Cascade nitration/cyclization of 1,7-enynes with tBuONO and H2O: One-pot self-assembly of pyrrolo[4,3,2-de]quinolinones

Article abstract of DOI:10.1002/anie.201404192

Here we describe the one-pot construction of the pyrrolo[4,3,2-de] quinolinone scaffold by a cascade nitration/cyclization sequence of 1,7-enynes with tBuONO and H₂O. The cascade proceeds through alkene nitration, 1,7-enyne 6-exo-trig cyclization, C-H nitrations, and redox cyclization, and exhibits excellent functional group tolerance. The mechanism was investigated using in situ high-resolution mass spectrometry (HR-MS).

Full text of DOI:10.1002/anie.201404192

Angewandte
Chemie
DOI: 10.1002/anie.201404192
Cascade Reaction
Cascade Nitration/Cyclization of 1,7-Enynes with tBuONO and H2O:
One-Pot Self-Assembly of Pyrrolo[4,3,2-de]quinolinones**
Yu Liu, Jia-Ling Zhang, Ren-Jie Song, Peng-Cheng Qian, and Jin-Heng Li*
Abstract: Here we describe the one-pot construction of the
pyrrolo[4,3,2-de]quinolinone scaffold by a cascade nitration/
cyclization sequence of 1,7-enynes with tBuONO and H2O.
The cascade proceeds through alkene nitration, 1,7-enyne 6-
of the pyrrole ring,[3] or by construction of the indole scaffold
followed by the piperidine ring.[4] However, in all cases the
construction of the pyrrolo[4,3,2-de]quinolinone scaffold
requires several steps with rather low overall yields.[3–5]
Thus, it would be highly desirable to exploit new routes and
especially one-pot strategies toward these structures.
Cascade reactions have proven to be a powerful shortcut
for the assembly of complex ring systems in organic syn-
thesis.[6] Among these processes, the cyclization of 1,n-enynes
is a particularly effective and atom-economical step for
constructing the ring systems.[6,7] Herein, we report an
unprecedented cascade nitration/cyclization of N-(2-(ethy-
nyl)aryl)acrylamides (1) with tBuONO and H2O for the one-
pot synthesis of pyrrolo[4,3,2-de]quinolinone architectures
under metal-free conditions (Scheme 1);[8] this is realized
through a cascade of alkene nitration, 1,7-enyne 6-exo-trig
À
exo-trig cyclization, C H nitrations, and redox cyclization, and
exhibits excellent functional group tolerance. The mechanism
was investigated using in situ high-resolution mass spectrom-
etry (HR-MS).
P
yrrolo[4,3,2-de]quinolinone represents an essential part of
the scaffold of numerous natural compounds and pharma-
ceuticals with remarkable biological and medicinal properties
(Figure 1) and is widely used as a valuable functional
intermediate.[1,2] For these reasons, considerable efforts have
been devoted to the development of new and simple methods
for the total synthesis of pyrrolo[4,3,2-de]quinolinone and its
derivatives.[3–5] Generally, these methods proceed either by
construction of the quinoline system followed by elaboration
À
cyclization, C H nitrations, and redox cyclization, represent-
ing the first example of a one-pot assembly of the pyrrolo-
[4,3,2-de]quinolinone scaffold.
Scheme 1. Cascade nitration/cyclization of 1,7-enynes.
We commenced our studies by exploring the reaction
between N-methyl-N-(2-(phenylethynyl)phenyl)methacryl-
amide (1a) with tBuONO to optimize the reaction conditions
(Table 1). After extensive screening of different reaction
parameters, the desired pyrrolo[4,3,2-de]quinolinone 2a was
formed with the highest yield from the reaction of 1,7-enyne
1a with 4 equiv tBuONO in dimethyl sulfoxide (DMSO) at
508C for 24 h (entry 1). Encouraged by these results, we
examined the effect of the reaction temperature (entries 1–3):
whereas at a reaction temperature of 508C product 2a was
isolated in 78% yield (entry 1), the yield decreased to 63%
when the temperature was increased to 808C (entry 2), and to
45% at a reaction temperature of 308C (entry 3). It has
been reported that 2,2,6,6-tetramethylpiperidin-1-yl)oxy
(TEMPO), a radical initiator, proved beneficial in some
nitration reactions using tBuONO.[8] However, the presence
of TEMPO suppressed the current reaction (entries 4 and 5):
the yield of product 2a decreased from 78% to 67% with
20 mol% TEMPO and to 53% with 100 mol% TEMPO.
Notably, a good yield was even achieved under N2 atmosphere
when DMSO was purged with N2 (entry 6). This suggests that
Figure 1. Examples of important pyrrolo[4,3,2-de]quinolinones.
[*] Y. Liu, J.-L. Zhang, Dr. R.-J. Song, P.-C. Qian, Prof. Dr. J.-H. Li
State Key Laboratory of Chemo/Biosensing and Chemometrics
College of Chemistry and Chemical Engineering
Hunan University, Changsha 410082 (China)
E-mail: jhli@hnu.edu.cn
Prof. Dr. J.-H. Li
State Key Laboratory of Applied Organic Chemistry Lanzhou
University
Lanzhou 730000 (China)
[**] We thank the Natural Science Foundation of China (No. 21172060),
Specialized Research Fund for the Doctoral Program of Higher
Education (No. 20120161110041), and Hunan Provincial Natural
Science Foundation of China (No. 13JJ2018) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404192.
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Table 1: Screening of the reaction conditions.[a]
Table 2: Nitration/cyclization of 1,7-enynes (1) with tBuONO.[a]
Entry
Variation from the standard conditions
Yield [%][b]
1
2
3
4
none
at 808C
at 308C
78
63
45
67
53
75
19
31
26
16
76
65
76
TEMPO (20 mol%)
TEMPO (100 mol%)
DMSO purged with N2
MeCN instead of DMSO
DMF instead of DMSO
CH2ClCH2Cl instead of DMSO
toluene instead of DMSO
0.9 mmol H2O in anhydrous DMSO
2.8 mmol H2O in anhydrous DMSO
none for 72 h
5
6[c]
7
8
9
10
11
12
13[d]
[a] Reaction conditions: 1a (0.2 mmol), tBuONO (4 equiv), and DMSO
(3 mL) at 508C under air atmosphere for 24 h. DMSO contains about
0.5% w/w H2O (about 0.92 mmol H2O in 3 mL DMSO). [b] Yield of the
isolated product. [c] Under N2 atmosphere. [d] 1a (1 g, 3.64 mmol).
the presence of O2 only slightly affects the reaction in terms of
yield (entry 1 versus entry 6). A series of solvents, including
MeCN, DMF, CH2ClCH2Cl, and toluene, were also tested,
and the results showed that they were less effective than
DMSO (entry 1 versus entries 7–10). We found that the
amount of H2O had a fundamental influence on the reaction
(entry 1 versus entries 11 and 12): a good yield was still
achieved at a loading of 0.9 mmol H2O using anhydrous
DMSO, but the yield decreased from 78% to 65% when the
amount of H2O was increased to 2.8 mmol. It is noteworthy
that the reaction could also be performed in good yield at
a scale of 1 g (3.64 mmol) 1,7-enyne 1a (entry 13).
[a] Reaction conditions: 1 (0.2 mmol), tBuONO (4 equiv), and DMSO
(3 mL) at 508C under air atmosphere for 24 h. [b] More than 95% of
substrate 1aa was recovered.
With the optimized reaction conditions in hand, a variety
of 1,7-enynes 1 were reacted with tBuONO to investigate the
scope of this nitrative cyclization protocol (Table 2). Initially,
the effect of substituents on the nitrogen atom was examined
1u and 1v were also suitable for this nitrative cyclization
transformation, giving products 2u and 2v in 60% and 75%
yield, respectively. Unfortunately, aliphatic alkynes were not
viable for this reaction.
À
(products 2b–g). Whereas substrate 1b with a free N H bond
Several substituents on the aromatic ring of the N-phenyl
moiety, including Me and Cl, were also compatible with the
optimized reaction conditions (products 2w–aa). Treatment
of substrates 1w or 1x, bearing a Me or a Cl group at the 5-
position, with tBuONO and H2O afforded the corresponding
products 2w and 2x in high yield. The bulky substrate 1y with
a Me group at the 5-position was successfully transformed
into product 2y in 70% yield. Substrate 1z having a Me group
on the 4-position of the N-phenyl moiety gave two regiose-
lective nitration isomers 2z in 41% yield; however, substrate
1aa with a NO2 group showed no reactivity (product 2aa).
Gratifyingly, this nitrative cyclization protocol could also be
applied to substrates 1ab and 1ac with a Bn or a Ph group at
the 2-position of the acrylamide moiety, providing products
2ab and 2ac in 88% and 75% yield, respectively.
was not viable for this nitrative cyclization reaction (product
2b), substrates 1c–g, having an allyl or a substituted benzyl
group on the nitrogen atom, were successfully converted into
the corresponding pyrrolo[4,3,2-de]quinolinones 2c–g in good
yields. The results showed that several substituents, including
MeO, Me, F, Cl, Br, CN, COCH(CH3)2, and CH2OMe, on the
aryl ring at the alkyne were well-tolerated under the
optimized conditions, and the substituents at the ortho-,
meta-, or para-position have no distinct influence on the
reaction. For example, substrates 1h–j with a MeO group
were transformed into products 2h–j with similar yields.
Importantly, halogen functional groups, F, Cl, and Br, were
compatible with the optimized reaction conditions, thereby
enabling subsequent modifications at the halogenated posi-
tions (products 2e, 2l–n, 2r, and 2t). Application of substrates
1o and 1p with an electron-withdrawing group, delivers the
desired products 2o and 2p in good yields. Heteroaryl alkynes
As can be seen in Table 1, the amount of H2O had an
effect on the reaction, suggesting that the oxygen atoms in
two NO2 groups may be from H2O. To verify this, we
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conducted an 18O-labeling experiment with H218O [Eq. (1)].[9]
The results showed that a mixture of product [18O1]-2a
containing one oxygen atom and and product [18O2]-2a
containing two oxygen atoms was observed with a 1:1 ratio.
Within the redox process, the oxygen atom of the nitro group
on the aryl ring can be derived from tBuONO and H2O, which
is the reason why this reaction can be performed without
addition of O2 (entry 6 versus entry 1; Table 1).
In summary, we have developed a novel metal-free
cascade reaction for the synthesis of pyrrolo[4,3,2-de]quino-
linone derivatives from readily available N-(2-(ethynyl)aryl)-
acrylamides and tBuONO in good yields. In this cascade,
which proceeds through alkene nitration, 1,7-enyne 6-exo-trig
À
cyclization, C H nitrations, and redox cyclization, two nitro
groups and an amine group are incorporated into the product.
This method provides a valuable one-pot shortcut for the
assembly of pyrrolo[4,3,2-de]-quinolinone derivatives and
exhibits a broad enyne scope with excellent functional
group tolerance. Detailed studies on the mechanism and the
extension of this cascade method are currently underway in
our laboratory.
This suggests that nitration at the 4-position of the N-phenyl
moiety is crucial for the described reaction, and that H2O is
not the sole source of the oxygen atoms of the nitro groups.
To understand the mechanism, three radical inhibitors
(4 equiv), TEMPO, hydroquinone, and butylhydroxytoluene
(BHT), were added to the reaction, leading to its inhibition.
With PhNO2 as the radical inhibitor, the yield of product 2a
decreased from 78% to 69%.[10] This implies that the reaction
proceeds via a radical intermediate, which is also supported
by the intermolecular kinetic isotope effect experiment (kH/
kD = 1).[9]
Proposed reaction mechanisms based on the above
results[9] and previous reports[6–8,11,12] are shown in Scheme 2.
Initially, addition of NO2, which is generated in situ from
tBuONO,[8,11,12] to the carbon–carbon double bond of the 1,7-
enyne 1a gives alkyl radical intermediate A, which cyclizes to
form intermediate B. The reaction of intermediate B with NO
or NO2 affords the corresponding intermediates C and C’,
which was supported by HRMS analysis.[9] Cationic inter-
mediates D and D’ are formed by electrophilic addition of the
Received: April 10, 2014
Published online: && &&, &&&&
Keywords: cascade reactions · cyclization · enynes · nitration ·
.
nitrogen heterocycles
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Cascade Reaction
Y. Liu, J.-L. Zhang, R.-J. Song, P.-C. Qian,
J.-H. Li*
&&&& — &&&&
Cascade Nitration/Cyclization of 1,7-
Enynes with tBuONO and H2O: One-Pot
Self-Assembly of Pyrrolo[4,3,2-
de]quinolinones
Nitration cascade: The pyrrolo[4,3,2-de]-
quinolinone scaffold was synthesized by
a metal-free reaction of N-(2-(ethynyl)-
aryl)acrylamides, tert-butyl nitrite and
H2O. This cascade reaction is triggered by
alkene nitration followed by 1,7-enyne 6-
À
exo-trig cyclization, C H nitrations, and
redox cyclization and forms the product
in good yields.
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