New domino heteroannulation of enaminones: Synthesis of diverse fused naphthyridines

Article abstract of DOI:10.1039/c2ob25349f

A series of new poly-functionalized fused naphthyridine derivatives were synthesized via a three-component reaction of aldehyde, 2-aminoprop-1-ene-1,1,3- tricarbonitrile and enaminone in EtOH using EtONa as a base. During these reaction processes, the domino construction of fused naphthyridine skeleton with concomitant formation of two new pyridine rings was readily achieved via base promoted three-component reactions in a one-pot operation. The procedures are facile, avoiding time-consuming and costly syntheses, tedious work-up and purifications of precursors.

Full text of DOI:10.1039/c2ob25349f

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New domino heteroannulation of enaminones: synthesis of diverse fused
naphthyridines†
Jing Li, Yan Yu, Man-Su Tu, Bo Jiang,* Shu-Liang Wang and Shu-Jiang Tu*
Received 18th February 2012, Accepted 11th April 2012
DOI: 10.1039/c2ob25349f
A series of new poly-functionalized fused naphthyridine derivatives were synthesized via a three-
component reaction of aldehyde, 2-aminoprop-1-ene-1,1,3-tricarbonitrile and enaminone in EtOH using
EtONa as a base. During these reaction processes, the domino construction of fused naphthyridine
skeleton with concomitant formation of two new pyridine rings was readily achieved via base promoted
three-component reactions in a one-pot operation. The procedures are facile, avoiding time-consuming
and costly syntheses, tedious work-up and purifications of precursors.
Multicomponent domino reactions (MDRs) have gained con-
siderable popularity in the synthetic community. They provide
efficient access to complex molecules from readily available
Introduction
Functionalized naphthyridines and their benzo/heterofused ana-
logues are present in numerous products of marine origin1 and
possess a wide range of biological activities such as antiproli-
ferative activity2 and HIV-1 integrase inhibition.3 They can also
act as allosteric inhibitors of Akt1 and Akt2,4 antitumor agents,5
and selective antagonists of 5-HT4 receptors.6 Because of the
biological activities they exhibit, these compounds have distin-
guished themselves as heterocycles of profound chemical and
biological significance. Thus the synthesis of these molecules
has attracted considerable attention.7,8 A survey of the literature
shows that the majority of the strategies involve either multistep
sequences,7 or expensive catalysts,7c–f,8 inert atmosphere,7b,c,e,8a
anhydrous conditions, lengthy reaction times,7c,d and laborious
workup.7b–d Therefore, a simple and novel method for synthesiz-
ing naphthyridines, especially polysubstituted fused naphthyri-
dines, is highly desirable and has practical benefits.
On the other hand, the derivatives of tetronic acid (tetrahydro-
furan-2,4-dione) are yet another nucleus known to be part of
many biologically active compounds, occupying a unique pos-
ition in medicinal and pharmaceutical chemistry by serving as
anti-inflammatory,9 antifungal,10 antibiotic,11 insecticidal,12
analgesic,13 anticoagulant,14 and antiepileptic15 agents. Thus, a
hybrid of these two motifs could potentially lead to a series of
structurally and biologically interesting compounds.
starting materials.16 They also form an ideal platform for rapid
generation of both complexity and diversity in a collection of
compounds with predefined functionality, e.g., ligands for cataly-
sis or bioactive compounds. Recently, we have been engaging in
the development of new multi-component ‘domino’ reactions
that can provide easy access to new core structures of chemical
and pharmaceutical interest.17,18 During our continuous efforts
on the development of useful multi-component domino reac-
tions, herein, we would like to report another highly efficient
approach to multi-functionalized furo[3,4-b][1,8]naphthyridine
derivatives through new three-component domino reactions of
aldehyde 1 with 2-aminoprop-1-ene-1,1,3-tricarbonitrile 2 and
enaminones 3 under microwave (MW) heating (Scheme 1). The
great aspect of the present domino reaction is shown by the fact
that the domino construction of fused naphthyridine skeleton
with concomitant formation of two new pyridine rings was
readily achieved via base promoted three-component reactions in
a one-pot operation. To the best of our knowledge, the one-pot
formation of two new pyridine rings and construction of such
furonaphthyridine skeleton with two NH2 groups residing in 2
and 4-positions of fused naphthyridine nucleus, respectively,
have not been achieved so far.
School of Chemistry and Chemical Engineering, and Jiangsu Key
Laboratory of Green Synthetic Chemistry for Functional Materials,
Jiangsu Normal University, Jiangsu, P.R. China. E-mail: jiangchem@
xznu.edu.cn, laotu@xznu.edu.cn; Fax: +86(516) 83500065; Tel: +86
(516) 83500065
†Electronic supplementary information (ESI) available: CCDC reference
number 868426. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2ob25349f
Scheme 1 The synthesis of furo[3,4-b][1,8]naphthyridines.
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versatile and readily obtainable reagents, and their chemistry has
received considerable attention in recent years.17a,b,19 We started
this study by subjecting a preformed enaminone 3a and 4-bro-
mobenzaldehyde 1a to the reaction with 2-aminoprop-1-ene-
1,1,3-tricarbonitrile 2 in ethanol at 100 °C under microwave
irradiation, using different bases (1.5 equiv.). Unfortunately, the
reaction scarcely proceeded in the presence of KOH or NaOH.
The incomplete reaction was observed using Et3N or NaOMe as
a base catalyst. Sodium ethylate (EtONa) was proven to be the
best base (Table 1, entry 5). Subsequently, the reaction catalyzed
by NaOEt was performed and repeated many times in different
temperatures in a sealed vessel under microwave irradiation for
16 min. The best yield of product 4a (83%) was obtained in
ethanol as the reaction temperature was increased to 120 °C.
With this result in hand, we went on to study the scope of the
methodology. Using the optimized reaction conditions, a variety
of structurally diverse aromatic aldehydes and enaminones were
investigated, and a series of new multi-functionalized furo[3,4-b]-
[1,8]naphthyridines were afforded in good yields. As shown in
Table 2, at the beginning, we made a search for the aldehyde
Results and discussion
We have planned to link two biologically important nuclei,
naphthyridines and furans, to generate a new set of compounds,
furo[3,4-b][1,8]naphthyridine analogues, using three-component
domino [3 + 2 + 1]/intramolecular six-membered heterocycliza-
tion of aldehydes 1 with 2-aminoprop-1-ene-1,1,3-tricarbonitrile
2 and enaminones 3. It should be mentioned that enaminones are
Table 1 Optimization of the catalyst in the synthesis of 4a under MW
Entry
Base
T (°C)
Time (min)
Yielda (%)
1
2
3
4
5
6
NaOH
KOH
MeONa
Et3N
EtONa
EtONa
100
100
100
100
100
120
16
16
16
16
16
16
Trace
Trace
38
42
57
83
a Isolated yield.
Table 2 Three-component domino synthesis of products 4 and 5 under MW irradiation
Entry
Product
3
Ar
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
4-Chlorophenyl (3a)
4-Chlorophenyl (3a)
4-Chlorophenyl (3a)
4-Chlorophenyl (3a)
4-Bromophenyl (3b)
4-Bromophenyl (3b)
4-Bromophenyl (3b)
4-Bromophenyl (3b)
4-Tolyl (3c)
4-Bromophenyl (1a)
4-Chlorophenyl (1b)
2,3-Dichlorophenyl (1c)
Phenyl (1f)
4-Bromophenyl (1a)
4-Chlorophenyl (1b)
Phenyl (1f)
16
18
20
20
18
20
22
20
18
20
22
18
19
18
20
17
18
20
24
20
22
16
25
24
26
26
18
17
18
17
22
20
83
81
80
78
84
82
79
80
79
78
75
82
80
78
83
81
76
72
80
74
77
82
84
81
71
69
76
72
75
79
80
79
4-Tolyl (1g)
9
4-Bromophenyl (1a)
4-Chlorophenyl (1b)
Phenyl (1f)
4-Bromophenyl (1a)
4-Chlorophenyl (1b)
4-Tolyl (1g)
4-Bromophenyl (1a)
3,4-Dichlorophenyl (1d)
4-Chlorophenyl (1b)
4-Tolyl (1g)
4-Chlorophenyl (1b)
Phenyl (1f)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
4j
4-Tolyl (3c)
4-Tolyl (3c)
4k
5a
5b
5c
5d
5e
5f
5g
5h
5i
4-Bromophenyl (3d)
4-Bromophenyl (3d)
4-Bromophenyl (3d)
4-Chlorophenyl (3e)
4-Chlorophenyl (3e)
4-Chlorophenyl (3e)
4-Chlorophenyl (3e)
Phenyl (3f)
Phenyl (3f)
Phenyl (3f)
5j
5k
5l
4-Tolyl (1g)
4-Fluorophenyl (3g)
4-Fluorophenyl (3g)
4-Chlorophenyl (3h)
4-Chlorophenyl (3e)
4-Chlorophenyl (3e)
4-Chlorophenyl (3e)
4-Bromophenyl (3i)
4-Bromophenyl (3i)
4-Bromophenyl (3i)
4-Tolyl (3j)
4-Chlorophenyl (1b)
4-Bromophenyl (1a)
4-Chlorophenyl (1b)
2,3-Dichlorophenyl
2-Chlorophenyl (1e)
Phenyl (1f)
2,3-Dichlorophenyl (1c)
Phenyl (1f)
4-Tolyl(1g)
5m
5n
5o
5p
5q
5r
5s
5t
3,4-Dichlorophenyl (1d)
4-Chlorophenyl (1b)
5u
4-Tolyl (3j)
a Isolated yield.
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Scheme 2 The synthesis of benzo[b][1,8]naphthyridines.
substrate scope, enaminone 3a and 2-aminoprop-1-ene-1,1,3-tri-
carbonitrile 2 were used as model substrates (Table 2, entries
1–4), and the results indicated that aromatic aldehydes bearing
chloro, or bromo group were suitable for the synthesis of com-
pound 4. The bulky o-substituted aldehyde 1c was converted
into the corresponding furo[3,4-b][1,8]naphthyridines 3c in 80%
yield. Subsequently, the enaminone scope of this interesting
transformation was investigated (Table 2). Several different
N-substituents were compared and substituents bearing electron-
withdrawing (4-bromophenyl, 3b) or electron-donating (4-tolyl,
3c) groups were found to be suitable for this domino reaction.
Furthermore, using various aldehydes together with different
N-substituted enaminones 3d–j resulted in the cyclization to cor-
responding substituted benzo[b][1,8]naphthyridine 5a–u
smoothly (Scheme 2). The results exhibit the scope and general-
ity of the novel multicomponent domino reaction with respect to
a range of enaminone and aldehyde substrates. Indeed, the proto-
col provides a straightforward pathway to construct highly sub-
stituted fused naphthyridine, which are generally prepared via
multi-step reactions.7
Fig. 1 X-Ray structural of 4a.
Scheme 3 The proposed mechanism for the formation of products 4.
Similar to our previous multi-component domino process,17,18
the present reaction also showed the following attractive charac-
teristics: (1) fast reaction rates which enable the reaction to be
completed within 16–26 min; (2) the environmentally friendly
process in which water is the major by-product; (3) the con-
venient work-up which only needs simple filtration since the pro-
ducts directly precipitate out after the reaction is finished and
when its mixtures are diluted with cold water; (4) readily avail-
able starting materials of aldehydes, 2-aminoprop-1-ene-1,1,3-
tricarbonitrile and preformed enaminone. Moreover, during these
domino processes, up to two pyridine rings and four sigma-
bonds were formed accompanied by CvO bond cleavage of the
aryl aldehydes, and two cyano groups of compound 2 were con-
verted into two amine groups in a one-pot intermolecular
manner. This observation is very interesting and important in
organic chemistry. Only microwave irradiation can make the
present multi-component domino reaction to occur rapidly and
efficiently, while normal heating diminished both yield and
speed. The structures of all the obtained compounds were based
on their spectroscopic data. Furthermore, the structure of the
product 4a has been unequivocally determined by X-ray struc-
tural analysis (Fig. 1).
Conclusion
In summary, we have described three-component domino [3 + 2
+ 1]/intramolecular six-membered heterocyclization as an
alternative method for the synthesis of a set of furo[3,4-b][1,8]-
naphthyridines and benzo[b][1,8]naphthyridines with concomi-
tant formation of two new pyridine rings in one-pot manner.
This MDR provides a general and efficient strategy for construc-
tion of structurally diverse fused naphthyridine skeleton. The
ready accessibility of the starting materials, the broad compatibil-
ity of N-substituted enaminone substrates, and the generality of
this process make the reaction highly valuable in view of the
synthetic and medicinal importance of fused heterocycles of this
type. The features of this strategy include mild conditions, con-
venient one-pot operation, short reaction periods of 16–26 min,
and excellent atom economy. Further investigations are in pro-
gress in our laboratory to improve our understanding of this
asymmetric version, to evaluate the process with a broader range
of substrates, and to synthesize more complex products and test
their biological activity.
Experimental
On the basis of all the above results, possible mechanism has
been proposed for the formation of furo[3,4-b][1,8]naphthyri-
dines as shown in Scheme 3. The reaction involves the ring
closure cascade reactions that consist of initial Knoevenagel con-
densation (2 to A), Michael addition (A to B), intramolecular
cyclization (B to C), and intramolecular second cyclization and
isomerization (C to 4).
General
Microwave irradiation was carried out with Initiator from
Biotage Company, Sweden. Melting points were determined in
open capillaries and are uncorrected. IR spectra were taken on a
FT-IR Tensor 27 spectrometer in KBr pellets and reported in
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inverse centimeters. 1H NMR spectra were measured on a
Bruker DPX 400 MHz (13C NMR, 100 MHz) spectrometer in
DMSO-d6 with chemical shifts (δ) given in parts per million
relative to TMS as an internal standard. HRMS (ESI) was deter-
mined by using microTOF-Q II HRMS/MS instrument (Bruker).
X-Ray crystallographic analysis was performed with a Siemens
SMART CCD and a Siemens P4 diffractometer.
2,4-Diamino-5,10-bis(4-chlorophenyl)-5,6,7,8,9,10-hexahydro-
8,8-dimethyl-6-oxobenzo[b][1,8]naphthyridine-3-carbonitrile (5m)
Pale yellow solid, mp: >300 °C. IR (KBr, ν, cm−1): 3446
(νNH2), 3397 (νNH2), 3343 (νNH2), 3237 (νNH2), 2198 (νCN),
1
1609 (νCvO). H NMR (400 MHz, DMSO-d6) (δ, ppm): 7.57
(d, J = 8.4 Hz, 2H, ArH), 7.43 (d, J = 8.4 Hz, 2H, ArH), 7.30
(q, 4H, ArH), 6.30 (s, 2H, NH2), 5.93 (s, 2H, NH2), 5.25 (s, 1H,
CH), 2.20 (q, 2H, CH2), 1.99 (d, J = 16.0 Hz, 1H, CH2), 1.77
(d, J = 17.2 Hz, 1H, CH2), 0.87 (s, 3H, CH3), 0.67 (s, 3H, CH3).
13C NMR (100 MHz, DMSO-d6) (δ, ppm): 194.0, 158.3, 155.2,
151.8, 151.3, 145.6, 138.1, 132.5, 129.1, 127.9, 127.4, 126.0,
111.9, 93.2, 70.3, 49.5, 41.4, 32.7, 31.9, 29.2, 25.9. HRMS
(ESI) m/z: calc. for C27H22Cl2N5O: 502.1201; [M − H−] found:
502.1193.
General procedure for the synthesis of compounds 4 and 5
Microwave heating: in a 10 mL reaction vial, an aldehyde
(1 mmol), 2-aminoprop-1-ene-1,1,3-tricarbonitrile (1 mmol),
EtOH (1.5 mL) were mixed and stirred at room temperature for
3 min. Then an enaminone (1 mmol) and EtONa (1.5 mmol)
was added into the mixture, and the system was heated for given
time at 120 °C under microwave irradiation. The automatic mode
stirring helped the mixing and uniform heating of the reactants.
Upon completion, monitored by TLC, the reaction mixture was
cooled to room temperature. The resulting suspension was neu-
tralized with diluted hydrochloric acid solution. Then the
mixture was stirred for 5 min. The solid product was collected
by Büchner filtration and washed with water and EtOH (95%),
and subsequently dried and recrystallized from EtOH (95%) to
give the pure product.
Acknowledgements
We are grateful for financial support from the National Science
Foundation of China (21072163 and 21102124), the Priority
Academic Program Development (PAPD) of Jiangsu Higher
Education Institutions, Science Foundation in Interdisciplinary
Major Research Project of Xuzhou Normal University (No.
09XKXK01), the NSF of Jiangsu Education Committee
(11KJB150016), and Jiangsu Science and Technology Support
Program (No. BE2011045), and Doctoral Research Foundation
of Xuzhou Normal Univ. (XZNU, No. 10XLR20).
2,4-Diamino-9-(4-chlorophenyl)-5,6,8,9-tetrahydro-6-oxo-5-
phenylfuro[3,4-b][1,8]naphthyridine-3-carbonitrile (4d)
Notes and references
Pale yellow solid, mp: 243–244 °C. IR (KBr, ν, cm−1): 3468
(νNH2), 3357 (νNH2), 3244 (νNH2), 2206 (νCN), 1748
(νCvO). 1H NMR (400 MHz, DMSO-d6) (δ, ppm): 7.56 (d, J =
8.8 Hz, 2H, ArH), 7.46 (d, J = 8.8 Hz, 2H, ArH), 7.41 (d, J =
7.2 Hz, 2H, ArH), 7.29 (t, J = 7.6 Hz, 2H, ArH), 7.19 (t, J = 7.2
Hz, 1H, ArH), 6.20 (s, 2H, NH2), 6.17 (s, 2H, NH2), 5.08 (s,
1H,CH), 4.58 (d, J = 16.0 Hz, 1H, CH2), 4.50 (d, J = 16.4 Hz,
1H, CH2). 13C NMR (100 MHz, DMSO-d6) (δ, ppm): 174.1,
158.8, 158.7, 156.0, 151.7, 144.2, 137.7, 132.1, 129.8, 128.4,
128.2, 127.5, 118.6, 116.5, 100.0, 92.0, 70.6, 65.2, 33.8. HRMS
(ESI) m/z: calc. for C23H16ClN5O2: 428.0914; [M − H−] found:
428.0947.
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Pale yellow solid, mp: >300 °C. IR (KBr, ν, cm−1): 3473
(νNH2), 3441 (νNH2), 3362 (νNH2), 3241 (νNH2), 2200 (νCN),
1
1607 (νCvO). H NMR (400 MHz, DMSO-d6) (δ, ppm): 7.68
(d, J = 7.6 Hz, 2H, ArH), 7.41 (d, J = 7.2 Hz, 2H, ArH), 7.35
(d, J = 7.6 Hz, 2H, ArH), 7.27 (d, J = 7.2 Hz, 2H, ArH), 6.29 (s,
2H, NH2), 5.93 (s, 2H, NH2), 5.26 (s, 1H, CH), 2.18 (s, 3H,
CH2), 1.94 (d, J = 17.2 Hz, 1H, CH2), 1.79 (s, 1H, CH2), 1.57
(s, 1H, CH2). 13C NMR (100 MHz, DMSO-d6) (δ, ppm): 194.3,
158.4, 155.3, 153.5, 151.6, 145.3, 138.5, 131.9, 130.8, 129.8,
121.1, 119.0, 116.7, 112.5, 92.5, 70.3, 36.1, 32.1, 28.3, 20.7.
HRMS (ESI) m/z: calc. for C25H19Br2N5O: 563.9854; [M − H−]
found: 563.9844.
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