Synthesis of Dihydrophenanthridines and Oxoimidazolidines from Anilines and Ethylglyoxylate via Aza Diels-Alder Reaction of Arynes and KF-Induced Annulation

Article abstract of DOI:10.1021/acs.orglett.6b02186

The transition-metal-free multicomponent coupling of arynes, anilines, and ethylglyoxylate, proceeding via an inverse electron-demand aza Diels-Alder cycloaddition and N-arylation, has been demonstrated. This protocol allows rapid access to N-aryl dihydrophenanthridine derivatives in moderate to high yields at room temperature from readily available starting materials. In addition, an unprecedented fluoride induced annulation of ethyl(arylimino)acetates led to the formation of highly functionalized oxoimidazolidine derivatives in good yields under mild conditions.

Full text of DOI:10.1021/acs.orglett.6b02186

Letter
pubs.acs.org/OrgLett
Synthesis of Dihydrophenanthridines and Oxoimidazolidines from
Anilines and Ethylglyoxylate via Aza Diels−Alder Reaction of Arynes
and KF-Induced Annulation
R. Santhosh Reddy,† Chandraiah Lagishetti,† Shuo Chen, I. N. Chaithanya Kiran, and Yun He*
School of Pharmaceutical Sciences and Innovative Drug Research Centre, Chongqing University, 55 Daxuecheng South Road,
Shapingba, Chongqing 401331, P. R. China
S
* Supporting Information
ABSTRACT: The transition-metal-free multicomponent cou-
pling of arynes, anilines, and ethylglyoxylate, proceeding via an
inverse electron-demand aza Diels−Alder cycloaddition and
N-arylation, has been demonstrated. This protocol allows
rapid access to N-aryl dihydrophenanthridine derivatives in
moderate to high yields at room temperature from readily
available starting materials. In addition, an unprecedented
fluoride induced annulation of ethyl(arylimino)acetates led to the formation of highly functionalized oxoimidazolidine derivatives
in good yields under mild conditions.
Scheme 1. Inverse Electron-Demand Aza Diels−Alder
he construction of nitrogen-containing heterocycles using
arynes as building blocks1 has become increasingly popular
Reaction of Arynes with Electron-Poor 2-Azadienes
T
after the development of the new synthetic method to generate
aryne by Kobayashi et al.2 The electrophilic coupling,3 [4 + 2]
cycloaddition,4 and 1,3-dipolar cycloaddition5 of arynes, in
particular, as opposed to transition-metal-catalyzed reactions,
have produced a number of very useful biologically active
heterocycles through multiple bond-forming transformations
(MBFTs). However, the use of an aza Diels−Alder reaction
(ADAR) of arynes has received only scant attention and remains
largely unexplored,6 presumably due to the aryne reactivity
toward imines and 2-aza-dienes to provide the corresponding
benzazetidines via [2 + 2] cycloaddition.7 Recently, (i) Wang and
co-workers reported a three-component ADAR for the
preparation of phenanthridines from electron-enriched anilines,
benzaldehyde derivatives, and benzenediazonium-2-carboxyla-
te;6b (ii) Coquerel/Rodriguez co-workers reported the ADAR
for the synthesis of functionalized isoquinolines and N-arylated
1,2-dihydroisoquinoline using electron-rich N-heteroaryl imines
and arynes with an additional oxidant or excess of aryne.6c,d All
these approaches mainly rely on the normal electron-demand
ADAR of highly electron-rich 2-aza-dienes with variation on
benzaldehyde derivatives (Supporting Information, Scheme 1).
In view of its efficiency in the rapid construction of synthetically
important structural motifs, further exploration of new
procedures with alternative substrates would be particularly
attractive. Moreover, an approach to access N-heterocycles via
this highly efficient ADAR with electron-poor 2-aza-dienes and
arynes in an inverse electron-demand (IED) manner has yet to
be established. As part of our continuing efforts on the transition-
metal-free synthesis of valuable bioactive scaffolds using arynes,8
we sought to address this problem by employing ethyl-
(arylimino)acetates as an electron-poor dienes (Scheme 1).9
The presence of a carboxylate group not only increases the
reactivity of the aldimine (2-aza-diene) but also opens a new
entry to the preparation of polycyclic heterocyclic compounds,
such as 5,6-dihydrophenanthridine derivatives containing a
carboxyl group in position 6. While the dihydrophenanthridine
derivatives are present in the skeleton of a wide number of
biologically active compounds,10 only a few synthetic methods
for this class of compounds have been reported in the literature.
Furthermore, the reported methods generally suffer from low
yields due to the limited functional group tolerance and the
undesired in situ oxidation.11
Herein we report a mild and novel multicomponent cascade
reaction initiated by an inverse electron-demand ADAR of
arynes, ethylglyoxylate, and anilines for the construction of 5,6-
dihydrophenanthridine derivatives with a broad substrate scope.
In addition, an unexpected annulation of ethyl(arylimino)-
acetates leading to the formation of highly functionalized
oxoimidazolidine derivatives in good yields under mild
conditions from simple and easily available starting materials is
also reported.
Our study was initiated with the optimization of reaction
conditions for the multicomponent inverse electron-demand
Received: July 25, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02186
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Organic Letters
Letter
ADAR of arynes. In an initial experiment, treatment of anisidine
(1a) and ethylglyoxylate (2) with aryne (1.2 equiv) generated in
situ from the 2-(trimethylsilyl)aryl triflate (3a) using KF and
MeCN as solvent resulted in the formation of N-arylated Diels−
Alder adduct 5a in 42% yield along with imine 6a (38%).
Surprisingly, the corresponding 5,6-dihydrophenanthridine 4a
and phenanthridines were not observed under this reaction
condition. Notably, when the reaction was performed using
Coquerel/Rodriguez aromatization reaction conditions (KF/18-
crown-6, MnO2 (10 equiv)) with aryne (1.2 equiv) at rt or 70 °C,
it furnished only the N-aryl- dihydrophenanthridine 5a. The
aromatization product was not observed as expected for a
“push−pull” system,6d which could be attributed to the fact that
the carboxylate group at position 6 of 5,6-dihydrophenanthridine
4a favors N-arylation over aromatization by increasing the
nucleophilicity of nitrogen through hydrogen bonding. The
effects of solvent, fluoride source, temperature, additive, and
stoichiometry were systematically studied (Supporting Informa-
tion, Table S1). Interestingly, when the reaction was performed
using 3.0 equiv of 1a and 5.0 equiv of KF/18-crown-6, N-aryl-
dihydrophenanthridine 5a was formed in an improved yield of
88% at rt in 3 h (entry 7, Table 1). Other fluoride sources
Scheme 2. Substrate Scope of the Inverse Electron-Demand
ADAR: Variation of Anilines
a
a
Reaction conditions: 1 (0.20 mmol), 2 (0.24 mmol), MgSO4 (3.0
equiv), MeCN:PhMe (1:1) (2.0 mL), 1 h then KF (1.0 mmol), 18-
crown-6 (1.0 mmol), 3a (0.6 mmol), 25 °C, 2 h. Yields of the isolated
b
1
a
products with respect to 1. Regioisomeric ratio determined by H
Table 1. Optimization of the ADAR Conditions
NMR analysis.
Scheme 3. Substrate Scope of the Inverse Electron-Demand
a
ADAR: Variation of Aryne Precursors
b
yield [%]
3a
F−
[equiv]
additive
[equiv]
entry [equiv]
solvent
5a
6a
c
c
1
2
3
4
1.2
1.2
1.2
1.2
KF (2.5) 18-c-6 (2.5) MeCN
CsF (2.5) MeCN
KF (2.5) 18-c-6 (2.5) THF
42
36
38
38
38 (9)
40 (9)
−
d
c
40 (12)
c
KF
(2.5)
18-c-6 (2.5) MeCN
36 (9)
de
,
f
c
c
5
6
7
1.2
2.0
3.0
KF (2.5) 18-c-6 (2.5) MeCN
40
48
88
41 (12)
32 (12)
e
KF (4.0) 18-c-6 (4.0) THF
c
KF (5.0) 18-c-6 (5.0) MeCN/
4 (4)
g
PhMe
a
Standard conditions: 1a (0.20 mmol), 2 (0.24 mmol), MgSO4 (3.0
b
equiv), solvent (2.0 mL), 1 h then fluoride source, 3a, rt, 2 h. Isolated
yields. (4-OMeC6H4)PhNH. MnO2 (10.0 equiv). Reaction temper-
ature 70 °C. O2 (balloon). MeCN/PhMe (1:1).
c
d
e
f
g
a
Reaction conditions: 1a (0.20 mmol), 2 (0.24 mmol), MgSO4 (3.0
including CsF, TBAT, and TBAF did not improve the yield. A
brief screening of the reaction media proved that MeCN/PhMe
(1:1) was the best choice with respect to yields. It is noteworthy
to mention that isolated ethyl (arylimino) acetate 6a also reacted
well with aryne 3a under the same ADAR conditions to furnish
the desired product without a significant difference in yield or
reaction time.
The generality of this reaction was subsequently explored by
varying the electronic and steric properties of both anilines 1 and
arynes 3 under the optimized condition (Schemes 2 and 3). As
shown in Scheme 2 the multicomponent ADAR of benzyne 3a,
substituted anilines 1, and ethylglyoxylate (2) proceeded
effectively at rt furnishing dihydrophenanthridine carboxylates
derivatives 5 with moderate to excellent yields. The substituted
anilines containing electron-neutral and -donating groups such as
equiv), MeCN/PhMe (1:1) (2.0 mL), 1 h then KF (1.0 mmol), 18-
crown-6 (1.0 mmol), 3 (0.6 mmol), 25 °C, 2 h. Yields of the isolated
b
1
products with respect to 1a. Regioisomeric ratio determined by H
NMR analysis.
ethoxy, methyl, ethyl, and isopropyl at the para position afforded
the desired products (5b−f) in high yields. Interestingly, the
presence of the 2-methyl and 2,4-dimethyl substituent in the N-
aryl group had no steric effect on the reaction outcome and
provided the desired products 5g−h. Moreover, electron-
withdrawing substituent fluoro and chloro groups at the para
position were also tolerated and provided desired products 5i−j
in moderate yields. We reasoned that the formation of imine and
its stability lowered the overall yield in the case of aniline
containing electron-withdrawing substituents compared to
B
DOI: 10.1021/acs.orglett.6b02186
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
electron-releasing substituents.9a It is noteworthy that the 3,4-
dimethoxy substituent in the N-aryl group reacted regioselec-
tively and gave only one isomer of the desired product (5k). The
regioselectivity could be explained in terms of the electronic and
steric factors that favor the ADAR from the less hindered
position. However, the 3,4-dimethyl substituent in the N-aryl
group resulted in the formation of an inseparable mixture of
regioisomers 5l/5l′ in a 1.2:1 ratio with a 78% combined yield.
To further understand the scope of this novel multicomponent
ADAR, a wide range of β-trimethylsilyl triflates 3 were then
examined with 1a as the amine source and the results are
summarized in Scheme 3. Interestingly, various electronically
different symmetrical arynes generated from the corresponding
precursors readily underwent multicomponent ADAR with
anisidine (1a) and ethylglyoxylate (2) to form the N-aryl
dihydrophenanthridine carboxylates (5a, m−o) smoothly in
good yields. Moreover, the extended π conjugate naphthalene
and the sterically hindered 2,5-dimethyl derived benzynes also
afforded the desired compounds (5p−q). Most notably,
unsymmetrical 3-methoxybenzyne resulted in the formation of
a 3:1 regioisomeric mixture of products 5r. Furthermore, 4-
methylbenzyne furnished almost equal amounts of all four
possible regioisomeric products (5s) in 82% yield, suggesting
that the steric effect of aryne in this reaction is negligible.
During our studies on the multicomponent ADAR of the
electron-withdrawing substituents, we unexpectedly observed
the formation of oxoimidazolidine derivatives 7 in moderate
yields under identical conditions (KF/18-crown-6, MeCN, rt) in
the absence of benzyne.
The scope and limitation of this unexpected annulation were
explored by subjecting various substituted anilines (1) to the
optimized reaction conditions. Compared with the result
obtained for the reaction between aniline and ethylglyoxylate
(Table 2, entry 1), reactions of anilines with para substituted
Table 2. Substrate Scope of Fluoride Induced Annulation of
a
Ethyl(arylimino)acetates 6
b
entry
Ar
product
yield [%]
1
C6H5
7a
7b
7c
7d
7e
7f
76
78
76
78
78
75
2
p-FC6H4
3
p-BrC6H4
4
p-ClC6H4
5
p-CF3C6H4
p-CO2EtC6H4
p-MeC6H4
p-MeOC6H4
m-FC6H4
6
c
7
7g
7h
7i
49
8
0
83
80
81
9
10
11
m-BrC6H4
m-CF3C6H4
7j
7k
a
Reaction conditions: 1a (0.20 mmol), 2 (0.12 mmol), MgSO4 (5.0
equiv), PhMe (2.0 mL), 1 h then filtered and filtrate was evaporated,
followed by 4 Å MS (300 mg/mmol of 1), KF (0.3 mmol), 18-crown-6
b
(0.3 mmol), MeCN (2 mL), 25 °C, 2 h. Yields of the isolated
Inspired by this result, to identify the optimal conditions for
the formation of the desired product 7a, we investigated the
effect of aniline and ethylglyoxylate stoichiometry, fluoride salts,
bases, solvents, reaction temperature, and time on the reaction
yield (Supporting Information, Table S2). Based on these results,
we found that the sequential reactions with 1.0 equiv of aniline
and 0.6 equiv of ethylglyoxylate for the in situ imine formation,
followed by treatment with KF (1.5 equiv)/18-crown-6 (1.5
equiv) in MeCN, provided the functionalized oxoimidazolidine
7a in 76% yield (Scheme 4).12 Imidazolidin-4-one derivatives are
c
products with respect to 1. 5 h.
electron-deficient groups such as F, Br, Cl, CF3, and CO2Et
showed similar results with respect to yield (Table 2, entries 2−
6). Notably, electron-rich p-toluidine gave the desired product in
moderate yield with 22% of recovered imine (Table 2, entry 7).
Unfortunately, more electron-rich p-anisidine did not react
under the optimized conditions (Table 2, entry 8). Gratifyingly,
anilines bearing meta substituted electron-deficient groups such
as F, Br, and CF3 resulted in the formation of desired products
with a significant increase in the yields (Table 2, entries 9−11).
Based on the literature precedence and our optimization
results, a probable mechanism for the fluoride induced
annulation reaction is outlined in Scheme 5. Reaction between
ethylglyoxylate (2) and an excess of aniline (1) generates
diamine adduct B which exists in equilibrium with imine and
aniline (A).9a The presence of an electron-withdrawing group on
the aryl nucleus of adduct B facilitates14a a proton abstraction by
fluoride salt14b to afford a nitrogen anion, which subsequently
undergoes intermolecular formal [3 + 2] annulation (C) with
Scheme 4. Fluoride Induced Synthesis of Functionalized
Oxoimidazolidines 7a
found to have a large spectrum of biological activities such as
antibacterial,13a cell inhibitor,13b antimalarial agents,13c anti-
proliferative,13d and anticancer.13e Despite a large and diverse set
of synthetic methods reported for imidazole ring structures,
methods for the synthesis of functionalized oxoimidazolidine
analogs are yet surprisingly limited. In 2008, Shi and co-workers
reported phosphine-catalyzed annulation of ethyl(arylimino)-
acetates to polysubstituted oxoimidazolidine derivatives using
methyl vinyl ketone (MVK).12 However, rapid synthesis of a
functionalized oxoimidazolidine framework under milder
reaction conditions are still desirable. In this context, we further
explored this fluoride induced rapid synthesis of functionalized
oxoimidazolidines from anilines 1 and ethylglyoxylate (2) under
mild conditions.
Scheme 5. Proposed Mechanism of the Cascade Annulation
C
DOI: 10.1021/acs.orglett.6b02186
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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nucleophilic nitrogen first adds across the C−N double bond of
imine, followed by a subsequent intramolecular nucleophilic
attack onto the ester carbonyl group to produce the desired
product 7.15
In conclusion, we have developed a multicomponent coupling
of arynes, anilines, and ethylglyoxylate, allowing the rapid
synthesis of N-aryl dihydrophenanthridine derivatives at rt. The
reaction between easily accessible electron-poor 2-aza-dienes
and arynes proceeds via an inverse electron-demand aza Diels−
Alder cycloaddition reaction and subsequent N-arylation in a
single step with two C−C and two C−N bond formations. In
addition, we have also developed a fluoride induced novel
method for the synthesis of polysubstituted oxoimidazolidine
derivatives from simple and easily available starting materials.
Further studies in related inverse electron-demand reactions of
arynes are ongoing in our laboratory and will be reported in due
course.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02186.
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■
S
Experimental procedures and spectral data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yun.he@cqu.edu.cn.
Author Contributions
†R.S.R. and C.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Science Foundation of China (Nos. 21572027 and 21372267)
and the Postdoctoral Research Grant (2014M562278).
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DOI: 10.1021/acs.orglett.6b02186
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