Synthesis of 2,6-diaryl-1,2-dihydropyridines through a 6π-electrocyclization of N-sulfonylazatrienes

Article abstract of DOI:10.1016/j.tetlet.2014.10.034

The development of an efficient synthetic method toward substituted 1,2-dihydropyridines from cinnamylideneacetophenones is reported. The key intermediates N-sulfonylazatrienes were synthesized through a TiCl₄-mediated direct condensation of primary sulfonamides with the substituted (E,E)-cinnamylideneacetophenones. The 6π-electrocyclization of these intermediates, catalyzed by a Lewis acid, selectively afforded the desired products in good yields.

Full text of DOI:10.1016/j.tetlet.2014.10.034

Accepted Manuscript
Synthesis of 2,6-diaryl-1,2-dihydropyridines through a 6π-electrocyclization of
N-sulfonylazatrienes
Diana I.S.P. Resende, Samuel Guieu, Cristina G. Oliva, Artur M.S. Silva
PII:
S0040-4039(14)01717-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.10.034
TETL 45263
Reference:
Tetrahedron Letters
To appear in:
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Revised Date:
Accepted Date:
29 August 2014
2 October 2014
7 October 2014
Please cite this article as: Resende, D.I.S., Guieu, S., Oliva, C.G., Silva, A.M.S., Synthesis of 2,6-diaryl-1,2-
dihydropyridines through a 6π-electrocyclization of N-sulfonylazatrienes, Tetrahedron Letters (2014), doi: http://
dx.doi.org/10.1016/j.tetlet.2014.10.034
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Synthesis of 2,6-diaryl-1,2-dihydropyridines through
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Diana I. S. P. Resende, Samuel Guieu, Cristina G. Oliva and Artur M. S. Silva

1
Tetrahedron Letters
journal homepage: www.els evier.com
Synthesis of 2,6-diaryl-1,2-dihydropyridines through a 6π-electrocyclization of N-
sulfonylazatrienes
Diana I. S. P. Resende,a Samuel Guieu,a,b Cristina G. Olivaa,¥ and Artur M. S. Silvaa,∗
a Department of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal.
b Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal.
¥ Actual address: Medicinal Chemistry Section, Experimental Therapeutics Programme, CNIO (Spanish National Cancer Research Centre), Madrid, Spain.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The development of an efficient synthetic method toward substituted 1,2-dihydropyridines from
cinnamylideneacetophenones is reported. The key intermediates N-sulfonylazatrienes were
synthesized through a TiCl4-mediated direct condensation of primary sulfonamides with the
substituted (E,E)-cinnamylideneacetophenones. The 6π-electrocyclization of these
intermediates, catalyzed by a Lewis acid, selectively afforded the desired products in good
yields.
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
1,2-dihydropyridines
6π-aza-electrocyclization
N-sulfonylazatrienes
(E,E)-cinnamylideneacetophenones
MW-irradiation
———
∗ Corresponding author. Tel.: +351-234-370-714; fax: +351-234-370-084; e-mail: Artur.silva@ua.pt

2
Tetrahedron
sulfonylazatrienes, used as precursors in nucleophilic
1. Introduction
additions,26,41 Diels-Alder,29 aza-Friedel-Crafts28,31 and Strecker-
type reactions.30 The sulfonamides 4a-c were prepared from the
corresponding chlorides, commercially available with the
exception of the highly unstable 2-pyridylsulfonyl chloride which
had to be synthesized.42 TiCl4 acts as an acid catalyst that
coordinates with the carbonyl group directing the nucleophilic
attack towards this group. The N-sulfonylazatrienes 2a-f were
obtained in moderate to good yields, and the substituents clearly
affected the results (Table 1). If an electron-donating substituent
is introduced in the para-position of the ring B (R2 = OCH3) the
reactivity of the imine is extremely enhanced and the N-
sulfonylazatriene 2c cyclized almost immediately (Entry 3). On
the other hand, the presence of an electron-donating substituent
(R1 = OCH3) in the para-position of the ring A led to a lower
yield.
The synthesis of N-heterocycles is an important area of
research due to their presence in numerous natural products and
drugs.1 Into this family, dihydropyridine derivatives have been
recognized as important scaffolds and versatile precursors.2-7
Although 1,4-dihydropyridines are well documented in the
literature,5 their isomeric counterparts, 1,2-dihydropyridines, are
less studied. They are important intermediates in the preparation
of natural products such as isoquinuclidines8-11 and iboga
alkaloids.11-15 Several approaches have been reported for the
synthesis of 1,2-dihydropyridines, most of them involving the
direct reduction of pyridines or pyridinium salts.6,16 Another
approach is the cyclization of linear precursors.6 Although 6π-
azaelectrocyclization reactions are well documented in the
literature, their application to the synthesis of 1,2-
dihydropyridines remains scarce,17-25 and was mainly performed
by Katsumura19-22 and Okamura.17-18 Moreover, the examples
reported suffered important drawbacks, such as the need of high
temperatures and long reaction times. The recent work of Nemes
and coworkers24 describing the 6π-electrocyclization of 1-
azatrienes represents an attractive synthetic route toward 2,6-
diaryl-3-cyano-1,2-dihydropyridines. Nevertheless, the electron-
withdrawing cyano group used to facilitate the (E)/(Z)
isomerization and the difficult deprotection of the amine are
limiting factors. The use of a protecting group at the nitrogen
atom, which would be both electron-withdrawing and easily
removed, such as heteroarylsulfonyl,26,27 would be an attractive
alternative. In the last years N-heteroarylsulfonylimines have
been recognized as remarkable compounds in transition metal-
catalyzed transformations due to their metal-coordinating
capability.28-31 (E,E)-cinnamylidene-acetophenone derivatives are
α,β,γ,δ-diunsaturated ketones widely used as intermediates32-36 in
the synthesis of potentially bioactive compounds such as
pyrazoles,37-38 2-styrylchromones39 and xanthones.39 The
combination of the properties of this conjugated system with the
versatility of the sulfonyl group prompted us to idealize a new
methodology for the synthesis of substituted 1,2-
dihydropyridines. Herein we report the development of a new
methodology for the synthesis of 2,6-diaryl-1,2-dihydropiridines
1 through a 6π-electrocyclization of N-sulfonylazatrienes 2
(Figure 1).
O
O
O
S
R3
O
S
N
R3
NH2
+
R1
R2
O
, R3 = 2-pyridyl
, R3 = 8-quinolyl
, R3 = p-tolyl
R1
R2
4a
4b
4c
, R1 = H; R2 = H
, R1 = OCH3; R2 = H
3a
3b
3c
2a-f
, R1 = H; R2 = OCH3
4d, R3 = N-Me2
Scheme 1. Synthesis of the N-sulfonylazatrienes 2a-f. Reagents and
conditions: TiCl4, Et3N, CH2Cl2, 40ºC.
Table 1. Scope of the TiCl4-mediated condensationa
Entry
R1
R2
R3
2 (%)b
1
2
3
4
5
6
H
OCH3
H
H
H
H
H
OCH3
H
Pyridyl
Pyridyl
Pyridyl
Quinolyl
NMe2
2a (74)
2b (46)
2c (c)
2d (65)
2e (87)
H
H
H
p-Tolyl
2f (91)
a
Reactions were carried out using 3a-c (5.0 mmol), 4a-d (5.0 mmol), Et3N
(10 mmol) and TiCl4 (5.0 mmol) in CH2Cl2 (15 mL) under nitrogen
b
c
atmosphere at reflux overnight. Yield of isolated products. Isolation of
compound 2c was not possible, instead the corresponding 1,2-
dihydropyridine 1c was obtained in 73% yield, the 6π-electrocyclization
occurring spontaneously.
2.2. Electrocyclization
On the contrary to the synthesis carried out by Nemes and
coworkers,24 in our case and with the exception of compound 2c,
the cyclization did not occur spontaneously. So we decided to use
a Lewis Acid to activate the reactivity of the imine. It should
coordinate with the N-sulfonylimine group to enhance its
electron-withdrawing properties, thus promoting the 6π-
electrocyclization. First, the effect of different Lewis Acids and
solvents on the reaction was investigated. The reactions were
carried out with the N-pyridylsulfonylazatriene 2a and a catalytic
amount of the Lewis Acid at room temperature (Scheme 2). After
7 days, a large quantity of the starting material still remained in
the reaction mixture and the purification led to the 1,2-
dihydropyridine 1a in low yields independently of the solvent or
Lewis Acid employed (Table 2).
Figure 1. Synthetic strategy toward 1,2-dihydropyridines 1.
2. Results and discussion
2.1. Synthesis of the N-sulfonylimines
The N-sulfonylimines 2a-f were synthesized through a TiCl4-
mediated direct condensation40 of the primary sulfonamides 4a-d
with the substituted (E,E)-cinnamylideneacetophenones 3a-
c
35,36,39 (Scheme 1). This method has been successfully applied to
the synthesis of different N-sulfonylimines, including N-

3
of toluene (Entry 5), CH2Cl2 (Entries 6-7) or CH3CN (Entry 8)
lowered the reactivity of the imine, leading to a higher recovery
of starting material. On the other hand, when using methanol
(Entry 9) the starting material was almost completely hydrolyzed
and only a very small amount of dihydropyridine could be
isolated.
Overall, the best conditions, in term of both yields and
reaction time, were obtained with the use of Ni(ClO4)2.6H2O as
the catalyst in THF, at 120ºC for one hour under microwave
irradiation.
Scheme 2. 6π-electrocyclization of N-sulfonylazatriene 2a.
The formation of the 1,2-dihydropiridine 1a was demonstrated
by the presence of four non-aromatic signals (see Figure 1 for
numbering) in the NMR spectrum: two double doublets at δ =
5.74 and 5.92 ppm corresponding to H-3 and H-4 and two
doublets at δ = 6.05 and 6.24 ppm due to H-5 and H-2. Single
crystals suitable for X-ray diffraction were obtained by slow
evaporation of a saturated solution in dichloromethane (Figure
2).43 The compound was obtained as a racemic mixture: carbon
C-2 has a R- or S-configuration, and the sulfonyl group adopts a
trans-conformation relatively to the phenyl ring B. The 1,2-
dihydropyridine core deviates substantially from planarity and is
in a boat conformation. The carbon-carbon bonds lengths
confirmed the double bonds localizations: C2-C3: 1.502 Å, C3-
C4: 1.327 Å, C4-C5: 1.443 Å, C5-C6: 1.340 Å.
Table 3. Lewis Acid and solvent screening on the 6π-
electrocyclization of 2aa
Entry Solvent Lewis Acid Time
2a (%)b 3a (%)b
1a (%)b
THF
THF
THF
Cu(TfO)2
Sc(TfO)3
La(TfO)3
3 h
3 h
3 h
27
32
44
64
62
59
-
-
-
-
22
28
57
34
37
34
8
1
2
3
4
5
6
THF Ni(ClO4)2.6H2O 1 h
Toluenec Ni(ClO4)2.6H2O 3 h
CH2Cl2 Ni(ClO4)2.6H2O 2 h
13
CH2Cl2 Ni(ClO4)2.6H2O 3 h
CH3CN Ni(ClO4)2.6H2O 3 h
MeOH Ni(ClO4)2.6H2O 3 h
19
11
-
10
10
80
7
8
9
65
59
4
Table 2. 6π-Electrocyclization of N-sulfonylazatriene 2aa
a
Reactions were carried out using 2a (0.08 mmol) and Lewis Acid (0.008
mmol) in solvent (4 mL) under microwave irradiation at a variable power to
3a (%)b
Lewis Acid 1a (%)b 2a (%)b
b
c
Entry Solvent
maintain Tmax = 120ºC. Yield of isolated products after chromatography.
Due to the low dielectric constant of toluene, it was used a Weflon bar in the
reaction.
CH2Cl2 Ni(ClO4)2.6H2O
CH2Cl2 Cu(TfO)2
Toluene Ni(ClO4)2.6H2O
THF Ni(ClO4)2.6H2O
15
14
5
73
25
57
22
12
48
7
1
2
3
4
15
64
a Reactions were carried out using 2a (5.0 mmol) and Lewis Acid (0.5 mmol)
in solvent (4 mL) under nitrogen atmosphere at room temperature for 7 days.
b Yield of isolated products after chromatography.
Scheme 3. 6π-Electrocyclization of N-sulfonylazatriene 2a-f.
Scheme
3 and table 4 show the scope of the 6π-
electrocyclization employing the best reaction conditions
previously obtained. The presence of an aromatic substituent on
the sulfonyl moiety seems to increase the reactivity of the N-
sulfonylazatrienes, leading to higher yields of 1,2-
dihydropyridines. The best results were obtained in the presence
of the pyridyl group, probably because the Lewis Acid could also
coordinate with the pyridyl nitrogen. Also, the electron-
withdrawing property of this N-pyridylsulfonyl group probably
increases the reactivity of the imine for the 6π-electrocyclization.
On the other hand, changing the substituent on the sulfonyl
moiety to a non-aromatic substituent led to a lower yield (Table
4, Entry 5). The introduction of an electron-donating group on
the ring A lowered the yield of the cyclization (Table 4, Entries
2-3), probably because it counteracts the electron-withdrawing
effects of the sulfonyl and the Lewis acid. As said before, when
the electron-donating substituent was on the ring B, the 6π-
electrocyclization occurred almost immediately with no need of a
catalyst and the 1,2-dihydropiridine was formed instantly.
Figure 2. Molecular structure of 1a (the two enantiomers are shown
as they appear in the asymmetric unit of the crystal structure).
Thermal ellipsoids are shown at the 50% probability level, hydrogen
atoms are shown with an arbitrary radius (0.30Å). C, grey; O, red; N,
blue; S, yellow; H, white.
These results were encouraging but not satisfactory, so
classical heating conditions were tested. Unfortunately, after 7
days at reflux a large quantity of starting material still remained,
and a major hydrolysis of the imine was also observed. This
decomposition was probably due to the long reaction times
involved. This time could be diminished by the use of microwave
irradiation, so the reactivity of the N-sulfonylazatriene 2a under
microwave irradiation was investigated (Table 3). A control
experiment without Lewis Acid, using CH2Cl2 as solvent, proved
that a large amount of the starting material still remained after 3
hours. Upon the Lewis acids tested, Ni(ClO4)2.6H2O gave the
best results (Table 3, Entries 1-4). The influence of the solvent
was then studied with this catalyst (Table 3, Entries 4-9). The use

4
Tetrahedron
17. De Lera, A. R.; Reischl, W.; Okamura, W. H. J. Am. Chem. Soc.
1989, 111, 4051-4063.
Table 4. Scope of the 6π-electrocyclization of N-sulfonyl-
azatrienes 2a-fa
18. Maynard, D. F.; Okamura,, W. H. J. Org. Chem. 1995, 60, 1763-
1771.
Entry R1
R2
R3 Time (h)
2 (%)b 3 (%)b
1(%)b
19. Tanaka, K.; Mori, H.; Yamamoto, M.; Katsumura, S. J. Org.
Chem. 2001, 66, 3099-3110.
20. Tanaka, K.; Katsumura, S. J. Am. Chem. Soc. 2002, 124, 9660-
H
OCH3
OCH3
H
H
H
H
H
H
H
H
H
Pyridyl
1
1
2
1
2
1
2
2a (-) 3a(34)
2b (40) 3b (24)
2b (26) 3b (24)
2d (16) 3d (38)
2e (38) 3e (25)
2f (54) 3f (11)
1a (64)
1b (35)
1b (42)
1d (41)
1e (18)
1f (33)
1f (60)
1
Pyridyl
Pyridyl
Quinolyl
NMe2
p-Tolyl
p-Tolyl
2
3
4
5
6
7
9661.
21. Tanaka, K.; Kobayashi, T.; Mori, H.; Katsumura, S. J. Org. Chem.
2004, 69, 5906-5925.
22. Kobayashi, T.; Hatano, S.; Tsuchikawa, H.; Katsumura, S.
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H
H
23. Wei, H.; Wang, Y.; Yue, B.; Xu, P.-F. Adv. Synth. Catal. 2010,
2f (21) 3f (19)
a Reactions were carried out using 2 (5.0 mmol) and Ni(ClO4)2 6H2O (0.5
mmol) in THF (4 mL) under microwave irradiation at a variable power to
maintain Tmax = 120ºC. b Yield of isolated products after chromatography.
.
352, 2450-2454.
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3. Conclusions
26. Esquivias, J.; Arrayás, R. G.; Carretero, J. C. J. Org. Chem. 2005,
70, 7451-7454.
In summary, a general method for the synthesis of substituted
1,2-dihydropyridines using cinnamylidenacetophenone as the
starting material has been presented. The 6π-electrocyclization of
N-sulfonylazatrienes afforded the 1,2-dihydropyridines in
reasonable to good yields, with complete selectivity. This new
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dihydropyridines bearing different substituents on the amine and
on the 2 and 6 positions. An asymmetric version of this reaction
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Acknowledgments
Thanks are due to the University of Aveiro and the Portuguese
Fundação para a Ciência e a Tecnologia (FCT) for funding the
Organic
Chemistry
Research
Unit
(project
PEst-
C/QUI/UI0062/2013), the CICECO Associate Laboratory
(PEst-C/CTM/LA0011/2013) and the Portuguese National
NMR Network (RNRMN). DISPR and SG also thank the FCT
for a doctoral grant (SFRH/BD/62696/2009) and a postdoctoral
grant (SFRH/BPD/70702/2010), respectively.
39. Silva, A. M. S.; Pinto, D. C. G. A.; Tavares, H. R.; Cavaleiro, J.
A. S.; Jimeno, M. L.; Elguero, J. Eur. J. Org. Chem. 1998, 2031-
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5
- Novel 2,6-diaryl 1-substituted 1,2-dihydropyridines (1,2-DHP) were obtained in good yields.
- 1,2-DHP were obtained by a Lewis acid catalyzed 6π-electrocyclization of N-sulfonylazatrienes.
- 6π-Azaelectrocyclization is promoted by the electron-withdrawing N-sulfonyl group.
- The scope of the 6π-azaelectrocyclization was studied using microwave irradiation.