Development of metal/organo catalytic systems for direct vinylogous michael reactions to build chiral γ,γ-disubstituted butenolides

Article abstract of DOI:10.1002/chem.201204466

Vinylogous reactions: The direct asymmetric vinylogous conjugate additions of γ-aryl- and alkyl-substituted butenolides to enones was realized for the first time by a designed and tested metal/organo dual-activation strategy, in which two effective metal/organo cooperative catalytic systems have been built (see scheme). Copyright

Full text of DOI:10.1002/chem.201204466

COMMUNICATION
DOI: 10.1002/chem.201204466
Development of Metal/Organo Catalytic Systems for Direct Vinylogous
Michael Reactions to Build Chiral g,g-Disubstituted Butenolides
Dongxu Yang,[a] Linqing Wang,[a] Depeng Zhao,[a] Fengxia Han,[a] Bangzhi Zhang,[a] and
Rui Wang*[a, b]
Cooperative catalysis is emerging as a valuable tool in
asymmetric synthesis, fuelled by the identification of more
efficient catalytic systems with increased substrate scopes.[1]
Many endeavors have been devoted during the past few
years toward developing the right catalyst pairs giving rise
to powerful bifunctional systems, and many types of organic
catalysts have been used in combination with appropriate
metals to achieve unprecedented enantioselective process-
es.[2] In this field of research, cinchona-derived scaffolds
have been recognized as one of the privileged structures for
construction of new metal/organo binary catalytic systems,[3]
given their extraordinary ability to induce stereocontrol and
their commercial availability. Since Mukaiyama and co-
workers developed the first example of combining cincho-
easy access to chiral g,g-disubstituted butenolides from g-
monosubstituted butenolides.
In recent years, the generation of optically active buteno-
lides has attracted considerable attention owing to their syn-
thetic significance in natural products and medicinally im-
portant agents.[7] In terms of atom economy and efficiency,
tremendous excellent approaches have been accomplished
toward developing direct stereocontrolled g-functionaliza-
tion of furanones and g-butyrolactams to build the desired
g-monosubstituted adducts.[8] Despite these impressive con-
tributions, however, the further use of such g-monosubstitut-
ed butenolides to be rendered direct asymmetric transfor-
mations to g,g-disubstituted butenolides is still a dark side in
this area of research. The first breakthrough in the direct
enantioselective preparation of g,g-disubstituted butenolides
derivatives was reported by Chen and co-workers[9] in 2010
with further contributions by the groups of Alexakis, Feng,
and Mukherjee.[10] Very recently, a highly enantio- and dia-
stereoselective asymmetric addition reaction between g-sub-
stituted butenolides and electron-deficient olefins containing
an oxazolidinone moiety has been reported by Huang, Tan,
and Jiang et al. during the preparation of our manuscript.[11]
In their work, the use of a tert-leucine-derived amine–thio-
nine with SnACHTUNGTRENNUNG(OTf)2 and applied it to the cyanation of ali-
phatic aldehydes,[4] several combinatorial catalytic strategies
concerning cinchona alkaloid derivatives have been identi-
fied that promote addition reactions through dual activation
of both substrates in nucleophilic–electrophilic additions.[5]
These well-documented pioneering studies highlight the po-
tential of this new research area and have brought us inspi-
ration. Given these outstanding ideas and considering our
continuous endeavors for designing metal/organo coopera-
tive catalytic systems,[6] we believe that the design and oper-
ation of such systems with a high efficiency in promoting
still challenging reactions plays a pivotal role in the discov-
ery of useful synthetic pathways. Herein, we have tried to
engage a series of Lewis acids to work together with the cin-
chona alkaloid and have developed two effective coopera-
tive Brønsted base/Lewis acid catalytic systems that allow
AHCTUNGTREGuNNUN rea catalyst could promote either g-aryl- or alkyl-substitut-
ed butenolides as nucleophiles. Nevertheless, the direct
asymmetric vinylogous conjugate addition of g-substituted
buACTHNUTRGNENGtU enCAHTUNGTRENNoUGN lides to the model substrate, such as enones, still rep-
resents a long-standing challenge. Moreover there is still a
general deficiency of catalytic methods that tolerate varia-
tions of both the g-aryl- and alkyl-substituted butenolides.
Herein, we disclose our own solution to these problems by
exploiting complementary activation modes of binary hybrid
catalytic systems, which enabled a highly direct enantioselec-
tive vinylogous addition of both the g-aryl- and alkyl-substi-
tuted butenolides to a series of aromatic and aliphatic
enones.
To establish the method we investigated the addition of g-
phenyl-substituted butenolide 1a with enone 2a in detail.
The initial trail under the catalysis of quinine (B1) failed to
bring about bond formation after 24 h of stirring at room
temperature (Table 1, entry 1). When the reaction was per-
formed at 608C, the conversion improved a little and the
product was obtained in 14% yield with 72% ee (ee=enan-
tiomeric excess, entry 2). We hypothesized that the poor cat-
alytic efficiency may be due to inefficient activation of the
[a] D. Yang, L. Wang, Dr. D. Zhao, F. Han, Dr. B. Zhang,
Prof. Dr. R. Wang
Key Laboratory of Preclinical Study for New Drugs of
Gansu Province, Institute of Biochemistry and Molecular Biology
School of Life Sciences and State Key Laboratory of
Applied Organic Chemistry
Lanzhou University, Lanzhou, 730000 (P.R. China)
[b] Prof. Dr. R. Wang
State Key Laboratory of Chiroscience
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University, Kowloon (Hong Kong)
E-mail: wangrui@lzu.edu.cn
bcrwang@polyu.edu.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204466.
Chem. Eur. J. 2013, 19, 4691 – 4694
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4691

Table 1. Optimization of the direct vinylogous conjugate addition of g-
substituted butenolide 1a to chalcone 2a.[a]
acid catalytic systems with perfect compatibility and that
would produce the desired levels of stereoselectivity and
catalytic efficiency (entries 6, 10). As illustrated in Table 1,
the product 3a could be obtained in 81% yield and 93% ee
by using the B1/Al
result (88%, 90% ee) could be achieved catalyzed by the
B1/La(OiPr)3/L1 system in a slightly higher yield with ac-
ceptable enantioselectivity. On the contrary, the other metal
ions, such as boron(III), indium(III), and tin(IV) were not
compatible to generate desirable catalytic systems in this
asymmetric vinylogous conjugate addition. Additionally, we
found that the catalytic systems needed properly compatible
pairs between the Brønsted base and the ligand of the metal
ions (entries 11–17),[5 h] and the reaction performed without
diphenol ligand could also generate the product 3aa in rela-
tively lower yield and with a slightly decreased ee value.
(entry 6 vs. 18).
ACHTUNGTERNN(UNG OiPr)3/L1 catalytic system, and a similar
AHCTUNGTRENNUNG
Entry
B[b]
L[b]
M
Yield [%][c]
d.r.[d]
ee [%][e]
A
ACHTUNGTRENNUNG
1
B1
B1
B1
B1
B1
B1
B1
B1
B1
B1
B2
B3
B4
B1
B1
B1
B1
B1
–
–
–
–
–
M
trace
14
–
–
–
72
–
2[f]
3
7:1
–
>20:1
16:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
–
ACHTUNGTRENNUNG
4
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
–
Ti
T
22
45
81
25
52
34
88
95
63
87
28
21
62
82
72
86
69
93
91
88
59
90
À77
À74
88
–
5[f]
6
Ti
ACHTUNGTRENNUNG
G
7
8
9
BACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
10
11
12
13
14
15
16
17
18
ACHTUNGTRENNUNG
Al
Al
Al
Al
Al
Al
Al
Al
ACHTUNGTRENNUNG
After establishing[16] the two optimized conditions for the
efficient cooperative catalysis directed asymmetric vinylo-
gous conjugate addition of g-substituted butenolides to
enones, we undertook an extensive survey of the scope of
the reaction with respect to the substrate. As summarized in
Table 2, aromatic enones 2 bearing various substituents and
enones with condensed-ring or heteroaryl units were exam-
ined with 1a (Table 2, entries 1–27). Excellent results were
achieved except for 2c, which produced a relatively lower
yield (entry 5). The reaction was also conducted with ali-
phatic enones, such as 2t and 2u, and reasonable results
were observed (entries 28, 29). Then the substrate scope of
the vinylogous reaction of various g-substituted butenolides
1 was examined. Excellent enantioselectivities and moderate
to good yields (77–97%) were obtained for butenolides 1b–
d bearing different g-aryl groups in the Michael reaction
with enones 2 (entries 30–34), and it was pleasing that the
reaction could be conducted with g-alkyl butenolides 2e de-
spite 20% mol of the binary catalytic systems being required
under a higher temperature (entries 35, 36). Finally, the data
constructing the substrate scope in Table 1 suggests that
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
R
–
–
N
>20:1
>20:1
>20:1
88
75
91
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] Unless otherwise noted, reactions were carried out with 1a (0.2 mmol,
1.0 equiv), 2a (0.3 mmol, 1.5 equiv), and B/M/L (10 mol%) in toluene
(2.0 mL) at room temperature for 24 h. [b] B1=quinine, B2=quinidine,
B3=cinchonine, B4=cinchonidine; L1=(R)-Binol, L2=3,3’-Br,Br-(R)-
Binol, L3=3,3’-I,I-(R)- Binol, L4=3,3’-(2-napthyl),(2-napthyl)-(R)-Binol;
L5=(S)-Binol. [c] Yield of isolated product. [d] Determined by 1H NMR
spectroscopic analysis of the crude reaction mixture. [e] Enantiomeric
excess was determined by HPLC analysis. [f] Reactions were carried out
at 608C.
g-substituted butenolide by a single Brønsted base. To im-
prove the rate of the transformation, we explored the possi-
bility of promoting a Lewis acid assisted Brønsted base
system by incorporating an appropriate metal ion capable of
activating the butenolide more effectively by enhancing the
acidity of the a-C H after the complexation. Meanwhile,
the LUMO activation of the enone by the same Lewis acid
could also smooth the bond-forming event. To test the con-
À
using system B (B1/La
with slightly lower enantioselectivity, whereas system A (B1/
Al(OiPr)3/L1) led to higher asymmetric induction. The abso-
ACHTUNGTNER(NUNG OiPr)3/L1) resulted in higher yield
cept, we initially examined a series of Lewis acids, such as
ACHTUNGTRENNUNG
[12]
M
N
lute configuration of 3 was determined by X-ray crystallo-
graphic analysis of the product 3ah (see the Supporting In-
formation for details).[17]
conjugate addition did not proceed as anticipated. We
speculated that these strong metal ions may bind too tightly
to the nitrogen atom, which could originally act as an organ-
ic base in the reaction. Considering the aforementioned hy-
pothesis, we turned our attention to other metal ions that
might be compatible with the precatalyst. Studies with well-
developed titanium(IV) complexes[13] provided relatively
higher reaction efficiency and better enantioselectivity (en-
tries 4, 5). Given and encouraged by this finding, we
deemed that we should broaden our views and look into po-
tential cooperative catalytic systems that could better har-
ness the reactivity of the substrates. After careful screening
Finally, we have explored some concise synthetic transfor-
mations as shown in Scheme 1. The product 3ab can be
readily converted into g,g-disubstituted-lactone 5 in the
presence of hydrogen gas and Pd/C. Then, lactone 5 can be
easily transformed into the corresponding esters 6 by a
Baeyer–Villiger oxidation. Alternatively, a highly stereose-
lective dihydroxylation of 3ab furnished compound 4 when
using RuCl3·H2O and NaIO4. A single isomer of 4 was ob-
tained and its relative configuration was confirmed by
NOESY experiment (see the Supporting Information for de-
tails).
of a series of Lewis acids of this type, including Al
(OnBu)3, In(OiPr)3, Sn(OiPr)4, and La
(OiPr)3,[15] we were
pleased to observe a couple of binary Brønsted base/Lewis
(OiPr)3,[14]
ACHTUNGTRENNUNG
B
A
G
G
N
At the end of this work, an interesting phenomenon was
observed.
A retro-g-functionalization was occasionally
4692
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Chem. Eur. J. 2013, 19, 4691 – 4694

Development of Metal/Organo Catalytic Systems
Table 2. Scope of the direct vinylogous Michael addition.
COMMUNICATION
Entry[a]
1
R1, R2
A/B[b] Yield [%][c] d.r.[d]
ee [%][e]
1
2
3
4
5
6
7
8
1a p-Me-Ph, Ph (2a)
1a p-Me-Ph, Ph (2a)
1a Ph, Ph (2b)
A
B
A
B
A
A
B
A
A
B
A
A
B
A
A
81
88
82
91
55
77
89
84
72
84
85
92
94
87
99
>20:1 93
>20:1 90
>20:1 99
>20:1 93
>20:1 99
>20:1 91
>20:1 91
16:1 93
Scheme 1. Transformation of the vinylogous conjugate adducts.
a) RuCl3·H2O (0.1 equiv), NaIO4 (2.5 equiv), CH3CN/H2O (5:1, v/v), 08C,
15 min, 52%; b) Pd/C, H2 (balloon), MeOH, RT, overnight, 75%;
c) KH2PO4, meta-chloroperoxybenzoic acid (mCPBA), dichloromethane,
608C, 24 h, 72%.
1a Ph, Ph (2b)
1a o-Me-Phe, Ph (2c)
1a m-Me-Ph, Ph (2d)
1a m-Me-Ph, Ph (2d)
1a p-MeO-Ph, Ph (2e)
1a o-MeO-Ph, Ph (2 f)
1a o-MeO-Ph, Ph (2 f)
9
11:1 95
10:1 92
10
11
12
13
14
15
1a m-MeO-Ph, Ph
N
>20:1 97
>20:1 98
>20:1 92
>20:1 97
>20:1 85
1a p-Cl-Ph, Ph (2h)
1a p-Cl-Ph, Ph (2h)
1a p-Br-Ph, Ph (2i)
1a p-CN-Ph, Ph (2j)
16
17
1a
1a
A
B
90
95
>20:1 98
>20:1 96
Scheme 2. Observed reversibility of the g-functionalization of the carbon-
yl group.
site of the carbonyl group of the g,g-disubstituted buteno-
lides took place and a new bond was built up with another
nucleophile.[18] A detailed study to expand the scope of the
reaction to encompass additional nucleophilic reagents is
currently underway in our laboratory.
18
1a
A
69
>20:1 92
19
20
1a
1a
A
B
83
96
17:1 91
17:1 90
In conclusion, we have developed two combinational cata-
lytic systems that enable the efficient direct asymmetric vi-
nylogous conjugate additions of g-aryl-substituted buteno-
lides to enones in good yields with high diastereoselectivi-
ties, and excellent enantioselectivities. In particular, the re-
versibility of g-functionalization of carbonyl compounds was
also discovered. Further research on the application of the
combinatorial catalyst to other reactions and the synthetic
utilities of the retrovinylogous additions are currently under-
way.
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35[f]
36[f]
1a 2-furyl, Ph (2n)
1a 2-thienyl, Ph (2o)
1a 2-thienyl, Ph (2o)
1a Ph, p-Me-Ph (2p)
1a Ph, p-Cl-Ph (2q)
1a Ph, p-MeO-Ph (2r)
1a Ph, 2-furyl (2s)
1a nHex, Ph (2t)
1a Ph, Me (2u)
1b Ph, p-Me-Ph (2p)
1b Ph, Ph (2b)
1c Ph, Ph (2b)
1d Ph, p-Me-Ph (2p)
1d Ph, Ph (2b)
1e Ph, Ph (2b)
A
A
B
82
93
98
91
97
86
92
67
85
86
86
97
77
81
57
68
>20:1 91
15:1 89
14:1 87
>20:1 95
>20:1 90
>20:1 93
>20:1 91
>20:1 86
>20:1 72
19:1 94
17:1 92
>20:1 98
18:1 97
10:1 92
>20:1 82
>20:1 75
A
A
A
A
A
A
A
A
A
A
A
A
B
Acknowledgements
1e Ph, Ph (2b)
[a] Unless otherwise noted, reactions were carried out with 1 (0.2 mmol,
1.0 equiv), 2 (0.3 mmol, 1.5 equiv), and the catalytic system A (B1/Al-
ACHTUNGTRENNUNG(OiPr)3/L1) or B (B1/LaAHCUTTGNREN(NUNG OiPr)3/L1) (10 mol%) in toluene (2 mL) at
room temperature for 24 h. [b] This column indicates the catalytic system
(A or B) used. [c] Yield of isolated product. [d] Determined by 1H NMR
spectroscopic analysis of the isolated product. [e] Enantiomeric excess
was determined by HPLC analysis. [f] The reactions were carried out by
using 20 mol% of the catalytic system at 608C for 48 h.
We are grateful for grants from the National Natural Science Foundation
of China (nos. 91213302, 20932003, and 21272102) and the Key National
S&T Program “Major New Drug Development” of the Ministry of Sci-
ence and Technology (2012ZX09504001-003).
Keywords: asymmetric synthesis · butenolides · catalysis ·
enones · organometallics
found, as shown in Scheme 2. When the adduct 3ab was
treated with a simple organic base, such as 1,8-diazabicyclo-
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À
N
Chem. Eur. J. 2013, 19, 4691 – 4694
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4693

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A
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A
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Received: December 14, 2012
Revised: January 30, 2013
Published online: March 4, 2013
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