Recyclization of 2-Iminochromenes via Ketene Aminal Intermediates: One-Pot, Three-Component Reaction for Synthesis of 1,4-Dihydropyridines-fused-1,3- diazaheterocycles

Article abstract of DOI:10.1080/00397911.2012.714040

An efficient, one-pot synthesis of 4-aryl-1,4-dihyropyrido[1,2-a]-fused-1, 3-diazaheterocycles by three-component reaction of 1,n-diamine, nitroketene dithioacetal (1,1-di(methylthio)-2-nitroethene), and 2-iminochromenes in EtOH is reported. This new protocol has the advantages of good yields, short reaction times, and convenient operation. The structures were corroborated spectroscopically (infrared, ¹H and ¹³C NMR, and electron impact-mass spectrometry) and by elemental analyses. A plausible mechanism for this type of recyclization is proposed. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.714040

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Recyclization of 2-Iminochromenes via
Ketene Aminal Intermediates: One-Pot,
Three-Component Reaction for Synthesis
of 1,4-Dihydropyridines-fused-1,3-
diazaheterocycles
Abdolali Alizadeh a , Rashid Ghanbaripour a & Long-Guan Zhu b
a Department of Chemistry , Tarbiat Modares University , Tehran ,
Iran
b Department of Chemistry , Zhejiang University , Hangzhou , China
Published online: 26 Jun 2013.
To cite this article: Abdolali Alizadeh , Rashid Ghanbaripour & Long-Guan Zhu (2013): Recyclization
of 2-Iminochromenes via Ketene Aminal Intermediates: One-Pot, Three-Component Reaction for
Synthesis of 1,4-Dihydropyridines-fused-1,3-diazaheterocycles, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:19, 2575-2582
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Synthetic Communications1, 43: 2575–2582, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.714040
RECYCLIZATION OF 2-IMINOCHROMENES VIA
KETENE AMINAL INTERMEDIATES: ONE-POT,
THREE-COMPONENT REACTION FOR SYNTHESIS
OF 1,4-DIHYDROPYRIDINES-FUSED-1,3-
DIAZAHETEROCYCLES
Abdolali Alizadeh,1 Rashid Ghanbaripour,1 and
Long-Guan Zhu2
1Department of Chemistry, Tarbiat Modares University, Tehran, Iran
2Department of Chemistry, Zhejiang University, Hangzhou, China
GRAPHICAL ABSTRACT
Abstract An efficient, one-pot synthesis of 4-aryl-1,4-dihyropyrido[1,2-a]-fused-1,3-
diazaheterocycles by three-component reaction of 1,n-diamine, nitroketene dithioacetal
(1,1-di(methylthio)-2-nitroethene), and 2-iminochromenes in EtOH is reported. This
new protocol has the advantages of good yields, short reaction times, and convenient oper-
ation. The structures were corroborated spectroscopically (infrared, 1H and 13C NMR, and
electron impact–mass spectrometry) and by elemental analyses. A plausible mechanism for
this type of recyclization is proposed.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords 1,4-Dihydropyridines; 2-iminochromene; ketene aminal; three-component
reaction
Received June 17, 2012.
Address correspondence to Abdolali Alizadeh, Department of Chemistry, Tarbiat Modares
University, P.O. Box 14115-175, Tehran, Iran. E-mail: aalizadeh@modares.ac.ir
2575

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A. ALIZADEH, R. GHANBARIPOUR, AND L.-G. ZHU
INTRODUCTION
Nitrogen-containing heterocyclic skeletons are prevalent in pharmaceuticals
and biologically functional molecules. The 1,4-dihydropyridine (1,4-DHP) skeleton
is one of the most versatile pesticidal, herbicidal, veterinary (as anthelmintic, anti-
bacterial, and antiparasitic), and heterocyclic pharmacophores.[1,2] The most notable
pharmacological feature of 1,4-DHPs are those acting as calcium channel blockers.
After the synthesis and development of a set of 1,4-DHPs that have antihypertensive
and antianginal activities,[3,4] the interest is growing toward pharmacological activi-
ties that are not connected with their calcium antagonist properties, such as neuro-
tropic, anticancer, and antiviral activities.[5,6] For instance, nifedipine is widely used
as an antianginal, felodipine is used to control hypertension and cerebrocrast has
antidiabetic, memory-enhancing, and neuroprotective effects (Fig. 1). The most
classical synthesis of symmetrical 1,4-DHPs was reported in 1882 by Hantzsch.[7]
Despite the long history, the wide-ranging biological activity of 1,4-dihydropyridine
derivatives, both naturally occurringe and synthetic, ensures that the synthesis of this
important ring system is still of interest and has instituted a diverse array of synthetic
approaches to these heterocycles.[8]
Chromene derivatives are an important class of compounds that widely present
in plants, vegetables, and fruits.[9] There are some natural products with
chromene-based structure that are able to prevent disease.[10] Some of them have
pharmaceutical effects, including antibiotic,[11] antifungicide,[12] and antitumor.[13]
They also have been used as photosensitizers in laser dyes and biological and medical
fluorescent research.[14,15] The 2-iminochromenes include most important protein
tyrosine kinase inhibitors (PTK) categories and they have attracted attention as
anticancer agents.[16,17] Some 2-iminochromenes were also studied for applications
in laser dyes[18] and synthesis.[19]
Heterocyclic ketene aminals (HKAs) are powerful and versatile intermediates
in heterocyclic synthesis. Reactions of cyclic ketene aminals with a number of elec-
trophilic reagents have been successfully used to synthesize five- and six-membered
Figure 1. Examples of some 1,4-dihydropiridines drugs.

1,4-DIHYDROPYRIDINES-FUSED-1,3-DIAZAHETEROCYCLES
2577
and fused heterocycles in recent years. As a part of our ongoing efforts to explore a
new route for the synthesis of a variety of active biologically fused polycyclic nitro-
gen heterocycles in our laboratory via one-pot synthesis and reaction of aminals,[20]
herein we report an efficient synthesis of 4-aryl-1,4-dihyropyrido[1,2-a]-fused-
1,3-diazaheterocycles derivatives via a new, and one-pot, three-component reaction
of a nitro ketene aminal derived from the addition of various 1,n-diamines to nitro
ketene dithioacetal (l,l-bis(methylthio)-2-nitroethene) with 2-iminochromenes.
RESULTS AND DISCUSSION
Our new synthetic route is shown in Scheme 1. The reaction of 1,n-diamine 1,
nitro ketene dithioacetal (l,l-bis(methylthio)-2-nitroethene), and 2-iminochromenes
within 5 h in EtOH at reflux conditions produced 4-aryl-1,4-dihyropyrido[1,2-a]-
fused-1,3-diazaheterocycles derivatives 3 in excellent yields (Scheme 1).
The structures of compounds 3a–f were deduced from their elemental analysis,
infrared (IR), and high-field 1H and 13C NMR spectra (see Supporting Information).
The structure of 3a was also confirmed by x-ray crystallography (Fig. 2). The single
crystal has been prepared from methanol solution and there are two molecules of
methanol in the crystal structure. The mass spectrum of 3a has displayed the molecu-
lar ion peak at m=z 299, which is in agreement with the proposed structure. The IR
spectrum of this compound showed absorption bands due to the OH, NH, and
NH2 stretching frequency at 3427, 3338, and 3223 cmꢀ1. Absorption bands at 2179,
1
1644, 1469, 1364 and 1511 cmꢀ1 are due to the CN, C C, and NO2 groups. The H
=
Scheme 1. Synthesis of 4-aryl-1,4-dihyropyrido[1,2-a]-fused-1,3-diazaheterocycles derivatives.
Figure 2. Molecular structure of compound 3a. (Figure is provided in color online.)

2578
A. ALIZADEH, R. GHANBARIPOUR, AND L.-G. ZHU
3
NMR spectrum of 3a showed one triplet signal (d ¼ 3.78, J ¼ 8.6 Hz), two quartet
signals (d ¼ 3.92, 3J ¼ 8.6 Hz and d ¼ 4.04, 3J ¼ 8.6 Hz) for CH2 groups because these
protons are diastereotopic, two doublets (d ¼ 6.69, 3J ¼ 8.7 Hz and d ¼ 6.69,
3J ¼ 6.9 Hz), and two triplets (d ¼ 6.66, 3J ¼ 7.1 Hz and d ¼ 6.95, 3J ¼ 6.9 Hz) for aro-
matic hydrogens and four singlets for CH, NH2, OH, and NH groups (d ¼ 4.85, 6.28,
9.24, and 9.42 ppm). The NH2, NH, and OH peaks are also exchanged with deu-
1
terium. The H-decoupled 13C NMR spectrum of 3a showed 14 distinct resonances,
in agreement with the suggested structure.
A possible mechanism is proposed in Scheme 2. The well-established
compound 4 could result from the initial addition of the 1,n-diamines to
Scheme 2. Plausible mechanism for the formation of products 3a–f.

1,4-DIHYDROPYRIDINES-FUSED-1,3-DIAZAHETEROCYCLES
2579
Table 1. Prepared 4-aryl-1,4-dihyropyrido[1,2-a]-fused-1,3-diazaheterocycles
Entry
Diamine
R
Product 3
Yield %
=
R
=
R
=
R
a
H
83
79
75
b
H
H
c
=
d
e
R
R
OMe
70
85
=
Br
=
R
f
Br
80
l,l-bis(methylthio)-2-nitroethene and subsequent attack of the resulting reactive
ketenaminals 4 on the 2-iminochromenes 2 to yield intermediate 5. This intermediate
could cyclization by two paths. The IR, 1H and 13C NMR spectra and x-ray crystal-
lography show that the product of path A was not created and another considerable
event occurs (path B) (Table 1).
EXPERIMENTAL
The diamines and l,l-bis(methylthio)-2-nitroethene were obtained from Merck
(Germany) and Fluka (Switzerland) and were used without further purification.
2-Iminochromenes were prepared according to literature.[21] Melting points were

2580
A. ALIZADEH, R. GHANBARIPOUR, AND L.-G. ZHU
measured on an Electrothermal 9100 apparatus. Elemental analyses for C, H, and N
were performed using a Heraeus CHN–O–Rapid analyzer. Mass spectra were
recorded on a Finnigan-Matt 8430 mass spectrometer operating at an ionization
1
potential of 70 eV. H and 13C NMR spectra were measured (dimethylsulfoxide
DMSO-d6 solution) with a Bruker DRX-500 Avance spectrometer at 500.13 and
125.8 MHz, respectively. IR spectra were recorded on a Shimadzu IR-460 spectro-
meter and absorbencies are reported in cmꢀ1
.
Typical Procedure for Preparation of 5-Amino-7-(2-hydroxyphenyl)-
8-nitro-1,2,3,7-tetrahydroimidazo[1,2-a]pyridine-6-carbonitrile (3a)
A solution of l,l-bis(methylthio)-2-nitroethene (0.165 g, 1 mmol) and 1,2-etha-
nediamine (0.074 g, 1 mmol) in ethanol (5 mL) was magnetically stirred for 4 h at
reflux. Then 2-iminochromenes (0.170 g, 1 mmol) was added to solution, and the
mixture was stirred at reflux for 1 h. Then the 2-iminochromene was solved immedi-
ately, and new precipitate began to appear slowly. After 1 h, the mixture was filtered
and the precipitate was washed with cold ethanol to afford the pure products 3a–f.
Complete experimental details are available online in the Supplementary
Materials.
5-Amino-7-(2-hydroxyphenyl)-8-nitro-1,2,3,7-tetrahydroimidazo[1,2-
a]pyridine-6-carbonitrile (3a)
Yield: 0.248 g (83%); yellow powder; mp ¼ 205–210 ꢁC (decomp). IR (KBr):
=
3427, 3338, and 3223 (NH2, NH, and OH), 2179 (CN), 1644 (C C), 1469 (CH2),
1364 (NO2), 1225 (C-N), 1103 cmꢀ1(C-O). 1H NMR (500.13 MHz, DMSO-d6):
3
dH ¼ 3.78 (2H, t,3JHH ¼ 8.6 Hz, CH2NH), 3.92 (1H, q, JHH ¼ 8.6 Hz, CH2N), 4.04
3
(1H, q, JHH ¼ 8.6 Hz, CH2N), 4.85 (1H, s, CH), 6.28 (2H, s, NH2), 6.66 (1H, t,
3
3
3JHH ¼ 7.1 Hz, Ar), 6.69 (1H, d, JHH ¼ 8.7 Hz, Ar), 6.94 (1H, d, JHH ¼ 6.9 Hz,
3
1
Ar), 6.95 (1H, t, JHH ¼ 6.9 Hz, Ar), 9.24 (1H, s, OH), 9.42 ppm (1H, s, NH). H
NMR (500.13 MHz, DMSO-d6=D2O): dH ¼ 3.78 (2H, t,3JHH ¼ 8.6 Hz, CH2NH),
3.92 (1H, q,3JHH ¼ 8.6 Hz, CH2N), 4.04 (1H, q, JHH ¼ 8.6 Hz, CH2N), 4.85 (1H,
3
3
3
s, CH), 6.66 (1H, t, JHH ¼ 7.1 Hz, Ar), 6.69 (1H, d, JHH ¼ 8.7 Hz, Ar), 6.94 (1H,
d, JHH ¼ 6.9 Hz, Ar), 6.95 ppm (1H, t, JHH ¼ 6.9 Hz, Ar). 13C NMR
(125.75 MHz, DMSO-d6): dC ¼ 36.0 (CH), 43.1 (CH2NH), 44.5 (CH2N), 58.7
(CCN), 105.5 (CNO2), 115.3 (CH of Ar), 118.5 (CH of Ar), 120.9 (CN), 127.1
3
3
(CH of Ar), 128.9 (CH of Ar), 130.4 (Cipso-CH), 149.5 (NCNH2), 152.3 (Cipso
-
OH), 154.8 ppm (NCNH). MS (EI, 70 eV): m=z (%) ¼ 299 (Mþ, 2), 236 (5), 149
(11), 137 (11), 123 (10), 109 (10), 95 (24), 81 (49), 69 (100), 55 (36). Anal. Calcd.
for C14H13N5O3 (299.29): C, 56.18; H, 4.38; N, 23.40%. Found: C, 56.12; H, 4.41;
N, 23.42%. Crystal data for 3a C14H13N5O3 (CCDC 846788): MW ¼ 2202.78,
˚
˚
˚
3
a ¼ 17.196(5) A, b ¼ 9.551(5) A, c ¼ 22.326(5) A, a ¼ 90.00 b ¼ 112.586(5),
3
ꢁ
˚
c ¼ 90.00 , V ¼ 3386(2) A , Z ¼ 1, Dc ¼ 1.080 mg=m , F (000) ¼ 1103, radiation,
˚
Mo Ka (k ¼ 0.71073 A), 3.6 ꢂ 2h ꢂ 25.2. Intensity data were collected at 295(2) K
with a Bruker APEX area-detector diffractometer and employing x=2h scanning
technique in the range of ꢀ20 ꢂ h ꢂ 20, ꢀ11 ꢂ k ꢂ 11, ꢀ26 ꢂ l ꢂ 26. The structure
was solved by a direct method, all nonhydrogen atoms were positioned, and

1,4-DIHYDROPYRIDINES-FUSED-1,3-DIAZAHETEROCYCLES
2581
anisotropic thermal parameters were refined from 3831 observed reflections with R
(into) ¼ 0.1201 by a full-matrix least-squares technique converged to R ¼ 0.0836 and
raw ¼ 0.2400 [I > 2 sigma (I)].
CONCLUSION
In summary, we have disclosed a concise approach to the synthesis of substi-
tuted 4-aryl-1,4-dihyropyrido[1,2-a]-fused-1,3-diazaheterocycles from a one-pot
reaction of 1,n-diamines, l,l-bis(methylthio)-2-nitroethene, and 2-iminochromenes.
Excellent yields of the products, fairly fast reaction times, and use of simple and inex-
pensive starting materials characterize this method. The present method carries the
advantage that not only is the reaction performed under neutral conditions, but
the substances have reacted without any activation or modification. The simplicity
of the present procedure makes it an interesting alternative to complex multistep
approaches.
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