Enantioselective intermolecular all-carbon [4+2] annulation: Via N-heterocyclic carbene organocatalysis

Article abstract of DOI:10.1039/c7cc08680f

The highly stereoselective intermolecular all-carbon [4+2] annulation between in situ generated acyclic dienolates and α,β-unsaturated acyl azoliums is disclosed. The identification of 2-acyloxy-3-butenones as suitable diene precursors is the key to the s

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Enantioselective Intermolecular All-Carbon [4+2] Annulation via
N-Heterocyclic Carbene Organocatalysis
Guoxiang Zhang,‡a Weici Xu,‡a Jian Liu,a Deb Kumar Das,a Shuang Yang,a Saima Perveen,a Hao
Zhang,a Xinglong Li,a and Xinqiang Fang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The highly stereoselective intermolecular all-carbon [4+2]
annulation between in situ generated acyclic dienolates and α,β-
unsaturated acyl azoliums is disclosed. The identification of 2-
acyloxy-3-butenones as suitable diene precursors is key to the
success of this transformation. The corresponding highly
functionalized cyclohexene products, which are inaccessible from
Diels-Alder reactions, were delivered with high levels of diastereo-
and enantioselectivities. A series of further transformations based
on the product showed the potential of this reaction.
R2
R2
O
Ar
NHC
(a)
ref 7
OTMS
Ar
F
+
R1
O
R1
R1
O
NC
R2
NC
R2
R1
NHC
[O]
(b)
(c)
ref 8
Ar
Ar
H
+
Ar
R1
O
O
O
O
ROCO
R2
NHC
R1
+
this work
H
The [4+2] annulation between a diene and a dienophile (e.g.,
the Diels-Alder reaction) is arguably one of the most powerful,
versatile, and reliable synthetic strategies for rapidly
generating structurally complex molecular architectures.1
Various transition-metals and organocatalysts have been
explored to facilitate highly stereoselective [4+2] annulations,
providing six-membered rings in concise and atom-economic
fashions.2 In principle, the normal-electron-demand α,β-
unsaturated acyl azoliums3 formed via N-heterocyclic carbene
(NHC) catalysis4 can be employed as dienophiles to react with
dienes, providing a new solution for the enantioselective [4+2]
annluations. However, despite the great advances in NHC-
catalyzed [3+3] and [2+3] reactions via α,β-unsaturated acyl
azoliums,5 enantioselective [4+2] annulations remain
underdeveloped.6 In 2011, a non-asymmetric all-carbon [4+2]
annulation using cyclic silyl dienol ethers and α,β-unsaturated
acyl fluorides was reported by Lupton and co-workers (Scheme
1a).7 The reactions of cyano-substituted dienolates with α,β-
unsaturated acyl azoliums affording benzene derivatives were
demonstrated by Ye and Wang, et al.8 However, attempts to
develop a highly enantioselective intermolecular version of the
all-carbon [4+2] annulation have been unsuccessful. The
Br
OCOR
Ar
R2
Scheme 1 Selected intermolecular [4+2] annulations via α,β-unsaturated acyl azoliums.
challenges arise mainly from the following aspects: (1)
According to the study from Studer and Mayr et al.,9 α,β-
unsaturated acyl azoliums are 103‒106 less electrophilic than
the iminium ions derived from Hayashi‒ Jorgenson pyrrolidine
or MacMillan’s imidazolidinones. This leads to the fact that
many widely-used dienes in Diels-Alder reactions fail to
undergo annulations with α,β-unsaturated acyl azoliums (see
Scheme 2 for more details), thus rendering the identification of
suitable dienes or diene precursors a huge challenge. (2) Since
the [4+2] annulation is hard to occur, the unwanted O-
acylation reactions leading to esters and [3+3] annulations
forming dihydropyranones and cyclohexanones are easy to be
predominated.5a,k,7 (3) The annulation products were
amenable to undergo decarboxylation or aromatization
processes, thus erasing two or all stereogenic centers within
the products.7,8,10 (4) Furthermore, to make the reaction
practically
valuable,
the
regio-,
diastereo-,
and
enantioselectivities must be well controlled. In this context,
the Lupton group took advantage of the reversible NHC
activation of dienyl esters to realize the asymmetric formation
of cyclohexyl β-lactones.11 A recent advance from the same
group is that they reported one example using an N-tBu-
substituted carbene catalyst in the reaction shown in Scheme
1a to afford the annulation product with 40% ee.3d Overall, a
series easily occurred side reactions have make the highly
†Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures and
spectroscopic data for all new compounds. CCDC 1569976. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
‡These two authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 20xx
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Table 1 Substrates scope of asymmetric [4+2] reactiona
Cl
Mes
O
N
N
O
Mes
Mes
C1 (20 mol%)
O
N
Nu
O
diene
+
Ph
H
via
N
N
O
BF4
R1 OH
K2CO3, THF
w/ or w/o MeOH
C14
(20 mol%)
Ph
Br
O
Ph
2a
N
N
1)
Mes
ROCO
CO2Me
Mes
ROCO
1
R1
+
R3
H
Bn
Cs2CO3, THF, 4 Å MS, 30
C
(a) dienes examined:
O
OTMS
OEt
Br
2
R3
OMe
OTMS
OEt
OTMS
Ph
4
2) Na2CO3, MeOH
R2
> 20:1 dr
R2
Ph
EtO
TMSO
TMSO
TMSO
OH
Ph
OH
Ph
4a, R = H
4b, R = F
4c, R = Cl
4d, R = Br
4e, R = OMe
4f, R = Me
80% yield, 93% ee
64% yield, 89% ee
80% yield, 91% ee
64% yield, 90% ee
60% yield, 97% ee
57% yield, 97% ee
D1
D2
D3
D4
D5
D6
D7
D8 Ph
PhOCO
CO2Me
PhOCO
CO2Me
4g
Ph
O
O
64% yield
99% ee
products:
O
O
Me
EtO
O
Me
Ph
O
O
O
O
Ph
Ph
O
O
Ph
OMe
OEt
P2
Ph
OMe
R
Ph
Ph
EtO
O
Ph
O
Ph
Me
P1
P3
P4
P5
Ph
P6
OH
Ph
R
(b)
PhOCO
OH
Ph
PhOCO
CO2Me
4l, R = Br
O
O
Ph OH
PhOCO
CO2Me
O
Ph
O
4i, R = Cl
O
PhOCO
PhOCO
CO2Me
Ph
PhOCO
OH
77% yield, 95% ee
(69% yield, 93% eeb)
76% yield, 81% ee
Ph
Ph
Ph
2a
base
MeOH
PhOCO
CO2Me
4j, R = Br
77% yield, 83% ee
4m, R = Me
74% yield, 87% ee
Ph
Ph
3a
53% yield
rac-
1a
Ph
D9 Ph
4h
Ph
Ph
3a'
Ph
43% yield
73% ee
4k
, R = Me
4n, R = OMe
66% yield, 93% ee
R
Ph
55% yield, 91% ee
possible byproducts:
Ph
O
Ph
Ph
PhOC
PhOCO
OCOPh
O
PhOCO
PhOCO
Cl
Br
Ph
Ph OH
CO2Me
Ph
Ph
Ph
Ph
B1
O
PhOCO
Ph
Ph
Ph
B2
B3
OH
OH
OH
B4
Ph
PhOCO
PhOCO
CO2Me PhOCO
Ph
CO2Me
CO2Me
Scheme 2 Evaluation of dienes in NHC-catalyzed [4+2] annulations.
F
Ph
Ph
F
S
Br
enantioselective intermolecular [4+2] annulation a formidable
challenge and has not been addressed to date.
We envisioned that the identification of a suitable
4q
65% yield
82% ee
4p
56% yield
80% ee
4r
4o
84% yield
97% ee
55% yield
92% ee
Me
Cl
Cl
Ph
OH
OH
R
diene/dienolate that could undergo selective all-carbon [4+2]
annulation with α,β-unsaturated acyl azoliums might be the
key point. As indicated in Scheme 2a, we commenced our
R
O
PhOCO
CO2Me
Ph
CO2Me
OH
OH
PhOCO
CO2Me
Me
O
CO2Me
Ph
O
Ph
O
Ph
Ph
4t, R = Me
40% yield, 72% ee
Ph
4v, R = Me, 41% yield, 83% ee
4w, R = tBu, 56% yield, 91% ee
4x, R = OEt, 35% yield, 86% ee
Cl
study by evaluating a series of dienes D1‒D7. The α,β-
4z
4s
4u
, R = iPr
41% yield
84% ee
70% yield
85% ee
unsaturated acyl azolium was generated in situ from α-
bromoenal 2a under NHC catalysis, a process that has been
well-established by Ye, Yao, Biju, and others.12 However, the
results were frustrated because no any desired products were
gotten. Contrarily, a series of undesired products were
Me
15% yield, 99% ee
4y, R = OtBu, 33% yield, 81% ee
O
O
OH
Ph
OH
Ph
PhOCO
O
PhOCO
Bn
4ab
64% yield
88% ee
4aa
52% yield
90% ee
N
N
H
Ph
Ph
Ph
Ph
detected. For instance, when cyclic dienes D1/D2, acyclic diene
a All reactions were run on a 0.2 mmol scale; all yields were of isolated products;
dr values were determined via 1H NMR analysis of the reaction mixtures; ee
values were determined via HPLC analysis on a chiral stationary phase. b 2 mmol
scale.
D3, and Denishefsky’s diene D4 were used, only ester P1 was
obtained. Brassard’s diene D5 also failed to afford the desired
product, with both esters P1 and P2 formed. [3+3] Annulation
product P3 was isolated when silyl dienol ether D6 was used,
and the reaction using D7 afforded dihydropyranone P4 and
annulation product B4 were not detected. Additionally, we
found that an isomerization process, which was not observed
in the previous reports of intermolecular annulations (Scheme
1a),7 occurred efficiently and made 3a a product inaccessible
from conventional Diels-Alder reactions.
ester P5
.
Considering the failure of the above widely-used dienes, we
transferred our focus to in situ generated dienolates, which
might undergo direct vinyloguous Michael addition‒
cyclization13 with α,β-unsaturated acyl azoliums. Then D8, a
dienolate derived from the corresponding 3-butenone, was
tested. Unfortunately, α-selective annulation predominated
and only dihydropyranone P6 was obtained (Scheme 2a).
Encouragingly, our further seeking of qualified dienolates
finally led to the discovery of D9, a dienolate derived from α-
benzoyloxy ketone 1a (Scheme 2b). Under the conditions used
in Scheme 2, the corresponding annulation product 3a’ was
detected, and after ring opening by MeOH,5s cyclohexene 3a
was obtained in 53% yield. To the best of our knowledge, 1a
and its analogues have not been used as dienolate precursors.
It is worthwhile to mention that only one diastereoisomer was
perceived, and the possible byproducts such as ester B1/diene
B2, benzene derivative B3, and [3+3]
Having successfully identified an acyclic dienolate species in
this challenging annulation, we then started to develop an
asymmetric version of this transformation, a challenge that
has not been fully addressed to date. After screening more
than forty sets of conditions (See the Supplementary
Information for more details), we were pleased to find that
chiral carbene C14, first used by Scheidt et al.,14 could afford
annulation product 3a in good yield, with >20:1 dr and 93% ee.
With these optimized conditions, we then evaluated the
generality and limitations of the reaction. As outlined in Table
1, a series of aromatic α-bromoenals having both electron-
donating and -withdrawing substituents on the phenyl rings
were compatible in the annulations, releasing the products
with 89‒97% ee (Table 1, 4b‒f). The single-crystal X-ray
2 | J. Name., 2012, 00, 1-3
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structure analysis of 4d confirmed the configuration of the
products.15 In addition, naphthyl and furyl enals could produce
4g and 4h with 99% and 73% ee, respectively (Table 1, 4g and
4h). Having tested the scope of enals, we then checked a series
of α-acyloxy-butenones. First, the introduction of different
substituents into the phenyl ring at the ketone moiety proved
successful, affording 4i‒k in 55‒77% yields with 81‒91% ee.
Second, we studied the vinyl substituents and found that
differently substituted phenyl rings (i.e., 4-Br-, 4-Me-, and 4-
OMe-C6H4) or thienyl group all showed little effects on the
outcomes, with 87‒97% ee detected (Table 1, 4l‒o). A big-
scale reaction was also conducted and only a slight decrease of
the ee was observed (Table 1, 4l). In addition, simultaneous
variations on both enals and vinyl ketones were also tolerated
(Table 1, 4p‒s). The product derived from aliphatic methyl
ketone was also obtained, albeit with moderate 72% ee (Table
1, 4t), and isopropyl ketone could afford the annulation
product with excellent 99% ee (Table 1, 4u). Products 4k and
4t should be emphasized, because cyclic dienolates with
similar substituents (i.e., electron-rich aryl groups or aliphatic
Scheme 4 Proposed reaction mechanism.
cyclohexanes 5e and 5f with six contiguous stereocenters,
respectively.17 In all cases, no any erosion of the ee values was
detected.
A plausible mechanism of this [4+2] annulation was
proposed do demonstrate the formation of the products. The
reaction of carbene with enal 2a affords α,β-unsaturated acyl
azolium
I; meanwhile, dienolate II is produced from ketone 1a
.
i
A vinylogous Michael addition of II to
I
produces enolate III
,
groups smaller than Pr) could not lead to the corresponding
[4+2] annulation products.3d,7 In addition,
and IV is generated after the olefin isomerization. Then an
intramolecular aldol reaction-lactonization occurs to release β-
lactone VI, and after the ring opening, cyclohexene 4a is finally
obtained. The acyloxy group may play two roles: precluding
the [3+3] annulation by blocking the α-position of dienolate II
a series of
butenones with different O-substituents (i.e., -OCOMe, -
t
OCOtBu, -OCO2Et, and -OCO2 Bu) all allowed access to the
products with good to excellent enantioselectivities (Table 1,
4w‒4z). Moreover, the ring opening can also be realized via
other nucleophiles, thus releasing amide 4aa and Weinreb
amide 4ab with 90% and 88% ee, respectively (Table 1, 4aa
and 4ab). Noteworthy is that, in most cases, byproducts such
as esters were not observed. Unfortunately, when R2 or R3 was
an aliphatic group, the annulation did not occur under the
optimal conditions.
and facilitating the aldol process (IV to
V) by pushing the
ketone moiety close to the azolium enolate.
In conclusion, we have successfully addressed the challenging
NHC-catalyzed asymmetric intermolecular all-carbon [4+2]
annulation. The identification of 2-acyloxy-3-butenones as
dienolate precusors played essential role in this
transformation. A series of highly functionalized cyclohexenes
were obtained with high to excellent enantioselectivities, and
the products are not accessible from conventional Diels-Alder
reactions. The synthetic potential of this reaction has been
demonstrated by a series of attractive transformations of the
products.
The densely functionalized products could enable a series of
further transformations, and the acyloxy group proved
particularly useful (Scheme 3). For instance, the hydrolysis of
4l under basic conditions generated cyclohexenol 5a, and the
further oxidation of 5a afforded cyclohexanone 5b
Additionally, the α-ketal rearrangement of 5b under copper
catalysis could afford cyclopentene 5c 16
Moreover, the
.
.
oxidation of 4l produced epoxide 5d, and the following ring
Acknowledgements
opening under basic and acidic conditions provided
This work was supported by the strategic priority research
program of the Chinese Academy of Sciences (XDB20000000),
National Science Foundation of China (21402199 and
21502192), and the Chinese Recruitment Program of Global
Experts. We thank Professor Daqiang Yuan for the X-ray data
analysis.
Notes and references
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Scheme 3 Transformations of the products.
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