Divergent Synthesis of Bicyclo[3.2.1]octenes and Cyclohexenes via Catalytic Annulations of Nazarov Reagent and Vinyl 1,2-Diketones

Article abstract of DOI:10.1021/acs.orglett.0c02763

Bicyclo[3.2.1]octanes and related structures are unique units that widely exist in natural products, but the rapid and stereoselective construction of this skeleton is a challenging issue. We report the stereodivergent synthesis of bicyclo[3.2.1]octenes u

Full text of DOI:10.1021/acs.orglett.0c02763

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Letter
Divergent Synthesis of Bicyclo[3.2.1]octenes and Cyclohexenes via
Catalytic Annulations of Nazarov Reagent and Vinyl 1,2-Diketones
Wenjun Liu, Shengtong Niu, Zhifei Zhao, Shuang Yang, Jinggong Liu,* Yongjin Li,* and Xinqiang Fang*
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ABSTRACT: Bicyclo[3.2.1]octanes and related structures are
unique units that widely exist in natural products, but the rapid
and stereoselective construction of this skeleton is a challenging
issue. We report the stereodivergent synthesis of bicyclo[3.2.1]-
octenes using Nazarov reagents and alkenyl 1,2-diketones with
Brønsted base catalysis under mild conditions. Both stereoisomers
of the bridged products can be obtained by tuning the reaction
conditions, and cyclohexene product can also be selectively formed.
Scheme 1. Background and Work Introduction
he rapid construction of bridged-ring scaffolds is an
important research topic in organic synthesis, owing to
T
the ubiquity of such units in a large amount of bioactive
molecules, natural products, and approved drugs.1 However,
the issue is not easily addressable, and in many cases, multiple
steps are needed and harsh conditions are employed to
produce a bridged-ring compound owing to the necessity of
overcoming the energy barrier; as a consequence, the yields are
usually low, and the stereoselectivity control is poor. For
instance, bicyclo[3.2.1]octanes or their related structures
widely exist as the key skeletons in many natural products
such as bisorbicillinolide, adenostemmoic acid D, trichorabdo-
nin, fujenal, fujenoic acid, and velloziolone (Scheme 1a),2 but
the rapid construction of such unit is still a challenging issue to
date.3 In this context, Snider and co-workers developed
systematically a radical cascade strategy for the total synthesis
of velloziolone (Scheme 1b),4a−d and the Carreira group
achieved the elegant total synthesis of (+)-dendrowardol C
using a intramolecular aldol reaction to form the bridged
structure (Scheme 1c).4e A wonderful work using highly
twisted bicyclo[3.2.1]oct-1-en-3-one intermediate in natural
products synthesis was developed by the Ma group (Scheme
1d).4f All these works have shown the unique importance of
bicyclo[3.2.1]octane construction in organic synthesis,
although the yields, the stereoselectivities, and the step
economy of the methods are left to be further improved.
On the other side, since its discovery in 1953,5a the Nazarov
reagent has attracted continuous and extensive studies owing
to its irreplaceable roles in the synthesis of cyclic molecules
through various annulation modes.5 However, the use of
Nazarov reagent in the assembly of bridged structures has been
extremely underdeveloped to the best of our knowledge, thus
leaving a big space to further promote the value of this named
reagent. Here, we report for the first time the one-step and
stereodivergent synthesis of bicyclo[3.2.1]octenes using
Nazarov reagent and alkenyl diketones by only tuning the
reaction conditions. Moreover, the reaction can also afford
cyclohexene compounds in a highly stereoselective manner. All
products can be obtained under mild conditions with good
Received: August 18, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02763
© XXXX American Chemical Society
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Table 1. Condition Optimization for Divergent Formation of Products
yields (%)
entry
base (mol %)
solvent
temp (°C)
time (h)
additive (mol %)
3a
4a
5a
1
2
3
4
5
6
7
8
none
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
toluene
DMF
DMSO
MeOH
EtOH
iPrOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMSO
DMSO
DMSO
DMSO
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
40
40
0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
24
24
10
10
10
6
none
none
none
none
none
none
none
none
none
none
none
none
none
none
0
0
0
0
0
0
0
0
0
0
33
32
0
0
0
0
28
30
31
trace
25
39
30
Cs2CO3 (20)
K2CO3(20)
CsOAc (20)
DMAP (20)
DBU (20)
Et3N (20)
K2CO3(20)
K2CO3(20)
K2CO3(20)
K2CO3(20)
K2CO3(20)
K2CO3(20)
K2CO3(10)
Cs2CO3(20)
Na2CO3(20)
NaOtBu(20)
KOH(20)
NaOH (20)
LiOH (20)
NaOH (20)
NaOH(20)
LiOH(20)
LiOH (20)
Cs2CO3(20)
Cs2CO3(20)
Cs2CO3(20)
Cs2CO3(20)
19
18
0
0
0
0
11
15
25
24
42
25
40
47
46
49
47
62
21
73
70
trace
trace
12
12
22
19
9
trace
14
42
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
trace
trace
trace
trace
trace
trace
trace
13
trace
11
14
trace
trace
51
none
none
none
none
none
none
none
TBAI (20)
none
TBAI (20)
TBAI (20)
BTEAI (20)
BTEAI (20)
BTEAI (20)
28
31
21
26
26
64
14
10
70
75
trace
trace
trace
trace
0
rt
rt
40
rt
56
47
60
a
All reactions were run on a 0.2 mmol scale under argon atmosphere. The diastereomeric ratio was determined via 1H NMR analysis of the reaction
mixtures. All yields were isolated yield. TBAI = tetra-n-butylammonium iodide. BTEAI = benzyltriethylammonium iodide.
stereoselectivity control; up to five stereocenters are
constructed, and three new C−C bonds are formed in the
process. This work provides a relatively easy path for the
synthesis of the bridged compounds.
(Table 1, entry 19). We then found that slightly increasing the
temperature could further improve the formation of 3a to 73%
yield (Table 1, entry 21), and the addition of tetra-n-
butylammonium iodide (TBAI) did not affect the result
(Table 1, entry 22). To our pleasure, based on the results
shown in entry 20, we further decreased the temperature and
prolonged the reaction time, and 5a could be obtained as the
major product (Table 1, entry 23), and the addition of catalytic
amount of TBAI further promoted the yield of 5a to 75%
(Table 1, entry 24). The final purpose is to produce bridged
compound 4a in a practical yield, but the task proved to be
more difficult. Using DMSO as the solvent and Cs2CO3 as the
base, we examined a series of other reaction parameters (Table
1, entries 25−28) and finally found that 4a can be formed in
60% yield at room temperature using benzyltriethylammonium
iodide (BTEAI) as the additive (Table 1, entry 28). Further
efforts to increase the yield of 4a did not give better results.
Having achieved the selective formation of two bridged
compounds and one [4 + 2] annulation product, we then
evaluated the generality and limitations of the protocol, with
the formation of product 3 first. The reaction showed good
tolerance to the R2 group in diketone substrates, and phenyl
rings bearing electron-withdrawing or electron-donating
We commenced by surveying the reaction using Nazarov
reagent 1a and alkenyl diketone 2a as the model substrates. No
reaction occurred without base (Table 1, entry 1), and the
addition of a catalytic amount of Cs2CO3 or K2CO3 led to the
formation of both 3a and 5a, with the latter as the major one
(Table 1, entries 2 and 3). The reactions using CsOAc,
DMAP, DBU, and Et3N all resulted in no observation of the
annulation products (Table 1, entries 4−7). Then, using
K2CO3, we screened a series of solvents, and THF and toluene
showed similar results compared to that using CH2Cl2 (Table
1, entries 8 and 9). To our pleasure, strong polar solvents of
DMF and DMSO allowed access to 4a, with DMSO showing
better selectivity (Table 1, entries 10 and 11). Reactions in
i
protic solvents such as MeOH, EtOH, and PrOH showed
better yields, and 3a was the major product when MeOH was
used (Table 1, entries 12−14). Then with MeOH as the
solvent, we tested a series of bases including Cs2CO3, Na2CO3,
NaOtBu, KOH, NaOH, and LiOH (Table 1, entries 15−20),
and NaOH showed better results concerning the yield of 3a
B
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a
groups had little impact on the outcomes (Scheme 2, 3b and
3c). When the R3 group was an electron-rich aromatic ring or
Scheme 3. Scope of Bridged Compound 4 Synthesis
a
Scheme 2. Scope of Bridged Compound 3 Synthesis
a
Reactions conditions: 1 (0.2 mmol), 2 (0.2 mmol), Cs2CO3 (20 mol
%), BTEAI (0.2 equiv), DMSO (1 mL), rt, 6 h, under argon
atmophere; all yields were of isolated products; dr values were
b
1
determined via H NMR analysis of the reaction mixtures. 0.5 g of
c
4h was obtained. 200 mg of 4 Å M.S. was added.
a
Reactions conditions: 1 (0.2 mmol), 2 (0.2 mmol), NaOH (20 mol
corresponding products could be formed efficiently (Scheme
3, 4g and 4h). Finally, alkyl-substituted diketones or Nazarov
reagent were also found to be compatible to the reaction,
producing 4i−4k in 51−88% yields (Scheme 3, 4i−4k). The
reaction could be scaled up to produce 0.5 g of 4h, and the
structures of the products shown in this table were confirmed
via single-crystal X-ray structure analysis of 4g.
The rapid synthesis of highly functionalized cyclohexenes is
also an important mission in organic synthesis, as witnessed by
the widely studied [4 + 2] annulations.6 In this context, we
continued to examine the formation of 5. To our pleasure, the
protocol is compatible to a series of Nazarov reagents and
diketones with different aryl groups, releasing 5a−5f in 73−
76% yields (Scheme 4, 5a−5f). Product 5a could be produced
on a large scale (0.6 g), and the configuration of the products
was confirmed by single-crystal X-ray structure analysis of 5f.
Furthermore, the diketone moieties within the products were
readily transferred to N-heterocyclic ring compounds such as
5aa and 5ab (Scheme 4, 5aa and 5ab), which further expanded
the utility of this method.
%), MeOH (1 mL), 40 °C, 12 h, under argon atmophere; all yields
were of isolated products; dr values were determined via H NMR
analysis of the reaction mixtures. 0.6 g of 3j was obtained.
1
b
aliphatic group, the reaction could also proceed to produce the
corresponding products, albeit in diminished yields (Scheme 2,
3d and 3e). Moreover, the use of alkyl R2 group also proved
possible, affording 3f in 66% yield (Scheme 2, 3f). When both
R2 and R3 were furan groups, the reaction could also work well
(Scheme 2, 3g). Substrates 1 with differently substituted R1
groups were also examined, and the corresponding products
3h−3k were all obtained in acceptable yields (Scheme 2, 3h−
3k). A Nazarov reagent with an aliphatic R1 group was also
tested, and the product could also be formed with moderate
yield (Scheme 2, 3l). Noteworthy is that the reaction affording
3j can be run on a gram scale, and the structure of 3a was
confirmed unambiguously via single-crystal X-ray structure
analysis.
Having gotten a variety of bridged compounds 3, we then
focused on the production of the isomeric product 4.
Unsaturated diketones with different R2 groups were all
tolerated under the optimal conditions, delivering 4b−4d
selectively (Scheme 3, 4b−4d). When both R2 and R3 are 4-
ClC6H4 or furyl groups, the corresponding products were
obtained in moderate to good yields (Scheme 3, 4e and 4f).
Moreover, the combinations of differently substituted R1, R2,
and R3 groups were also tested, and in all cases, the
A series of preliminary studies to achieve the asymmetric
catalysis of the reaction have also been conducted using chiral
tertiary amine catalysts and chiral phase-transfer catalysts (see
Table S1 in the Supporting Information for the details), but
less satisfactory results were observed, and more efforts are
needed to solve the problem.
The reaction between Nazarov reagent 1a and chalcone 6a
under the conditions in Scheme 2 only resulted in 7a in 42%
C
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a
the reaction using 5a as the substrate can afford 3a as the
major product under the standard conditions employed in
Scheme 2, and 4a was obtained as the major product using the
conditions described in Scheme 3 (Scheme 5b). Accordingly, a
postulated mechanism was proposed as shown Scheme 5c.
Initially, an addition occurs between the deprotonated Nazarov
reagent and 1a to give enolate I, which is followed by the
second Michael addition to form six-membered intermediate
II. There should be equilibrium between enolates II, III, and
IV, and the protonation of IV affords 5a. Conversely, the
further deprotonation of III will lead to intermediate V with a
double enolate structure, and 3a and 4a then can be selectively
generated via an aldol process. Although at the current stage
the origin of the diastereoselective formation of the two
bridged products is not very clear, we think that the solvents
might play crucial roles in forming different stereoisomers in
the reactions, which can be clearly observed from the results
shown in Table 1, entries 15, 19, and 25. The protic solvent of
MeOH might interact with intermediate V through hydrogen
bonding, which leads to different diastereoselectivity.7
Scheme 4. Formation of 5 and Further Transformations
In summary, we have achieved the rapid and divergent
construction of bicyclo[3.2.1]octenes and cyclohexenes under
mild conditions with good to high levels of stereocontrol using
Nazarov reagents and vinyl diketones. Three different products
can be formed through the slight modifications of the reaction
conditions, thus achieving the molecular diversity from the
same set of starting materials. The protocol provides a new
choice for the synthesis of natural products and bioactive
compounds containing bicyclo[3.2.1]octane units. The unique
reactivity of unsaturated diketones is also highlighted in this
work.
a
Reactions conditions: 1 (0.2 mmol), 2 (0.2 mmol), LiOH (20 mol
%), TBAI (0.2 equiv), MeOH (1 mL), 0 °C, 24 h, under argon
atmophere; all yields were of isolated products; dr values were
b
determined via 1H NMR analysis of the reaction mixtures. 0.6 g of 5a
was obtained.
yield (Scheme 5a), and no bridged product was formed,
indicating the unique reactivities of unsaturated diketones
compared to the chalcone-type compounds. Mechanistically,
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02763.
Scheme 5. Control Experiments and Postulated Mechanism
Experimental procedures, spectroscopic data for all new
compounds, and crystallographic data for 3a, 4g, and 5f
(PDF)
Accession Codes
CCDC 2015422−2015424 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Xinqiang Fang − State Key Laboratory of Structural Chemistry,
and Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Center for Excellence in Molecular Synthesis, Fujian
Institute of Research on the Structure of Matter, University of
Chinese Academy of Sciences, Fuzhou 350100, China;
orcid.org/0000-0001-8217-7106; Email: xqfang@
fjirsm.ac.cn
Yongjin Li − Orthopedics Department, Guangdong Provincial
Hospital of Traditional Chinese Medicine, The Second Affiliated
Hospital of Guangzhou University of Chinese Medicine,
Guangzhou 510120, China; Email: lyj2106@126.com
D
https://dx.doi.org/10.1021/acs.orglett.0c02763
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Jinggong Liu − Orthopedics Department, Guangdong Provincial
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Authors
Wenjun Liu − State Key Laboratory of Structural Chemistry,
and Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Center for Excellence in Molecular Synthesis, Fujian
Institute of Research on the Structure of Matter, University of
Chinese Academy of Sciences, Fuzhou 350100, China
Shengtong Niu − State Key Laboratory of Structural Chemistry,
and Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Center for Excellence in Molecular Synthesis, Fujian
Institute of Research on the Structure of Matter, University of
Chinese Academy of Sciences, Fuzhou 350100, China
Zhifei Zhao − State Key Laboratory of Structural Chemistry,
and Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Center for Excellence in Molecular Synthesis, Fujian
Institute of Research on the Structure of Matter, University of
Chinese Academy of Sciences, Fuzhou 350100, China
Shuang Yang − State Key Laboratory of Structural Chemistry,
and Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Center for Excellence in Molecular Synthesis, Fujian
Institute of Research on the Structure of Matter, University of
Chinese Academy of Sciences, Fuzhou 350100, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02763
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSFC (21871260, 21502192,
21402199), the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB20000000), Fujian Natural
Science Foundation (2018J05035), and China Postdoctoral
Science Foundation (2018M630734).
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