Metal-Free Synthesis of Polycyclic Quinazolinones Enabled by a (NH4)2S2O8-Promoted Intramolecular Oxidative Cyclization

Article abstract of DOI:10.1002/ejoc.201900469

An efficient metal-free, (NH₄)₂S₂O₈ mediated intramolecular oxidative cyclization for the construction of polycyclic heterocycles was disclosed. A series of polycyclic quinazolinone derivatives with good functional group tolerance were obtained in high yields. The natural products tryptanthrin and rutaecarpine, as well as their derivatives, were easily synthesized by this strategy. A preliminary mechanism study suggested the carbon-centered radical was involved in the catalytic cycle.

Full text of DOI:10.1002/ejoc.201900469

A Journal of
Accepted Article
Title: Metal-free synthesis of polycyclic quinazolinones enabled by a
(NH4)2S2O8-promoted intramolecular oxidative cyclization
Authors: Lijuan Xie, Cong Lu, Dong Jing, Xinrui Ou, and Ke Zheng
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900469
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10.1002/ejoc.201900469
European Journal of Organic Chemistry
COMMUNICATION
Metal-free synthesis of polycyclic quinazolinones enabled by a
(NH4)2S2O8-promoted intramolecular oxidative cyclization
Lijuan Xie, Cong Lu, Dong Jing, Xinrui Ou, and Ke Zheng*
Dedication ((optional))
Abstract:
An
efficient
metal-free,
(NH4)2S2O8 mediated
intramolecular oxidative cyclization for the construction of polycyclic
heterocycles was disclosed. A series of polycyclic quinazolinone
derivatives with good functional group tolerance were obtained in
high yields. The natural products tryptanthrin and rutaecarpine, as
well as their derivatives, were easily synthesized by this strategy. A
preliminary mechanism study suggested the carbon-centered radical
was involved in the catalytic cycle.
Introduction
The quinazolinone is an attractive scaffold that widely exists in
many pharmaceutical relevant natural products and assigned as
privileged structure in medicinal chemistry (Figure 1).[1-2]
Hundreds of natural alkaloids containing the quinazolinone core
have been found and many of them play a significant role in
drug discovery for their versatile biological activities, such as
antibacterial,
antifungal,
antiviral,
antimalarial,
and
antiparkinsonian.[2-3] Over the past decades, the synthesis of
these privileged scaffolds has attracted a lot of interest for both
synthetic and medicinal chemists owing to their important
applications, and a variety of methods have been developed.[4]
The traditional synthetic routes to quinazolinone compounds are
based on 2-aminobenzoic acids or their derivatives as starting
materials.[5] Typically, 2-aminobenzamides react with one-carbon
reagents, such as aldehydes, aromatic alcohols, toluene, tertiary
amines and methyl radical sources, to form the iminium
intermediates, followed by the intramolecular nucleophilic
addition to construct quinazolinone framework is one of the most
common methods (Scheme 1-A1).[6-8] The Ullmann-type
cascade reactions by using 2-halobenzamides or 2-halobenzoic
Scheme 1. Pathways to synthesize quinazolinones.
acids as the starting materials also have been proven highly
efficient for the synthesis of quinazolinones by Fu and other
research groups.[9] Compared to the amine, the formation of
iminium ion from amide is more challenging and still far less
studied due to its delocalization of the nitrogen lone pair to the
carbonyl oxygen of amide.[10a] Recently, the CuII/O2 and I2/DTBP
catalysis were demonstrated as efficient systems for the
intramolecular C-O or C-N cross dehydrogenative coupling via
amide iminium intermediates to synthesize benzoxazinones and
quinazolinones under high temperature (130 oC) by the Maiti
group and the Wang group, respectively (Scheme 1-A2).[10]
Despite significant advances that have been made in this field,
the method for the synthesis of fused polycyclic quinazolinone
derivatives is rare, especially in a metal-free fashion.[11] Herein,
we developed a metal-free, (NH4)2S2O8 mediated C-H/N-H cross
dehydrogenative coupling via amide iminium intermediate under
mild condition, providing an efficient and facile approach to
synthesize a series of polycyclic quinazolinone derivatives (>40
examples), as well as the natural products tryptanthrin,
rutaecarpine and their analogues (Scheme 1B).
Figure 1. Natural products and drug molecules containing quinazolinone.
[*]
Prof. K. Zheng
Key Laboratory of Green Chemistry & Technology, Ministry of
Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China
E-mail: kzheng@scu.edu.cn
Supporting information for this article is given via a link at the end
of the document.
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10.1002/ejoc.201900469
European Journal of Organic Chemistry
COMMUNICATION
Results and Discussion
Table 2. Scope for the synthesis of quinazolinone derivatives. [a]
Initially, the 2-aminobenzamide 1a was selected as the model
substrate for the optimization studies. As shown in Table 1, the
reaction proceeded smoothly when using 2.0 equiv of K2S2O8 as
the oxidant in DMSO at 60 °C, furnished the quinazolinone
product 2a in 30% yield (Table 1, entry 1). The yield was further
improved, when Na2S2O8 or (NH4)2S2O8 was used as oxidant
and the (NH4)2S2O8 gave the best result (77% yield, entries 2-3).
Other oxidants such as DDQ and DTBP were also explored and
no target product was formed (entries 4-5). The solvents had a
great influence on the reaction, and the DMSO had been
indentified as optimal for the reaction (entries 6-10). The yield of
2a decreased to 55% when reducing the amount of (NH4)2S2O8
to 1.0 equiv (entry 11). Finally, we found that the best result was
obtianed by using 4.0 equiv of (NH4)2S2O8 as the oxidant at
60 °C for 2.5 h, furnishing the product 2a in 92% yield (entries
12-13).
Table 1. Optimization of reaction conditions.[a]
Entry
Oxidant
Solvent
Yield (%)[b]
1
2
K2S2O8 (2.0 equiv)
Na2S2O8 (2.0 equiv)
(NH4)2S2O8 (2.0 equiv)
DDQ (2.0 equiv)[c]
DMSO
DMSO
DMSO
DMSO
DMSO
CH3CN
DCE
30
70
77
NR
NR
27
20
NR
11
20
55
86
92
[a] 1 (0.05 mmol), 4.0 equiv (NH4)2S2O8, solvent (1.0 mL), under an air
atmosphere. Isolated yield. [b] 2.0 equiv (NH4)2S2O8.
3
substrates were also tolerable in the reaction, affording the
desired polycyclic quinazolinones 2t and 2u in 54% and 41%
yields, respectively. Notably, the natural product rutaecarpine 2v
and its analogues (2w-2x) were achieved in 47-60% yields
under the optimal conditions, which provided an efficient and
facile approach to construct rutaecarpine analogues. More
significantly, this method can be applied to the synthesis of N-
substituted quinazolinones, which represent new chemical
entities and never been achieved before, demonstrating the
generality of this method (2y-2aa, Table 2). Next, we further
4
5
TBHP (2.0 equiv)[c]
6
(NH4)2S2O8 (2.0 equiv)
(NH4)2S2O8 (2.0 equiv)
(NH4)2S2O8 (2.0 equiv)
(NH4)2S2O8 (2.0 equiv)
(NH4)2S2O8 (2.0 equiv)
(NH4)2S2O8 (1.0 equiv)
(NH4)2S2O8 (3.0 equiv)
(NH4)2S2O8 (4.0 equiv)
7
8
Dioxane
DMF
9
10
11
12
13
Toluene
DMSO
DMSO
DMSO
Table 3. Scope for the synthesis of quinazolinone derivatives. [a]
[a] 1a (0.05 mmol), solvent (1.0 mL), under an air atmosphere. [b] Isolated
yield. [c] DDQ: 2,3-Dichloro-5,6-Dicyano-1,4-Benzoquinone, TBHP: Tert-
Butyl Hydroperoxide. NR = no reaction.
Based on the optimal conditions, the substrates scope of
the reaction was investigated. As shown in Table 2, this method
exhibited excellent functional group tolerance, and a wide range
of 2-aminobenzamides were compatible in the reaction, yielding
the polycyclic quinazolinone derivatives in good to high yields
(up to 92% yield, 2a-2aa). Both the electron-withdrawing and
electron-donating substituents at different positions of benzene
ring A furnished the desired products 2b-2i in good yields, while
the 5-OMe substrate 1e gave a low yield. Similarly, benzene ring
B of the tetrahydroisoquinoline moiety bearing various electronic
properties at different positions also be successfully applied to
the reaction, leading to the corresponding products 2j-2r in good
yields. The 2-aminonaphthamide furnished the product 2s in
high yield (70% yield, Table 2). In addition, the heterocyclic
[a] 3 (0.05 mmol), 4.0 equiv (NH4)2S2O8, solvent (1.0 mL), under an air
atmosphere. Isolated yield. [b] Isolated combined yields. Isomer ratios were
determined by analysis of the 1H NMR spectra of isolated products.
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10.1002/ejoc.201900469
European Journal of Organic Chemistry
COMMUNICATION
Table 4. Scope for the synthesis of imidazole compounds. [a]
Scheme 3. Proposed mechanism.
radical anion. Sequentially, further oxidation of intermediate I by
.
-
SO4
radical anion could generate an amide iminium
intermediate III. Finally, an intramolecular nucleophilic addition
of intermediate III, followed by further oxidation gave the final
product 2 and acyl substituents when employed did not need the
further oxidation. In a competitive pathway, the recombination of
the radical intermediate I and SO4 radical anion could generate
the sulfate intermediate II, followed by the formation of
intermediate III, which can easily transform to quinazolinone
product 2.
[a] 5 (0.05 mmol), 4.0 equiv (NH4)2S2O8, solvent (1.0 mL), under an air
atmosphere. Isolated yield. [b] 2.0 equiv (NH4)2S2O8.
applied this system to the cyclization of 2-amino-N-
benzylbenzamides, furnished the corresponding products 4a-4c
in moderate yields (Table 3). To our delight, it was found that the
cyclization of the isoindoline derivatives was also tolerated，
providing the desired products in high yields (65-89% yields, 4d-
4h). To our surprise, the natural product tryptanthrin 4i was
observed in moderate yield by the sequentially oxidation of
cyclization product under the optimal conditions (Table 3).
Furthermore, this method could be further extended to the
synthesis of polycyclic benzimidazole derivatives by using 2-
(3,4-dihydroisoquinolin-2(1H)-yl)aniline 5 as the substrates. As
shown in Table 4, the corresponding tetracyclic imidazole
products 6a-6g were observed in good to high yields (62-91%
yields) under the optimal conditions, offering further synthetic
utility of this method.
To identify the mechanism of this reaction, control
experiments were performed (Scheme 2). When 4.0 equiv of
TEMPO (2,2,6,6-tetramethylpiperidin-1-oxy) or BHT (2,6-di-tert-
butyl-4-methylphenol) was added under the standard condition,
only trace amount of product 2a was obtained. These results
suggested that the radical intermediates were involved in the
catalytic cycle. Similar result was observed under N2
.
-
Conclusions
In conclusion, we have developed a simple, metal-free protocol
for the intramolecular dehydrogenative C-H/N-H cross-coupling
reactions under mild condition. This method offered an efficient
way to construct the fused polycyclic quinazolinone derivatives.
The synthetic utility of this method was further demonstrated by
the synthesis of the natural products tryptanthrin, rutaecarpine
and their analogues. Preliminary mechanism studies suggested
that the carbon-centered radical and amide iminium ion were the
key intermediates in the catalytic cycle. The biological activity
test of these polycyclic quinazolinone derivatives and further
application of this strategy is ongoing in our laboratory.
atmosphere compared to that performed under air conditions, Experimental Section
indicating that the O2 was not necessary for this transformation.
The oven-dried Schlenk tube (10 mL) containing a stirring bar was
Based on these experimental results and literature
precedences,[12]
proposed mechanism involving an
charged with 0.05 mmol 1a and 0.2 mmol (NH4)2S2O8, 1.0 mL DMSO,
and heated to 60 °C for 2-6 h. After the reaction was completed, the
product was directly purified by column chromatography (PE/EA) to
afford the desired products.
a
intramolecular nucleophilic addition of amide iminium ion was
depicted in Scheme 3. First, the homolysis of S2O82- leads to the
.
-
formation of SO4 radical anion under thermolysis. Then the
carbon-centered radical intermediate I was generated from the
.
-
2-aminobenzamide 1 by single-electron-transfer (SET) of SO4
Acknowledgments
We thank the National Natural Science Foundation of China
(Nos. 21602142), and Sichuan University for financial support.
We thank the Xiaoming Feng laboratory (SCU) for access to
equipment. We also thank the comprehensive training platform
of the Specialized Laboratory in the College of Chemistry at
Sichuan University for compound testing.
Scheme 2. Control experiments.
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10.1002/ejoc.201900469
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Conflict of interest
The authors declare no conflict of interest.
Keywords: quinazolinones • metal-free • carbon radical •
tryptanthrin • rutaecarpine
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Key Topic* Synthetic Methods
Lijuan Xie, Cong Lu, Dong Jing, Xinrui
Ou, and Ke Zheng*
An efficient metal-free, (NH4)2S2O8
mediated intramolecular oxidative
cyclization for the construction of fused
polycyclic quinazolinone derivatives
under mild condition was disclosed.
This method provided an efficient and
facile approach to the synthesis of
polycyclic quinazolinone derivatives
(>40 examples), as well as the nature
products tryptanthrin, rutaecarpine and
their analogues.
Page No. – Page No.
Title : Metal-free synthesis of
polycyclic quinazolinones enabled by
a (NH4)2S2O8‑promoted intramolecular
oxidative cyclization
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