A facile synthesis of tetrahydroimidazo[1,2-a]pyridines and tetrahydrobenzo[b]imidazo[1,2,3-ij][1,8]naphthyridines through NHC-catalyzed cascade annulations

Article abstract of DOI:10.1039/c3ra41935e

A new efficient approach for the synthesis of tetrahydroimidazo[1,2-a] pyridines and tetrahydrobenzo[b]imidazo[1,2,3-ij][1,8]naphthyridines with biological significance was successfully developed through a NHC-catalyzed cascade C-C bond and C-N bond formation process. This new alternative approach for the assembly of these multi-functionalized nitrogen bridgehead-fused heterocycles features mild conditions, moderate to high yields and operational simplicity. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41935e

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A facile synthesis of tetrahydroimidazo[1,2-a]pyridines
and tetrahydrobenzo[b]imidazo[1,2,3-
ij][1,8]naphthyridines through NHC-catalyzed cascade
annulations3
Cite this: RSC Advances, 2013, 3,
10801
Changsheng Yao,*a Weihui Jiao,b Zhaoxin Xiao,b Yuanwei Xie,b Tuanjie Li,b
Xiangshan Wang,b Rui Liub and Chenxia Yub
A new efficient approach for the synthesis of tetrahydroimidazo[1,2-a]pyridines and tetrahydrobenzo
[b]imidazo[1,2,3-ij][1,8]naphthyridines with biological significance was successfully developed through a
NHC-catalyzed cascade C–C bond and C–N bond formation process. This new alternative approach for the
assembly of these multi-functionalized nitrogen bridgehead-fused heterocycles features mild conditions,
moderate to high yields and operational simplicity.
Received 16th March 2013,
Accepted 23rd April 2013
DOI: 10.1039/c3ra41935e
www.rsc.org/advances
The imidazo-fused bridgehead-nitrogen scaffold is an impor-
tant structural motif in pharmacological molecules displaying
a wide range of bioactivities like antimicrobial,1 anti-rhino-
viral2 and anticoccidial3 activities. Therefore, the fabrication of
these nitrogen bridgehead-fused heterocycles incorporating an
imidazole ring has attracted considerable attention during
recent decades.4
manufacture a wide variety of N-bridgehead-fused heterocycles
in the past few decades. Previous studies delineated the
successful synthesis of bicyclic pyridones A through the
reaction of HKAs with a number of biselectrophilic reagents,
including a,b-unsaturated carboxylic acid derivatives,4c,6b,6c
Meldrum’s acid and aryl aldehydes,9 itaconic anhydride,10
propiolic acid ester11 and allenic esters.12 In general, all of
these methods involving C–N bond-formation rely on activated
carboxylate derivatives as coupling partners. Although much
progress has been achieved, the preparation of some of these
specific carboxylate derivatives remains difficult and, to date,
the use of activated carboxylate derivatives is to some extent
limited. Herein, this motivated us to develop a new, effectively
catalytic, method to meet the increasing scientific demands.
Due to the unique inversion of the classical reactivity
(Umpolung) in organic synthesis, the direct in situ activation of
aldehydes by N-heterocyclic carbenes (NHCs) have drawn
considerable attention from organic chemists.13 Umpolung
of aldehydes to acyl nucleophiles (a1 A d1 umpolung, a3 A d3
umpolung) has become an intense topic of current research.14
Recently, according to the present state of NHC organocata-
lysis, another field of NHC-catalyzed redox reactions to yield
acyl azolium has experienced tremendous growth in this
As an important type of imidazo-fused bridgehead-nitrogen
heterocycle, bicyclic pyridone motifs A (Fig. 1) and their
substituted variants (n = 1, 2) have been reported as the basis
for analgesics, anti-inflammatory agents and inhibitors of
pilus assembly in uropathogenic Escherichiacoli (so-called
pilicides).5 The bicyclic pyridone framework can be con-
structed via three approaches: the cyclocondensation of
ketenaminals,6 the addition of diamines to cyanobutenoic
esters7 and the addition of diamines to 1-halo or methylthio-
pyridones.8 However, considering the biological significance
and synthetic efficiency of these scaffolds, it is still highly
desirable to develop new strategies to install them rapidly.
Because of the conjugation effect of the electron-donating
amino groups and the electron-withdrawing aroyl substituent,
the electron density on the a-carbon is enhanced leading to
higher nucleophilicity than that of nitrogen. Therefore,
heterocyclic ketene aminals (HKAs) B (Fig. 2) have been
successfully used as powerful and versatile bisnucleophiles to
aSchool of Chemistry and Engineering, Key Laboratory of Biotechnology for Medicinal
Plant, Jiangsu Normal University, Xuzhou, 221116, P. R. China.
E-mail: csyao@jsnu.edu.cn
bSchool of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou
221116, P. R. China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
Fig. 1 Bicyclic pyridone motifs A.
c3ra41935e
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Table 1 Optimization of the reaction conditions
Fig. 2 Heterocyclic ketene aminals (HKAs) B.
area.15 In this field the acyl azolium has been exploited as a
novel synthon in new reactions to give a variety of highly
functionalized products from simple starting materials.
Furthermore, several literature works showed NHC-catalyzed
reactions could generate a,b-unsaturated acyl azolium C as
biselectrophiles from readily available starting material
including alkynyl aldehydes,15a,15e,15f a,b-unsaturated aldehy-
des15a,15c and a-bromoenal15b,15d via internal or external redox
processes. Moreover, a,b-unsaturated acid fluorides could also
be transformed into C in the presence of a NHC (Fig. 3).15g
Since HKAs are highly reactive bisnucleophiles, we
hypothesized that they would react with 1,3-biselectrophilic
a,b-unsaturated acyl azolium efficiently to give bicyclic
pyridones through C–C bond and C–N bond formations. To
continue our work on the NHC-catalyzed cascade synthesis of
heterocycles,15b,16 we herein wish to describe a new NHC-
catalyzed C–C bond and C–N bond formation strategy for
assembling the scaffold of imidazo(pyrido)[1,2-a]pyridine and
tetrahydrobenzo[b]imidazo[1,2,3-ij][1,8]naphthyridine via the
reactions of HKAs with bromoenals.
a
Entry Precat. 1a (mol%) Solvent Base (mol%) T (uC) Yield (%)b
1
2
3
4
5
6
7
8
4a
4b
4c
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
150
200
150
THF
THF
THF
THF
THF
DCM
K2CO3 (110) 60
K2CO3 (110) 60
K2CO3 (110) 60
K2CO3 (110) 60
K2CO3 (110) 60
K2CO3 (110) 60
ND
ND
ND
45
4d
4e
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
—
40
ND
ND
55
50
43
60
35
55
65
60
57
70
71
CH3CN K2CO3 (110) 60
Toluene K2CO3 (110) 60
Toluene NEt3 (110)
Toluene t-BuOK (110) 60
Toluene Cs2CO3 (110) 60
9
60
10
11
12
13
14
15
16
17
18
19
Toluene DBU (110)
60
Toluene Cs2CO3 (110) 50
Toluene Cs2CO3 (110) 65
Toluene Cs2CO3 (110) 70
Toluene Cs2CO3 (110) 80
Toluene Cs2CO3 (160) 65
Toluene Cs2CO3 (210) 65
Toluene Cs2CO3 (160) 65
ND
a
b
Precatalyst (10 mol%). Isolated yields.
Initially, several readily available NHC precursors were
tested as a model reaction, in which a-bromocinnamic
aldehyde 1a, 2-(imidazolidin-2-ylidene)-1-phenylethanone 2a
and imidazolium salt 4d were found to be the most efficient
combination affording a promising 45% yield of the desired
product 3a at 60 uC in THF (Table 1, entry 4). We then
examined the reaction using acetonitrile, methylene dichlor-
ide (DCM), tetrahydrofuran (THF), and toluene as solvents.
Among the solvents tested, toluene was found to be preferable
for the reaction giving 55% yield (Table 1, entry 8). Several
Table 2 Substrate scope for the synthesis of tetrahydroimidazo[1,2-a]pyridines
Entry R1
R2
R3
n
Producta Yield (%)b
1
2
3
4
5
6
7
8
C6H5
4-ClC6H4
C6H5
4-ClC6H4
C6H5
3-ClC6H4
4-ClC6H4
C6H5
H
H
H
H
H
H
H
H
H
H
H
C6H5
2,4-Cl2C6H3
C6H5
1
1
2
1
1
1
1
1
1
1
1
1
3a
3b
3c
3d
3e
3f
3g
3h
3i
71
75
73
72
72
74
70
71
75
72
60
50
C6H5
2,4-Cl2C6H3
2,4-Cl2C6H3
2,5-Cl2C6H3
2-ClC6H4
4-BrC6H4
2,4-Cl2C6H3
C6H5
9
C6H5
4-CH3OC6H4
CH3
10
11
12
3j
3k
3l
CH3
CH3 C6H5
a
Products 3a–3k were obtained using a-bromoenal (1.5 mmol), HKA
(1.0 mmol), catalyst 4d (34 mg) and Cs2CO3 (520 mg) in toluene (7
mL). Product 3l was synthesised using a-bromoenal (2 mmol), HKA
(1.0 mmol), catalyst 4d (34 mg) and triethylamine (112 mg) in
b
toluene (7 mL). Isolated yields.
Fig. 3 Reported methods for generating a,b-unsaturated acyl azolium.
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typical bases including K2CO3, Cs2CO3, DBU, NEt3 and t-BuOK
were tested for this reaction. As Table 1 showed, Cs2CO3 was
found to be the best among the bases tested (Table 1, entry 11).
Temperature screening revealed that 65 uC was the optimal
reaction temperature. It was worth noting that the use of 1.5
equiv. of 1a resulted in the best yield after examination. To
validate the catalysis of NHC in this reaction, a further blank
experiment was processed in the absence of the precatalyst 4d,
but none of the desired product was obtained.
Scheme 1 The cascade synthesis of tetrahydrobenzo[b]imidazo[3,2,1-
ij][1,8]naphthyridines.
Encouraged by these results, we then focused on the
substrate scope of this catalytic method for the synthesis of
tetrahydroimidazo[1,2-a]pyridines. Firstly, we studied the
reaction using a series of HKAs (Table 2). It was noticeable
that varying the ring size of the cycles in the heterocyclic
ketene aminals (five-membered HKAs or six-membered HKAs)
had no obvious effect on yield. Furthermore, a variety
substituents on the phenyl rings of the HKAs were well
tolerated in the reaction. Additionally, it was found that a-
bromoenals bearing electron-rich or electron-deficient phenyl
groups could participate in the reaction and afford the desired
products in favorable yields. Interestingly, we discovered that
simple 2-bromobut-2-enal could also participate in this
reaction giving 60% yield. On the basis of the previous results,
2-bromo-3-methylbut-2-enal was used in this reaction.
Excitingly, we successfully obtained the product in 50% yield.
Although the yield was not satisfying, it provided a promising
avenue for the enantioselective assembly of this framework
containing a quaternary carbon catalyzed by chiral NHCs.20a
Table 3 One-pot synthesis of tetrahydrobenzo[b]imidazo[1,2,3-
ij][1,8]naphthyridines from a variety of a-bromoenals and HKAs
Entry
X
R1
Product
Yield (%)a
1
2
3
4
5
6
7
5-Cl
4-Cl
5-Cl
5-Cl
4-Cl
4-Cl
4-Cl
4-CH3OC6H4
4-CH3OC6H4
3-CH3OC6H4
3-ClC6H4
3-ClC6H4
4-ClC6H4
5a
5b
5c
5d
5e
5f
65
60
64
65
63
60
62
C6H5
5g
a
Total isolated yields.
Scheme 2 Proposed mechanism (e.g., product 5g).
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Recently, Li et al.17 reported that the condensation of an
aldehyde, Meldrum’s acid and a functionalized HKA could give
rise to an imidazo(pyrido)[1,2-a]pyridine intermediate
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bearing a highly polarized push-pull interaction C–C double
bond and a Cl atom as a leaving group. The subsequent
intramolecular cyclization readily provided fused heterocycles
via an SNAr process (Scheme 1).17 To further expand the
applications and scope of the above-mentioned NHC-catalyzed
cascade cyclization, the following SNAr reaction was explored
(Table 3). After consumption of the HKAs with a chlorine atom
at the ortho-position with respect to the carbonyl group, the
solvent was replaced with DMF and the reaction mixture was
kept at 100 uC, yielding tetrahydrobenzo[b]imidazo[1,2,3-
ij][1,8]naphthyridines 5 in a smooth one-pot process.20b
On the basis of all the above results, two reasonable
pathways were proposed to account for the cyclization as
illustrated in Scheme 2.12,15c,15f,15g,17–19 Both pathways
involved in the formation of E at the beginning. On the one
hand, H was proposed as an intermediate derived from the
Claisen rearrangement of G, which was generated by the 1,2-
addition of E to F.15c,15f,19 On the other hand, the reaction
between E and F could lead to the formation of H directly,
through a 1,4-addition. The following intramolecular acylation
could then liberate the NHC catalyst and generate 3e.15g,18,19
Finally, an intramolecular SNAr reaction displacing the
o-chlorine of the aryl group due to attack of the NH group
gave the ultimate product 5g.12,17
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domino procedure as an alternative method for the synthesis
of bicyclic pyridones with different substituted patterns and
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a
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one-pot process starting from a variety of HKAs and a-
bromoenals. A wide range of potential biologically active
nitrogen bridgehead-fused heterocycles for biomedical screen-
ing have been synthesized successfully.
Acknowledgements
We are grateful for financial support of NSFC (No. 21242014), a
project funded by the Priority Academic Program Development
of Jiangsu Higher Education Institutions, and the Major Basic
Research Project of the Natural Science Foundation of the
Jiangsu Higher Education Institutions (09KJA430003). This
project was also sponsored by Qing Lan Project (10qlg005).
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