Stereospecific Decarboxylative Nazarov Cyclization Mediated by Carbon Dioxide for the Preparation of Highly Substituted 2-Cyclopentenones

Article abstract of DOI:10.1002/anie.201705909

Highly substituted 2-cyclopentenones were stereospecifically and regioselectively constructed with high catalytic efficiency through Lewis-acid catalyzed decarboxylative Nazarov cyclization of the cyclic carbonate derivative, which is prepared by reacting the propargyl alcohol with carbon dioxide in the presence of a silver catalyst. The stereochemistry of the 2-cyclopentenone is strictly controlled by the geometry of the alkene in the starting material. This method is applicable for various substrates.

Full text of DOI:10.1002/anie.201705909

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Accepted Article
Title: Stereospecific Decarboxylative Nazarov Cyclization Mediated
by Carbon Dioxide for Preparation of Highly-Substituted 2-
Cyclopentenones
Authors: Keiichi Komatsuki, Yuta Sadamitsu, Kohei Sekine, Kodai
Saito, and Tohru Yamada
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705909
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10.1002/anie.201705909
Angewandte Chemie International Edition
COMMUNICATION
Stereospecific Decarboxylative Nazarov Cyclization Mediated by
Carbon Dioxide for Preparation of Highly-Substituted 2-
Cyclopentenones
Keiichi Komatsuki, Yuta Sadamitsu, Kohei Sekine, Kodai Saito and Tohru Yamada*
This paper is dedicated to Professor Emeritus Teruaki Mukaiyama of The University of Tokyo in celebration of his 90th birthday.
Abstract:
Highly-substituted
2-cyclopentenones
were
high
Main issues of Nazarov cylization:
stereospecifically and regioselectively constructed with
a
(1) Catalytic efficiency
(2) Regioselectivity of elimination of proton
catalytic efficiency through the Lewis-acid catalyzed decarboxylative
Nazarov cyclization of the cyclic carbonate derivative, prepared by
the reaction of the propargyl alcohol with carbon dioxide in the
presence of a silver catalyst. The stereochemistry of the 2-
cyclopentenone could be strictly controlled by the geometry of the
alkene in the starting material. This method was applicable for
various substrates.
(3) Construction of stereochemistry
Representative solution
: Substrate control
LA
EDG
O
O
O
5
4
cat. LA
EWG
EWG
R1
EDG
EWG
EDG
R2
- LA
R2
R1
R2
H
Cationic intermediate
R1
trans
LA: Lewis acid
Nazarov cyclization is a conrotatory 4π-ring-closure reaction
of divinyl ketones through a pentadienyl cation intermediate in
the presence of a Lewis acid or Brønsted acid, producing highly-
This study
: Decarboxylative Nazarov cyclization
LA
O
O
substituted
2-cyclopentenone
derivatives.[1]
The
2-
O
O
O
O
O
Nazarov
cyclization
cyclopentenones could be used as an attractive building block,
and applicable for the synthesis of biologically-active
compounds and pharmaceuticals.[2] There are two problematic
points about the Nazarov cyclization, generally; (1) more than a
stoichiometric amount of an acid is required for an efficient
conversion, and (2) the regioselectivity of the proton elimination
from a cyclic carbocation intermediate can be poor, resulting in
the formation of several cyclopentenone isomers. To solve these
issues, Frontier’s group reported that the introduction of an
electron-donating group at the α-position and an electron-
withdrawing group at another α-position of the divinyl ketones
R1
R3
R5
R4
R1
LA
R1
R5
R4
R5
R4
E/Z
- CO2
R3
R3
R2
- LA
R2
R2
Cyclic carbonates
2-Cyclopentenones
[trans/cis]
(1) Ease of elimination of CO2 from the carbonate
Efficient catalytic cycle
(2) Decarboxylation - ring-closure
Regio- and Stereospecificity
Scheme 1. Background and concept.
dramatically
improved
the
catalytic
efficiency
and
regioselectivity.[3] Since their report, this strategy has been
frequently applied by many research groups to develop efficient
Nazarov cyclizations (Scheme 1). However, this method is
based on the substrate control and rapid epimerization due to
the existence of the electron-withdrawing group at the α-position
of the product induces a formation of the thermodynamically-
stable trans-product, mainly. Therefore, developing a widely
decarboxylative reaction of carbonates has been often employed
for the palladium-catalyzed allylic substitution reaction using
enol carbonates,[7] the Lewis acid catalyzed decarboxylation was
also recently developed.[8] The present Nazarov reaction-based
approach to obtain 2-cyclopentenones involves decarboxylation
from the cyclic enol carbonate derivative bearing a substituted-
allyl group using a Lewis acid catalyst (Scheme 1). It was initially
expected that the decarboxylation of the cyclic carbonate was
evidently irreversible and would easily proceed, resulting in the
realization of the efficient catalytic cycle. Although this reaction
is based on the Lewis acid activation, a side-reaction for the
applicable reaction with
stereodivergence would be quite attractive.[4,5]
a
broad substrate scope and
Various carbonates are readily prepared and extensively
employed for further transformations.[6] Among them, the
transition metal-catalyzed decarboxylative reaction from
carbonates is one of the most representative methods to
achieve various transformations. While the
production of the regioisomer, derived from
a cationic
intermediate, could not be found. Additionally, we envisioned
that the decarboxylation and 4π-ring-closure steps would quickly
proceed and the stereochemistry of the 2-cyclopentenone could
strictly be regulated by the geometry of the substituted allyl
group in the cyclic carbonate, stereospecifically.
[*]
K. Komatsuki, Y. Sadamitsu, Dr. K. Sekine, Dr. K. Saito,
Prof. T. Yamada
The preparation of cyclic carbonate derivatives was
developed based on the silver-catalyzed CO2 incorporation
reaction into propargyl alcohol derivatives.[9,10] It was found that
the cyclic carbonate (E)-2a was successfully formed
quantitatively when (E)-1a was treated with AgOAc (10 mol%),
Department of Chemistry, Keio University
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522 (Japan)
E-mail: yamada@chem.keio.ac.jp
[**]
The authors declare no competing financial interest.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201705909
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COMMUNICATION
O
DBU (1 equiv) in toluene at 25 ºC under CO2 (1 MPa) (Scheme
2).[11]
O
O
BF3·Et2O (10 mol%)
R1
R3
O
R5
R4
trans-3
R1
R5
R4
DCE(0.1 M)
rt
R3
R2
R2
2
O
O
O
Ph
Ph
Me
PMP
p-CF3C6H4
Me
Me
Ph
Ph
trans-3c
81%[c]
trans-3a
95%
trans-3b
80%
Scheme 2. Preparation of cyclic carbonate.
O
O
O
Ph
Ph
Ph
Me
Me
Me
The carbonate (E)-2a was then treated with a catalytic
amount of various Lewis acids in 1,2-dichloroethane at room
temperature. When this reaction was carried out using Cu(OTf)2,
and Yb(OTf)3, the Nazarov cyclization proceeded to afford trans-
3a with low conversions (Table 1, entries 1-2). When this
reaction proceeded using BiCl3, cationic Au(I) catalyst and Ag(I),
trans-3a were obtained in 79%, 85% and 86% yields,
respectively (entries 3-5). When Sc(OTf)3 was employed for this
reaction, the full conversion of (E)-2a occurred to give (E)-3a in
95% yield (entry 6). FeCl3 and MgBr2 were also quite effective
for this reaction (entries 7 and 8). While
Ph
trans-3f
Me
PMP
trans-3d
p-CF3C6H4
trans-3e
97%[c]
71%
95%
O
O
O
Ph
Me
Ph
Ph
Ph
Ph
Ph
iPr
tBu
Ph
Ph
trans-3h
trans-3i
96%[c,d]
trans-3g
97%
92%
O
O
O
Ph
Me
Me
nBu
Me
nBu
trans-3j
94%
trans-3k
95%
trans-3l
96%
O
Table 1. Screening of lewis acid catalyst.
O
O
Ph
Me
Ph
Me
Ph
Me
Entry
Lewis acid
Time
RSM[c]
Yield of 3a (%)[a]
3m
Me
1
2
Cu(OTf)2
Yb(OTf)3
BiCl3
12
12
12
12
12
1
60
52
37
40
Me
3n
87%
87%[c]
3o
96%
3
14
79
General tendency
O
This work
O
4
(PPh3)AuCl - AgSbF6
AgSbF6
3
85
Rx
Rx
Ry
Ry
5
ND
ND
ND
ND
ND
ND
86
Rx > Ry (cation-stabilizing ability)
Rx: Aryl group, long alkyl chain
6
Sc(OTf)3
FeCl3
95
7
1
96
[a] Reactions were carried out on a 0.15 mmol scale with substrate (0.150
mmol), BF3⋅OEt2 (0.015 mmol) in 1,2-dichloroethane (0.1 M) at room
temperature. Yields of
propargyl alcohols 1. [b] Isolated yields. [c] BF3⋅OEt2 (30 mol%) was used. [d]
Carbonate (E)-2i was prepared using DBU (2 equiv) and AgOAc (20 mol%)
under CO2 (1 MPa). PMP = 4-methoxyphenyl.
8
MgBr2
1
96
3 were calculated based on the corresponding
9
B(C6F5)3
1
96
10
BF3⋅OEt2
1
96 (95)[b]
Conditions: Reactions were carried out on a 0.1 mmol scale with (E)-2a
(0.150 mmol), Lewis acid (0.015 mmol) in 1,2-dichloroethane (0.1 M) at
room temperature. Yields of 3a were calculated based on (E)-1a. [a]
Determined by 1H NMR. [b] Isoated yields. [c] Recovery of starting material.
DCE = 1,2-dichloroethane. ND = not detected.
Scheme 3. Application of various substrates.[a,b]
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10.1002/anie.201705909
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COMMUNICATION
a boron Lewis acid also worked well, it was found that BF3⋅OEt2,
which is one of the most common Lewis acids, was the catalyst
of choice for this reaction (entry 10).
The reaction using (Z)-2a as a substrate under the optimized
conditions was then examined to confirm the outcome of the
stereoselectivity at the 4,5-positions of the 2-cyclopentenones.
Carbonate (Z)-2a was synthesized from (Z)-1a by a similar
silver-catalyzed CO2 fixation reaction. When (Z)-2a was
employed for the Nazarov reaction, the corresponding product
cis-3a was obtained in 89% yield, involving the formation of a
small amount trans-3a.[15,16] This result indicated that the
stereochemistry of the 2-cyclopentenone could clearly be
controlled by the geometry of the olefin part in the starting
material, stereospecifically.
In order to evaluate the scope of this Nazarov cyclization,
the substituent effect on the exo-olefin part was investigated
(Scheme 3). Electron-donating (2b) and electron-withdrawing
(2c) group substituted cyclic carbonates were smoothly
converted to the corresponding cyclopentenones, trans-3b and -
3c in 80% and 81% yields, respectively.[12] Substrates bearing
various aryl groups at the olefin termini were also applicable for
this reaction to afford trans-3d and -3e in 71% and 97% yields,
respectively. When substrates having a tri-substituted olefin
were employed for this method, trans-3f and -3g were
successfully isolated in high yields. The introduction of other
alkyl groups at the tether part (R5) was also suitable for this
transformation to give trans-3h and -3i in high yields.[13] Alkyl
group substitution at R1 (trans-3j) showed almost the same
reactivity as with the aryl group substituted analogues.
Cyclopentenones fused 6- and 7-membered carbocycles, trans-
3k and -3l, were successfully constructed using substrates 2k
and 2l under the same reaction conditions. In the case of the
reaction of 2m bearing an exo-methylene, the di-substituted
cyclopentenone 3m was obtained in 87% yield using 30 mol% of
BF3⋅OEt2. Substrates 2n and 2o bearing another type of tri-
substituted olefin unit were subjected to this Nazarov cyclization,
and the corresponding products 3n and 3o having quaternary
carbon center at the 4-position were efficiently constructed in
87% and 96%, respectively. It is generally known that the
formation of double bond of the cyclopentenones preferentially
occurs at the carbocation-stabilizing group-substituted side in
the typical Nazarov reaction.[14] Thus, it is noteworthy that the 2-
cyclopentenone, in which the substitution pattern is different
from a usual Nazarov-cyclization product, could be accessed by
this method. (depicted below in Scheme 3).
In conclusion, we have developed an efficient Nazarov
cyclization triggered by the Lewis acid-promoted decarboxylation
of the cyclic carbonate derivative, prepared from the reaction of
the silver-catalyzed CO2 fixation for the propargyl alcohol
derivative. This Nazarov cyclization produces various-highly
substituted 2-cyclopentenone derivatives as a single isomer,
which have a totally different substitution pattern from the
classical Nazarov cyclization product. It was elucidated that the
relative configuration of the product is attributed to the geometry
of the substituted-allyl group in the cyclic carbonate derivative.
Investigations of the mechanistic insights and applications to the
synthesis of more complex molecules are currently under way.
Experimental Section
The reaction was performed using
a pressure-resistant test-tube
equipped with a stirring bar in a 30 mL autoclave. To a mixture of AgOAc
(0.015 mmol) and 1 (0.15 mmol) in dehydrated toluene (1.5 mL) in the
test-tube was added DBU (0.15 mmol). The test-tube containing the
reaction mixture was placed in the autoclave. The autoclave was purged
with CO2 and the reaction mixture was stirred at 25 ºC for 4 h under CO2
atmosphere (1 MPa). After CO2 was carefully released, the reaction
mixture was diluted with EtOAc (10 mL), and washed with water (10 mL),
then brine (10 mL). The organic phase was dried over Na2SO4, and the
solvent was removed under reduced pressure and dried in vacuo to
obtain the corresponding cyclic carbonate 2. The cyclic carbonate 2 was
immediately used for the next step without further purification. The cyclic
carbonate 2 was dissolved in dehydrated DCE (1.5 mL) and BF3⋅Et2O
(0.015 mmol) was added. The reaction mixture was stirred at room
temperature under N2. After 1 h, the reaction was quenched with brine
(10 mL), and extracted three times with EtOAc. The combined organic
layer was dried over N2SO4, then concentrated in vacuo and purified by
flash column chromatography to afford the 2-cyclopentenone 3.
Received: ((will be filled in by the editorial staff))
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Keywords: carbon dioxide • C-C bond formation • stereospecific
reaction • electrocyclic reaction • silver catalyst
[a] Reactions were carried out on a 0.15 mmol scale with substrate (0.150
mmol), BF3⋅OEt2 (0.030 mmol) in 1,2-dichloroethane (0.1 M) at room
temperature for 1 h. Yields of 3a were calculated based on the corresponding
propargyl alcohol (Z)-1a. [b] Isolated yields. [c] NMR yield.
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Scheme 4. Investigation of the stereospecificity of this Nazarov cyclization.[a]
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10.1002/anie.201705909
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COMMUNICATION
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Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn, Angew. Chem. Int.
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Tsuji, T. Fujihara, Chem. Commun. 2012, 48, 9956; f) J. Rintjema, A. W.
Kleij, Synthesis 2016, 48, 3863.
[5]
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catalytic Nazarov-type cyclization, see: a) X. Shi, D. J. Gorin, F. D.
Toste, J. Am. Chem. Soc. 2005, 127, 5802; b) L. Zhang, S. Wang, J.
Am. Chem. Soc. 2006, 128, 1442; c) B. G. Pujanauski, B. A. B. Prasad,
R. Sarpong, J. Am. Chem. Soc. 2006, 128, 6786; d) G. Lemière, V.
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[11] Carbonate (E)-2a was less-stable during silica-gel column
chromatography. Thus, after the CO2 fixation reaction, the mixture was
diluted with ethyl acetate and was washed with H2O to remove DBU.
After drying, we used the crude product for the next step without further
purification. See the Supporting information for details.
[12] The relative configuration of 3c was evidently determined to be trans by
a single crystal X-ray structural analysis. CCDC 1553266 (trans-3c)
contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
The relative configurations of all the other derivatives were determined
by the analogy to trans-3c.
[13] Although the Wagner-Meerwein rearrangement on the carbocationic
intermediate could occur during the Nazarov reaction, we did not
observe this rearrangement of the methyl group derived from the t-butyl
group. Therefore, we consider that the decarboxylation step and 4π
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ring-closure step might proceed via
a concerted mechanism. For
examples of the 1,2-carbon shift during the Nazarov cyclization, see: a)
J. Huang, D. Lebœuf, A. J. Frontier, J. Am. Chem. Soc. 2011, 133,
6307; b) D. Lebœuf, J. Huang, V. Gandon, A. J. Frontier, Angew. Chem.
Int. Ed. 2011, 50, 10981; c) D. Lebœuf, V. Gandon, J. Ciesielski, A. J.
Frontier, J. Am. Chem. Soc. 2012, 134, 6296.
[14] As the related reaction, Piancatelli rearrangement, which is also acid-
catalyzed 4π-ring-closure reaction using 2-furylcarbinol derivative to
afford 2-cyclopentenone derivatives, is known. Review for the
Piancatelli rearrangement, see: C. Piutti, F. Quartieri, Molecules 2013,
18, 12290.
[15] From the 1H NMR study, it was confirmed that the formation of trans-3a
resulted from the isomerization of cis-3a during the reaction. For
detailed information, see the Supporting Information. Additionally, the
substrate (Z)-2a has a lower reactivity for these reaction conditions.
Thus, BF3⋅OEt2 (20 mol%) was used to accelerate the reaction.
[16] The relative configuration of 3a was determined to be cis by a single
crystal X-ray structural analysis. CCDC 1553268 (cis-3a) contains the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
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K. Komatsuki, Y. Sadamitsu, K. Sekine,
K. Saito, T. Yamada*
Page No. – Page No.
Stereospecific Decarboxylative
Nazarov Cyclization Mediated by
Carbon Dioxide for Preparation of
Highly-Substituted 2-
Highly-substituted 2-cyclopentenones were stereospecifically and regioselectively
constructed with a high catalytic efficiency through the Lewis-acid catalyzed
decarboxylative Nazarov cyclization of the cyclic carbonate derivative, prepared by
the reaction of the propargyl alcohol with carbon dioxide in the presence of a silver
catalyst. The stereochemistry of the 2-cyclopentenone could be strictly controlled by
the geometry of the alkene in the starting material.
Cyclopentenones
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