Asymmetric Formal [3+2] Cycloaddition Reaction of Succinaldehyde and Nitroalkene Catalyzed by Diphenylprolinol Silyl Ether

Article abstract of DOI:10.1002/ejoc.201500623

The enantioselective domino Michael/Henry reaction of nitroalkenes with succinaldehyde was found to proceed efficiently upon using diphenylprolinol silyl ether as the organocatalyst. The reaction affords cis-disubstituted nitropentenes with excellent diastereoselectivities and enantioselectivities after treatment of the Michael product with Ac₂O and pyridine. cis-Disubstituted nitropentenes are obtained with excellent diastereoselectivities and enantioselectivities by a formal [3+2] cycloaddition reaction. The first reaction is a domino reaction composed of the diphenylprolinol silyl ether mediated Michael reaction of nitroalkene with succinaldehyde followed by a Henry reaction. The next reaction is a dehydration achieved by using Ac₂O and pyridine.

Full text of DOI:10.1002/ejoc.201500623

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500623
Asymmetric Formal [3+2] Cycloaddition Reaction of Succinaldehyde and
Nitroalkene Catalyzed by Diphenylprolinol Silyl Ether
Shigenobu Umemiya[a] and Yujiro Hayashi*[a]
Keywords: Organocatalysis / Michael addition / Domino reactions / Cycloaddition / Nitro compounds
The enantioselective domino Michael/Henry reaction of
nitroalkenes with succinaldehyde was found to proceed
tenes with excellent diastereoselectivities and enantio-
selectivities after treatment of the Michael product with
efficiently upon using diphenylprolinol silyl ether as the or- Ac2O and pyridine.
ganocatalyst. The reaction affords cis-disubstituted nitropen-
cycloaddition reaction for the three-pot synthesis of prosta-
Introduction
glandin A1 and E1 methyl esters.[15]
The field of organocatalysis has grown rapidly, and many
organocatalysts have been developed.[1] Diphenylprolinol
silyl ether 1,[2] which was independently developed by our
group[3] and Jørgensen’s group,[4] has been employed suc-
cessfully in many asymmetric, catalytic reactions involving
enamines and an iminium ion as a reactive intermediate.
The cycloaddition reaction is a powerful method for the
construction of carbocycles,[5] and diphenylprolinol silyl
ether 1 is an effective catalyst for a number of asymmetric
cycloaddition reactions: [2+1] cycloaddition reactions for
the synthesis of cyclopropane derivatives,[6] [2+2] cycload-
dition reactions for cyclobutane derivatives,[7] and [4+2] cy-
cloaddition reactions (Diels–Alder reaction) for cyclohex-
ene derivatives.[8] The [6+2] cycloaddition reaction can also
be catalyzed by the diphenylprolinol silyl ether catalyst.[9]
Formal [3+2] cycloaddition is a useful method for the syn-
thesis of cyclopentane derivatives, for which diphenylproli-
nol silyl ether can also be utilized effectively.[10]
(1)
(2)
We already reported the use of a formal [4+2] cycload-
dition reaction for the formation of cyclohexanecarbal-
dehyde by a domino Michael/intramolecular Henry reac-
tion of 2,5-dihydroxy-3,4-dihydropyran (glutaraldehyde
equivalent) and β-nitrostyrene catalyzed by diphenylproli-
nol silyl ether [Equation (1)].[11,12] We anticipated that a for-
mal [3+2] cycloaddition reaction would proceed if succin-
aldehyde[13,14] was employed instead of glutaraldehyde in a
similar domino reaction to afford chiral cyclopentane-
carbaldehydes [Equation (2)]. Herein, we describe the suc-
cessful realization of this scenario. We previously reported
a preliminary result involving the use of the formal [3+2]
Results and Discussion
We found that succinaldehyde was easily prepared from
2,3-dimethoxytetrahydrofuran upon treatment with water
in the presence of Amberlyst 15 [Equation (3)]. Initially, we
selected β-nitrostyrene as a model nitroalkene, and the reac-
tion was investigated by using diphenylprolinol trimethyl-
silyl ether 1 (10 mol-%) as the catalyst [Equation (4)]. The
Michael reaction followed by intramolecular Henry reac-
tion proceeded in CH2Cl2 to afford substituted cyclopen-
tanecarbaldehyde 2. To prevent facile cis/trans isomeriza-
tion, product 2 was reduced with NaBH4 in situ to provide
diol 3 in 62% yield in a one-pot operation. Several dia-
stereomers (ratio 37:34:16:13) were observed by analysis of
the crude mixture by 1H NMR spectroscopy. Upon treating
the mixture of diastereomers of diol 3 with Ac2O and pyr-
[a] Department of Chemistry, Graduate School of Science, Tohoku
University,
6-3 Aramaki-Aza Aoba, Aoba-ku, Sendai 980-8578, Japan
E-mail: yhayashi@m.tohoku.ac.jp
http://www.ykbsc.chem.tohoku.ac.jp/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500623.
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Asymmetric Formal [3+2] Cycloaddition Reaction
idine, nitrocyclopentene derivative 4 was obtained in 75% Table 1. Effect of the solvent in the formal [3+2] cycloaddition reac-
tion of β-nitrostyrene and succinaldehyde.[a]
yield. The diastereomeric ratio was excellent (cis/trans ratio
Ͼ95:5) and the enantioselectivity of the reaction for the
major cis isomer was also excellent (91%ee). Although ex-
cellent diastereoselectivity was obtained in the previous for-
mal [4+2] cycloaddition reaction of glutaraldehyde [Equa-
tion (1)], the diastereoselectivity of the first domino
Michael/Henry reaction of succinaldehyde was not high
[Equation (4)]. The excellent diastereoselectivity obtained
upon treatment of diol 3 with Ac2O and pyridine indicates
that the Michael reaction proceeds with excellent diastereo-
selectivity but that the subsequent intramolecular Henry re-
action is not diastereoselective.
Entry
Solvent
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
CH2Cl2
MeOH
toluene
THF
hexane
CH3CN
CH2Cl2
10
50
48
42
45
18
1.5
62
Ͻ5
50
46
47
55
74
91
n.d.[d]
90
n.d.[d]
n.d.[d]
n.d.[d]
95
5
6
7[e]
[a] Unless otherwise noted, the reaction was performed with succin-
aldehyde (0.75 mmol), nitrostyrene (0.30 mmol), and organocata-
lyst 1 (0.03 mmol, 10 mol-%) in solvent (0.60 mL) at room tempera-
ture for the indicated time. See the Supporting Information for de-
tails. [b] Yield of isolated diol as a mixture of several diastereomers.
[c] Determined by HPLC analysis on a chiral phase after treatment
of the diol with Ac2O and pyridine to the corresponding mono-
ester. See the Supporting Information for details. [d] n.d.: not deter-
mined. [e] The reaction was performed in the presence of p-nitro-
phenol (0.03 mmol).
(3)
Table 2. Effect of the solvent in the formal [3+2] cycloaddition reac-
tion of 4-phenyl-1-nitro-1-butene and succinaldehyde.[a]
(4)
To improve the enantioselectivity, a range of solvents was
screened (Table 1). The reaction barely proceeded in MeOH
(Table 1, entry 2) and was very slow in toluene, THF, and
hexane (Table 1, entries 3–5). A lower yield was obtained
upon conducting the reaction in CH3CN (Table 1, entry 6).
We previously reported that the presence of acid accelerates
the Michael reaction of aldehydes and nitroalkenes,[16] and
we were pleased to find that the inclusion of p-nitrophenol
as an additive increased both the yield and the enantio-
selectivity (Table 1, entry 7). That is, if the reaction was per-
formed in the presence of p-nitrophenol in CH2Cl2, the
product was obtained in 74% yield with 95%ee (cis/trans
ratio Ͼ95:5).
With excellent results achieved in the model reaction of
β-nitrostyrene, we then applied the reaction conditions to a
range of 1-alkyl-2-nitroethene derivatives and found that
the yields were moderate. The reaction of 4-phenyl-1-nitro-
1-butene afforded not only desired cyclopentylmethanol 5
but also uncyclized nitro derivative 6 in yields of 60 and
25%, respectively (Table 2, entry 1). Considering that the
Entry Solvent
Time Yield [%][b] of Yield [%][c] of ee [%][d]
[h]
5
6
1
2
3
CH2Cl2
toluene
THF
12
15
8
6
6
60
46
61
73
82
25
33
28
16
Ͻ 5
94
n.d.[e]
93
94
94
4
CH3CN
CH3CN
5[f]
[a] Unless otherwise noted, the reaction was performed with succin-
aldehyde (0.75 mmol), nitrostyrene (0.30 mmol), and organocata-
lyst 1 (0.03 mmol, 10 mol-%) in solvent (0.60 mL) at room tempera-
ture for the indicated time. See the Supporting Information for de-
tails. [b] Yield of isolated diol 5 as a mixture of several dia-
stereomers. [c] Yield of 6. [d] The ee of diol 5, as determined by
HPLC analysis on a chiral phase after treatment of 5 with Ac2O
and pyridine to the corresponding monoester. See the Supporting
Information for details. [e] n.d.: not determined. [f] The reaction
was performed in the presence of iPr2EtN (0.33 mmol).
First, a range of solvents was screened in the reaction
best reaction conditions for 1-aryl- and 1-alkyl-2-nitroeth- (Table 2). Among those investigated, including CH2Cl2, tol-
ene substrates could be different, optimization of the reac- uene, THF, and CH3CN, the reaction was the fastest in
tion conditions was performed with 4-phenyl-1-nitro-1-but- CH3CN and afforded the product in good yield (73%);
ene as a representative 1-alkyl-2-nitroethene.
however, uncyclized nitro derivative 6 was also found in
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S. Umemiya, Y. Hayashi
SHORT COMMUNICATION
16% yield (Table 2, entry 4). Byproduct 6 was assumed to
be derived from reduction of Michael product 7 (Figure 1),
which indicates that the subsequent intramolecular Henry
reaction is slow. To facilitate the second Henry reaction, we
added base after the Michael reaction. It was found that
the addition of iPr2EtN to the Michael product improved
Figure 1. Structure of Michael product 7.
Table 3. Formal [3+2] cycloaddition reaction of succinaldehyde catalyzed by 1.[a]
[a] Conditions A: For the first reaction, succinaldehyde (0.75 mmol), nitroalkene (0.30 mmol), and organocatalyst 1 (0.03 mmol, 10 mol-
%) in CH2Cl2 (0.60 mL) at room temperature for the indicated time. For the second reaction, Ac2O (0.75 mmol) and pyridine (1.0 mL)
for 16 h. Conditions B: For the first reaction, succinaldehyde (0.75 mmol), nitroalkene (0.30 mmol), and organocatalyst 1 (0.03 mmol,
10 mol-%) in CH2Cl2 (0.60 mL) at room temperature for the indicated time; then iPr2NEt (0.33 mmol) was added. For the second reaction,
Ac2O (0.75 mmol) and pyridine (1.0 mL) for 16 h. See the Supporting Information for details. [b] Yield of isolated diol as a mixture of
1
several diastereomers. [c] Yield of isolated cyclopentene derivative. [d] Determined by analysis by H NMR spectroscopy. [e] Determined
by HPLC analysis on a chiral phase. See the Supporting Information for details.
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Asymmetric Formal [3+2] Cycloaddition Reaction
the yield, and desired product 5 was obtained in 82% yield Michael reaction is known to proceed with high syn selec-
with 94%ee, with the generation of a trace amount of uncy- tivity,[3] the cis-cyclopentane derivative with regard to the
clized product 6 (Table 2, entry 5). These reaction condi- formyl and R groups would be generated before isomeriza-
tions were employed in the first step of our total synthesis tion to the thermodynamically more stable trans isomer.
of prostaglandin A1 and E1 methyl esters.[15]
Having optimized the reaction conditions for both 1-
aryl-2-nitroethenes (conditions A) and 1-alkyl-2-nitroeth-
enes (conditions B), the generality of the reaction was inves-
tigated (Table 3).
In the reaction of 1-aryl-2-nitroethenes, not only was the
phenyl group suitable, but naphthyl, electron-deficient p-
bromophenyl, electron-rich p-methoxyphenyl, and het-
eroaromatic groups such as furyl substituents were also suc-
cessfully employed to provide the cis-disubstituted cyclo-
pentene derivatives in good yields with excellent enantio-
selectivities and perfect diastereoselectivities (Table 3, en-
tries 2–5). The reaction was slower with the naphthyl-sub-
stituted substrate because of the low solubility of the start-
ing material in CH2Cl2, and the reaction proceeded grad-
ually as it dissolved (Table 3, entry 2).
A
tert-
butoxycarbonyl-substituted nitroalkene was also a suitable
substrate, and this compound provided the cyclopentene de-
Scheme 1. Reaction mechanism of the formal [3+2] cycloaddition
reaction.
rivative in
a short reaction time in a nearly dia-
stereomerically and enantiomerically pure form (Table 3,
entry 6). The reaction of 1-alkyl-2-nitroethene with 2-phen-
ylethyl, 2-triisopropylsiloxyethyl (CH2CH2OTIPS), and cy-
clohexyl groups as alkyl substituents proceeded efficiently
to afford the products with excellent cis selectivities and
enantioselectivities (Table 3, entries 7–9).
Hong and co-workers reported an excellent [3+2] cyclo-
addition reaction[10f] that was performed by using 4-hy-
droxybut-2-enal as a succinaldehyde surrogate in the pres-
ence of catalyst 1 (20 mol-%) with CH3CO2H to afford
trans isomers with respect to the formyl and aryl groups in
moderate yields (51–65%) with excellent enantioselectivities
[Equation (5)]. This is complementary to our reaction,
which affords the thermodynamically unstable cis isomer.
Succinaldehyde, which is easily prepared [Equation (3)], is
more reactive than 4-hydroxybut-2-enal; consequently, the
reaction requires a lower catalyst loading and gives better
yields.
Conclusion
In summary, we developed a domino Michael/Henry re-
action of readily available succinaldehyde and nitroalkenes
catalyzed by diphenylprolinol silyl ether 1 to afford nitrocy-
clopentene derivatives in good yield with excellent enantio-
selectivities, in which the thermodynamically unstable cis
isomers are obtained selectively. Given that the nitroalkene
moiety can be converted into several functional groups,[17]
the obtained cis-disubstituted nitropentenes are syntheti-
cally useful compounds, as we demonstrated in our recent
three-pot synthesis of prostaglandin A1 and E1 methyl es-
ters.[15]
Acknowledgments
This work was supported by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan, through a Grant-
in-Aid for Scientific Research on Innovative Areas “Advanced Mo-
lecular Transformations by Organocatalysts” (grant number
23105010).
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Published Online: June 5, 2015
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