Tandem Access to Acridones and their Fused Derivatives: [1+2+3] Annulation of Isocyanides with Unsaturated Carbonyls

Article abstract of DOI:10.1002/adsc.201901560

A wide range of acridones and their cyclo[b]-fused derivatives are efficiently constructed by a double annulation of o-enoyl arylisocyanides with α, β-unsaturated carbonyls under simple metal-free condition. This protocol is general, efficient and practical, featuring the successive formation of two rings by a one-pot domino transformation. A tandem process involing an isocyanide-based [1+4] cycloaddition, an aminofuran-based intramolecular [4+2] cycloaddition, ring opening and aromatization is proposed for the transformation. (Figure presented.).

Full text of DOI:10.1002/adsc.201901560

Title: Tandem Access to Acridones and their Fused Derivatives: [1 + 2
+ 3] Annulation of Isocyanides with Unsaturated Carbonyls
Authors: Lingjuan Zhang, Wenhui Yang, Zhongyan Hu, Xian-Ming
Zhang, and Xianxiu Xu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901560
Link to VoR: http://dx.doi.org/10.1002/adsc.201901560

10.1002/adsc.201901560
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Tandem Access to Acridones and their Fused Derivatives: [1 + 2
+ 3] Annulation of Isocyanides with Unsaturated Carbonyls
Ling-Juan Zhang,+a Wenhui Yang,+a Zhongyan Hu,b Xian-Ming Zhanga* and Xianxiu
Xub*
a
Key Laboratory of Magnetic Molecules & Magnetic Information Materials (Ministry of Education), School of
Chemistry & Material Science, Shanxi Normal University, Linfen, Shanxi, 041004, P. R. China.
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized
b
Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of
Education, Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014, P. R. China.
E-mail: zhangxm@dns.sxnu.edu.cn; xuxx677@sdnu.edu.cn
+
These authors contributed equally to this work
((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract: A wide range of acridones and their cyclo[b]- cycloaddition, an aminofuran-based intramolecular [4 + 2]
fused derivatives are efficiently constructed by a double cycloaddition, ring opening and aromatization is proposed
annulation of o-enoyl arylisocyanides with α, β-unsaturated for the transformation.
carbonyls under simple metal-free condition. This protocol is
Keywords: [1 + 2 + 3] annulation; acridones; isocyanides;
general, efficient and practical, featuring the successive
tandem reaction; unsaturated carbonyls
formation of two rings by a one-pot domino transformation.
A tandem process involing an isocyanide-based [1 + 4]
Introduction
Acridones and their fused frameworks[1] are
prominent moieties in many natural products,[2]
anticancer
drugs,[3]
anti-malarials[4]
and
biomaterials.[2a,5] Moreover, the acridones are also
served as useful building blocks for the construction
of organic semiconductors,[6] dyestuffs and
fluorescent labels.[7] As a consequence, great effort
has been devoted to developing new protocols for
their preparation. Up to now, the synthetic methods
mainly rely on (i) the intramolecular cyclization of
the corresponding N-arylsubstituted acids, amides,
and ketones (Scheme 1, path a)[8] as well as 2-
aminobenzophenones (Scheme 1, path b and c),[9] (ii)
the
palladium/copper
co-catalyzed
oxidative
carbonylation of diphenylamines with CO (Scheme 1,
path d);[10] and Cu(II)-mediated cascade reaction of o-
amino acetophenones and phenylboronic acids
(Scheme 1, path e);[11] (iii) the annulation of
anthranilic acid derivatives with aryne or
aryliodonium (Scheme 1, path f[12] and g[13]). Despite
these achievements, most of the existing synthetic
approaches focus on the closure of the central
pyridone ring from two preformed benzenoids.
Therefore, the development of conceptually new
methodologies is still highly demanded.
Scheme 1. The synthesis of acridones.
1
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10.1002/adsc.201901560
Advanced Synthesis & Catalysis
Table 1. Screening of reaction conditions.[a,b]
Functionalized isocyanides, as versatile
building blocks, have been widely applied in
synthesis of N-containing heterocycles.[14] During
the past decade, our research group has been
working on the double annulation of
functionalized isocyanides, such as active
methylene isocyanides,[15] trifluoromethylated
isocyanides[16] o-alkenylarylisocyanides,[17] and o-
enoyl arylisocyanides,[18] as a novel strategy for
the efficient construction of polycyclic
azaheterocycles. Last year, we developed a
formal [1 + 2 + 3] annulation of o-alkenylaryl
isocyanides with unsaturated ketones as a new
strategy for the expeditious synthesis of carbazole
derivatives.[19] In this domino process, an
aminofuran intermediate was in situ generated
and converted to carbazoles via a facile
intramolecular Diels-Alder reaction of furan
(IMDAF). While the IMDAF reaction has been
proven to be useful for the construction of
complex frameworks for years,[20] a domino
T
(h)
7
7
7
7
18
5
5
Entry
Solvent
Base
T (oC)
3aa
1
2
3
4
5
6
7
8
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMF
--
--
--
--
130
90
21%
14%
22%
10%
20%
78%
74%
71%
63%
74%
150
130
130
130
130
130
130
130
DCE
--
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DABCO
DBN
DBU
DABCO
DABCO
5
5.5
3
9[c]
10[d]
[a]Reaction conditions: 1a (0.25 mmol), 2a (0.3 mmol),
base, solvent (2 mL).
process
involving
the
generation
and
[b] Yields of isolated products.
transformation of furan intermediate is still
apprealing due to its high synthetic efficiency and
ready availability of starting materials. Herein,
we expand the [1 + 2 + 3] annulation to o-enoyl
arylisocyanides, whereby a wide range of
polysubstituted acridone derivatives have been
efficiently synthesized from readily available
[c] DABCO (5 mol%) was added.
[c] DABCO (20 mol%) was added.
With the optimal conditions in hand (Table 1, entry
6), the scope of o-enoyl arylisocyanides 1 was first
surveyed. Fortunately, most of the desired acridone
products were obtained in moderate to good yields
(Table 2). In the double annulation, various o-enoyl
arylisocyanides 1 bearing diverse R1 groups such as
phenyl (1a), electron-rich aryls (1b, 1c, 1g-i),
electron-deficient aryls (1d-f) and heteroaryls (1j)
were all effective substrates. The reactions were
proceeded smoothly, when the R2 groups of
isocyanides were electron-deficient groups such as 5-
chloro, 4-chloro and 4-bromo (1k-m). Notably, the
steric effect of substituents on the benzene ring of
isocyanides 1 has no obvious influence on product
yields.
isocyanides and
,-unsaturated carbonyls
(Scheme 1, bottom). This domino transformation
allows the successive formation of two rings
under metal-free conditions in a single operation,
which represents a new synthetic strategy for
acridone synthesis.
Results and Discussion
In 2018, we prepared various o-enoyl arylisocyanides
from readily available 1-(2-aminophenyl)prop-2-en-
1-ones, and used them as versatile synthons for the
efficient synthesis of phenanthridines.[18] Herein, we
chose o-cinnamoyl arylisocyanide 1a and 5-
benzylidene dimethylbarbituric acid 2a as model
substrates to screen the reaction conditions (Table 1).
As expected, when a mixture of arylisocyanide 1a
(0.25 mmol) and 2a (1.2 equiv.) in 1,4-dioxane was
stirred at 130 °C, the acridone product 3aa was
obtained in 21% yield (Table 1, entry 1). There was
no significant improvement of the yield of 3aa when
changing the reaction temperature and the solvent
(Table 1, entries 2-5). To our delight, the yields of
3aa was increased to 78% when using DABCO (10
mol%) as the catalyst (Table 1, entry 6). DBN and
DBU were found to be less effective than DABCO in
terms of yields (Table 1, entries 7-8). Increasing or
decreasing the loading of DABCO led to a slightly
lower yield of 3aa (Table 1, entries 9-10). Hence, the
optimal conditions were determined as treatment of
the reaction mixture with 10 mol% DABCO at
130 °C in 1,4-dioxane (Table 1, entry 6).
Furthermore, the scope of α,β-unsaturated
diketones
2
was investigated under identical
conditions. Firstly, we made a study for the scope of
5-substituted vinyl dimethylbarbituric acids. As
depicted in Table 2, a wide range of R3 groups on
vinyl dimethylbarbituric acids such as phenyl (2a),
electron-rich aryls (2b, 2c), electron-poor aryls (2d,
2e, 2g) and β-naphthyl group (2i) were compatible
and various acridones 3 were prepared in good yields
in the cascade reaction.[21] More steric hindrance of
the substituent was unfavourable to the reaction (2f).
By contrast, the vinyl dimethylbarbituric acids 2
bearing heteroaryl and aliphatic groups at β-position
gave lower yields of acridones 3ah and 3aj.
Moreover, this tandem reaction was proved to be of
high practical and was amenable to gram scale up
under the same reaction condition (3ai). In addition,
using unprotected vinyl barbituric acid 2k as
substrate, acridone derivative 3ak was constructed
with moderate yield. Furtherly, β-benzylidene 5/7-
membered cyclic 1,3-diketones (2l and 2n) were also
2
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10.1002/adsc.201901560
Advanced Synthesis & Catalysis
transformed smoothly into the corresponding
acridones with high yields (3al and 3an). It was
worth mentioning that the three component reaction
of dimedone, benzaldehyde and o-cinnamoyl
arylisocyanide 1a was feasible for the synthesis of
3am in moderate yield. Additionally, a three
component double [1 + 2 + 3] annulation of diketone
2o with isocyanide 1a was successfully developed for
the
highly efficient
construction of
the
polysubstituted diacridone 3ao in 81% yield.
Table 2. Scope of isocyanides and cyclic α,β-unsaturated carbonyls[a,b]
[a]Unless noted, the reactions were carried out under conditions A. Conditions A: 1a (0.25 mmol), 2a (0.3 mmol), DABCO
(0.025 mmol), 1,4-dioxane (2 mL), 130 °C.
[b] Yields of isolated products.
[c] Conditions B: 1) 1a (0.25 mmol), 2a (0.3 mmol), DABCO (0.025 mmol), 1,4-dioxane (2 mL), 130 °C, stirring for 5 h; 2)
Tf2O (30 mol%), 130 °C, stirring for 0.5 h.
[d] Three-component reaction, dimedone (0.3 mmol), benzaldehyde (0.3 mmol) and isocyanide 1a (0.25 mmol).
[e] 1.25 g of 3ai was obtained.
3
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10.1002/adsc.201901560
Advanced Synthesis & Catalysis
Subsequently, the scope of acyclic α,β-unsaturated
compounds 4 was examined for the efficient
synthesis of polyfunctionalized acridones (Table 3).
The reaction of isocyanide 1a with a wide range of
acyclic α,β-unsaturated compounds 4 resulted the
tricyclic frameworks bearing various R4 groups
(phenyl, methyl and tert-butyl), R5 groups (cyano,
and nitro) and R3 groups, such as phenyl (5aa, 5ag-
5ai), para-substituted aryl (5ab, 5ac), β-naphthyl
group (5ae). The acridone derivatives 5 were
generally obtained in good to high yields, except
heteroaryl- and styryl-substituted 5ad and 5af. These
results indicated that the electronic and steric effects
have no obvious influence on the [1 + 2 + 3]
annulation, which is also suitable for the efficient
preparation of polysubstituted acridones.
Table 3. Scope of non-cyclic α,β-unsaturated
compounds.[a,b]
Scheme 2. Control Experiments
7al in 90% yield (Scheme 2c). The aromatization
product 3al was achieved after treating with Tf2O (30
mol%) in 1,4-dioxane at 130 °C (Scheme 2d). These
results clearly showed that aminofuran and
hydroxydihydroacridone
are
the
possible
intermediates. Therefore, the [1 + 2 + 3] annulation
strategy involves the [1 + 4] annulation of
isocyanides with α,β-unsaturated compounds
followed by an intramolecular Diels-Alder reaction,
rearrangement and dehydration sequence.
Based on the experimental results and our previous
research,[19] a possible mechanism for the synthesis of
acridone derivatives 3 and 5 is proposed (see Scheme
3, exemplified with the formation of 3aa). At the
beginning, a [1 + 4] annulation, initiated by
nucleophilic addition of isocyanide 1a on to β-carbon
[a]Conditions B: 1) 1a (0.25 mmol), 2a (0.3 mmol), DABCO
(0.025 mmol), 1,4-dioxane (2 mL), 130 °C, stirring about 5
h; 2) Tf2O (30 mol%), 130 °C, stirring about 0.5 h.
[b]Yields of isolated products.
In order to understand the mechanism of this
double annulation, control experiments were
conducted. When o-cinnamoyl arylsocyanide 1a and
5-vinyl dimethylbarbituric acid 2a were treated with
DABCO (10 mol%) in 1,4-dioxane at 130 °C for 1 h,
the cyclization aminofuran 6aa was isolated in 74%
yield (Scheme 2a). Next, it was found that the
acridone 3aa was obtained quantitatively from
aminofuran 6aa under the optimized condition
(Scheme 2b). Moreover, o-cinnamoyl arylsocyanide
1a and 2-benzylidene-1,3-indandione 2l gave the
11b-hydroxy-11b,12-dihydro-indeno[1,2-b]acridone
Scheme 3. Proposed mechanism
4
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10.1002/adsc.201901560
Advanced Synthesis & Catalysis
of Michael acceptor 2a and followed by cyclization,
takes place to form the intermediate I. Then an
aminofuran intermediate II is generated through
isomerization of I. Subsequently, an intramolecular
Diels-Alder reaction of furan (IMDAF) proceeds
smoothly to produce the oxygen-bridged tetracyclic
intermediate III, from which an oxabicyclic ring-
opening reaction happens to give the hydroxylated
intermediate IV. Finally, the acridone 3aa is obtained
via dehydration. The role of DABCO is not very clear
at this stage, it probably promotes both the IMDAF
and the dehydration steps. In some cases (conditions
B in Tables 2 and 3), trifluoromethanesulfonic
anhydride is used to promote the dehydration
reaction.
9-aryl acridine framework is often found in
electrically/photochemically active catalysts[22]. As
shown in Scheme 4, 9-aryl acridine derivatives were
conveniently prepared from acridone 3aa in two
steps. Treatment of 3aa with iodomethane afforded
N-methyl acridone 8 in quantitative yield, then 9-aryl
acridines 9a[23] and 9b were obtained in moderate
mmol, 58.25 mg) and α,β-unsaturated ketone 2a (0.3
mmol, 73.2 mg) in 1,4-dioxane (2 mL) was added
DABCO (0.025 mmol, 2.8 mg) and set in a pre-
o
heated (130 C) metal block. After the reaction was
finished as indicated by TLC (reaction time, 4~5 h),
the reaction mixture was cooled to room temperature
and concentrated in vacuo. Purification of the crude
product with flash column chromatography (silica
gel; DCM : EtOAc = 300:7-250:7) to give acridone
3aa (87.5 mg, 78% yield) as a yellow solid.
Acknowledgements
Financial support provided by NSFC (21672034 and 21402112),
the 1331 Engineering, Technological Innovation Programs of
Higher Education Institutions in Shanxi ( 2019L0453) and the
postgraduate innovation project(2019SY297) of Shanxi Province,
the China Postdoctoral Science Foundation Funded Project
(2018M640650 and 2019T120605) is gratefully acknowledged.
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In conclusion, a novel base-catalyzed [1 + 2 + 3]
annulation of o-enoyl arylisocyanides and α,β-
unsaturated carbonyls has been successfully
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10.1002/adsc.201901560
Advanced Synthesis & Catalysis
FULL PAPER
Tandem Access to Acridones and their Fused
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Adv. Synth. Catal. 2019, Volume, Page – Page
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