Biomimetic asymmetric aldol reactions catalyzed by proline derivatives attached to β-cyclodextrin in water

Article abstract of DOI:10.1016/j.tetlet.2012.04.140

Two proline derivatives, (S)-2-aminomethylpyrrolidine and (R)-2-aminomethylpyrrolidine modified β-CD (CD-1, CD-2) were synthesized in the yields of 31% and 14%. Their self-inclusion conformations were characterized by ¹H ROESY NMR studies and q

Full text of DOI:10.1016/j.tetlet.2012.04.140

Tetrahedron Letters 53 (2012) 3541–3545
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Biomimetic asymmetric aldol reactions catalyzed by proline derivatives
attached to b-cyclodextrin in water
⇑
Hai-Min Shen, Hong-Bing Ji
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two proline derivatives, (S)-2-aminomethylpyrrolidine and (R)-2-aminomethylpyrrolidine modified b-
CD (CD-1, CD-2) were synthesized in the yields of 31% and 14%. Their self-inclusion conformations were
characterized by 1H ROESY NMR studies and quantum calculation. When CD-1 was applied to
asymmetric aldol reactions, up to 94% ee was obtained. Substrate selectivity was also observed in these
asymmetric aldol reactions.
Received 18 February 2012
Revised 12 April 2012
Accepted 24 April 2012
Available online 8 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
b-Cyclodextrin
Proline derivative
Asymmetric aldol reaction
Quantum calculation
b-Cyclodextrin (b-CD) is a water-soluble macrocyclic oligomer
possessing seven -(+)-glucopyranosyl units linked by -1,4-glyco-
applied in the asymmetric aldol reactions.8 Although, so many al-
dol organocatalysts derived from proline were reported, the Lewis
bases to activate aldol donors in these organocatalysts were the
same.9 The secondary amine in the pyrrolidine ring is the Lewis
base to activate aldol donors, acting as catalytic active center. In
addition, to perform as effective aldol organocatalysts, most of
them must be employed in organic solvents, although some inclu-
sion complexes of b-CD and proline derivatives have been applied
in the asymmetric aldol reactions as catalysts in water.10 Our initial
motivation was to combine b-CD and proline derivatives together
to form aldol organocatalysts covalently, and apply these organo-
catalysts as aldolase enzyme mimics to catalyze asymmetric aldol
reactions in water, which is still a very challenging objective at
present.
In this research, two proline derivatives, (S)-2-aminomethyl-
pyrrolidine and (R)-2-aminomethylpyrrolidine modified b-CD
(CD-1 and CD-2) were synthesized and applied as aldolase enzyme
mimics in asymmetric aldol reactions in water. Up to 94% ee was
achieved. Besides, a method to design and synthesize artificial en-
zymes based on b-CD was presented.
D
a
sidic bonds.1 For its ready availability in commerce, b-CD has be-
come the most widely utilized member in the cyclodextrin
family. Its unique property to have a hydrophilic exterior and a
hydrophobic cavity makes it an attractive unit in the construction
of artificial enzymes in organic synthesis.2 In the artificial enzymes
based on b-CD, b-CD can mimic the hydrophobic pockets in en-
zyme to bind the hydrophobic substrates in its hydrophobic cavity
and offer the hydrophobic substrates a hydrophobic environment,
and the modifying group of b-CD plays the role of catalytic active
site. The formation of inclusion complex between b-CD unit and
substrate increases the local concentration of substrate and immo-
bilizes the substrate near the catalytic active center, thus an
obvious acceleration in reaction rate,3 excellent substrate selectiv-
ity,3b,4 regioselectivity,5 and enantioselectivity6 can usually be
achieved. Hu and co-workers reported the cyclohexane-1,2-dia-
mine modified b-CD as aldolase enzyme mimic, with excellent
enantioselectivity.6d To our best knowledge, the artificial enzymes
based on b-CD mostly focus on the acceleration of reaction rate and
the enhancement of regioselectivity, reports on the enhancement
of enantioselectivity are very limited.
Aldol reactions are an effective method to form carbon–carbon
bond in organic synthesis, especially the asymmetric aldol reac-
tions, being an attractive field all the time. Since List et al., reported
the asymmetric aldol reactions catalyzed by proline (Scheme 1) in
2000,7 numerous proline derivatives have been synthesized and
COOH
N
H
O
OH
O
O
H
30 mol%
DMSO
+
NO2
NO2
68%(76% ee)
⇑
Corresponding author. Tel.: +86 20 8411 3658; fax: +86 20 8411 3654.
E-mail address: jihb@mail.sysu.edu.cn (H.-B. Ji).
Scheme 1. Asymmetric aldol reaction of 4-nitrobenzaldehyde and acetone cata-
lyzed by proline.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.140

3542
H.-M. Shen, H.-B. Ji / Tetrahedron Letters 53 (2012) 3541–3545
CH3COONa–HCl buffer solution. CD-1 and CD-2 exhibited different
O
catalytic activity and enantioselectivity as shown in Table 1 and
Table 2. Compared with CD-2, CD-1 showed excellent catalytic
activity and enantioselectivity. When the pH of the CH3COONa–
HCl buffer solution was kept at 4.5 or lower, more than 80% ee
could be obtained. When the pH increased from 4.5 to 7.0, the
ee% decreased from 81% to 7%. This phenomenon could be attrib-
uted to the protonation of the secondary amine linked to the C-6
of b-CD. At lower pH, the secondary amine linked to the C-6 be-
came protonated, and accordingly, did not possess the catalytic
activity. The aldol reactions took place in the hydrophobic cavity
of CD-1, catalyzed by the secondary amine in the pyrrolidine ring,
which was included in the hydrophobic cavity and not protonated,
more than 80% ee could be obtained. When the pH increased, the
protonation of the secondary amine linked to the C-6 became
weaker, and the aldol reactions could be catalyzed by the second-
ary amine linked to the C-6, giving a racemic aldol product, result-
ing in lower ee%. The enantioselective aldol reactions occurred in
the hydrophobic cavity of CD-1 that had also been confirmed by
the inhibition experiment and control experiment. When the same
mol of 1-adamantanecarboxylic acid as CD-1 was added, the aldol
reaction was inhibited obviously. The yield of the aldol product de-
creased from 86% to 15%, and the ee% decreased from 79% to 25%,
as shown in Table 1 (entry 11). When proline derivative (S)-2-
[(benzylamino)methyl]pyrrolidine (PD-1) was employed as cata-
lyst for the asymmetric aldol reactions, because of the protonation
of both the secondary amines in PD-1, no catalytic activity was ob-
served (see Table S1 in Supplementary data).
As for CD-2, the poor catalytic activity and enantioselectivity
could be attributed to the conformation of the modifying group
in the hydrophobic cavity of CD-2. Unlike CD-1, in the self-inclu-
sion conformation of CD-2, the secondary amine in the pyrrolidine
ring cannot be included in the hydrophobic cavity of CD-2 effec-
tively as shown in Figure 3. In pH 5.5 and lower pH, both the sec-
ondary amine in the pyrrolidine ring and the secondary amine
linked to the C-6 became protonated, possessing no catalytic activ-
ity as shown in Table 2. In pH 6.0 and higher pH, the protonation of
both the secondary amines became weaker, and both of them pos-
sessed the ability to catalyze the aldol reactions, but giving lower
ee%. In this reaction system, the ee% of the aldol products derived
from the aldol reactions that occurred in the hydrophobic cavities
of CD-1 and CD-2. The products obtained from the aldol reactions
occurred outside the hydrophobic cavities of CD-1 and CD-2, cata-
lyzed by the secondary amine that linked to the C-6 of b-CD are
racemic.
N
H
N
H
CD-1
CD-2
NH2
NH2
NH2
LiAlH4, THF
0°C to Reflux
6-OTs-β-CD
DMF,80°C
O
N
H
N
H
NH2
N
H
N
H
NH
NH
CD-1
CD-2
Scheme 2. Schematic synthesis of CD-1 and CD-2.
The synthesis of CD-1 and CD-2 is shown in Scheme 2. (S)-2-
Aminomethylpyrrolidine and (R)-2-aminomethylpyrrolidine were
available from reduction of (S)-prolinamine and (R)-prolinamine
by LiAlH4. Then CD-1 and CD-2 were readily synthesized by the
nucleophilic substitution of mono(6-O-p-tolylsulfonyl)-b-CD with
an excess of (S)-2-aminomethylpyrrolidine and (R)-2-aminometh-
ylpyrrolidine. 1H ROESY NMR spectra of CD-1 and CD-2 showed
that the modifying groups in CD-1 and CD-2 possessed the config-
uration to be included in the hydrophobic cavities of CD-1 and CD-
2, forming self-inclusion configuration which is a common phe-
nomenon in the b-CD derivatives possessing hydrophobic modify-
ing groups.11 Obvious correlation peaks between the H atoms
located in the pyrrolidine ring of the modifying groups and the
H-3, H-5 in the cavity of the parent b-CD can be observed
(Fig. 1). The self-inclusion configuration had also been confirmed
by the quantum calculation. The conformations of CD-1 and CD-2
were optimized by Gaussian 03 program at the level of PM3.12
The optimal conformations of CD-1 and CD-2 are shown in Figure 2
and Figure 3, in which the modifying groups of CD-1 and CD-2
were included in the hydrophobic cavities of CD-1 and CD-2, pos-
sessing the self-inclusion configurations. The results of 1H ROESY
NMR and quantum calculation were in good consistency with each
other.
With CD-1 and CD-2 as aldolase enzyme mimics in hand, the
asymmetric aldol reactions were conducted in 0.5 M aqueous
The substrate scope for the artificial aldolase CD-1 was ex-
tended to a variety of aromatic aldehydes. The results are pre-
sented in Table 3. CD-1 not only exhibits a good tolerance to a
wide range of substrates but also demonstrates some substrate
selectivity to both the aldol acceptors and the aldol donors. For
example, 2-naphthaldehyde could undergo the asymmetric aldol
reactions smoothly, giving 94% ee in the yield of 46%, but 1-naph-
thaldehyde could not arrive at it. Proper orientation and conforma-
tion of the substrate in the hydrophobic cavity of CD-1 are
essential for the effective catalysis. Compared with 2-substituted
benzaldehydes, when 3-substituted and 4-substituted benzalde-
hydes were employed as substrates, better ee% could be achieved.
Aromatic aldehydes with large aromatic rings, such as 2-naphthal-
dehyde and 4-phenylbenzaldehyde, seem to be the very favorable
substrates for CD-1. To some extent, the ee% increased with the in-
crease in the molecular volume of the aromatic aldehydes. At last,
based on our research above and the work reported by Hu et al.,6d
the origin of enantioselectivity induced by CD-1 can be summa-
rized as illustrated in Figure 4. When 4-nitrobenzaldehyde ap-
proaches the enamine in the hydrophobic cavity of CD-1 to form
a carbon–carbon bond, there are two face choices to the enamine,
H atoms belong to thepyrrolidine ring
3.60
3.65
3.70
3.75
H-5
H-3
3.80
3.85
3.90
3.95
4.00
4.05
4.10
4.15
4.20
4.25
3.4
3.0
2.6
2.2
1.8
1.4
δ(ppm)
Figure 1. Partial 1H ROESY NMR spectrum of CD-1.

H.-M. Shen, H.-B. Ji / Tetrahedron Letters 53 (2012) 3541–3545
3543
CD-1
CD-1(a)
CD-1(b)
Figure 2. Optimal conformation of CD-1 (a, side view; b, top view) obtained at the level of PM3.
CD-2
CD-2(a)
CD-2(b)
Figure 3. Optimal conformation of CD-2 (a, side view; b, top view) obtained at the level of PM3.
Table 1
Table 2
Asymmetric aldol reactions of 4-nitrobenzaldehyde and acetone catalyzed by CD-1
Asymmetric aldol reactions of 4-nitrobenzaldehyde and acetone catalyzed by CD-2
Entry
pH
Yieldd (%)
eed (%)
Configuration
Entry
pH
Yielda (%)
eea (%)
Configuration
1
2
3
4
5
6
7
8
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
4.5
4.5
4.5
23
30
40
47
52
60
91
91
63
86
15
81
81
81
78
76
62
24
7
R
R
R
R
R
R
R
R
R
R
R
1
2
3
4
5
6
7
8
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Trace
Trace
Trace
Trace
Trace
20
N.D.
N.D.
N.D.
N.D.
N.D.
32
—
—
—
—
—
R
31
64
23
13
R
R
9a
10b
11c
80
79
25
Condition:
CD-2
0.02 mmol,
4-nitrobenzaldehyde
0.20 mmol,
acetone
0.2 mL + 0.1 mL, solvent (0.5 mol/L CH3COONa–HCl) 1 mL, 25 °C, 48.0 h; N.D. (ee):
The ee of the product was not determined.
a
Condition:
CD-1
0.02 mmol,
4-nitrobenzaldehyde
0.20 mmol,
acetone
Determined by HPLC analysis with a Chiralcel AS-H column. The absolute
0.2 mL + 0.1 mL, solvent (0.5 mol/L CH3COONa–HCl) 1 mL, 25 °C, 48.0 h.
configurations of the aldol products were determined by comparison of the eluting
a
CD-1 0.04 mmol, 4-nitrobenzaldehyde 0.20 mmol, acetone 0.2 mL + 0.1 mL,
sequence of the enantiomers with literature.6d
solvent (0.5 mol/L CH3COONa–HCl) 1 mL, 25 °C, 48.0 h.
b
CD-1 0.04 mmol, 4-nitrobenzaldehyde 0.20 mmol, acetone 0.2 mL + 0.1 mL,
solvent (0.5 mol/L CH3COONa–HCl) 1 mL, 25 °C, 96.0 h.
c
CD-1 0.04 mmol, 4-nitrobenzaldehyde 0.20 mmol, 1-adamantanecarboxylic
In conclusion, following our initial motivation to construct
artificial enzyme mimics through attaching the catalytic active
center to b-CD and to conduct organic reactions in water, we have
successfully designed and synthesized two proline derivatives,
(S)-2-aminomethylpyrrolidine and (R)-2-aminomethylpyrrolidine
modified b-CD, CD-1 and CD-2, as aldolase enzyme mimics. When
applied to the asymmetric aldol reactions in water, CD-1 exhibited
excellent catalytic activity and enantioselectivity, up to 94% ee was
achieved in the yield of 46%. We have also provided a principle in
the design and synthesis of artificial enzymes based on b-CD in this
letter, to attach appropriate small organic molecules which possess
catalytic activity to the appropriate position of b-CD.
acid 0.04 mmol, acetone 0.2 mL + 0.1 mL, solvent (0.5 mol/L CH3COONa–HCl) 1 mL,
25 °C, 96.0 h.
d
Determined by HPLC analysis with a Chiralcel AS-H column. The absolute
configurations of the aldol products were determined by comparison of the eluting
sequence of the enantiomers with literature.6d
but one face is more accessible because of the steric hindrance gen-
erated from the limited cavity of CD-1, giving the enantioselective
aldol product. Herein, the (R)-configuration in the aldol product is
more favorable.

3544
H.-M. Shen, H.-B. Ji / Tetrahedron Letters 53 (2012) 3541–3545
Table 3
Asymmetric aldol reactions of a variety of substrates catalyzed by CD-1
Entry
Aldehyde
CHO
Ketone
Yielda (%)
51
syn/antia
eea (%)
84
Configuration
O
1
—
R
CH3CCH3
CHO
O
CH3CCH3
2
3
4
62
77
86
—
—
—
24
76
79
R
R
R
NO2
CHO
O
CH3CCH3
NO2
CHO
CHO
O
CH3CCH3
O2N
O
CH3CCH3
5
6
89
70
—
—
75
0
R
NC
CHO
O
CH3CCH3
—
Cl
CHO
O
CH3CCH3
7
42
—
78
R
Cl
CHO
CHO
O
CH3CCH3
8
9
62
38
32
—
—
—
86
86
84
R
R
R
Cl
O
CH3CCH3
H3C
CHO
O
CH3CCH3
10
H3CO
CHO
O
CH3CCH3
11
12
37
—
—
91
R
CHO
O
CH3CCH3
Trace
N.D.
—
CHO
O
13
14
46
57
—
94
R
CH3CCH3
CHO
CHO
CHO
O
CH3CCH2CH3
96/4
39/29
R/R
O2N
O2N
O2N
O
O
15
16
90
36
56/44
6/94
63/28
38/54
R/S
R/R
CHO
CHO
O
CH3CCH2CH3
17
18
56
89
93/7
48/56
57/19
R/R
R/S
NC
NC
O
66/34
CHO
CHO
O
19
20
39
31
8/92
97/3
32/49
59/4
R/R
R/R
NC
O
CH3CCH2CH3

H.-M. Shen, H.-B. Ji / Tetrahedron Letters 53 (2012) 3541–3545
3545
Table 3 (continued)
Entry
Aldehyde
Ketone
Yielda (%)
67
syn/antia
eea (%)
87/52
Configuration
CHO
CHO
O
21
22
70/30
R/S
O
Trace
N.D.
N.D.
—
CHO
CHO
O
23
24
Trace
47
N.D.
N.D.
—
CH3CCH2CH3
O
60/40
73/38
R/S
CHO
O
25
N.D.
—
—
—
Condition: CD-1 0.04 mmol, aldehyde 0.20 mmol, acetone 0.2 mL + 0.1 mL, solvent(pH 4.5, 0.5 mol/L CH3COONa–HCl) 1 mL, 25 °C, 96.0 h; N.D. (Yield, ee): The yield or ee of
the product was not determined.
a
Determined by HPLC analysis with a Chiralcel AS-H column. The absolute configurations of the aldol products were determined by comparison of the eluting sequence of
the enantiomers with literature.6d
Christensen, B.; Bols, M. Eur. J. Org. Chem. 2005, 2734; (d) Bjerre, J.; Rousseau,
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N
OH
O
O
OH
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N
O2N
NO2
(S)-
(R)-
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H
O
H
O
NO2
major
NO2
minor
Figure 4. Schematic origin of enantioselectivity induced by CD-1.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Nos. 21176268 and 21036009), higher-level talent project
for the Guangdong Provincial Universities and the Fundamental
Research Funds for the Central Universities for providing financial
support to this project.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
04.140.
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