Diastereoselective Construction of Cyclopropane-Fused Tetrahydroquinolines via a Sequential [4 + 2]/[2 + 1] Annulation Reaction

Article abstract of DOI:10.1021/acs.orglett.0c00874

A sequential [4 + 2]/[2 + 1] annulation of α-aryl vinylsulfoniums with 2-aminochalcones and 2-(2-aminobenzylidene)-1H-indene-1,3(2H)-dione is reported that affords a series of cyclopropane-fused tetrahydroquinolines. The salient features of this novel and practical transformation include high efficiency, transition-metal-free nature, operational simplicity, and outstanding functional group tolerance.

Full text of DOI:10.1021/acs.orglett.0c00874

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Letter
Diastereoselective Construction of Cyclopropane-Fused
Tetrahydroquinolines via a Sequential [4 + 2]/[2 + 1] Annulation
Reaction
Zhen-Hua Wang,* Li-Wen Shen, Ke-Xin Xie, Yong You, Jian-Qiang Zhao, and Wei-Cheng Yuan*
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ABSTRACT: A sequential [4 + 2]/[2 + 1] annulation of α-aryl
vinylsulfoniums with 2-aminochalcones and 2-(2-aminobenzyli-
dene)-1H-indene-1,3(2H)-dione is reported that affords a series of
cyclopropane-fused tetrahydroquinolines. The salient features of
this novel and practical transformation include high efficiency,
transition-metal-free nature, operational simplicity, and outstanding
functional group tolerance.
etrahydroquinolines, a kind of privileged structural
scaffold in the family of nitrogen-containing heterocycles,
are widely distributed in a number of pharmaceutically relevant
bioactive molecules, such as galipinine, nicainoprol, and
oxamniquine (Figure 1).1 Access to functionalized tetrahy-
nolines (Scheme 1a).6a Soon thereafter, Doyle’s group realized
a novel Lewis acid-catalyzed Povarov reaction that provides
T
Scheme 1. Strategy for the Construction of Cyclopropane-
Fused Tetrahydroquinolines
Figure 1. Natural products containing tetrahydroquinoline or
cyclopropane and products in this work.
droquinolines has long attracted the interest of synthetic
chemists and pharmacologists.2 Moreover, as omnipresent and
ubiquitous subunits, cyclopropanes not only show great
potential for development in medicinal chemistry (Figure 1)3
but also act as all-purpose and important intermediates in the
construction of series of key framework structures.4 It also
deserves to be mentioned that researchers have found that
cyclopropane-fused tetrahydroquinolines could serve as potent
HIV-1 non-nucleoside reverse transcriptase inhibitors.5 Driven
by the structural complexity and remarkable biological
properties of cyclopropane-fused tetrahydroquinolines, intense
interest has been triggered within both the synthetic and
medicinal communities, and several efficient approaches have
been developed to date.6 In 2012, Cramer’s group reported
palladium-catalyzed C−H arylation of cyclopropanes to
achieve the construction of structurally diverse tetrahydroqui-
cyclopropane-fused tetrahydroquinolines (Scheme 1b).6b
Although great progress has been achieved, the construction
of cyclopropane-fused tetrahydroquinolines has been restricted
to metal catalysis until now. Hence, it is necessary to develop
efficient transformations without metal catalysis to synthesize
cyclopropane-fused tetrahydroquinolines.
Received: March 9, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00874
© XXXX American Chemical Society
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A

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Cyclopropanation of alkenes as an efficient means could lead
to synthetically useful and biologically important cyclo-
propane-fused heterocyclic ring systems.7α-Substituted vinyl-
sulfonium tetraphenylborates as unique vinylsulfoniums have
better stability and accessibility due to the existence of the
tetraphenylborate counterion.8 To the best of our knowledge,
the use of α-aryl vinylsulfonium tetraphenylborates as materials
for the preparation of cyclopropane-fused tetrahydroquinolines
has not been exploited up to now. On the other hand, 2-
aminochalcones and 2-(2-aminobenzylidene)-1H-indene-
1,3(2H)-dione are useful synthons and could be used to
construct kinds of functionalized tetrahydroquinolines.9 In this
context and in conjunction with the long-term interest of our
group directed toward the exploitation of novel reactions for
the construction of heterocyclic compounds,10 in this work we
developed a novel transition-metal-free sequential [4 + 2]/[2 +
1] annulation reaction of α-substituted vinylsulfoniums with 2-
aminochalcones and 2-(2-aminobenzylidene)-1H-indene-
1,3(2H)-dione that delivers valuable cyclopropane-fused
tetrahydroquinolines (Scheme 1c).
Cs2CO3 was used, an improvement in the yield to 97% was
achieved (Table 1, entry 12).
After we obtained the optimized reaction conditions, we
inspected the scope and generality of the sequential [4 + 2]/[2
+ 1] annulation reaction by reacting 1a with 2-aminochalcones
2. As shown in Scheme 2, the results showed that the
a b c
, ,
Scheme 2. Substrate Scope of 2-Aminochalcones
We carried out the preliminary optimization experiments
employing the α-phenylvinylsulfonium tetraphenylborate (1a)
and 2-aminochalcone 2a as the model substrates to verify the
feasibility of the transformation. The reaction proceeded
smoothly in the presence of tetramethylguanidine, yielding
cyclopropane-fused tetrahydroquinoline 3a in 46% yield with
excellent diastereoselectivity. To improve the synthetic
efficiency, some other bases were evaluated, including DBU,
DABCO, Na2CO3, Cs2CO3, K2CO3, and t-BuOK. It was found
that reaction activity is strongly influenced by the base, and the
reaction with Cs2CO3 as the base gave the best results, yielding
3a with >20:1 dr in 90% yield (Table 1, entry 6). In order to
increase the efficiency of synthetic process, the medium was
investigated, and the results showed that THF was the most
suitable solvent, in which the expected sequential reaction was
complete within 2 h and afforded product 3a in excellent yield
(96%; Table 1, entry 10). In addition, when 1.5 equiv of
a
Reactions were performed with 0.12 mmol of 1a, 0.10 mmol of 2,
b
and 0.15 mmol of Cs2CO3 in THF (1.0 mL). The dr values were
determined by H NMR analysis. Isolated yields are shown.
c
1
electronic properties and positions of substituent groups on the
aromatic skeletons made no negative impression on the
diastereoselectivities, and all of the desired products were
obtained with >20:1 dr. We first examed substrates 2 bearing
variation of the substituent R1 at C3 or C5 on the aniline
moiety, and the reaction yielded products 3b−e in good yields
(83−97%). Then the substituent on the aryl ring (R2) was
explored, and irrespective of the substituent position and
electronic properties, all of them could be used in the
developed annulation, furnishing the cyclopropane-fused
tetrahydroquinoline derivatives 3f−l in moderate to excellent
yields (57−99%). What is more, the sequential transformation
also could tolerate heteroaromatic rings (R2), generating 3m
and 3n in 99% and 97% yield, respectively. It is worth
mentioning that 2-naphthyl-substituted 2-tosylaminochalcone
2o also could successfully be converted into the corresponding
cyclopropane-fused tetrahydroquinoline 3o in 91% yield.
Next, more in-depth exploration and study of the sequential
transformation generality was our focus. First, diverse
substitutions on the benzene ring of α-substituted vinyl-
sulfonium tetraphenylborates 1 were explored. When vinyl-
sulfonium tetraphenylborates 1b−h bearing substituents at the
para or meta position possessing electron-rich or electron-
deficient character were employed, the reactions took place
smoothly to afford the desired annulation products 3p−v in
62−99% yield with >20:1 dr (Scheme 3). However, vinyl-
sulfonium tetraphenylborates 1i and 1j bearing a chlorine or
bromine atom, respectively, at the ortho position could not give
the desired products. Additionally, α-(2-naphthyl)-
vinylsulfonium tetraphenylborate (1k) and α-biphenylvinylsul-
fonium tetraphenylborate (1l) were tolerated by the desired
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
base
solvent
time (h)
yield (%)
dr
1
2
3
4
5
6
7
8
9
TMG
DBU
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
MeOH
THF
2
2
12
48
2
2
2
3
96
2
46
69
9
>20:1
>20:1
n.d.
DABCO
Na2CO3
K2CO3
Cs2CO3
t-BuOK
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
trace
81
90
69
95
n.r.
96
94
97
n.d.
>20:1
>20:1
>20:1
>20:1
n.d.
>20:1
>20:1
>20:1
10
11
12
Toluene
THF
2
0.5
d
a
Unless specified otherwise, reactions were performed with 0.12
mmol of 1a, 0.10 mmol of 2a, and 0.12 mmol of base in DCM (1.0
b
c
d
mL). Isolated yields. Determined by 1H NMR analysis. 0.15 mmol
of Cs2CO3 was used.
B
https://dx.doi.org/10.1021/acs.orglett.0c00874
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corresponding annulation product 5a in excellent yield (99%).
Then the reactions of various α-aryl-substituted vinylsulfonium
tetraphenylborates 1 with 4 were conducted to test the
generality of the sequential reaction. It was found that the
transformations were well-tolerated and furnished the desired
annulation products 5b−i in 54% to >99% yield. Notably, the
reaction also worked with the bulky 2-naphthylvinylsulfonium
tetraphenylborate 4j, delivering product 5j in 96% yield. We
unambiguously determined the relative configuration of 5b by
single-crystal X-ray analysis (CCDC 1955197).
Scheme 3. Substrate Scope of α-Substituted
Vinylsulfoniums
a b c
, ,
In order to further investigate the generality and scope of
this approach, some other anilines bearing different Michael
acceptors were employed as reaction partners (Scheme 5). The
a b c
, ,
Scheme 5. Experimental Results with Other Substrates
a
b
The reaction conditions were the same as in Scheme 2. The dr
c
values were determined by 1H NMR analysis. Isolated yields are
shown.
a
Reaction conditions: 1a (0.24 mmol), 6 (0.20 mmol), and Cs2CO3
b
1
(0.30 mmol) in 2.0 mL of THF. Determined by H NMR analysis.
transformation and led to interesting polycyclic adducts 3w
and 3z in yields of >99% and 99%, respectively, with >20:1 dr.
From X-ray diffraction analysis and NMR data, we
unambiguously verified the structures of 3v (CCDC
1955198) and 3z (CCDC 1954605).
Spirocyclopropane motifs as core structures are widely
present in many biological compounds and natural products.11
The construction of spirocyclopropanes has provoked
researchers’ interest.12 Encouraged by the success of the
above synthesis of cyclopropane-fused tetrahydroquinolines,
we tried to further investigate the formation of spirocyclopro-
pane-fused tetrahydroquinoline structural motifs with the
reaction of 1a and 2-(2-aminobenzylidene)-1H-indene-
1,3(2H)-dione (4) under the standard conditions (Scheme
4). The transformation occurred successfully and afforded the
c
Isolated yields are shown.
sequential [4 + 2]/[2 + 1] reaction was applicable to nitro-
bearing substrate 6a, giving the corresponding adduct 7a in
66% yield with >20:1 dr. With 2-aminocinnamaldehyde 6b as a
reaction partner, only a small amount of product was obtained.
Michael acceptors containing an ester group (6c) or cyano
group (6d) showed low reactivity, and we did not observe the
corresponding desired products in the current study.
In the presence of optimal reaction conditions, we
performed a gram-scale reaction of α-(3-bromophenyl)-
vinylsulfonium tetraphenylborate (1h) with 2-aminochalcone
2a (3.0 mmol, 1.13 g), and cyclopropane-fused tetrahydroqui-
noline 3v was obtained in 98% yield (Scheme 6). Moreover,
Scheme 6. Gram-Scale Reaction and Derivatization of 3v
Scheme 4. Experimental Results with 2-(2-
Aminobenzylidene)-1H-indene-1,3(2H)-dione
a b
,
derivatization of 3v with 1-naphthaleneboronic acid was
conducted by a palladium-catalyzed C−C bond coupling
reaction, affording the corresponding product 8 in 97% yield.
On the basis of our experimental results, HRMS analysis (for
details, see the Supporting Information) and the related
literature,7,8 a possible reaction pathway for sequential [4 + 2]/
[2 + 1] reaction is proposed in Figure 2. In the presence of
a
b
The reaction conditions were the same as in Scheme 2. Isolated
yields are shown.
C
https://dx.doi.org/10.1021/acs.orglett.0c00874
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Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Zhen-Hua Wang − Institute for Advanced Study, Chengdu
University, Chengdu 610106, China; orcid.org/0000-0002-
0603-707X; Email: wangzhenhua@cdu.edu.cn
Wei-Cheng Yuan − Institute for Advanced Study, Chengdu
University, Chengdu 610106, China; National Engineering
Research Center of Chiral Drugs, Chengdu Institute of Organic
Chemistry, Chinese Academy of Sciences, Chengdu 610041,
China; orcid.org/0000-0003-4850-8981;
Email: yuanwc@cioc.ac.cn
Authors
Figure 2. Proposed plausible pathway.
Li-Wen Shen − National Engineering Research Center of Chiral
Drugs, Chengdu Institute of Organic Chemistry, Chinese
Academy of Sciences, Chengdu 610041, China
Ke-Xin Xie − Chengdu Institute of Biology, Chinese Academy of
Sciences, Chengdu 610041, China
Yong You − Institute for Advanced Study, Chengdu University,
Chengdu 610106, China
Jian-Qiang Zhao − Institute for Advanced Study, Chengdu
University, Chengdu 610106, China; orcid.org/0000-0002-
4444-9630
Cs2CO3, 2-aminochalcone 2a is deprotonated to generate
nucleophilic anion A, which attacks α-phenyl-substituted
vinylsulfonium tetraphenylborate 1a to give intermediate B.
Probably because of the π−π-stacking interaction between the
phenyl groups of the vinylsulfonium and the aniline in the 2-
aminochalcone, the subsequent intramolecular Michael
addition preferentially takes place via intermediate B-2,
resulting in the formation of intermediate C. Subsequently,
perhaps as a result of the steric hindrance effect from the
benzoyl and phenyl groups, the intramolecular SN2 reaction
preferentially occurs via intermediate C-1 to yield the expected
cyclopropane-fused tetrahydroquinoline product 3a with
simultaneous release of dimethyl sulfide.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00874
Notes
In conclusion, our group has developed a practical and
efficient Cs2CO3-promoted sequential [4 + 2]/[2 + 1]
annulation reaction of α-substituted vinylsulfoniums with 2-
aminochalcones and 2-(2-aminobenzylidene)-1H-indene-
1,3(2H)-dione. Under mild conditions, structurally diverse
cyclopropane-fused tetrahydroquinolines and spirocyclopro-
pane-fused tetrahydroquinolines could be efficiently furnished
in excellent yields with this protocol. What is more, the
underlying synthetic application of the reported transformation
was proved through scale-up verification and chemical
conversion. In addition, a plausible pathway was proposed to
rationalize the stereoselective outcome of the sequential
annulation process. We expect these cyclopropane-fused
tetrahydroquinoline products to have good prospects for
chemical biology and drug discovery.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (21801026, 21572223,
21871252, and 21801024), the National Key R&D Program
of China (2018YFC0807301-3), the Project of Youth Science
and Technology Innovation Team of Sichuan Province
(2016TD0027), and the Startup Fund of Chengdu University
(2081917039).
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ASSOCIATED CONTENT
* Supporting Information
■
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