Gold-Catalyzed Carbocyclization/C=N Bond Formation Cascade of Alkyne-Tethered Diazo Compounds with Benzo[c]isoxazoles for the Assembly of 4-Iminonaphthalenones and Indenes

Article abstract of DOI:10.1002/adsc.202100602

A gold-catalyzed carbocyclization/C=N bond formation cascade reaction has been developed for the synthesis of polyfunctionalized 4-iminonaphthalenones and indenes in good to high yields under mild reaction conditions. The reaction goes through 5/6-endo-di

Full text of DOI:10.1002/adsc.202100602

RESEARCH ARTICLE
doi.org/10.1002/adsc.202100602
Gold-Catalyzed Carbocyclization/C=N Bond Formation Cascade
of Alkyne-Tethered Diazo Compounds with Benzo[c]isoxazoles for
the Assembly of 4-Iminonaphthalenones and Indenes
Ming Bao,+a Xiongda Xie,+a Wenhao Hu,a,* and Xinfang Xua,*
a
Guangdong Key Laboratory of Chiral Molecule and Drug Discovery, School of Pharmaceutical Sciences, Sun Yat-sen
University, Guangzhou, Guangdong 510006, China
E-mail: huwh9@mail.sysu.edu.cn; xuxinfang@mail.sysu.edu.cn
+
These authors contributed equally to this work.
Manuscript received: June 18, 2021; Revised manuscript received: June 25, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100602
Abstract: A gold-catalyzed carbocyclization/C=N bond formation cascade reaction has been developed for
the synthesis of polyfunctionalized 4-iminonaphthalenones and indenes in good to high yields under mild
reaction conditions. The reaction goes through 5/6-endo-dig diazo-yne carbocyclization to form the endocyclic
vinyl carbene species from corresponding alkyne-tethered diazo compounds, followed by electrophilic
addition/NÀ O bond cleavage/aromatization sequence with benzo[c]isoxazoles, which features a C=N bond
formation process. In addition, the resulting products could be converted into multi-substituted 2-naphthol
derivatives in high yields.
Keywords: Gold-Catalysis; Alkyne Carbocyclization; Endocyclic Vinyl Carbene; Benzo[c]isoxazoles; 4-
Iminonaphthalenones
Introduction
transformations,[7] including CÀ H/XÀ H insertion,[8]
cycloaddition,[9] cyclization,[10] ylide interception,[11]
The 4-iminonaphthalenone motif is a unique and oxidation,[12] rearrangement,[13] and many others
valuable skeleton found in a variety of natural products (Scheme 1a).[14] On the other hand, instead of using
and bioactive molecules.[1] During the past decade, a oxidative reagents to form α-carbonyl or α-imino
variety of catalytic approaches have been reported for carbene species, the catalytic transformations of diazo
construction of 4-iminonaphthalenones, such as con- group embodied alkynes via carbene-alkyne metathesis
densation of quinone with arylamine,[2] naphthol (CAM) process, which in situ lead to the vinyl carbene
oxidation[3] and aromatic electrophilic substitution with intermediates, has provided a complemental approach
nitrosoarenes.[4] Nevertheless, most of these methods in this area (Scheme 1b).[15–17] Recently, our group has
are less efficient for the synthesis of multi-substituted been focused on the catalytic diazo-yne carbocycliza-
analogous. Therefore, the development of effective and tion for direct construction of a variety of carbocyclic
novel synthetic methods for the assembly of highly frameworks.[18] For example, the gold-catalyzed 6-
substituted 4-iminonaphthalenone frameworks with endo-dig carbocyclization of alkynes with the pendent
readily available substrates under mild conditions diazo unit in situ led to endocyclic vinyl carbene
would certainly be in demand.
intermediates, providing practical approaches for the
In the last two decades, the homogeneous gold synthesis of multi-functionalized[18a] and fused[18b]
catalysis comprises one of the most effective methods naphthalene derivatives with corresponding nucleo-
for the alkyne functionalization.[5,6] In particular, the philic species (Scheme 1c, path a and path b). Inspired
gold-catalyzed transformations of alkynes that involve by these advances and as our ongoing interest in
carbene intermediates are of particular interest due to catalytic alkyne functionalization,[15,19] we envisioned
the divers possibilities for the effective construction of that a general approach for the construction of 4-
CÀ C and CÀ X bonds through metal carbene iminonaphthalenone frameworks could be realized by
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Table 1. Optimization of the reaction conditions.[a]
°
entry cat (x mol%)
T( C) Yield (%)[b]
3a/4a/5a
1
2
3
4
5
6
7
8
PPh3AuNTf2 (5.0)
IPrAuNTf2 (5.0)
Me4tBuXPhosAuNTf2 (5.0)
Me3(OMe)tBuXPhosAuNTf2 (5.0) 80
BrettPhosAuNTf2 (5.0)
JohnPhosAuSbF6 (5.0)
JohnPhosAuSbF6 (5.0)
Rh2(OAc)4 (1.0)
Cu(MeCN)4PF6 (5.0)
AgSbF6 (5.0)
60
60
80
35/22/–
45/17/–
48/23/–
55/22/–
53/18/–
82/11/–
88/<10/–
–/67/–
80
80
60
80
60
80
80
9
10
11
–/88/–
<10/53/–
–/<5/65
ZnCl2 (20.0)
[a] The reaction was carried out on a 0.1 mmol scale: catalyst (x
mol%), 1a (31.8 mg, 0.1 mmol), and 2a (14.3 mg,
0.12 mmol, 1.2 equiv.) in DCE (2.0 mL) under argon
atmosphere.
Scheme 1. Catalytic alkyne transformations via carbene inter-
mediates.
[b] Isolated yields.
interception of the endocyclic vinyl carbene intermedi-
ates with N-nucleophiles to form the C=N bond.[20] (entries 8–10).[22] This prodcut was genarated via CAM
Herein, we report our recent results on this direction, a process followed by formal [3+2]-cyclization.[15–17]
gold-catalyzed 6-endo-dig diazo-yne carbocyclization Whereas, the ZnCl2 (20 mol%) led to the naphthalene-
of diazo compounds 1, followed by a stepwise C=N 1,4-diol 5a in 65% yield, which was derived from
bond formation cascade with benzo[c]isoxazoles, addition of endocyclic vinyl carbene intermediate with
providing an expeditious synthesis of poly-functional- unintentionally
contaminated
with
water
ized 4-iminonaphthalenone and indene derivatives (entry 11).[18,23]
from corresponding diazo compounds (n=0 or 1) in
With the optimal reaction conditions established,
good to high yields under mild conditions. (Scheme 1c, the substrate scope with respect to the diazo com-
path c).[21]
pounds was investigated (Table 2). A variety of
substituents on the different positions of terminal aryl
ring were all well tolerated, delivering the 4-imino-
naphthalenone products 3a–3h in 63–95% yields. The
Results and Discussion
Initially, the alkyne tethered diazo compound 1a and 1-naphthyl, 1-thienyl, and cyclopropyl substituted
benzo[c]isoxazole 2a were used as the model sub- alkynes also underwent the reaction smoothly, leading
strates (Table 1). A variety of gold-complexes, which to the corresponding products 3i–3k in good to high
are pontent π-acids and could selectively activate the yields. The impact of the ester part was then evaluated.
alkynes,[7,18] were investigated in dichloroethane (DCE) It was found that the reaction could be applied to
°
°
at 60 C or 80 C to ensure the complete conversion benzyl, 3-phenylallyl, diphenylmethyl, and alkyl esters
(entries 1–7). Among them, JohnPhosAuSbF6 showed without a noticeable yield deterioration (3l–3o, 71%–
°
best results at 60 C in term of yield and selectivity, 94% yields). Notably, L-menthol and citronellol
providing the iminonaphthalenone 3a in 88% yield derived esters were also effective, providing product
(entry 7). Encouraged by these results, other different 3p and 3q in 78% and 75% yields, respectively. Diazo
metal catalysts were studied (entries 8–11). In the cases compounds with substituents on the aryl linkage were
with Rh2(OAc)4, Cu(MeCN)4PF6, or AgSbF6 as the suitable substrates, affording the corresponding prod-
catalyst, the tricyclic product 4a was obtained selec- ucts 3r–3t in 50%–89% yields. The structure of
tively in 67%, 88%, and 53% yields, respectively product 3g was confirmed by single-crystal X-ray
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Table 2. Substrate scope of diazo compounds 1.[a]
Table 3. Substrate scope of benzo[c]isoxazoles.[a]
[a] Reaction conditions: 1d (34.8 mg, 0.1 mmol), 2 (0.12 mmol,
1.2 equiv.), and JohnPhosAuSbF6 (3.9 mg, 5.0 mol%) in
°
DCE (2.0 mL) at 30 C under an argon atmosphere for 24 h.
The yields are given in isolated yields.
ing group at the 5- or 6-position of aryl ring proceeded
smoothly to afford the corresponding products 6a–6h
in 80–93% yields. Moreover, the cascade reaction also
occurred well with alkyl or phenyl substitution at the
3-position of benzo[c]isoxazoles, forming 6i and 6j in
50% and 82% yields, respectively.
Furthermore, this method could be extended to
donor-acceptor type of diazo compounds 7. After a
brief optimization (see Table S1 in SI for details), the
best results were obtained by using BrettPhosAuNTf2
°
(5.0 mol%) as the catalyst in DCE at 60 C for 24 hours
(Table S1, entry 4). The substrate scope was then
examined under these optimal reaction conditions. As
shown in Table 4, the reaction occurred smoothly with
benzo[c]isoxazoles that have various substitutions on
the different positions of aryl ring, including electron-
neutral, electron-withdrawing, and electron-donating
groups, leading to 8a–8i in good to high yields.
Moreover, comparable high yields were obtained in the
cases with 3-methyl and 3-phenyl substituted ones,
finishing 8j and 8k in 55% and 85% yields,
respectively. The structure of these generated indenes
was confirmed by single-crystal X-ray diffraction
analysis of its bromo-derivative 8l (90% yield).[24]
To gain insight into the reaction mechanism, control
experiments with alkynes 9 or 11, which did not have
[a] Reaction conditions: 1 (0.1 mmol), 2a (14.3 mg, 0.12 mmol,
1.2 equiv.), and JohnPhosAuSbF6 (3.9 mg, 5.0 mol%) in
°
DCE (2.0 mL) at 30 C under an argon atmosphere for 24 h.
The yields are given in isolated yields.
diffraction analysis, and other products were assigned
by analogy.[24]
The reaction scope with respect to benzo[c] the diazo unit, with benzo[c]isoxazole 2a were
isoxazoles was then examined under standard con- conducted under standard conditions (Scheme 2).
ditions (Table 3). The reaction with benzo[c]isoxazoles
Both of these reactions only led to the cyclization
containing an electron-donating or electron-withdraw- products 10 and 12 in 90% and 85% yields,
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Table 4. Substrate scope for the synthesis of indenes.[a]
Scheme 3. Proposed reaction mechanism.
the diazo group to generate the vinyl gold carbene
II,[18a,b] which might also existing as a cationic form
II’.[15] Both of these species, II and II’, could be
terminated by an electrophilic addition/ring-opening/
aromatization sequence with benzo[c]isoxazole 2a to
form the 4-iminonaphthalenone product 3a via III
with E-geometry.[20] The formation of indenes 8 could
go through an analogous process involving a 5-endo-
dig carbocyclization.[18c] In the cases with Rh, Cu or
Ag catalysts, the formation of carbene intermediate IV
would occur preferentially, and the tricyclic product 4a
was generated predominantly via carbene/alkyne
mathesis process.[15] Moreover, the intermolecular
OÀ H insertion/aromatization of the vinyl carbene II
would lead to the formation of naphthalene-1,4-diol 5a
in the presence of ZnCl2.[23]
[a] Reaction conditions:
1.2 equiv.), and BrettPhosAuNTf2 (5.0 mg, 5.0 mol%) in
7
(0.1 mmol),
2
(0.12 mmol,
°
DCE (2.0 mL) at 30 C under an argon atmosphere for 24 h.
The yields are given in isolated yields.
To demonstrate the synthetic utility of current
method, we performed the reaction on a gram scale to
provide 2.1 g 3d in 90% yield. Then, the generated
products were subjected to further transformation
(Scheme 4). The epoxidation product 13 was isolated
as a single isomer in 90% yield after treating 3d with
H2O2 in acetone.[24] The 2,3-disubstituted naphthoqui-
none 14 was afforded in the presence of CF3CO2H in
80% yield. Moreover, the reduction product 15 could
be obtained smoothly by treating 6f with NaBH4 at
°
Scheme 2. Control experiments.
respectively; and no reaction with benzo[c]isoxazole 0 C in 90% yield. These postsynthetic modifications
2a was occurred. These results implied that this enhanced the potential value of this method for the
reaction might go through intramolecular 6-endo-dig expeditious synthesis of naphthalene derivatives with
carbocyclization to form the endocyclic vinyl carbene structural diversity.
species initially, rather than intermolecular electro-
philic addition with 2a which is well known for the
Conclusion
formation of α-imino carbene.[21] Based on these results
and previously reported work,[18,20,21] a proposed mech- In summary, we have reported a gold-catalyzed diazo-
anistic pathway has been illustrated in Scheme 3. yne carbocyclization/C=N bond formation cascade of
Initially, the gold catalyst coordinates the π-bond of alkyne-tethered diazo compounds with benzo[c]
alkyne to form a gold π-complex I, followed by a 6- isoxazoles that provides an expeditious synthesis of
endo-dig cyclization with the nucleophilic carbon on polyfunctionalized 4-iminonaphthalenones and indenes
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Acknowledgements
Support for this research from the National Natural Science
Foundation of China (21971262, 81973176), National Postdoc-
toral Program for Innovative Talents (BX20190399), Guang-
dong Provincial Key Laboratory of Chiral Molecule and Drug
Discovery (2019B030301005), The Program for Guangdong
Introducing Innovative and Entrepreneurial Teams (No.
2016ZT06Y337) is greatly acknowledged.
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General Procedure for the Synthesis of
4-iminonaphthalenones 3and 6 (Table 2 and 3)
To a 10-mL oven-dried vial with a magnetic stirring bar,
JohnPhosAuSbF6 (3.9 mg, 5.0 mol%),
1
(0.1 mmol),
2
(0.12 mmol, 1.2 equiv.) and DCE (2.0 mL) was added in
°
sequence at 30 C under argon atmosphere, and the reaction
mixture was stirred for 24 h under these conditions. When the
reaction was completed (monitored by TLC), the crude reaction
mixture was purified by flash column chromatography on silica
gel (Hexanes:EtOAc=20:1 to 15:1) to give the pure products 3
or 6 in 50%–95% yields.
General Procedure for the Synthesis of Indenes 8
(Table 4)
To a 10-mL oven-dried vial with a magnetic stirring bar,
BrettPhosAuSbF6 (5.0 mg, 5.0 mol%),
7
(0.1 mmol),
2
(0.12 mmol, 1.2 equiv.) and DCE (2.0 mL) was added in
°
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mixture was stirred for 24 h under these conditions. When the
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contain the supplementary crystallographic data for 3g,
8l, and 13, respectively. These data can be obtained free
of charge from the Cambridge crystallographic data
centre via www.ccdc.cam.ac.uk/data_request/cif.
Adv. Synth. Catal. 2021, 363, 1–7
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RESEARCH ARTICLE
Gold-Catalyzed Carbocyclization/C=N Bond Formation
Cascade of Alkyne-Tethered Diazo Compounds with Benzo
[c]isoxazoles for the Assembly of 4-Iminonaphthalenones
and Indenes
Adv. Synth. Catal. 2021, 363, 1–7
M. Bao, X. Xie, W. Hu*, X. Xu*