Highly enantioselective direct allylic alkylation of butenolides with Morita-Baylis-Hillman carbonates catalyzed by chiral squaramide-phosphine

Article abstract of DOI:10.1039/c5ra14667d

An efficient asymmetric vinylogous allylic alkylation of β,γ-butenolides with Morita-Baylis-Hillman carbonates has been developed. With a chiral cyclohexane-based squaramide-phosphine catalyst 5e, optically active γ,γ-disubstituted butenolides containing

Full text of DOI:10.1039/c5ra14667d

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Highly enantioselective direct allylic alkylation of
butenolides with Morita–Baylis–Hillman
carbonates catalyzed by chiral squaramide-
phosphine†
Cite this: RSC Adv., 2015, 5, 74170
Received 24th July 2015
Accepted 21st August 2015
*
*
Tian-Chen Kang, Xuan Zhao, Feng Sha and Xin-Yan Wu
DOI: 10.1039/c5ra14667d
www.rsc.org/advances
An efficient asymmetric vinylogous allylic alkylation of b,g-butenolides
Recently, the enantioselective allylic alkylation with MBH
with Morita–Baylis–Hillman carbonates has been developed. With a adducts catalyzed by Lewis basic tertiary amines or phosphines
chiral cyclohexane-based squaramide-phosphine catalyst 5e, optically has emerged as a powerful strategy to deliver multifunctional
active g,g-disubstituted butenolides containing adjacent quaternary compounds.6 Among these examples, the asymmetric vinyl-
and tertiary chiral centers have been constructed in good-to-excellent ogous reaction involving MBH carbonates catalyzed by tertiary
yields (up to 98%) and excellent stereoselectivities (87 : 13–99 : 1 dr, phosphines has not been well developed.7 To the best of our
96–99% ee).
knowledge, the only example is the reaction between 2-trime-
thylsilyloxy furan and MBH carbonates reported by Shi's group
(Scheme 1).5a Despite their ability to render g,g-stereogenic
quaternary centers, which are challenging motifs in organic
synthesis, the application of g-substituted b,g-butenolides in
the asymmetric allylic alkylation is still in its infancy4a,d prob-
ably due to the low reactivity. Considering the higher nucleo-
philicity of tertiary phosphines than the corresponding amines,
we envisioned that chiral tertiary phosphines could be more
effective for the direct vinylogous allylic alkylation between
g-substituted butenolides and MBH carbonates. Herein, we
report the rst direct asymmetric vinylogous allylic alkylation of
butenolides with MBH carbonates to access g,g-disubstituted
butenolides using chiral tertiary phosphine as organocatalyst.8
We began our investigation with the vinylogous allylic
alkylation of b,g-butenolide 1a with MBH carbonate 2a in the
presence of 10 mol% rac-4a in CH2Cl2 at ambient temperature.
To our delight, the vinylogous reaction completed in 4 hours
and the regioselective g,g-disubstituted butenolide was
obtained with excellent diastereoselectivity in 92% yield. Then
we examined the chiral cyclohexane-based bifunctional phos-
phine organocatalysts at 25 ꢀC (Fig. 1), and the results are
summarized in Table 1.9 In general, the aliphatic thioureas
provided higher yields than the aromatic thioureas with a
similar diastereoselectivity (entries 1–4 vs. entries 5 and 6). The
b,g-Unsaturated butenolides have attracted great attention
recently due to their utility for the direct construction of
g,g-disubstituted butenolide derivatives, which are important
motifs in biologically active natural compounds and pharma-
ceutically useful molecules.1–3 Many endeavors have been
devoted towards developing efficient protocols to access
optically active g,g-disubstituted butenolides in the past
decade.4 In 2010, Chen and co-workers4a have developed the
rst direct asymmetric allylic alkylation of g-substituted b,g-
butenolides with Morita–Baylis–Hillman (MBH) carbonates to
access g,g-disubstituted butenolides containing adjacent
quaternary and tertiary chiral centers, catalyzed by cinchona
alkaloid derivatives. Since Chen's pioneering work, various
direct vinylogous reactions with b,g-butenolides as the nucleo-
philes have been reported, and the electrophiles involve a,b-
unsaturated carbonyl compounds,4b,f–k nitroolens,4e,l aldimi-
nes4c and MBH carbonates.4a,d It is worth noting that the prog-
ress has been mainly focused on Michael reactions. To the best
of our knowledge, enantioselective vinylogous allylic alkylation
has been rarely described.4a,d,5 Therefore, the development of a
simplied protocol to allow easy access to diverse g,g-disub-
stituted butenolides from b,g-unsaturated butenolides via
vinylogous allylic alkylation is still challenging.
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, Shanghai 200237, P. R. China. E-mail:
xinyanwu@ecust.edu.cn; Tel: +86 21 64252011
† Electronic supplementary information (ESI) available: Experimental procedures,
NMR spectra and HPLC chromatograms of the products. See DOI:
10.1039/c5ra14667d
Scheme 1 Previous work with chiral phosphine catalysts.
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As shown in Table 2, in halogenated solvents g,g-disubstituted
butenolides 3a could be obtained in excellent yield, diaster-
eoselectivity and enantioselectivity (entries 1–3). Lower yields
were produced with other polar solvents surveyed due to the low
conversion of the substrates (entries 4–6). Change of the ratio of
1a/2a from 2 : 1 to 1 : 1.5 led to higher reaction rate with a
similar yield, but the enantioselectivity was slightly lower
(entries 2 and 7–9). It was found that higher reaction tempera-
ture resulted in an increase of reaction rate with a decrease of
diastereoselectivity, without compromising the enantiose-
lectivity (entries 2, 10 and 11). Change of substrate concentra-
tion in CHCl3 did not affect the yield and diastereoselectivity.
Meanwhile, a slightly higher enantioselectivity was achieved by
proceeding the reaction in a lower concentration (entries 2 and
12–15). Further decrease of the concentration from 0.1 M to
0.025 M had no signicant impact on stereoselectivity. The
decrease of catalyst loading from 10 mol% to 5 mol% led to a
signicant decrease in yield due to a lower conversion (entry 2
vs. entry 16). Therefore, the optimal reaction conditions have
been identied as follows: 1.5 equiv. of b,g-butenolide 1 and
Fig. 1 Structures of the chiral bifunctional phosphines screened.
bifunctional phosphines with an additional chiral group could
enhance the enantioselectivity, and a chirality match existed
between the chiral backbone and the additional group (entries 3
and 4). To improve the stereoselectivity, chiral phosphines with
other H-bonding donators were examined. Squaramide 5c gave
better enantioselectivity but lower yield than the corresponding
thiourea 4c (entry 9 vs. entry 3). Therefore squaramide-phos-
phines10 containing different scaffolds were evaluated (entries
7–11). Organocatalyst 5e bearing aromatic scaffold exhibited
better yield and enantioselectivity than 5b and 5c bearing
aliphatic scaffold. Using 6a with NH2 group as the H-bonding
donor, the vinylogous reaction rate was increased, but poor
enantioselectivity obtained. Amide-phosphine 6b with a single
ꢀ
MBH carbonate 2 in CHCl3 (0.1 M) at 25 C using 10 mol% of
H-bonding donator exhibited
a lower reactivity, albeit
squaramide-phosphine 5e as the chiral catalyst.
producing the same level of enantioselectivity as the
squaramide-phosphines (entry 13). Therefore squaramide-
phosphine 5e was selected as the chiral organocatalyst to
further optimize the reaction conditions.
Subsequently, the optimization of other reaction parameters
(solvent, ratio of substrates, temperature and substrate
concentration) was carried out with squaramide-phosphine 5e.
Under the optimized reaction condition, the substrate scope
of the vinylogous allylic alkylation using a simple g-methyl-
substituted butenolide 1a and differently substituted MBH
carbonates was examined (Table 3, entries 1–16). Excellent
enantioselectivities were observed for MBH carbonates derived
Table 2 Optimization of the reaction conditionsa
Table
1 Screening of the chiral bifunctional phosphine
organocatalystsa
Ratio
(1a/2a) (M)
Conc. Time Yieldb
eec
(%)
Entry Solvent
(h)
(%)
drc
1
2
3
4
5
6
7
8
CH2Cl2
CHCl3
ClCH2CH2Cl 1.5 : 1
1.5 : 1
1.5 : 1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.4
0.1
0.05
12
12
12
96
96
96
3
9
24
4
97
98
92
52
57
Trace
98
95
97
98
98
98
98
98
98
37
98 : 2 98
98 : 2 98
97 : 3 98
29 : 1 96
34 : 1 97
Time Yieldb
eec
(%)
Entry Catalyst
(h)
(%)
drc
THF
1.5 : 1
1.5 : 1
1.5 : 1
1 : 1.5
1 : 1
1
2
3
4
5
6
7
8
4a (R ¼ Bn)
4
94
72
93
52
27
97 : 3 59
97 : 3 73
97 : 3 82
97 : 3 60
97 : 3 82
EtOAc
CH3CN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
ndd
nd
4b (R ¼ n-C12H25
)
24
12
12
144
24
24
3
36
48
12
3
4c (R ¼ (S)-1-phenylethyl)
4d (R ¼ (R)-1-phenylethyl)
4e (R ¼ Ph)
98 : 2 96
98 : 2 98
98 : 2 98
97 : 3 98
99 : 1 98
98 : 2 97
98 : 2 99
98 : 2 99
98 : 2 99
98 : 2 99
9
2 : 1
10e
11f
12
13
14
15
16g
1.5 : 1
1.5 : 1
1.5 : 1
1.5 : 1
1.5 : 1
1.5 : 1
1.5 : 1
4f (R ¼ 3,5-(CF3)2C6H3)
5a (R ¼ Bn)
Complex ndd
nd
nd
NRe
97
84
31
97
83
85
nd
48
12
12
12
5b (R ¼ n-C12H25
)
97 : 3 97
97 : 3 90
97 : 3 95
98 : 2 98
96 : 4 44
91 : 9 92
9
5c (R ¼ (S)-1-phenylethyl)
5d (R ¼ Ph)
10
11
12
13
5e (R ¼ 3,5-(CF3)2C6H3)
6a (R ¼ H)
0.025 12
0.2 96
6b (R ¼ Bz)
72
a
Unless otherwise noted the reactions were performed with 1a, 2a, and
a
b
c
The reactions were performed with 0.3 mmol of 1a, 0.2 mmol of 2a, 10 mol% of 5e in the solvent at 25 ꢀC. Isolated yield. The dr and ee
b
and 10 mol% of catalyst in 1 mL CH2Cl2 at 25 ꢀC. Isolated yield. values were based on chiral HPLC analysis, and the given ee data were
c
d
e
f
g
The dr and ee values were based on chiral HPLC analysis, and the for 3a. Not determined. At 40 ꢀC. At 0 ꢀC. The catalyst loading
given ee data were for 3a. Not determined. e No reaction.
was 5 mol%.
d
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from different alkyl acrylates and methyl vinyl ketone, while 18 vs. entry 17). To our delight, the reaction tolerated all the b,g-
n-butyl acrylate and t-butyl acrylate provided lower yields butenolides bearing different g-aryl groups with either an
probably due to the steric effect (entries 1–5). MBH carbonates electron-withdrawing or an electron-donating group, affording
with a strong electron-withdrawing group at the 4- and 3-posi- the corresponding adducts 3 in excellent yields, diastereo- and
tion of aromatic ring gave better yields than those with a weak enantioselectivities (Table 3, entries 17–22).
electron-withdrawing group or without substituent (entries 1
To determine the absolute conguration, a vinylogous allylic
and 6–14). MBH carbonates bearing two electron-withdrawing alkylation was performed with DHQD(PYR)2 as chiral catalyst,
groups at both 3- and 4-positions of phenyl also afforded the according to the literature report.4a Compound 3p was assigned
desired products in good yields and enantioselectivities (entries as (R,S) by comparing the HPLC spectra and optical rotation
15 and 16). The MBH carbonate with substituent at the 2-posi- values with the product resulting from DHQD(PYR)2 catalysis.
tion of phenyl failed to produce the desired product probably The absolute congurations of other products were assigned by
due to an ortho effect. To our disappointment, MBH carbonates analogy.
with an electron-donating group such as methyl and methoxyl
at phenyl ring, and MBH carbonates of alkyl aldehydes are organocatalyzed enantioselective vinylogous allylic alkylation of
unreactive substrates under the typical reaction condition. MBH carbonates with b,g-butenolides. In the presence of
In conclusion, we have developed the rst chiral phosphine-
Further exploration of the substrate scope has been focused 10 mol% chiral cyclohexane-based squaramide-phosphine 5e,
on the b,g-butenolides bearing different g-aryl groups. the corresponding g,g-disubstituted butenolides containing
Considering the reaction rate, excess MBH carbonate adjacent quaternary and tertiary stereogenic centers were
(1.5 equiv.) was used, and the vinylogous reaction was accom- obtained in up to 98% yield with excellent diastereo- and
plished within 3 hours attaining the same enantiocontrol (entry enantioselectivities. This asymmetric reaction provides an effi-
cient approach toward the synthesis of chiral g,g-disubstituted
butenolides.
Table 3 Substrate scope of the direct asymmetric allylic alkylation of
b,g-butenolides with MBH carbonatesa
Acknowledgements
We are grateful for the nancial support from the National
Natural Science Foundation of China (21242007, 21102043),
Science and Technology Commission of Shanghai Municipality
(15ZR1409200), and the Fundamental Research Funds for the
Central Universities.
Time Yieldb
eec
(%)
Notes and references
Entry R1
R2
R3
(h)
(%)
drc
1 For recent reviews of natural products containing
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
3-NO2
4-CN
3-CN
4-CF3
3-CF3
4-Br
OMe
OEt
12
12
3a, 98
3b, 92
3c, 56
3d, 52
3e, 95
3f, 98
3g, 91
3h, 93
3i, 94
3j, 87
3k, 72
3l, 81
98 : 2
98 : 2
99 : 1
99 : 1
99 : 1
87 : 13 98
97 : 3 98
90 : 10 98
96 : 4 98
90 : 10 97
99
99
99
99
99
´
butenolides, see: (a) A. D. Rodrıguez, Tetrahedron, 1995, 51,
OnBu 120
`
4571; (b) A. Bermejo, B. Figadere, M.-C. Zafra-Polo,
OtBu 120
Me
OMe
OMe
OMe
OMe
OMe
OMe 120
OMe 96
OMe 120
OMe 120
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12
12
12
72
72
9
10
11
12
13
14d
15
16
17
18e
19e
20e
21e
22e
96 : 4
96 : 4
98
98
97
96
98
98
4-Cl
4-F
H
3m, 74 97 : 3
3n, 67
3o, 92
3p, 91
3q, 94
3q, 94
3r, 97
3s, 78
3t, 98
3u, 94
96 : 4
94 : 6
94 : 6
88 : 12 99
88 : 12 99
93 : 7
98 : 2
91 : 9
94 : 6
3,4-Cl2 OMe
12
48
14
3
3
6
3,4-F2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
OMe
OMe
OMe
OMe
OMe
OMe
OMe
C6H5
C6H5
4-FC6H4
4-ClC6H4
4-MeC6H4
98
98
99
98
2
3
4-MeOC6H4 4-NO2
a
Unless otherwise noted, the reactions were performed with 0.3 mmol
b
of 1, 0.2 mmol of 2, 10 mol% of 5e in 2 mL CHCl3 at 25 ꢀC. Isolated
yield. The dr and ee values were based on chiral HPLC analysis, and
c
d
the given ee data were for 3. The catalyst loading was 20 mol%.
e
With 0.2 mmol of 1 and 0.3 mmol of 2.
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