Asymmetric cascade annulation based on enantioselective oxa-diels-alder cycloaddition of in situ generated isochromenyliums by cooperative binary catalysis of Pd(OAc)2 and (S)-trip

Article abstract of DOI:10.1021/ja405764p

An asymmetric cascade annulation between 2-hydroxystyrenes and 2-alkynylbenaldehyes or 1-(2-alkynylphenyl)ketones has been established with good to excellent enantioselectivities (up to >99.5% ee), on the basis of an enantioselective oxa-Diels-Alder cycloaddition of in situ generated metallo-isochromenylium intermediates, by cooperative binary catalysis of Pd(OAc)₂ and (S)-Trip. The developed methodology is workable for a broad spectrum of substrates and shows great efficiency in establishing dense multiple chiral centers including quaternary carbons of variable bridged ring systems. The mechanism study suggests that (S)-Trip plays multiple roles in assembling the reactants and controlling the stereoselectivity.

Full text of DOI:10.1021/ja405764p

Article
pubs.acs.org/JACS
Asymmetric Cascade Annulation Based on Enantioselective Oxa-
Diels−Alder Cycloaddition of in Situ Generated Isochromenyliums by
Cooperative Binary Catalysis of Pd(OAc)2 and (S)‑Trip
Shu-Yan Yu, Hao Zhang, Yang Gao, Lei Mo, Shaozhong Wang, and Zhu-Jun Yao*
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing, Jiangsu 210093, China
S
* Supporting Information
ABSTRACT: An asymmetric cascade annulation between 2-
hydroxystyrenes and 2-alkynylbenaldehyes or 1-(2-alkynyl-
phenyl)ketones has been established with good to excellent
enantioselectivities (up to >99.5% ee), on the basis of an
enantioselective oxa-Diels−Alder cycloaddition of in situ
generated metallo-isochromenylium intermediates, by cooper-
ative binary catalysis of Pd(OAc)2 and (S)-Trip. The
developed methodology is workable for a broad spectrum of
substrates and shows great efficiency in establishing dense multiple chiral centers including quaternary carbons of variable
bridged ring systems. The mechanism study suggests that (S)-Trip plays multiple roles in assembling the reactants and
controlling the stereoselectivity.
hydroxylstyrene (2a, Figure 1, R3 = H) in organic media at
ambient temperature, providing high yield of rac-3aa (Figure 1,
INTRODUCTION
■
Isochromenyliums, a unique type of Huckel aromatic oxonium
̈
R1 = 4,5-(OCH3)2; R2 = Ph; R3 = H)6a via cascade
intermolecular oxa-Diels−Alder cycloaddition and intramolec-
ular nucleophilic substitution (Figure 1). Such tetrahydronaph-
thalene framework represents a new type of conformationally
constrained analogues of podophyllotoxin (PPT), a famous
anticancer natural drug lead.11 For exploring the biological
applications of natural product-like tetrahydronaphthalene
derivatives, development of enantioselective cascade trans-
formations via isochromenyliums is thus particularly important
and urgent.
intermediate, have attracted great attention of organic chemists
in recent years.1−3 These highly reactive intermediates were
often generated in situ for various transformations,4 and a
number of air-stable crystalline isochromenylium tetrafluor-
oborates (ICTBs) were also prepared, characterized and
applied by us.5,6 Rich cationic chemistries of isochromenyliums
have been demonstrated in the literature, including various
cascade reactions for constructing complex natural and
unnatural products.7 Because of their diverse and high
reactivity, enantioselective transformations based on the planar
isochromenyliums are still challenging organic chemists.
Tanaka group reported the first enantioselective Rh-catalyzed
reaction via isochromenylium intermediates between o-
alkynylbenzaldehydes and N-substituted isatin in the presence
of a complex ferrocenyl-phosphine ligand,8 and Slaughter and
co-workers developed an enantioselective acetalization of
isochromenylium intermediates with a gold(I) acyclic diami-
nocarbene complex.9 An asymmetric [3 + 2]-cycloaddition of
platinium-containing carbonyl ylide (a 2-oxonium-1,3-diene
equivalent similar to isochromenylium) in the presence of chiral
phosphine ligand has also been reported by Iwasawa group,
providing a series of potentially useful 8-oxabicyclo[3.2.1]-
octane derivatives.10 However, catalytic asymmetric cascade
reaction forming multiple C−C and C−X bonds via
isochromenyliums remains as a problem yet.
To achieve the above-mentioned purpose, controlling the
stereoselectivity of oxa-Diels−Alder reaction between isochro-
menylium diene and styrene-type dienophile is a crucial work.
Since isochromenylium intermediate could be generated in situ
by treatment of o-alkynylbenzaldehyde with a variety of metal
catalysts,4 we deduced that certain proper combination of a
metal catalyst and a chiral Brønsted acid might efficiently
organize both the reactants into one unified transition state,
delivering satisfactory enantioselectivity to the final product. In
the past decade, chiral Brønsted acids have emerged as the
versatile enantioselective catalysts in various enantioselective
procedures.12 Among them, BINOL-derived phosphoric acids
are commonly applied as superior bifunctional catalysts of
strong Brønsted acid (P−OH) and hydrogen-bond acceptor
(PO).13 According to our proposal (Figure 1), a cooperative
binary catalysis mode14,15 was expected to execute multiple
tasks, including converting o-alkynylbenzaldehyde 1 into the
During our recent studies on air-stable isochromenylium salts
and their cationic reactions,5,6 many cascade transformations
were found to show high efficiency in constructing complex
multiring structures with multiple stereogenic centers. For
instance, air-stable ICTB A could smoothly react with 2-
Received: June 9, 2013
© XXXX American Chemical Society
A
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a
Table 1. Screening of Binary-Catalyst Combinations
b
entry
BA
metal catal
temp
yield (ee) (%)
1
P-1
P-1
P-1
P-1
P-1
P-1
P-1
P-2
P-3
P-4
P-5
−
AgOTf
reflux
rt
88 (10)
97 (0)
2
3
AuCl3
rt
63 (0)
4
PtCl2
rt
96 (0)
5
Cu(OTf)2
Zn(OTf)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
rt
20 (20)
12 (0)
6
rt
7
rt
96 (26)
85 (43)
73 (15)
75 (79)
37 (10)
8
rt
9
rt
10
11
rt
rt
a
Reaction conditions: 1a (0.5 mmol) and 2a (0.75 mmol), metal
catalyst (10 mol %) and Brønsted acid (10 mol %) in 1,2-
b
dichloroethane (5 mL) under nitrogen atmosphere. Isolated yields,
and ee values were determined by chiral HPLC.
Figure 1. Rational design of asymmetric cascade annulation of o-
alkynylbenzaldehyde with 2-hydroxylstyrene; anticancer tetrahydro-
naphthalene natural product podophyllotoxin (PPT) and its bioactive
analogues PDE and GL-331.
identified to be the best acid catalyst for this cascade annulation
(entry 10, 75% yield, 79% ee).
With the catalyst combination of Pd(OAc)2 and (S)-Trip (P-
4), other reaction parameters were investigated (Table 2). The
reaction could be completed in many solvents, and DCM was
identified to be the best one (entry 7). Incomplete conversion
was observed in CH3CN, though the reaction afforded a higher
enantioselectivity (entry 4). Lowering the catalyst loadings did
not cause any detriment on the enantioselectivity of product
(entries 7 and 8). A slightly better result was observed when the
reaction was performed at 0 °C with a longer reaction time (21
h, entry 9). An improved enantioselectivity was achieved when
increasing the loading of (S)-Trip over that of Pd(OAc)2, and
50% excess (0.5 equiv more than Pd(OAc)2) was found to be
sufficient (entries 8, 10−11). Further reducing the catalyst
loadings did not significantly affect the enantioselectivity of
product but slowed down the reaction (entry 12). Though use
of as low as 1 mol % Pd(OAc)2 could provide similar
enantioselectivity (95% ee), a much longer reaction time was
required (30 h, entry 13). Water was found to be detrimental
but not fatal to this reaction (entries 14−15). Therefore,
anhydrous solvent is preferred for the reaction. The optimal
catalyst combination was finally determined as Pd(OAc)2 (2.5
mol %) and (S)-Trip (3.75 mol %), giving 3aa (in DCM, rt, 13
h) in 71% yield and 95% ee (entry 12).
corresponding metallo-isochromenylium intermediate, ex-
changing encounter anion with the chiral Brønsted acid,
assembling olefin 2 through hydrogen bond, and delivering the
enantio-differentiative environment to the key [4 + 2]-
cyclization.
RESULTS AND DISCUSSION
■
Reaction between o-alkynylbenzaldehyde 1a and 2-hydroxyl-
styrene 2a was chosen as the examination platform for the
above hypothesis (Table 1). No reaction was observed with
only 10 mol % of phosphoric acid P-1 at room temperature,
while heating the mixture to reflux resulted in the production of
3aa (88% yield) with low enantioselectivity (10% ee) (entry 1)
(the absolute structure of 3aa was assigned later according to
the X-ray single crystal structures of its analogues 3ac, 3aj, 3ak,
5ca, and 5fa). Further attempts to improve the enantiose-
lectivity all failed including increasing the loading of phosphoric
acid catalyst and using other phosphoric acids. Requirement of
high temperature to promote this reaction is owing to the weak
activating ability of phosphoric acid to the alkyne group of 1a,
and it is harmful for the stereochemical control. In order to
reduce the reaction temperature and examine the proposed
binary catalysis mode, a number of metal catalysts (10 mol %)
were further screened together with P-1. Most of the reactions
gave the racemic product 3aa in satisfactory yields except two
with Cu(OTf)2 and Pd(OAc)2. Combination of Pd(OAc)2 and
P-1 afforded 3aa in 96% yield and 26% ee at rt (entry 7).
Delightedly, varying the 3,3′-subsitituents of phosphoric acid P-
1 (P-2−P-5) was proven to be effective to improve the
enantioselectivity (entries 8−11), and (S)-Trip (P-4) was
Under the optimized conditions, generality of the reaction
was then explored (Tables 3−5). Significant electronic effect
was observed through the substituent(s) R1 of 1 (Table 3).
When the electron density of the phenyl ring of 1 decreased,
the enantioselectivity of products turned poor (entries 1−5,
3aa−3ea). The electron-donating substituent(s) R1 is believed
to stabilize the resulting palladium-containing isochromenylium
intermediate, an electron-deficient 2-oxonium-1,3-diene, and
thus favors the following cycloaddition. This well agrees with
B
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a
a
Table 2. Optimization of the Reaction Conditions
Table 3. Examination of the Reaction Scope of 1
b
Pd(OAc)2
(mol %)
(S)-Trip
(mol %)
time
(h)
yield (ee)
entry
solvent
(%)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
5
10
DCE
4
75 (79)
38 (78)
68 (79)
43 (87)
trace
10
THF
6
10
Toulene
CH3CN
DMF
4
10
19
19
6
10
10
Dioxane
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
41 (77)
71 (85)
65 (88)
79 (90)
69 (93)
67 (93)
71 (95)
69 (95)
68 (89)
69 (83)
10
6
5
7
c
9
5
5
21
7
10
11
12
13
5
10
5
7.5
3.75
1.5
3.75
3.75
7
2.5
1
13
30
13
13
d
14
2.5
2.5
e
15
a
Unless otherwise noted, the reaction was conducted between 1a (0.5
mmol) and 2a (0.75 mmol) with indicated catalysts in 5 mL of solvent
b
at room temperature (23 °C) under nitrogen atmosphere. Isolated
c
yields, and ee values were determined by chiral HPLC. The reaction
d
was performed at 0 °C. The reaction was performed without N2
e
protection. The reaction was conducted with the addition of water (1
equiv).
our previous observation on the air-stable ICTBs.5 Little effect
was observed when the alkyne substituent R2 of 1 was a phenyl
ring bearing various functionalities (such as OMe, CO2Me, CN,
CHO, t-Bu, NO2 etc.). All these reactions could be carried out
smoothly and gave the desired tricyclic products in good yields
and excellent enantioselectivities (3fa−3pa). Compared to the
weak electronic effect of R2, the hindrance effect seemed to be
more sensitive (3fa−3ha). Poor ees were observed in those
having a heteroaromatic subsitituent (3qa and 3ra). This might
be caused by unclear heteroatom-poisoning effects on Pd-
(OAc)2. Unsatisfactory results also happened to the reactions of
substrates 1 having an aliphatic or alicyclic substituent at R2
(3sa−3va). We guess that insufficient interactions between
substrate 1 and the phenyl ring of catalyst P-4 might be a
reason for the low ees of these reactions (for more discussion
on the mechanism, see below text).
To our delight, this binary catalytic system worked well for
the reactions with o-alkynylketones 4 (Table 4). The desired
products 5aa−5ca containing a newly formed quaternary
carbon were obtained in good yields and satisfactory
enantioselectivities under standard conditions (entries 1−3).
As for the aryl ketones, these transformations also could be
smoothly carried out and delivered the products with good
enantioselectivities using a higher catalyst loading (5 mol %
Pd(OAc)2 and 7.5 mol % (S)-Trip) (5da−5fa, entries 4−6).
Effect of the substituents of o-hydroxystyrene 2 was also
investigated (Table 5). Variation of the 5-substituent (R4) was
insensitive to the reaction (3ab−3ae, entries 1−4), while a 4-
CH3 at R4 brought significant decrease in both chemical yield
and enantioselectivity (3af, entry 5). No desired reaction
happened when the o-hydroxystyrene substrate had a 3-CH3
group (not shown in Table 5). A number of multisubstituted
a
Unless otherwise noted, the reaction was conducted with 1 (0.5
mmol) and 2a (0.75 mmol), Pd(OAc)2 (2.5 mol %), (S)-Trip (3.75
mol %) in anhydrous DCM (5 mL) at rt under nitrogen atmosphere.
b
Isolated yields, and ee values were determined by chiral HPLC.
c
When the reaction was conducted with Pd(OAc)2 (2.5 mol %) and
(R)-Trip (3.75 mol %), the desired compound 3aa was obtained with
75% yield and −96% ee. The yields of two diastereomers (dr = 3:1),
and the shown ee value is for the major isomer.
d
styrene-type olefins were also examined (entries 6−12). All of
them were found to undergo the cascade reactions smoothly
under the optimal catalytic conditions, except the one with a
CH3 group at R7 (entry 12, 3am). The hindrance of R7 might
interfere with the formation of hydrogen-bond in the transition
C
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a
Table 4. Examination of the Reactions of o-Alkynylketones
4
Table 5. Examination of the Scope of Substrate 2
a
b
time
(h)
5 (yield, ee)
entry
4 (R1/R3)
(%)
1
2
4a (R1 = 4,5-di-OCH3; R3 = CH3)
4b (R1 = 4,5-di-OCH3; R3 = i-Pr)
4c (R1 = 4,5-di-OCH3; R3 = c-C6H11)
4d (R1 = 4,5-di-OCH3; R3 = 4-CH3C6H4)
6
5aa (80, 93)
5ba (80, 85)
5ca (86, 93)
5da (78, 83)
5ea (87, 85)
46
54
46
96
c
3
d
4
d
5
4e (R1 = 4,5-di-OCH3; R3 = 4-
CH3OC6H4)
ce
,
6
4f (R1 = 4,5-OCH2O; R3 = 3,4,5-
54
5fa (77, −92)
(CH3O)3C6H2)
a
Unless otherwise noted, the reaction was conducted with 4 (0.5
mmol) and 2a (0.75 mmol), Pd(OAc)2 (2.5 mol %), (S)-Trip (3.75
mol %) in anhydrous DCM (5 mL) at rt under nitrogen atmosphere.
b
c
Isolated yields, and ee values were determined by chiral HPLC. Its
absolute structure was confirmed by X-ray crystal analysis; see Figure
d
2. Pd(OAc)2 (5 mol %) and (S)-Trip (7.5 mol %) were used.
e
Pd(OAc)2 (5 mol %) and (R)-Trip (7.5 mol %) were used.
state (see below text). Furthermore, reaction with the Z-isomer
of olefin 2g was surprisingly slow and gave 3ag (the same
product from the reaction with E-2g, entry 6) in a much lower
yield after 6 days. The Z-olefin seems to be unfavorable for
formation of the transition state (see below text for mechanism
discussion), and slow conversion of Z-isomer into E-isomer
took place prior to its reaction with 1a. More attractively, this
type of reaction showed great efficiency in generating four
densely continuous stereogenic centers, including two examples
giving a newly born all-carbon quaternary stereocenter (entries
10−11, 3ak and 3al).
In order to understand the reaction mechanism, several
control experiments were performed (Table 6). Failure of the
cascade transformation at rt in the absence of either Pd(OAc)2
or (S)-Trip reveals that both metal catalyst and organocatalyst
are needed for the reaction (entries 1 and 2), and it more likely
adopts a cooperative metal/organo binary catalysis mode. Only
a trace amount of product was detected after 3 days by
replacing Pd(OAc)2 with PdCl2 (entry 4). This suggests that
the suitable encounter anion of Pd(II) (AcO− vs Cl−) is also
very important for the reaction, and a weaker acid anion (AcO−
in this work) favors the anionic exchange with (S)-Trip upon
Pd(II) before the assembly of both reactants for the oxa-Diels−
Alder cycloaddition.
a
All the reaction was conducted with 1a (0.5 mmol) and 2 (0.75
mmol), Pd(OAc)2 (2.5 mol %) and (S)-Trip (3.75 mol %) in
b
anhydrous DCM (5 mL) at rt under nitrogen atmosphere. Isolated
yields, and ee values were determined by chiral HPLC. Its absolute
structure was determined by X-ray crystal analysis; see Figure 2.
scale-up reaction was performed with 1a (1.33 g, 5 mmol) and 2c
(0.90 g, 7.5 mmol), giving the desired product 3ac (2.04 g, 88%, 95%
ee). When the reaction was conducted with Pd(OAc)2 (2.5 mol %)
and (R)-Trip (3.75 mol %), the desired compound 3ak was obtained
with 78% yield and −96% ee.
c
d
A
e
Furthermore, masking the hydroxyl group of styrene 2a with
methoxyl group seriously affected the progress of this reaction,
giving the corresponding adduct in only 10% yield after 5 days
(Scheme 1). Shifting the ortho-phenolic hydroxyl group of 2a to
the meta- or para- position resulted in no reaction or much
lower yield of the corresponding adduct. Therefore, the ortho-
phenolic hydroxyl group in the styrene substrate is con-
servatively essential to achieve this type of highly enantiose-
lective cascade transformation.
On the basis of the results from the cascade reactions and
control experiments, a possible mechanism was proposed
(Figure 3). First, the alkyne bond of o-alkynylbenzaldehyde 1
(or o-alkynylketone 4) is coordinated and activated by
Pd(OAc)2 to initiate a cycloisomerization by attack of the
oxygen of the internal CO group, affording an Pd(II)-
isochromenylium intermediate A. After anionic exchange with
(S)-trip and release of a molecule of HOAc, the resulting
intermediate B grasps o-hydroxyl styrene 2 through hydrogen-
bond between the phenolic OH of styrene and the PO of
Trip.16 Such highly ordered supramolecular assembly of two
substrates and two catalysts perfectly devises a transition state
for an “intramolecular” oxa-Diels−Alder reaction17 under
asymmetric environment. After the asymmetric [4 + 2]-
cyclization, the resulting carbocation C (stabilized by the
oxygen atom of the internal carbonyl group) is quickly trapped
by the internal phenol hydroxyl to provide D. Finally, the C−
Pd bond of D is broken down by protonation with HOAc to
D
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Journal of the American Chemical Society
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Figure 2. X-ray structures of representative compounds 3ac, 3aj, 3ak,
5ca, and 5fa.
Figure 3. Proposed reaction mechanism.
a
Table 6. Control Experiments of the Cooperative Catalysis
net result, two new C−C bonds and one C−O bond are
stereoselectively generated from this cascade annulation, as well
as two new rings.
CONCLUSION
■
b
Pd(II) (2.5
mol %)
(S)-Trip (3.75
temp
(°C)
yield (ee)
(%)
The first successful example of catalytic asymmetric cascade
transformation has been achieved on the basis of an
enantioselective oxa-Diels−Alder reaction of metallo-isochro-
menylium intermediate with o-hydroxystyrene with good to
excellent enantioselectivities (up to >99.5% ee) in a broad
spectrum of substrates. The developed metallo/organo binary
catalytic methodology showed great efficiency in multiple-bond
formation and establishing bridged-ring systems with densely
multiple stereogenic centers including quaternary carbons, and
therefore provides a convenient one-step access to enantiopure
complex tetrahydronaphthalene natural product-like derivatives
of medicinal interest.
entry
mol %)
time
1
2
3
4
5
Pd(OAc)2
−
Pd(OAc)2
−
23
23
23
23
30
3 d
3 d
16 h
3 d
4 d
trace
(S)-Trip
(S)-Trip
(S)-Trip
(S)-Trip
NR
71 (95)
trace
PdCl2
PdCl2
60 (32)
a
All the reactions were carried out beween 1a (0.5 mmol) and 2a
(0.75 mmol) with the indicated catalysts in 5 mL of DCM under
nitrogen atmosphere. Isolated yields, and ee values were determined
by chiral HPLC.
b
Scheme 1. Examination of the Role of ortho-Phenolic
Hydroxyl Functionality of Styrene 2a
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and full characterizations, copies of
NMR spectra for new compounds, chiral HPLC analyses for
products 3 and 5 (PDF), and X-ray data of 3ac, 3aj, 3ak, 5ca,
and 5fa (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yaoz@nju.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
deliver product 3 (or 5), and the metal catalyst Pd(OAc)2 is
regenerated in the meanwhile. The high enantioselectivity is
considered to stem from both the steric hindrance of the bulky
1,3,5-triisopropyl phenyl group of Trip moiety and the
hydrogen-bond interactions between the phenolic hydroxyl
group of substrate 2 with the PO functionality of Trip. As a
Grants from MOST (Nos. 2010CB833202 and
2013AA092903), NSFC (Nos. 21272112 and 21032002),
National Science Fund for Talent Training in Basic Science
(J1103310), and State Key Laboratory of Phytochemistry and
Plant Resources in West China, Kunming Institute of Botany
are greatly appreciated.
E
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Journal of the American Chemical Society
Article
(13) Several recent reviews on chiral phosphoric acids, see:
(a) Connon, S.-J. Angew. Chem., Int. Ed. 2006, 45, 3909−3912.
(b) Adair, G.; Mukherjee, S.; List, B. Aldrichimica Acta 2008, 41, 31−
42. (c) Terada, M. Synthesis 2010, 1929−1982.
(14) For reviews on combinative binary catalytic system of transition
metals and organocatalysts, see: (a) Shao, Z.-H.; Zhang, H.-B. Chem.
Soc. Rev. 2009, 38, 2745−2755. (b) Koenigs, R.-M.; Atodiresei, I.;
Rueping, M. Chem.Eur. J. 2010, 16, 9350−9365. (c) Hubbert, C.;
Hashmi, A.-S.-K. Angew. Chem., Int. Ed. 2010, 49, 1010−1012. (d) Du,
Z.-T.; Shao, Z.-H. Chem. Soc. Rev. 2013, 42, 1337−1378.
(15) For representative works, see: (a) Hamilton, G.-L.; Kang, E.-J.;
Mba, M.; Toste, F.-D. Science 2007, 317, 496−499. (b) Antonchick,
A.-P.; Brinkmann, C.; Rueping, M. Angew. Chem., Int. Ed. 2007, 46,
6903−6906. (c) Lu, Y.-D.; Johnstone, T.- C.; Arndtsen, B.-A. J. Am.
Chem. Soc. 2009, 131, 11284−11285. (d) Muratore, M.-E.; Holloway,
C.-A.; Pilling, A.-W.; Storer, R.-I.; Trevitt, G.; Dixon, D.-J. J. Am. Chem.
Soc. 2009, 131, 10796−10797. (e) Han, Z.-Y.; Xiao, H.; Chen, X.-H.;
Gong, L.-Z. J. Am. Chem. Soc. 2009, 131, 9182−9183. (f) Mukherjee,
S.; List, B. J. Am. Chem. Soc. 2007, 129, 11336−11337. (g) Cai, Q.;
Zhao, Z.-A.; You, S.-L. Angew. Chem., Int. Ed. 2009, 48, 7428−7431.
(k) Terada, M.; Toda, Y. J. Am. Chem. Soc. 2009, 131, 6354−6355.
(h) Belot, S.; Vogt, K.-A.; Besnard, C.; Krause, N.; Alexakis, A. Angew.
Chem., Int. Ed. 2009, 48, 8923−8926. (i) Komanduri, V.; Krische, M.-J.
J. Am. Chem. Soc. 2006, 128, 16448−16449.
REFERENCES
■
(1) Kuznetsov, E.; Shcherbakova, I.-V.; Balaban, A.-T. Adv. Heterocycl.
Chem. 1990, 50, 157−254.
(2) (a) Swager, T.-M. J. Org. Chem. 1999, 64, 6499−6504. (b) Prauda,
I.; Kovesdi, I.; Trinka, P.; Reiter, J. J. Heterocycl. Chem. 2001, 38, 403−
414. (c) Suzuki, T.; Okada, C.; Arai, K.; Awaji, A.; Shimizu, T.;
Tanemura, K.; Horaguchi, T. J. Heterocycl. Chem. 2001, 38, 1409−
1418. (d) Zhu, J.-L.; Germain, A.-R.; Porco, J. A., Jr. Angew. Chem., Int.
Ed. 2004, 43, 1239−1243. (e) Marsini, M. A.; Gowin, K. M.; Pettus, T.
R. Org. Lett. 2006, 8, 3481−3483. (f) Zhu, J.-L.; Grigoriadis, N.-P.;
Lee, J.-P.; Porco, J. A., Jr. J. Am. Chem. Soc. 2005, 127, 9342−9343.
(g) Beeler, A.-B.; Su, S.; Singleton, C.-A.; Porco, J. A., Jr. J. Am. Chem.
Soc. 2007, 129, 1413−1419.
(3) (a) Wei, W.-G.; Yao, Z.-J. J. Org. Chem. 2005, 70, 4585−4590.
(b) Qian, W.-J.; Wei, W. -G.; Zhang, Y.-X.; Yao, Z.-J. J. Am. Chem. Soc.
2007, 129, 6400−6401. (c) Yao, Y.-S.; Yao, Z.-J. J. Org. Chem. 2008,
73, 5221−5225.
(4) (a) Asao, N.; Takahashi, K.; Yamamoto, Y. J. Am. Chem. Soc.
2002, 124, 12650−12651. (b) Asao, N.; Nogami, T.; Takahashi, K.;
Yamamoto, Y. J. Am. Chem. Soc. 2002, 124, 764−765. (c) Asao, N.;
Kasahara, T.; Yamamoto, Y. Angew. Chem., Int. Ed. 2003, 42, 3504−
3506. (d) Asao, N.; Menggenbateer, K.-S.; Yamamoto, Y. J. Org. Chem.
2005, 70, 3682−3685. (e) Kusama, H.; Funami, H.; Shido, M.;
Iwasawa, N. J. Am. Chem. Soc. 2005, 127, 2709−2716. (f) Kusama, H.;
Ishida, K.; Funami, H.; Iwasawa, N. Angew. Chem., Int. Ed. 2008, 47,
4903−4905. (g) Barluenga, J.; Vazquez-Villa, H.; Ballesteros, A.;
(16) (a) Mukherjee, S.; List, B. J. Am. Chem. Soc. 2007, 129, 11336−
11337. (b) Chai, Z.; Rainey, T.-J. J. Am. Chem. Soc. 2012, 134, 3615−
3618. (c) Tao, Z.-L.; Zhang, W.-Q.; Chen, D.-F.; Adele, A.; Gong, L.-
Z. J. Am. Chem. Soc. 2013, 135, 9255−9258.
(17) Although several previous works on the [4 + 2]-cyclization of
metallo-isochromenylium (see refs 4a, 4c, 4d, 5, 6a, and 6b)
mentioned this type of reaction as oxa-Diels−Alder cycloadditions,
we cannot fully exclude the possibility of a highly ordered cationic
stepwise process, an unconcerted formal [4 + 2]-cycloaddition.
Gonzal
́
ez, J.-M. J. Am. Chem. Soc. 2003, 125, 9028−9029.
(h) Barluenga, J.; Vazquez-Villa, H.; Ballesteros, A.; Gonzal
́
ez, J.-M.
Org. Lett. 2003, 5, 4121−4123. (i) Shin, S.; Gupta, A.-K.; Rhim, C.-Y.;
Oh, C.-H. Chem. Commun. 2005, 4429−4431. (j) Hsu, Y. C.; Ting, C.
M.; Liu, R. S. J. Am. Chem. Soc. 2009, 131, 2090−2091.
(5) Hu, Z.-L.; Qian, W.-J.; Wang, S.; Wang, S.-Z.; Yao, Z.-J. Org. Lett.
2009, 11, 4676−4679.
(6) For our previous studies on air-stable isochromenylium
tetrafluoroborates (ICTBs), see: (a) Hu, Z.-L.; Qian, W.-J.; Wang,
S.; Wang, S.-Z.; Yao, Z.-J. J. Org. Chem. 2009, 74, 8787−8793. (b) Hu,
Z.-L.; Yang, Z.-Y.; Wang, S.-Z.; Yao, Z.-J. Chem.Eur. J. 2010, 17,
1268−1274. (c) Hu, Z.-L.; Qian, W.-J.; Yao, Z.-J. Sci. China: Chem.
2010, 53, 869−876. (d) Liao, H.-Z.; Hu, Z.-L.; Cui, K.; Jiao, J.; Qin, Y.;
Yao, Z.-J. Synthesis 2010, 3474−3478. (e) Yu, S.-Y.; Hu, Z.-L.; Zhang,
H.; Yao, Z.-J. Tetrahedron Lett. 2012, 53, 2065−2068.
(7) (a) Dyker, G.; Hildebrandt, D. J. Org. Chem. 2005, 70, 6093−
6096. (b) Sato, K.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2005, 70,
8977−8981. (c) Yue, D.; Dellaca, N.; Larock, R.-C. Org. Lett. 2004, 6,
1581−1584. (d) Prauda, I.; Reiter, J. J. Heterocycl. Chem. 2001, 199−
204. (e) Asao, N.; Nogami, T.; Lee, S.-Y.; Yamamoto, Y. J. Am. Chem.
Soc. 2003, 125, 10921−10925.
(8) Hojo, D.; Noguchi, K.; Tanaka, K. Angew. Chem., Int. Ed. 2009,
48, 8129−8132.
(9) Handa, S.; Slaughter, L.-M. Angew. Chem., Int. Ed. 2012, 51,
2912−2915.
(10) Ishida, K.; Kusama, H.; Iwasawa, N. J. Am. Chem. Soc. 2010, 132,
8842−8843.
(11) (a) Ward, R.-S. Nat. Prod. Rep. 1999, 16, 75−96. (b) Wu, Y.;
Zhao, J.; Chen, J.; Pan, C.; Li, L.; Zhang, H. Org. Lett. 2009, 11, 597−
600. (c) Stadler, D.; Bach, T. Angew. Chem., Int. Ed. 2008, 47, 7557−
7559. (d) Reynolds, A.-J.; Scott, A.-J.; Turner, C.-I.; Sherburn, M.-S. J.
Am. Chem. Soc. 2003, 125, 12108−12109. (e) Bush, E.-J.; Jones, D. W.
J. Chem. Soc., Perkin Trans. 1 1996, 151−155. (f) Hadimani, S.-B.;
Tanpure, R.-P.; Bhat, S.-V. Tetrahedron Lett. 1996, 37, 4791−4794.
(g) Speybroeck, R.-V.; Guo, H.; Eycken, J.-V.; Vandewalle, M.
Tetrahedron 1991, 47, 4675−4682. (h) Andrews, R.-C.; Teague, S.-T.;
Meyers, A.-I. J. Am. Chem. Soc. 1988, 110, 7854−7858.
(12) For recent reviews on chiral Brønsted acid catalysis, see:
(a) Doyle, A.-G.; Jacobsen, E.-N. Chem. Rev. 2007, 107, 5713−5743.
(b) Akiyama, T. Chem. Rev. 2007, 107, 5744−5758. (c) Yu, X.; Wang,
W. Chem.Asian J. 2008, 3, 516−532. (d) Terada, M. Chem.
Commun. 2008, 4097−4112.
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