Diastereo- and enantioselective direct vinylogous Michael addition of γ-substituted butenolides to 2-enoylpyridines catalyzed by chiral bifunctional amine-squaramides

Article abstract of DOI:10.1039/c5cc06383c

The diastereo- and enantioselective direct vinylogous Michael addition reaction of γ-substituted butenolides to 2-enoylpyridines has been achieved. A range of γ,γ-disubstituted butenolide derivatives, bearing two consecutive tri- and tetrasubstituted stereogenic centers, were readily obtained in good yields with excellent stereoselectivities (up to >99:1 dr and >99% ee).

Full text of DOI:10.1039/c5cc06383c

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DOI: 10.1039/C5CC06383C
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COMMUNICATION
Diastereo- and Enantioselective Direct Vinylogous
Michael Addition of γ-Substituted Butenolides to 2-
Enoylpyridines Catalyzed by Chiral Bifunctional
Amine-Squaramides
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
Zhen-Hua Wang,ac Zhi-Jun Wu,b Deng-Feng Yue,ac Yong You,ac Xiao-Ying Xu,*a Xiao-Mei
Zhanga and Wei-Cheng Yuan*a
The diastereo- and enantioselective direct vinylogous
Michael addition reaction of γ-substituted butenolides to 2-
Asymmetric organocatalysis employing hydrogen bonding for
substrate activation has been demonstrated to be an effective and
enoylpyridines has been achieved.
disubstituted butenolide derivatives,
A
range of γ,γ-
bearing two
versatile strategy in facilitating
a
variety of organic
transformations.7 In this realm, chiral amine-squaramide catalysts,
possessing a squaramide moiety as a powerful hydrogen bond
donor and a tertiary amine moiety as the basic site, have been
identified as a family of chiral bifunctional catalysts for a range
consecutive tri- and tetrasubstituted stereogenic centers,
were readily obtained in good yields with exce llent
stereoselectivities (up to >99:1 dr and >99% ee).
γ,γ-Disubstituted butenolide skeletons represent a structural of asymmetric transformations.8 Meanwhile, we noticed that 2-
type of both synthetic and biological importance. A great number enoylpyridines, featuring α,β-unsaturated carbonyls attached to a
of biologically active natural products and pharmaceutically pyridine group, were excellent substrates for highly
relevant molecules contain the special butenolide motifs.1 Much enantioselective conjugated addition reactions.9 In particular, the
attention has been devoted to the synthesis of diverse γ,γ- 2-alkanoyl-pyridine moiety has been shown to be efficient
disubstituted butenolide compounds by synthetic chemists.2 In template for high reactivity and stereoselectivity due to readily
this research area, the most common synthetic methods for the coordination to hydrogen bonding (Scheme 1).9a However, an
optically active butenolides mainly confined to the catalytic asymmetric addition of γ-substituted butenolides to 2-
asymmetric Mukaiyama-type reactions by using the preformed enoylpyridines has not been reported yet, despite the popularity
silyloxyfurans.2,3 However, alternative approaches involving γ- of γ-substituted butenolides as versatile nucleophiles. Therefore,
substituted butenolides as pronucleophiles by functionalization at as our continuing program of research in asymmetric
the γ-position for generating enantiomerically pure γ,γ- organocatalysis,10 herein, we document a highly efficient
disubstituted butenolides recently have received more attention methodology for the diastereo- and enantioselective direct
due to atom and step economy. In this respect, various vinylogous Michael addition reaction of γ-substituted
transformations,such as the direct asymmetric allylic alkylation,4 butenolides to 2-enoylpyridines with chiral bifunctional amine-
vinylogous Mannich,5 and vinylogous Michael addition squaramide (Scheme 1), allowing the generation of γ,γ-
reactions6 of γ-substituted butenolides, have been well disubstituted butenolides possessing consecutive tri- and
documented. Even so, considering the ubiquitous nature of tetrasubstituted stereogenic centers.
butenolides in natural products and medicinally important agents,
developing new strategies for the efficient construction of the
usefulγ,γ-disubstituted butenolide compoundsis stilldesirable.
Scheme 1 Diastereo- and Enantioselective Direct Vinylogous Michael
a.National Engineering Research Center of Chiral Drugs,
Addition Reaction ofγ-SubstitutedButenolides to 2-Enoylpyridines.
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu 610041, China
Email: yuanwc@cioc.ac.cn (W .-C. Yuan); xuxy@cioc.ac.cn (X.-Y. Xu)
b.Chengdu Institute of Biology, Chinese Academy of Sciences,
Chengdu 610041, China
We started our studies with the reaction of γ-phenyl-
substituted butenolide 1a and 2-enoylpyridine 2a in the presence
of various chiral bifunctional organocatalysts A-E in
dichloromethane (Table 1). The reaction gave product 3a in
moderate yield and 77:23 diastereomeric ratio (dr) and 63% ee
c. University of Chinese Academy of Sciences, Beijing 100049, China.
†Electronic supplementary in formation (ESI) av ailable: Exp eri mental p rocedures,
spectral data o f new co mpounds, and crystallographic data. CCD C 1057095. For
ESI and crystallographic data in CIF or other electronic format see DO I:
xxxxxxxxxxx
This journal is © The Royal Society of Chemistry 20xx
J . Name., 2013, 00, 1-3 | 1
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with catalyst A (entry 1). Similar to catalyst A, bifunctional
Journal Name
Under the optimized experimental conditions, we then
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DOI: 10.1039/C5CC06383C
thiourea-tertiary amine catalysts B and C, possessing chiral 1,2- examined a variety of γ-substituted butenolides 1 and 2-
diamine skeleton, also afforded mediocre results (entries 2 and 3). enoylpyridines 2 to establish the general utility of this
Another thiourea-tertiary amine catalyst D derived from quinine asymmetric direct vinylogous Michael addition reaction (Table
could finish the reaction in 29 h to provide 3a in 78% yield with 2). We were pleased to find that a wide range of aromatic 2-
91:9 dr and 80% ee (entry 4). To our delight, the reaction enoylpyridines underwent reactions with 1a in high reactivities
proceeded to completion with chiral bifunctional quinine amine- and excellent diastereo- and enantioselectivities (entries 1-13).
squaramide catalyst E in 7 h, affording 3a in 75% yield with up Firstly, the Michael acceptors with electron-withdrawing groups
to 98:2 dr and 98% ee (entry 5). Having identified catalyst E as furnished the desired products in 73-88% yields and with up
the strongest candidate, we undertook a screen of solvents with to >99:1 dr and >99% ee values regardless of the kind and
10 mol % catalyst dosage at room temperature (entries 6-9). By position of the substituents in the benzene ring (entries 1-9).
comparison, excellent reaction rate and stereochemistry control Similarly, a series of satisfactory results could be readily
could be obtained when 1,2-dichloroethane (DCE) was used as a obtained with the 2-enoylpyridine possessing various electron-
solvent (entry 7). Subsequently, we screened reaction donating substitutions on the phenyl group (entries 10-13). Fused
o
temperature and catalyst loading. At 0 C, 3a was obtained in aromatic 2-enoylpyridine 2o also reacted efficiently with 1a,
85% yield with 99:1 dr and >99% ee after 6 h (entry 10). Further giving 3o in 65% yield and 98:2 dr and 98% ee (entry 14).
decreasing temperature to -10 oC, no any improvement on Nevertheless, as highlighted in entries 15 and 16, the 2-
reaction rate was observed (entry 11). With 5 mol % E, the enoylpyridines bearing a heteroaromatic substituent also were
reaction could complete in 8 h and furnish 3a in 82% yield and proven to be suitable reaction partner for giving the
excellent stereoselectivity (entry 12). However, with 1 mol % E, corresponding adducts 3pand 3qin excellent results. In addition,
a set of slightly worse results were observed (entry 13). Notably, not only aliphatic substituted but also ester group substituted 2-
when the reaction was carried out on a scale up to 30-fold enoylpyridine could participate in the reaction and furnish the
increase, there was no change in reactivity and stereoselectivity, expected adducts 3r and 3s in good yields and excellent
this suggests that this method is amenable to large scale stereoselectivities (entries 17 and 18). On the other hand, the
production(entry 14).
substrate scope for γ-substituted butenolide component was also
investigated. Excellent diastereo- and enantioselectivities and
good yields were able to be readily obtained with butenolides 1b-
e bearing different γ-aromatic substituents having an electronic
nature ranging from electron-withdrawing to electron-donating
(entries 19-22). Afterwards, a 2-naphthyl-based butenolide also
gave rise to the corresponding adduct 3x in 85% yield with 97:3
dr and 99% ee (entry 23). Unfortunately, in the case of aliphatic
substituted butenolide as a substrate, the catalyst system was
inefficient and no reaction occurred (entry24).
Table 1 Conditions Optimizationa
Table 2 Substrate Scope of Direct Vinylogous Michael Addition Reaction
between γ-SubstitutedButenolides and2-Enoylpyridinesa
Time
(h)
42
42
42
29
7
20
4
20
46
6
Yield
(%)b
45
60
51
78
75
56
75
63
78
85
83
82
74
76
ee
(%)d
63
57
72
80
98
88
99
Entry
x
Solvent
drc
cat.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
10
10
10
10
10
10
10
10
10
10
10
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DCE
toluene
Et2O
DCE
DCE
77:23
78:22
73:27
91:9
98:2
97:3
99:1
99:1
97:3
99:1
99:1
99:1
98:2
>99:1
A
B
C
D
E
E
E
E
E
E
E
E
E
E
Time
(h)
3/yield
ee
Entry
R2
drc
1
(%)b
(%)d
1
2
3
4
5
6
7
8
2-FC6H4 (2b)
3-FC6H4 (2c)
4-FC6H4 (2d)
2-ClC6H4 (2e)
4-ClC6H4 (2f)
2-BrC6H4 (2g)
3-BrC6H4 (2h)
4-BrC6H4 (2i)
12
12
11
12
11
15
12
11
3b/85
3c/74
3d/86
3e/76
3f/73
3g/74
3h/87
3i/75
>99:1
99:1
94:6
99:1
96:4
>99:1
>99:1
98:2
>99
>99
98
>99
99
>99
98
99
1a
1a
1a
1a
1a
1a
1a
1a
99
97
>99e
>99f
99e
98e
99e,g
10
8
22
13
DCE
DCE
DCE
1
5
4-NO2C H4
6
a Unless noted, the reactions were carried out with 1a (0.1 mmol), 2a (0.15
mmol), and a specified amount catalyst in 2.0 mL of solvent at room
9
11
3j/88
>99:1
99
1a
(2j)
3-MeC6H4
(2k)
4-MeC6H4 (2l)
4-MeOC6H4
(2m)
b
c
1
24
23
18
3k/84
3l/61
>99:1
>99:1
92:8
>99
99
10
11
12
1a
1a
1a
temperature. Isolated yields. Determined by H NMR analysis of crude
reaction mixture. d Ee of major diastereomer was determined by chiral HPLC
e
g
analysis. Run at 0 oC. f Run at -10 oC. Scale-up reaction with 30-fold
3m/71
96
increase was carriedout.
4-BnOC6H4
(2n)
13
34
3n/72
>99:1
99
1a
2 | J. Name., 2012, 00, 1-3
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1-naphthyl
(2o)
2-furyl (2p)
2-thienyl (2q)
Me (2r)
CO2Et (2s)
14
11
3o/65
98:2
>99:1e >99
96:4
97:3
>99:1 >99
98:2
98:2
97:3
98:2
97:3
nr
98
1a
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DOI: 10.1039/C5CC06383C
13
13
13
12
10
9
3p/78
3q/85
3r/84
3s/85
3t/83
3u/86
3v/77
3w/81
3x/85
nr
15
16
17
18
19
20
21
22
23
24
1a
1a
1a
1a
1b
1c
1d
1e
1f
97
>99
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
99
99
98
99
99
nr
9
13
13
72
1g
a Unless noted, the reactions were carried out with 1 (0.1 mmol), 2 (0.15
mmol), and catalyst E (5 mol %) in 2.0 mL of DCE at room temperature. b
c
Isolated yields. Determined by 1H NMR analysis of crude reaction
d
mixture. Ee of major diastereomer was determined by chiral HPLC.
analysis. e This dr value was determinedby HPLC analysis
The absolute configuration of the major isomer 3q was
unambiguously determined to be (4S, 11S) by single-crystal X-
ray analysis (See Supporting Information).11 Assuming a
common reaction pathway for the catalyst E-catalyzed direct
vinylogous Michael addition reaction of γ-substituted
butenolides and 2-enoylpyridines, the configurations of the other
products were assignedby analogy.
Scheme 3 Contrast Experiments with1a andDifferent Types ofSubstrates.
Based on the aforementioned clues and the well-documented
activation mode involving chiral bifunctional amine-squaramide
catalysts,8 a plausible transition state model is proposed to
account for the observed stereoselectivity. As shown in Fig. 1,
the tertiary amine moiety of chiral amine-squaramide catalyst E
deprotonates from γ-substituted butenolide to generate an
ammonium ion and an enolate. Simultaneously, dual hydrogen
bonding from squaramide unit of catalyst E to the 2-alkanoyl-
pyridine moiety of the 2-enoylpyridines not only contributes to
reducing its LUMO energy, but also directs the 2-enoylpyridines
for a face-selective nucleophilic attack. Preferentially, the
Michael attack of enolates from the Si face to the Re face of 2-
enoylpyridines thus leads to the formation of the major
stereoisomer.
The Michael adduct can undergo the further transformation.
As shown in Scheme 2, the product 3a could be hydrogenated by
Pd/C in methanol. The double bond and the carbonyl group were
both reduced, giving access to a pair of epimerides 4 and 4'
possessing three stereogenic centers with excellent ee values.
Importantly, 4 and 4' could be readily separated with simple
column chromatographyin 23% and 44% yield,respectively.12
Scheme 2 Reduction ofProduct 3a.
As the above-listed in Table 1, entry 12, the reaction of 1a and
2a could give 3a in 82% yield with 99:1 dr and 99% ee after 8 h
(Scheme 3, (1)). However, under the standard conditions, the
addition reaction of 1a and chalcone 5 exhibited very low
reactivity, product 6 was observed only in 28% yield with 88:12
dr and up to 96% ee even for 86 h (Scheme 3, (2)). Similarly, the
corresponding product 8 of 1a addition to 2-enoylpyridine-N-
oxide 7 could be obtained 67% yield with 99:1 dr and 90% ee
after the time for 48 h (Scheme 3, (3)). Furthermore, it was found
that the nitrogen atom in different positions of the pyridine also
obviously affected the reactivity and stereoselectivity (Scheme 3,
(4) and (5)). By comparison the results of these reactions, it
revealed that the pyridyl group of 2a and the position of nitrogen
atom in pyridine were crucial to the direct vinylogous Michael
addition of γ-substituted butenolides and 2-enoylpyridines,
especially in the enhancement of the reactivity and
stereoselectivity. These results suggest that the powerful
hydrogen-bonding interactions between the squaramide unit of
catalyst E and the 2-alkanoyl-pyridine moiety of the 2-
enoylpyridines play a very important role in the substrate
activation.
Fig. 1 A proposed transition state for the direct vinylogous Michael addition
reaction.
In conclusion, we have developed
a highly efficient
methodology for the diastereo- and enantioselective direct
vinylogous Michael addition reaction of γ-substituted
butenolides and 2-enoylpyridines with a chiral bifunctional
amine-squaramide as a catalyst. With the developed protocol, a
range of γ,γ-disubstituted butenolide derivatives, bearing two
consecutive tri- and tetrasubstituted stereogenic centers, were
readily obtained in good yields with almost diastereo- and
enantiomerical purities (up to 88% yield, >99:1 dr and >99% ee).
This catalytic system is well suitable for a wide scope of β-
substituted 2-enoylpyridines and γ-aryl-substituted butenolides.
The further transformation of the obtained adduct was also
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Chem. Eur. J., 2011, 17, 6890; (c) P. Chauhan, S. Mahajan, U. Kaya, D.
demonstrated. Preliminary experiments revealed that the pyridyl
group of the Michael acceptors is crucial to promoting this direct
vinylogous Michael addition reaction with high reaction activity
and stereoselectivity.
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Hack andD. Enders, Adv. Synth. Catal.,2015, 357,253.
DOI: 10.1039/C5CC06383C
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We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21372217), the CAS
western light program, and Sichuan Youth Science and
Technology Foundation (2013JQ0021, 2015JQ0041).
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