Rhodium(III)-catalyzed [4?+?2] annulation of N-arylbenzamidines with 1,4,2-dioxazol-5-ones: Easy access to 4-aminoquinazolines via highly selective C[sbnd]H bond activation

Article abstract of DOI:10.1016/j.cclet.2021.02.061

A novel approach for the synthesis of 4-aminoquinazolines has been developed via rhodium(III)-catalyzed [4 + 2] annulation of N-arylbenzamidines with 1,4,2-dioxazol-5-ones. This reaction features excellent regioselectivity, broad substrate scope and high step economy, which would provide the reference for the construction of the fused 4-aminoquinazolines with biologically and pharmacologically active compounds.

Full text of DOI:10.1016/j.cclet.2021.02.061

G Model
CCLET-6190; No. of Pages 5
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Rhodium(III)-catalyzed [4 + 2] annulation of N-arylbenzamidines with
1,4,2-dioxazol-5-ones: Easy access to 4-aminoquinazolines via highly
selective CꢀꢀH bond activation
*
*
Jie Ren, Yanzhen Huang, Chao Pi, Xiuling Cui , Yangjie Wu
Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities, Green Catalysis Center and
College of Chemistry, Zhengzhou University, Zhengzhou 450052, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel approach for the synthesis of 4-aminoquinazolines has been developed via rhodium(III)-
catalyzed [4 + 2] annulation of N-arylbenzamidines with 1,4,2-dioxazol-5-ones. This reaction features
excellent regioselectivity, broad substrate scope and high step economy, which would provide the
reference for the construction of the fused 4-aminoquinazolines with biologically and pharmacologically
active compounds.
Received 6 January 2021
Received in revised form 24 February 2021
Accepted 25 February 2021
Available online xxx
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
[4 + 2] Annulation
N-Arylbenzamidine
4-Aminoquinazoline
Excellent regioselectivity
High step economy
Quinazoline is a preeminent class of structural motif in natural
products, functional materials and bioactive compounds [1].
Among this family, 4-aminoquinazolines have drawn considerable
attention due to their extensive occurrence in many medicinal
molecules, such as inhibitors of breast cancer resistance protein
(ABCG2) [2], antihypertensive drugs [3], anti-cancer agents [4] and
importantly lapatinib [5] (Fig. 1). Because of their great value, the
synthesis of 4-aminoquinazolines has gained much attention [6].
The traditional approach to 4-aminoquinazolines includes nucle-
ophilic substitution of aryl or alkyl amines with 4-chloroquinazo-
lines, which requires multi steps from 2-aminobenzoic acids [7].
Recently, Zhu’s group developed a palladium-catalyzed intramo-
lecular C(sp2)-H amidation to construct 4-aminoquinazolines in
the presence of O2 as the terminal oxidant and starting from
isonitriles [8]. Consequently, the development of greener pathway
starting from readily available and easily handled reactants to
access such a motif is of great significance.
removal, multiple functions has been imparted to DGs so that they
could function as a nucleophilic or electrophilic reagent for the
postchemical transformations as well as being a chelating groupand
internal oxidant [10]. Based on that, the commercially available
imidamides have attracted intensive interest as substrates, because
NꢀꢀH imine could act as both directing groups and intramolecular
nucleophile or electrophile [11]. In which, CꢀꢀH regioselectivity
presented challenge, however afforded the opportunity to manipu-
late an array of different heterocycles. Li, Wu and our group have
developed Rh, Ir and Ru-catalyzed intramolecular annulations using
NꢀꢀH imine as directing group to construct indoles [12] and
benzimidazoles [13]. In these reactions, the C-alkyl group promoted
the selectivity and the metal-X (X
¼
C, N) underwent migratory
insertion into the C N bond (acting an electrophile), causing NH to
¼
be removed through subsequent elimination. Imidamide was
employed as four-atom synthon in the field of CꢀꢀH activation,
mainly occurred at N-phenyl ring instead of the C-phenyl ring
perhaps due to its low thermos-stability of the key intermediate. So
far,therearefewsuccessfulreportsfromLi,DuandLiugroupsonthe
Rh(III)-catalyzed intermolecular CꢀꢀH functionalization of N-
phenylbenzimidamides with alkynes, diazo compounds and sulfur
ylides to produce 1-aminoisoquinolines [14]. In these reactions, the
Rh preferred to afford a five-membered metallacyclo-intermediate
and the CꢀꢀH activation selectively occurred at C-phenyl ring. With
our ongoing efforts to build structurally divers heterocycles [15],
herein, we explored a Rh(III)-catalyzed [4 + 2] cyclization of N-
phenylbenzimidamides with 1,4,2-dioxazol-5-ones to synthesize
Transition-metal catalyzed annulation based on CꢀꢀH activation
has been considered as an efficient and atom-economic strategy for
the construction of heterocyclic compounds in past decade [9].
Directing groups (DGs) are generally required to resolve the regio-
selectivityand improve the catalytic activity of metal. To address the
limitation of the DGs, such as requirement of preinstallation and
* Corresponding authors.
E-mail addresses: cuixl@zzu.edu.cn (X. Cui), wyj@zzu.edu.cn (Y. Wu).
https://doi.org/10.1016/j.cclet.2021.02.061
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Ren, Y. Huang, C. Pi et al., Rhodium(III)-catalyzed [4 + 2] annulation of N-arylbenzamidines with 1,4,2-dioxazol-5-
ones: Easy access to 4-aminoquinazolines via highly selective CꢀꢀH bond activation, Chin. Chem. Lett., https://doi.org/10.1016/j.
cclet.2021.02.061

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Chinese Chemical Letters xxx (xxxx) xxx–xxx
(Table 1, entry 8). When the amount of 4 Å MS was increased to 2
equiv., 3aa could be afforded in 67% yield (Table 1, entry 9).
Meanwhile, the higher yield was obtained by increasing the ratio of
1a and 2a to 1:1.2 (Table 1, entry 10). When the reactions were
carried out in other medium, including EtOH (ethanol), DMSO
(dimethyl sulfoxide), DCM (dichloromethane) and CH3CN (aceto-
nitrile), the yield of the desired product was significantly decreased
(Table 1, entries 11–14). Finally, the optimal reaction conditions
were identified as follows: N-phenylbenzamidine (0.1 mmol), 3-
phenyl-1,4,2-dioxazol-5-one (0.12 mmol), [Cp*RhCl2]2 (10 mol%),
AgOTf (40 mol%), Zn(OAc)2 (25 mol%) and 4 Å MS (2 equiv.) in DCE
(2 mL) at 120 ꢁC under air atmosphere for 3 h.
With the optimized conditions in hand, the substituted N-
arylbenzamidines were first examined with 3-phenyl-1,4,2-diox-
azol-5-one 2a (Scheme 1). To our delight, electron-donating (Me,
OMe, Et, iPr, tBu) and electron-withdrawing groups (F, Cl, Br) at the
para-position of N-phenyl ring worked well in this transformation,
affording the desired products in 62%–75% yields (3aa-3ia).
However, only trace amount of the desired product was observed
for N-(4-NO2-phenyl)benzimidamide. The meta- (Me, OMe, F and
Cl) and ortho-substituted N-arylbenzamidines (Me, F) were also
compatible in this transformation, affording the corresponding
products in 53%–78% yields (3ja-3oa). Meanwhile, the disubsti-
tuted N-arylbenzamidines were found to be amenable substrates,
providing the products in 74%–76% yields (3pa-3qa). These results
indicated that the steric hindrance of N-phenyl ring did not
dramatically influence this transformation. N-Pyridyl and N-
phenyl(4-benzyl) benzimidamides were all converted into the
desired products in moderate yields (3ra-3sa). The N-alkylbenza-
midine was investigated, such as n-propyl. However, no desired
product was obtained. Moreover, substituents at the para and
meta-position of C-phenyl ring also performed well, affording the
product in 62%–78% yields (3ta-3wa). However, only trace
amounts of desired products were obtained for ortho-Cl and
ortho-Me substituted N-arylbenzamidines (3xa and 3ya), perhaps
due to the steric hindrance.
To further explore the scope of this transformation, various
1,4,2-dioxazol-5-ones were investigated under the optimized
conditions (Scheme 2). The various para- and meta-substituted
(including Me, OMe, Cl, Br and CF3) 1,4,2-dioxazol-5-ones could
couple successfully with N-phenylbenzamidine to deliver the
corresponding products in 52–86% yields (3aa-3ah). Meanwhile,
the ortho- and poly-substituted 1,4,2-dioxazol-5-ones also per-
formed well, affording the products in 81%–82% yields (3ai-3aj).
These results indicated that steric hindrance of 1,4,2-dioxazol-5-
ones did not dramatically influence this transformation. Satisfy-
ingly, the reaction also displayed an excellent tolerance toward the
group of adamantine (3ak). However, this catalytic system was not
applied to the 3-pentyl-1,4,2-dioxazol-5-one. When the substrates
with groups of naphthalene, bisphenyl, thiophene and benzothio-
phene, the transformation showed excellent tolerance and
afforded 64%–73% yields (3al-3ao).
Fig. 1. Examples of 4-aminoquinazolines with bioactivity.
Table 1
Optimization of the reaction conditions.a
Entry
Additive
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
AgSbF6
DCE
37
23
45
19
NR
52
Trace
60
67
74
AgNTf
DCE
AgOTf
DCE
AgOAc
DCE
AgOTf/Zn(OTf)2
AgOTf/Zn(OAc)2
AgOTf/CsOAc
DCE
DCE
DCE
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
AgOTf/Zn(OAc)2/4 Å MS
DCE
DCE
DCE
EtOH
DMSO
DCM
CH3CN
9c
10d
11
12
13
14
NR
NR
28
NR
NR: no reaction.
a
Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), [Cp*RhCl2]2 (10 mol%),
AgOTf (40 mol%), Zn(OAc)2 (25 mol%), 4 Å MS (1 equiv.) and solvent (2 mL), 120 ꢁC,
3 h, air.
b
Isolated yields.
4 Å MS (2 equiv.).
2a (0.12 mmol).
c
d
4-aminoquinazolines. CꢀꢀH activation occurred at C-phenyl ring
with excellent regio-selectivity. In this reaction, various 4-amino-
quinazoline derivatives could be easily obtained with high step
economy, and the title products could undergo further chemical
transformation for the synthesis of useful nitrogen-heterocycles.
We initiated our study by using N-phenylbenzamidine 1a and
3-phenyl-1,4,2-dioxazol-5-one 2a as the model substrates to
examine the reaction parameters. The desired [4 + 2] annulation
occurred at C-phenyl ring to afford the product 3aa in 37% yield in
the presence of [Cp*RhCl2]2 and AgSbF6 in DCE (1,2-dichloroeth-
ane) at 120 ꢁC under air for 3 h (Table 1, entry 1). The structure and
regioselectivity was unambiguously confirmed by the single-
crystal X-ray diffraction analysis of 3aa (Supporting information).
Screening the silver salts revealed that AgOTf was the best choice,
and produced the desired product in 45% yield (Table 1, entries 2–
4). Subsequently, a variety of additives were investigated and Zn
(OAc)2 gave better results (Table 1, entries 5–7). After introduction
of 4 Å MS as an additive, the yield of product reached to 60%
In addition, to demonstrate the synthetic utility of this
transformation in organic synthesis, the template reaction was
conducted with larger scale under the standard reaction con-
ditions, and afforded 65% of isolated yield, confirming the
robustness of these conditions (Scheme 3a). Meanwhile, we
investigated protection of NH group in 3aa via coupling with iodo
methane (Scheme 3b). The coupled product can be used as ligands
in polymer light-emitting diodes [16]. Notably, the benzimidazo
[1,2-c]quinazolines 5aa could also be synthesized via a phenyl-
iodine diacetate (PIDA)-mediated intramolecular CꢀꢀH cyclo-
amination of 3aa (Scheme 3c), and its derivatives exhibited
excellent antibacterial and antifungal activities [17]. These results
suggested that our strategy was a practicability synthetic method
and a late-stage modification tool.
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Scheme 1. Reaction conditions: 1 (0.1 mmol), 2a (0.12 mmol), [Cp*RhCl2]2 (10 mol%), AgOTf (40 mol%), Zn(OAc)2 (25 mol%), 4 Å MS (2 equiv.) and DCE (2 mL), 120 ꢁC, 3 h, air.
Isolated yields.
Scheme 2. 1a (0.1 mmol), 2 (0.12 mmol), [Cp*RhCl2]2 (10 mol%), AgOTf (40 mol%), Zn(OAc)2 (25 mol%), 4 Å MS (2 equiv.) and DCE (2 mL), 120 ꢁC, 3 h, air. Isolated yields.
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system, which is consistent with the results obtained. Meanwhile,
the kinetic isotope effect (KIE = 3.9) was obtained (Scheme 4b),
indicating that the CꢀꢀH activation might be involved in the
turnover-limiting step.
On the basis of above mechanistic investigations and the
previous reported literatures [11k,[14d], a plausible mechanism of
the [4 + 2] cycloaddition reaction is proposed in Scheme 5. Initially,
the ligand exchange of [Cp*RhCl2]2 in the presence of OTfꢀ and
OAc- yielded active [Cp*RhX2] (X = OTf-, OAc-) species. Subse-
quently, a reversible CꢀꢀH bond cleavage with N-phenylbenzami-
dine 1a afforded a five-membered cyclo-rhodium intermediate A
instead of A0. Next, coordination of 3-phenyl-1,4,2-dioazol-5-one
2a with intermediate A gave the intermediate B, which underwent
migratory insertion to generate the intermediate
C by the
elimination of CO2. Finally, protonation of C regenerated active
complex of [Cp*RhX2] for the next catalytic cycle, and simulta-
neously resulted in the amidated product D, which underwent
intramolecular nucleophile addition to give the desired product
3aa.
Scheme 3. Scale-up reaction and derivation.
In summary, we have developed a novel and highly efficient
Rhodium(III)-catalyzed [4 + 2] cyclization of N-arylbenzamidines
and 1,4,2-dioxazol-5-ones to synthesize 4-aminoquinazolines via
highly selective CꢀꢀH bond functionalization at C-phenyl ring. This
strategy proceeded with excellent regioselectivity, broad substrate
scope and high step economy. Moreover, two CꢀꢀN bonds were
formed in reaction. The products were also subjected to various
chemical transformations to afford the medicinal molecules, and
may provide wider application in organic synthesis. Further
research on the application of the 4-aminoquinazoline frameworks
is currently underway.
Scheme 4. Mechanistic studies.
Declaration of competing interest
The authors report no declarations of interest.
Acknowledgments
We greatly acknowledge partial financial support from the
Ministry of Science and Technology of China (No.
2016YFE0132600), Henan Center for Outstanding Overseas Scien-
tists (No. GZS2020001), and Zhengzhou University.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in
the
online
version,
at
doi:https://doi.org/10.1016/j.
cclet.2021.02.061.
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