Expeditious Synthesis of Multisubstituted Quinolinone Derivatives Based on Ring Recombination Strategy

Article abstract of DOI:10.1021/acs.orglett.9b04224

We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various quinolinone derivatives in good to excellent chemical yields (up to 98%).

Full text of DOI:10.1021/acs.orglett.9b04224

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Expeditious Synthesis of Multisubstituted Quinolinone Derivatives
Based on Ring Recombination Strategy
Kazuma Yokoo and Keiji Mori*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16
Nakacho, Koganei, Tokyo 184-8588, Japan
S
* Supporting Information
ABSTRACT: We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination
strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various
quinolinone derivatives in good to excellent chemical yields (up to 98%).
arbon−hydrogen bond functionalization is a powerful
Scheme 2. Two Strategies for Further Transformation from
the Internal Redox Adduct
C
tool in modern synthetic organic chemistry. Because of
its utility from the perspectives of atom economy and step
economy, many organic chemists have devoted time and effort
to the development of novel reactions.1 Recently, we have also
been interested in the development of a novel C−H bond
functionalization reaction via an intramolecular hydride shift/
cyclization process, as described in Scheme 1.2−7 This reaction
Scheme 1. C(sp3)−H bond Functionalization by Hydride
Shift/Cyclization Process (Internal Redox Process)
system has four features: (1) functionalization of an inert
C(sp3)−H bond, (2) without use of external oxidants
(reducing the amount of wastes), (3) simple Lewis and/or
Brønsted acids catalyzed reaction (without transition metal
catalyst), and (4) construction of a polycyclic framework. The
last point (point 4) is worth emphasizing. Various useful
heterocycles, such as tetrahydroquinolines,6 tetrahydroisoqui-
nolines,3e,g,6u quinazolines,3a,7e tetralins,3c,d,h,7m and indoles,3l
could be elegantly constructed.
Although this reaction system has high synthetic potential, it
has a major drawback as well: narrowness of the synthetic
transformation from the cyclized adduct (internal redox
adduct). In this context, we recently reported an effective
strategy, as shown in the upper part of Scheme 2. The key
point is the dual role of the heteroatom as (1) a driving force
of the hydride shift and (2) an activator of the resulting
cyclized adduct by acting as a leaving group, which enables the
highly diastereoselective synthesis of CF3-substituted spiroiso-
chromans.3m
Received: November 25, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04224
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
As an alternative strategy for achieving the goal, we focused
on the ring recombination of the cyclized adduct. The strategy
is described in the lower part of Scheme 2. Tetrahydroquino-
lines are selected as the suitable internal redox adduct due to
the presence of a relatively labile C−N bond at β-position to
the dicarbonyl moiety. We envisioned that if the target C−N
bond could be cleaved under an acid catalyst and a subsequent
cyclization reaction of the resulting aniline moiety and ester
portion occurred, another useful skeleton, quinolinone
derivatives, would be obtained. There are two challenges,
however: (1) the selective cleavage of the targeted C−N bond
and (2) preferential bond formation between the resulting
amine portion and ester portion over bond reformation.
Herein we wish to report the realization of this concept. The
reaction consists of seven transformations (condensation/
hydride shift/cyclization/C−N bond cleavage/cyclization/
decarboxylation/isomerization). Subjecting a mixture of
aldehyde 3 and Meldrum’s acid to neutral conditions (simple
heating), acidic conditions (1 mol % of BF3·OEt2), and basic
conditions (1.0 equiv of DBU) in succession in the same
reaction vessel enabled the above-described seven trans-
formations to afford various quinolinone derivatives 4 in
good to excellent chemical yields (Scheme 3).
Table 1. Examination of Reaction Conditions
b
yield (%)
entry
catalyst
4a
6a (E/Z)
5a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Sc(OTf)3
Yb(OTf)3
Gd(OTf)3
Mg(OTf)2
Sn(OTf)2
Zn(OTf)2
TiCl4
SnCl4
BF3·OEt2
TfOH
12
13
9
80 (24/76)
85 (19/81)
74 (14/86)
−
−
−
−
99
−
−
−
−
−
33
34
29
21
7
27
23
10
10
10
17
26
24
24
45 (0/100)
48 (0/100)
66 (38/62)
51 (41/59)
86 (17/83)
56 (34/66)
55 (38/62)
88 (28/72)
89 (28/72)
77 (34/66)
73 (21/79)
63 (41/59)
−
−
12
20
−
−
−
−
−
95
95
Tf2NH
c
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
d
de
,
df
,
dg
,
Scheme 3. One-Pot Synthesis of Quinolinone Derivatives
via Internal Redox Reaction/Ring Recombination/
Isomerization Sequence
dh
,
di
,
−
a
Unless otherwise noted, all reactions were conducted with 0.10
mmol of 5a in the presence of 5 mol % of catalyst in ClCH2CH2Cl
b
c
(1.0 mL) at refluxing temperature. Isolated yield. 3 mol % of acid
d
e
f
g
catalyst. 1 mol % of acid catalyst. In benzene. In toluene. In
h
i
xylene. In CH2Cl2. Dimethyl malonate derivative was used as the
starting material.
conditions (entries 12−17). Gratifyingly, the catalyst loading
could be reduced to 1 mol % without sacrificing chemical
yields (89%, entry 13). Examination of reaction solvents
suggested that most of the solvents could be employed in this
reaction, with ClCH2CH2Cl being the best. As expected, the
high reactivity of Meldrum’s acid moiety was important to
promote the desired reaction: a less reactive substrate with a
dimethyl malonate moiety did not afford the desired
quinolinone derivative, and complete recovery of starting
material was observed (entry 18).
Thus, we were able to develop an effective catalytic
transformation from the internal redox adduct. However, one
problem remained unsolved. Compounds 4a and 6a were
obtained as an inseparable mixture in all cases, which is the
major drawback of the present method for practical use.
Nevertheless, we were pleased to find that treatment of the
mixture of 4a and 6a in CH3CN with a stoichiometric amount
of DBU (1.0 equiv) induced the complete double-bond
isomerization in 6a to afford quinolinone 4a in excellent
chemical yield (95%).8 Importantly, this isomerization reaction
could also be conducted in ClCH2CH2Cl without sacrificing
the chemical yield (91%). Furthermore, not only the
condensation of aldehyde 3a and Meldrum’s acid but also
the [1,5]-hydride shift/cyclization process proceeded smoothly
by only heating a mixture of them in ClCH2CH2Cl. The
smooth progress of the three reactions [(1) 3a to 5a, (2) 5a to
4a and 6a, and (3) a mixture of 4a and 6a to 4a] in the same
solvent system provided motivation to conduct a one-pot
reaction. Refluxing a mixture of 3a and Meldrum’s acid in
Details of the screening for the reaction conditions are listed
in Table 1. We envisioned tetrahydroquinoline derivative 5a
with Meldrum’s acid moiety as the suitable substrate because
of its high susceptibility to nucleophilic attack of Meldrum’s
acid. At the outset, a solution of 5a in ClCH2CH2Cl was
treated with 5 mol % of Sc(OTf)3, which worked as an
effective catalyst in most of the internal redox reactions we had
developed.3 Gratifyingly, the desired ring recombination
process proceeded to give desired 4a in 12% chemical yield
(entry 1). In this case, two structural isomers (E)- and (Z)-6a
with an exo-double bond were obtained as the major product
(80% chemical yield) and their E/Z ratio was 24/76. Because
4a and 6a could not be separated by silica gel column
1
chromatography, their chemical yields were determined by H
NMR measurement. Examination of the acid catalysts
indicated that most of the acid catalysts were effective for
the desired process, although the ratios of 4a to 6a were
slightly different (entries 2−11). Various metal triflates, such as
Yb(OTf)3, Gd(OTf)3, Sn(OTf)2, and Zn(OTf)2, afforded
adducts 4a and 6a in good to excellent chemical yields (78−
98% combined chemical yields, entries 2−6). Excellent
conversion was also observed with commonly used strong
Lewis acids (TiCl4, SnCl4, BF3·OEt2, entries 7−9) and
Brønsted acids (TfOH and Tf2NH, entries 10 and 11).
Based on the above results, we set BF3·OEt2 as the optimal
acid catalyst because of low cost and ease of handling, and
conducted a more detailed screening for the reaction
B
DOI: 10.1021/acs.orglett.9b04224
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ClCH2CH2Cl for 12 h and additional reflux for 12 h after the
addition of 1 mol % of BF3·OEt2 followed by 1 h of stirring at
room temperature in the presence of DBU afforded
quinolinone 4a in excellent chemical yield (89%). Importantly,
this one-pot reaction could be conducted in 4.74 mmol scale
(81%), which clearly indicates the high practicability of the
present method (Scheme 4).
possessing various types of benzyl group participated in the
reaction, and corresponding adducts 4a−e were obtained in
good to excellent chemical yields (82−90%). N,N-Dibenzyl-
amine derivative 3f also afforded desired adduct 4f in excellent
chemical yield (89%). On the other hand, the chemical yield of
product 4g decreased to only 29% when N,N-diethylamine
substrate 3g was employed.
In sharp contrast to the limitation of the substituents on the
amine moiety, the substituent on the mother aromatic ring
(R1) was almost negligible (an amine group was fixed to the
N,N-dibenzyl amine group because of its easy preparation).
Various quinolinone derivatives 4h−j and tricyclic analogue 4k
were obtained in good to excellent chemical yields (89−98%).
There are two possible pathways for the formation of 4/6
from 5 (Figure 2). One possible pathway (path I) is the
Scheme 4. Toward a One-Pot Reaction
Figure 2. Two possible pathways for the formation of 4/6 from 5.
proposed mechanism described in Scheme 2, in which acid-
promoted C−N bond cleavage is involved as the key step.
Subsequent intamolecular cyclization followed by decarbox-
ylation affords 4/6. Ring opening initiated by the nucleophilic
addition of a nucleophile (H2O is most likely) followed by the
intramolecular amide formation (cyclization) is another
possible pathway (path II).
With the optimized one-pot protocol, the substrate scope of
this quinolinone synthesis was examined (Figure 1). Initially,
the substituent effect on the nitrogen atom was surveyed,
which suggested that employment of a benzyl group was
important to achieve good chemical yields. Substrates 3a−e
Additional experiments imply that path I would be a more
reliable pathway (Figure 3). Heating 5a in water-containing
reaction medium (H2O−dioxane system) in the absence of
BF3·OEt2 gave not quinolinone 4a/6a but simple lactam 79 in
moderate chemical yield (55%, eq 1). In addition, the reaction
in the presence of a dehydrating agent (MS5 Å) resulted in
Figure 1. Substrate scope.
Figure 3. Additional experiments for clarifying reaction mechanisms.
C
DOI: 10.1021/acs.orglett.9b04224
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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almost the same level of conversion as the optimized reaction
conditions (74%, eq 2. cf. 99%, entry 13 in Table 1). These
results would rule out the nucleophilic addition initiated
reaction pathway. The possibility of the C−N bond cleavage
pathway was further supported by a cation-trapping experi-
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of BF3·OEt2 and 5 equiv of NaBH4, saturated product 8 was
obtained in 12% chemical yield, accompanied by normal
products 4a and 6a (43%).10
In summary, we have developed an effective synthetic
method to generate multisubstituted quinolinone derivatives
via a C−N bond cleavage induced ring recombination strategy.
This method has two features: (1) accomplishment of seven
transformations including C(sp3)−H bond functionalization
and C−N bond cleavage in one pot, and (2) a simple reaction
procedure (only mixing the substrate, acid, and base).
Additional experiments suggested that the proposed acid-
promoted C−N bond cleavage mechanism would be the most
reliable pathway. Further investigation of the synthesis of more
complex cyclic structures starting from internal redox adducts
is underway.
(4) For the double C(sp3)−H bond functionalization by sequential
utilization of the internal redox reaction developed by our group, see:
(a) Mori, K.; Kurihara, K.; Yabe, S.; Yamanaka, M.; Akiyama, T. J.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04224.
■
S
Experimental procedures, analytical and spectroscopic
data for new compounds, copies of NMR spectra,
computational details, and Cartesian coordinates (PDF)
(5) Such reactions are classified as the “tert-amino effect.” For
reviews, see: (a) Meth-Cohn, O.; Suschitzky, H. Adv. Heterocycl.
Chem. 1972, 14, 211. (b) Verboom, W.; Reinhoudt, D. N. Recl. Trav.
Chim. Pays-Bas 1990, 109, 311. (c) Meth-Cohn, O. Adv. Heterocycl.
Chem. 1996, 65, 1. (d) Quintela, J. M. Recent Res. Dev. Org. Chem.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: k_mori@cc.tuat.ac.jp
ORCID
́
́
́
́
2003, 7, 259. (e) Matyus, P.; Elias, O.; Tapolcsanyi, P.; Polonka-
́
́
́
Balint, A.; Halasz-Dajka, B. Synthesis 2006, 2006, 2625.
(6) For selected examples of the internal redox reactions, see:
(a) Pastine, S. J.; McQuaid, K. M.; Sames, D. J. Am. Chem. Soc. 2005,
127, 12180. (b) Pastine, S. J.; Sames, D. Org. Lett. 2005, 7, 5429.
(c) Zhang, C.; Kanta De, C.; Mal, R.; Seidel, D. J. Am. Chem. Soc.
2008, 130, 416. (d) Che, X.; Zheng, L.; Dang, Q.; Bai, X. Synlett
Keiji Mori: 0000-0002-9878-993X
Notes
The authors declare no competing financial interest.
́
́
2008, 2008, 2373. (e) Polonka-Balint, A.; Saraceno, C.; Ludanyi, K.;
ACKNOWLEDGMENTS
■
́
́
Benyei, A.; Matyus, P. Synlett 2008, 2008, 2846. (f) Zhang, C.;
Murarka, S.; Seidel, D. J. Org. Chem. 2009, 74, 419. (g) McQuaid, K.
M.; Sames, D. J. Am. Chem. Soc. 2009, 131, 402. (h) Shikanai, D.;
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(i) Ruble, J. C.; Hurd, A. R.; Johnson, T. A.; Sherry, D. A.; Barbachyn,
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Am. Chem. Soc. 2009, 131, 3991. (j) Yang, S.; Li, Z.; Jian, X.; He, C.
Angew. Chem., Int. Ed. 2009, 48, 3999. (k) Mahoney, M. J.; Moon, D.
T.; Hollinger, J.; Fillion, E. Tetrahedron Lett. 2009, 50, 4706.
(l) McQuaid, K. M.; Long, J. Z.; Sames, D. Org. Lett. 2009, 11, 2972.
(m) Vadola, P. A.; Sames, D. J. Am. Chem. Soc. 2009, 131, 16525.
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Japan Society for the Promotion
of Science, and by grants from The Naito Foundation.
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finally cyclization and decarboxylation.
(10) Treatment of a mixture of 4a and 6a (4a/6a = 11/89) with 1
mol % BF3·OEt2 and 5 equiv of NaBH4 resulted in the recovery of
SMs (4a and 6a: 86%), which clearly indicates that saturated
compound 8 was produced by trapping the carbocation intermediate.
E
DOI: 10.1021/acs.orglett.9b04224
Org. Lett. XXXX, XXX, XXX−XXX