Divergent Synthesis of Quinazolin-4(3H)-ones and Tryptanthrins Enabled by a tert-Butyl Hydroperoxide/K3PO4-Promoted Oxidative Cyclization of Isatins at Room Temperature

Article abstract of DOI:10.1021/acs.orglett.6b01291

A synergetic tert-butyl hydroperoxide/K₃PO₄-promoted oxidative cyclization has been developed for the facile synthesis of various functionalized quinazolin-4(3H)-ones from commercially available isatins and amidine hydrochlorides at room temperature. The synthetic utility of this strategy was illustrated by the convenient synthesis of tryptanthrin derivatives via a self-dimerization of isatins under the same conditions.

Full text of DOI:10.1021/acs.orglett.6b01291

Letter
pubs.acs.org/OrgLett
Divergent Synthesis of Quinazolin-4(3H)‑ones and Tryptanthrins
Enabled by a tert-Butyl Hydroperoxide/K3PO4‑Promoted Oxidative
Cyclization of Isatins at Room Temperature
Feng-Cheng Jia,† Zhi-Wen Zhou,† Cheng Xu,† Yan-Dong Wu,*,† and An-Xin Wu*,†,‡
†Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Hubei, Wuhan 430079, P. R. China
‡State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: A synergetic tert-butyl hydroperoxide/K3PO4-
promoted oxidative cyclization has been developed for the
facile synthesis of various functionalized quinazolin-4(3H)-
ones from commercially available isatins and amidine hydro-
chlorides at room temperature. The synthetic utility of this
strategy was illustrated by the convenient synthesis of
tryptanthrin derivatives via a self-dimerization of isatins
under the same conditions.
he quinazolinone nucleus is exemplified as a privileged
Scheme 1. Synthetic Routes to Quinazolin-4(3H)-one and
Tryptanthrin Derivatives
T
structure that widely exists in natural products,1,2 such as
2-methyl-4(3H)-quinazolinone,1a luotonins A, B, E, and F,1b,c
tryptanthrin,1c and rutaecarpine1d (Figure 1). Quinazolinone
Figure 1. Quinazolinone skeleton containing natural products.
derivatives are also known to possess a range of biological and
medicinal activities including antibacterial, antiviral, anti-
inflammatory, and anticancer properties.2 Futhermore, quina-
zolinone derivatives are also used as inhibitors of various
enzymes.3
Because of their great value, the synthesis of quinazolinones
has attracted extensive attention. Representative synthetic
methods of this skeleton are mainly summarized as the
following five types: (i) condensation of 2-aminobenzamides
with aldehydes or acyl chlorides;4 (ii) oxidation/cyclization
reaction of 2-aminobenzamides with benzylamines, benzyl
alcohols, or benzyl halides;5 (iii) condensation/C−C bond
cleavage of 2-aminobenzamides with carbonyl compounds;6
(iv) coupling/cyclization reaction of o-halogen benzoic acid
derivatives with amidines or benzylamines;7 (v) domino
reaction using palladium-catalyzed carbon monoxide or
isocyanide insertion as the key step8 (Scheme 1a). Although
these reactions provide efficient accesses to quinazolin-4(3H)-
ones, their applications are limited by poor substitution
diversity or the requirement for harsh reaction conditions.
Therefore, the development of concise methods for the facile
construction of diverse quinazolin-4(3H)-ones is highly
desirable.
Tryptanthrin, as a kind of natural quinazoline alkaloid, was
isolated from the indigo plant Strobilanthes cusia (Acantha-
ceae).9 Various approaches10−12 were explored for efficient
construction of this significant skeleton because of its
Received: May 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01291
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Letter
widespread biological and pharmaceutical activities.13 Among
these, the oxidative cyclization of isatins or its analogues have
received considerable attention in light of the availability of
starting materials. Traditionally, such processes require highly
toxic oxidants such as KMnO4 or POCl3 and have low
efficiencies.10 Recently, Wang and co-workers proposed a
graceful synthesis of tryptanthrins from indoles via the copper-
catalyzed aerobic oxidation (Scheme 1b).11 In addition, Reddy
and co-workers12 developed a copper-catalyzed domino
oxidative cyclization strategy for the construction of tryptan-
thrins from 2-aminoaryl methyl ketones and isatins. Herein, we
present an ambient-temperature tert-butyl hydroperoxide
(TBHP)/K3PO4-promoted domino oxidative cyclization for
the divergent synthesis of quinazolin-4(3H)-ones and tryptan-
thrins (Scheme 1c). Advantages of this method include the use
of commercially available starting materials, mild reaction
conditions, and good functional group compatibility.
In our previous work, we have disclosed an efficient domino
protocol for the synthesis of 2-(1,3,4-oxadiazol-2-yl)aniline
derivatives by employing copper-catalyzed intramolecular
decarboxylative coupling as the key step.14 Then our attention
switched to develop new domino reaction involving copper-
catalyzed intermolecular decarboxylative coupling. Our study
commenced with the reaction of isatin (1a) and benzamidine
hydrochloride (2a) in the presence of CuI, TBHP, and K2CO3
in DMSO at room temperature (25−30 °C) in a sealed vessel
under air. Gratifyingly, the desired 2-phenylquinazolin-4(3H)-
one (3a) identified by NMR spectra was obtained in 82% yield.
To our surprise, a slight yield increase was observed in the
absence of CuI. After investigating various reaction parameters
including bases, oxidants, and solvents, the optimized reaction
conditions were eventually identified as 1a (0.5 mmol), 1.2
equiv of 2a, 1.5 equiv of TBHP, and 3 equiv of K3PO4 in 4 mL
of DMSO at room temperature for 6 h (Table 1 and see the
Supporting Information).
a
Table 1. Optimization of the Reaction Conditions
b
entry
base
oxidant
solvent
yield (%)
1
K2CO3
Na2CO3
Cs2CO3
K3PO4
KOH
TBHP
TBHP
TBHP
TBHP
TBHP
m-CPBA
DDQ
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
85
83
2
3
82
4
88
5
78
6
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
trace
trace
8
7
8
PIDA
9
K2S2O8
DTBP
DBP
trace
trace
trace
trace
trace
86
10
11
12
13
TBPB
TEMPO
TBHP
TBHP
TBHP
TBHP
TBHP
c
14
15
16
17
18
87
EtOH
61
CH3CN
H2O
75
trace
a
Reactions conditions: 1a (0.5 mmol), 2a (0.6 mmol), base (1.5
mmol), and oxidant (0.75 mmol) were added at room temperature
(25−30 °C) in 4 mL of solvent in a sealed vessel under air for 6 h.
b
c
Isolated yield. TBHP (5.5 M in decane).
a,b
Scheme 2. Scope of Isatins and Amidine Hydrochlorides
The substrate scope was subsequently examined, as shown in
Scheme 2. Various isatins (1a−m) with different electronic
properties effectively underwent the oxidative annulation
reactions with 2a to provide the corresponding products in
moderate to good yields (42−89%, 3a−m). Substitutions at 4-,
5-, 6-, and 7- positions were all tolerated. The greater reactivity
of the substrates containing an electron-withdrawing group at
the 5-position of the aryl ring could lead to formation of the
corresponding quinazolin-4(3H)-ones in higher yields (com-
pared 3b,c to 3d,e). Steric hindrance had an obvious effect on
this transformation (compared 3j−m to 3a−i). Notably, the
quinazoline scaffolds were formed with a diverse range of
halogen atoms, providing enormous possibilities for further
modification. The reaction was attempted with a range of
amidine hydrochlorides under the optimized conditions. It was
found that the reaction conditions were compatible with both
electron-rich (4-Me and 3-MeO) and halogenic (4-Cl)
substituents on the phenyl rings of the benzamidine hydro-
chloride, which yielded the corresponding products in
satisfactory yields (65−90%, 3n−p). The aliphatic and
heteroaryl amidine reagents (acetamidine hydrochloride, cyclo-
propane-1-carboximidamide hydrochloride, and isonicotinimi-
damide hydrochloride) were also found to be suitable for this
transformation (84−87%, 3q−s).
a
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), K3PO4 (1.5
mmol), and TBHP (0.75 mmol) in DMSO (4 mL) at room
remperature (25−30 °C) in a sealed vessel under air for 6 h. Isolated
yields.
b
Having established the scope of our new domino reaction,
the reaction mechanism was then evaluated. We initially
investigated the reaction of isatin (1a) under the optimized
conditions in the absence of benzamidine hydrochloride (2a),
which unexpectedly afford natural product tryptanthrin (4a) in
90% yield (Scheme 3a). Interestingly, when isatin (1a, 2 equiv)
B
DOI: 10.1021/acs.orglett.6b01291
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Organic Letters
Letter
Scheme 3. Control Experiments
Scheme 4. Possible Mechanism
To show the generality of this TBHP/K3PO4 system for the
synthesis of tryptanthrins, several representative isatins were
selected to construct symmetric tryptanthrin derivatives. Isatins
containing electron-neutral (5-Me), electron-rich (5-OMe),
and halogenated substituents (5-F, 5-Cl, 4-Cl) were converted
to the corresponding products in moderate to good yields
(Scheme 5, 4a−f, 66−90%). To further indicate the synthetic
was treated with benzamidine hydrochloride (2a, 1 equiv)
under the optimized conditions, 2-phenylquinazolin-4(3H)-one
(3a) and tryptanthrin (4a) were obtained in 42% and 18%
yield, respectively (Scheme 3b). When the reactions of isatin
(1a) with TBHP (5 equiv) and K3PO4 (3 equiv) were
conducted in DMSO at room temperature for 50 min, the
target product tryptanthrin (4a) was also isolated in 32% yield
along with the possible unstable byproducts tert-butyl 2-
aminobenzoperoxoate detected by MS (Scheme 3c). To further
demonstrate the possible isatoic anhydride intermediate
implied by the aforementioned three control experiments,
piperidine served as stronger electrophile to trap possible
isatoic anhydride generated in situ, which gave the desired
product 5 in 82% yield under standard conditions (Scheme 3d).
Furthermore, when the sterically hindered 7-methylindoline-
2,3-dione was conducted under the optimized conditions, only
a trace of oxidative product 8-methyl-1H-benzo[d][1,3]-
oxazine-2,4-dione was detected by MS (Scheme 3e). Next,
when the reaction of 1H-benzo[d][1,3]oxazine-2,4-dione (6)
and benzamidine hydrochloride (2a) in the presence of K3PO4
was performed in DMSO, the desired product 3a was isolated
in 85% yield (Scheme 3f). On the other hand, the radical
scavengers, TEMPO (2,2,6,6-tetramethylpiperidinooxy) and
HQ (hydroquinone), did not influence this reaction obviously
(see the SI), which indicated that a radical pathway might not
be involved in this reaction either. Taken together, these
control experiments clearly indicated that the isatoic anhydride
acted as the key intermediate involved in this domino process.
On the basis of the above observations and literature
precedent,15−17 a tentative reaction mechanism was proposed
as shown in Scheme 4. Initially, the intermolecular nucleophilic
attack of TBHP to isatin (1a) led to intermediate C, and
intramolecular cyclization followed by rearrangement would
afford the intermediate isatoic anhydride (6). Subsequently, the
decarboxylative nucleophilic attack of isatoic anhydride (6)
with amidine hydrochlorides (2a) provide the amide E, which
was converted to the target product 3a by deamination
cyclization. In the absence of amidine hydrochlorides, self-
dimerization of isatin would form tryptanthrin (4a) through an
analogous oxidative cyclization.
Scheme 5. Direct Synthesis of Tryptanthrins via a Self-
Dimerization of Isatins
a,b
a
Reaction conditions: 1 (1.0 mmol), TBHP (0.75 mmol), and K3PO4
(1.0 mmol) in DMSO (4 mL) at room temperature (25−30 °C) in a
b
sealed vessel under air for 12 h. Isolated yields.
practicality of the reaction, gram-scale syntheses of 2-phenyl-
quinazolin-4(3H)-one (3a) and tryptanthrin (4a) were
performed, and the desired products 3a and 4a were obtained
in 83% and 85% yield, respectively (Scheme 6).
In summary, a synergetic TBHP/K3PO4-promoted domino
oxidative cyclization of isatins with amidine hydrochlorides has
been developed, providing a rapid access to pharmaceutically
significant quinazolin-4(3H)-one derivatives. The advantages of
Scheme 6. Gram-Scale Experiment
C
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Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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elegant protocol for the concise synthesis of tryptanthrins via a
rare self-dimerization of isatins under the same conditions.
Further applications of this oxidative cyclization strategy for the
synthesis of other fascinating heterocycles are currently
underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01291.
Experimental procedures, product characterization, and
1H and 13C NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chwuyd@mail.ccnu.edu.cn.
*E-mail: chwuax@mail.ccnu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21272085 and 21472056) for financial
support. We also acknowledge an excellent doctorial dis-
sertation cultivation grant from Central China Normal
University (2015YBZD015).
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