(thiomethyl)-2-nitroethylene 4 and
substituted 2-aminophenols as start materials (Scheme 2).
Icopezil, a benzisoxazole derivative, is a very potent
and selective acetylcholinesterase (AChE) inhibitor
with an IC50 = 0.33 nM.10 Carreiras et al.13 described
Conclusions
We have shown that the double nucleophilic substitution
between hydroxylalkylamines or alkyldiamines with
1,1-bis(thiomethyl)-2-nitroethylene 4 employing
microwave heating furnished the corresponding nitroketene
N,O-aminoacetals and N,N-aminals with moderate to

Vol. 25, No. 5, 2014
Sangi et al.
889
Table 1. Inhibition percentage of N-heterocycles 5 and 7 against eelAChE
3. Kalisiak, J.; Ralph, E. C.; Cashman, J. R.; J. Med. Chem. 2012,
55, 465.
Compound
Inhibition at 200 µM / %
4. Bock, M. G.; Chikkanna, D.; McCarthy, C.; Moebitz, H.;
Pandit, C.; PCT Int. Appl. WO2011054828, 2011.
5. Schroeder, M. E.; Flattum, R. F.; Pestic. Biochem. Physiol. 1984,
22, 148.
5a
17.7
12.8
35.3
26.8
0.0
5b
5c
5d
5e
6. Foks, H.; Pancechowska-Ksepko, D.; Janowiec, M.;
Zwolska, Z.; Augustynowicz-Kopee, E.; Phosphorus, Sulfur
Silicon Relat. Elem. 2005, 180, 2291.
5f
27.5
15.3
19.5
53.5
70.0
0.0
7a
7. Lipnicka, U.; Jasztold-Howorko, R.; Witkiewicz, K.;
Machon, Z.; Acta Pol. Pharm. 2002, 59, 121.
7b
7c
7d
7e
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Ferreira,A. G.; Correa,A. G.; Paixao, M. W.; Tetrahedron 2013,
69, 9007.
tacrine
99.3
good yields, in very short reaction time. A small library
of benzoxazoles was also obtained with moderate to good
yields. The activity of these heterocycles againstAChE was
evaluated and benzoxazole 7d showed the best inhibition
profile. Further enzymatic studies of this new class of
benzoxazole will be reported in due course.
11. Villalobos, A.; Butler, T. W.; Chapin, D. S.; Chen, Y. L.;
DeMattos, S. B.; Ives, J. L.; Jones, S. B.; Liston, D. R.; Nagel,
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71, 2249.
Supplementary Information
Supplementary information is available free of charge
at http://jbcs.sbq.org.br as PDF file.
14. Zhu, J.; Wu, C.-F.; Li, X.; Wu, G.-S.; Xie, S.; Hu, Q.-N.;
Deng, Z.; Zhu, M. X.; Luo, H.-R.; Hong, X.; Bioorg. Med.
Chem. 2013, 21, 4218.
Acknowledgements
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TheauthorsaregratefultoFAPESP(grants2009/07281-0
and EMU 2009/54040-8 for HRMS analysis), CNPq
(INCT-INBEQMeDI), CAPES and GSK Trust in Science
Program for financial support and fellowships.
16. Anand, P.; Singh, B; Arch. Pharmacal Res. 2013, 36, 375.
17. Vanzolini, K. L.;Vieira, L. C. C.; Corrêa,A. G.; Cardoso, C. L.;
Cass, Q. B.; J. Med. Chem. 2013, 56, 2038.
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FAPESP has sponsored the publication of this article.