Construction of a benzo[b]azepine skeleton through decarboxylative ylide [6+1] annulations with modified vinyl benzoxazinanones

Article abstract of DOI:10.1021/acs.orglett.0c04041

A Lewis acid-promoted [6+1] annulation between sulfur ylides and modified vinyl benzoxazinanones was described. In this reaction, the newly designed vinyl benzoxazinanones could serve as a novel six-atom synthon, and the key to success is the installation of an electron-withdrawing group on the alkene moiety of the benzoxazinanones. A broad range of substrates are compatible with this mild reaction system, thereby providing a facile and practical approach for constructing a benzo[b]azepine skeleton.

Full text of DOI:10.1021/acs.orglett.0c04041

pubs.acs.org/OrgLett
Letter
Construction of a Benzo[b]azepine Skeleton through
Decarboxylative Ylide [6+1] Annulations with Modified Vinyl
Benzoxazinanones
Qing-Zhu Li,† Zhi-Qiang Jia,† Lin Chen, Xiang Zhang, Hai-Jun Leng, Rong Zeng, Yan-Qing Liu,
Wen-Lin Zou, and Jun-Long Li*
Cite This: Org. Lett. 2021, 23, 814−818
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ABSTRACT: A Lewis acid-promoted [6+1] annulation between
sulfur ylides and modified vinyl benzoxazinanones was described.
In this reaction, the newly designed vinyl benzoxazinanones could
serve as a novel six-atom synthon, and the key to success is the
installation of an electron-withdrawing group on the alkene moiety
of the benzoxazinanones. A broad range of substrates are
compatible with this mild reaction system, thereby providing a
facile and practical approach for constructing a benzo[b]azepine skeleton.
itrogen-containing heterocycles are key structural motifs
in numerous natural products, pharmaceuticals, and
N
agrochemicals.1 In particular, the benzo[b]azepine skeleton
represents an important class of benzannulated medium-sized
N-heterocycles due to its broad bioactivity. For example, drugs
such as Benazepril, Epinastine, and other related bioactive
compounds all contain the benzo[b]azepine ring system
(Figure 1).2 Because of their significant pharmaceutical value,
the development of straightforward synthetic methods for the
construction of benzo[b]azepine scaffolds is urgent.
Intermolecular cyclization is considered as one of the most
efficient methods for the construction of heterocycles, due to
the formation of at least two chemical bonds in a single step.3
Among various cyclization strategies, the ylide-mediated [n+1]
annulations are of particular interest in organic chemistry.4 In
the past several decades, remarkable progress has been made in
this field, and various annulation types, such as [2+1],5 [3+1],6
[4+1],7 and [5+1]8 cyclizations, were successively developed
by incorporating different n-atom reaction partners, thereby
providing efficient protocols for accessing structurally
complicated cyclic compounds. However, to the best of our
knowledge, the type of ylide [6+1] annulation9 that could
construct seven-membered rings is yet to be established thus
far. The main challenge of this annulation is probably the
presence of inherent entropic factors and transannular
interactions during the formation of a medium-sized ring
system.10 Another reason is the lack of an easily available and
bench-stable six-atom synthon with ambiphilic properties.
Theoretically, such 1,6-dipolar substrates might easily undergo
intramolecular cyclization to furnish a six-membered ring, and
therefore, the desired intermolecular [6+1] annulation path-
way with an ylide partner might be hindered (Scheme 1a). In
this scenario, it is of considerable significance to design a novel,
Figure 1. Selected examples of bioactive benzo[b]azepine derivatives.
bench-stable, and reactive six-atom synthon that could enable
the challenging [6+1] annulation reactions for the synthesis of
medicinally interesting seven-membered heterocycles.
Vinyl benzoxazinanones have been widely applied in
synthetic methodology development. They typically serve as
a transient 1,4-dipole in palladium-catalyzed cyclizations
through decarboxylative generation of an ambiphilic N-anion-
involved π-allyl palladium species.11 With this key intermedi-
ate, multifarious [4+n] annulations have been achieved for
Received: December 5, 2020
Published: January 27, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04041
© 2021 American Chemical Society
Org. Lett. 2021, 23, 814−818
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a
Scheme 1. Challenges in Ylide [6+1] Annulation and Our
Design
Table 1. Optimization Studies
b
entry
base
solvent
Lewis acid
yield (%)
1
2
3
4
5
6
7
8
toluene
MeCN
THF
DCM
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
−
−
−
−
−
−
−
−
50
47
25
45
66
50
<5
26
34
58
77
31
88
56
68
36
NaHCO3
Et3N
c
9
TMG
−
10
11
12
13
14
15
16
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Sc(OTf)3
Sn(OTf)2
Al(OTf)3
Fe(OTf)2
LiCl
LiClO4
BF3·Et2O
a
Reactions were performed with vinyl benzoxazinanones 1a (0.15
mmol), sulfonium salts 2a (0.1 mmol), a base (0.12 mmol), and a
Lewis acid (0.02 mol) in 1.0 mL of solvent at room temperature for
b
c
72 h. Isolated yield. Tetramethylguanidine.
as additives to improve the reaction efficiency, and Fe(OTf)2
was found to be quite compatible with this reaction system to
provide the highest yield (entries 10−16).
With the optimal conditions in hand, we set out to explore
the generality of this [6+1] annulation with a combination of
various substituted vinyl benzoxazinanones 1 and sulfonium
salts 2. As shown in Scheme 2, sulfonium salts featuring either
an electron-donating or electron-withdrawing group on the
benzene ring worked well under standard conditions,
generating a range of benzo[b]azepine derivatives 3a−3m in
generally good yields. The reaction proceeded smoothly with a
2-naphthyl-substituted sulfonium salt, and an 87% yield of 3n
was obtained. Heteroaromatic sulfonium salts were also
suitable substrates for producing furyl 3o or thienyl 3p in
high yields. Besides ketones, other electron-withdrawing group
on 2, such as esters, could also be compatible with this
reaction, delivering an ester-substituted product 3q in
moderate yield. Next, we examined the reaction of 2b with
various vinyl benzoxazinanones 1 under optimal conditions. As
shown in the latter part of Scheme 2, this annulation was
tolerant of electron-rich and electron-deficient substituents at
position 5 or 6 of vinyl benzoxazinanones. The corresponding
products 3r−3w were produced in satisfying yields. The
reactions also proceeded well for the 7,8-dimethyl-substituted
1, providing 3x in 85% yield. Furthermore, the different ester
group on 1 has proven to show limited effects on the reaction
outcome (3y).
assembling various interesting aza-heterocycles (Scheme 1b).12
For example, Xiao’s group pioneered the palladium-catalyzed
[4+1] annulation of vinyl benzoxazinanones and sulfur ylides.
To avoid the use of precious metal catalysts, we designed a
new type of building block by simply introducing an electron-
withdrawing group (EWG) on the olefin moiety, which might
mimic the reactivity of the well-established Morita−Baylis−
Hillman adducts.13 Thus, the activated alkene moiety could be
directly attacked by a nucleophile at the terminal carbon, and
an in situ-generated N-anion could further react with
electrophilic species. Notably, the electron-deficient alkene
and N-nucleophile were spatially separated by a rigid aromatic
structure, which could inhibit the intramolecular side reaction
and make the substrates bench-stable. On the basis of this
hypothesis, the newly designed vinyl benzoxazinanones could
serve as an excellent 1,6-dipole in a palladium-free annulation
(Scheme 1c). Herein, we report the design of modified vinyl
benzoxazinanone substrates and the successful application in a
Lewis acid-promoted decarboxylative [6+1] annulation with
sulfur ylides, which provides an efficient protocol for accessing
novel molecular structures (Scheme 1d).14
We started by investigating the reaction of the modified
vinyl benzoxazinanones 1a and sulfonium salts 2a in the
presence of a base at ambient temperature (Table 1).15 To our
satisfaction, the desired [6+1] annulation proceeded smoothly
by using K2CO3 in toluene, affording product 3a in 50% yield
under transition metal-free conditions (entry 1). After various
solvents had been screened, DCE led to the best result (entries
2−5). Moreover, the examination of bases resulted in inferior
yields (entries 6−9). Then, a series of Lewis acids were tested
Subsequently, we performed several experiments to
demonstrate the versatility of this method. First, the direct
sulfide-catalyzed [6+1] annulation of 1a with 2-bromoaceto-
phenone 4 was also achieved by employing tetrahydrothio-
phene 5 as the organocatalyst, giving the corresponding
product 3b in 65% yield (Scheme 3a). Then, synthetic
derivatization of product 3b was also extensively studied.
815
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a,b
Scheme 2. Substrate Scope of the [6+1] Annulation
Scheme 3. Synthetic Application of 3b
could easily convert to a CCR5 antagonist through several
established transformations (Scheme 3c).2f
The mechanism of this decarboxylative [6+1] annulation is
proposed in Scheme 4. Sulfur ylide was initially generated in
the presence of a base. With the assistance of a Lewis acid,
a
Reactions were performed with 0.15 mmol of 1, 0.10 mmol of 2,
0.12 mmol of K2CO3, and 0.02 mmol of Fe(OTf)2 in 1.0 mL of DCE
b
c
at rt for 72 h. Isolated yield. The structure of 3b was determined by
X-ray diffraction analysis.16
Scheme 4. Proposed Mechanism
Interestingly, the multifunctionalized quinoline 6 could be
easily obtained from 3b through a cascade reaction of aerobic
dehydrogenation and oxidative ring contraction17 in the
presence of 1,8-dazabicyclo[5.4.0]undec-7-ene (DBU). Upon
treatment of 3b with ammonium acetate, aromatic aldehyde,
and iodine in EtOH at 80 °C, imidazole-annulated product 7
was delivered in 70% yield.18 Moreover, the diastereoselective
reduction of 3b could afford alcohol 8, which could directly
undergo further cyclization with triphosgene to produce a
tricyclic product 9 in 66% total yield (Scheme 3b). In addition,
Corey-chaykovsky reagent 10 could also react with modified
vinyl benzoxazinanones, delivering the corresponding benzo-
[b]azepine derivatives 11a and 11b. Notably, product 11b
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vinyl benzoxazinanone 1a could be further activated and easily
attacked by sulfur ylides at the terminal alkene position,
thereby forming a betaine intermediate. Next, this intermediate
released CO2 gas to give an amino anion and then underwent
an intramolecular cyclization to deliver the final cyclized
product.
In conclusion, we have developed an unprecedented
decarboxylative [6+1] annulation of vinyl benzoxazinanones
and sulfur ylides. The success was greatly attributed to the
design of electron-deficient vinyl benzoxazinanone substrates,
which could serve as an excellent six-atom synthon. Promoted
by Lewis acids, this annulation features advantages of mild
conditions, high yields, and a broad substrate scope, thereby
providing a facile route for producing benzo[b]azepine
derivatives. In addition, this reaction pathway could also be
realized by using direct sulfide organocatalysis. The synthetic
utilization of the obtained benzene-annulated azepine products
was demonstrated in various functional group derivatizations.
Further investigations of the biological studies of these novel
molecules are currently underway in our laboratory.
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Hai-Jun Leng − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Rong Zeng − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Yan-Qing Liu − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Wen-Lin Zou − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04041
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04041.
■
Author Contributions
†Q.-Z.L. and Z.-Q.J. contributed equally to this work.
sı
Notes
The authors declare no competing financial interest.
Complete experimental procedures, characterization of
new products, NMR spectra, and HPLC chromatograms
(PDF)
FAIR data, including the primary NMR FID files, for
compounds 1a−1i, 3a−3y, 6, 7, 9, 11a, and 11b (ZIP)
ACKNOWLEDGMENTS
■
The authors are grateful for the financial support from the
NSFC (21871031, 21702021, and 22071011), the Thousand
Talents Program of Sichuan Province, and the Chengdu
Talents Program.
Accession Codes
CCDC 2000640 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
Jun-Long Li − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China; orcid.org/0000-0002-4700-
0142; Email: lijunlong709@hotmail.com
Authors
Qing-Zhu Li − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Zhi-Qiang Jia − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Lin Chen − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
of Antibiotics, School of Pharmacy, Chengdu University,
Chengdu 610052, China
Xiang Zhang − Antibiotics Research and Re-evaluation Key
Laboratory of Sichuan Province, Sichuan Industrial Institute
817
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