Influence of hyaluronan environments on the stereoselectivity of an aldol reaction

Article abstract of DOI:10.1002/jccs.201100564

The hyaluronan that is present in extracellular matrices forms chiral mesh-like structures in aqueous media. To understand the influence of specific environments on the stereoselectivity of chemical reactions, organocatalysis-mediated aldol reactions in t

Full text of DOI:10.1002/jccs.201100564

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Communication
Influence of Hyaluronan Environments on the Stereoselectivity of an Aldol Reaction
Takashi Ishikawa, Hiroto Noritake and Kiichiro Totani*
Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi,
Tokyo 180-8633, Japan
(Received: Oct. 27, 2011; Accepted: Nov. 28, 2011; Published Online: Dec. 19, 2011; DOI: 10.1002/jccs.201100564)
The hyaluronan that is present in extracellular matrices forms chiral mesh-like structures in aqueous me-
dia. To understand the influence of specific environments on the stereoselectivity of chemical reactions,
organocatalysis-mediated aldol reactions in the presence of hyaluronan were investigated in water. In
these experiments, changes in the diastereomeric ratio were observed in an aldol reaction under hya-
luronan conditions.
Keywords: Hyaluronan; Stereoselectivity; Aldol reaction.
INTRODUCTION
Stereocontrol methods for chemical reactions, so-
weight.6 Since HA dissolves easily and forms a mesh-like
structure in aqueous media, we attempted to apply this
HA-mesh as a chiral environment for a carbon-carbon bond
formation reaction in water.
called asymmetric synthesis, are divided into intramolecu-
lar asymmetric induction and intermolecular asymmetric
induction. In the former method the asymmetric environ-
ment is generated in the substrate,1 whereas in the latter
method the asymmetric environment is generated by the re-
agent, catalyst or ligand.2 During our analyses of biochemi-
cal reactions within an extracellular matrix, we found that
such stereocontrol reactions could be performed within the
chiral mesh-like structure that is constructed by collagen,
glycosaminoglycan and hyaluronan (HA).3 Although, the
enzyme was the main factor for stereoinduction in these re-
actions, the chiral mesh-like structure of the extracellular
matrix molecules used suggested the possibility that they
might function as a novel stereocontrol factor for chemical
reactions. The aim of this study was to investigate the influ-
ence of the high dimensional environment formed by a
chiral macromolecular matrix on the stereoselectivity of
chemical reactions. To this end, HA was selected as the
component for formation of the chiral high dimensional en-
vironment. HA structure was first determined in the 1930s
in the laboratory of Karl Meyer.4 This biomacromolecule is
a polymer of disaccharides consisting of D-glucuronic
(GlcA) acid and D-N-acetylglucosamoine (GlcNAc), linked
via alternating b-1,4 and b-1,3 glycosidic bonds.5 HA can
form a mesh-like structure in an aqueous media, and plays
an important role in stabilizing intercellular compartments.
The size of the grid that is formed in a HA-mesh is known
to depend on HA concentration but not on its molecular
The use of an organocatalyst is one way to hasten
chemical reactions in an aqueous media.7 In particular, var-
ious aldol reactions have been successfully carried out in
water by using various amino acid derivatives as organo-
catalysts.8 The aldol reaction is a powerful carbon-carbon
bond forming reaction that produces the b-hydroxyl car-
bonyl moieties found in natural products.9 Since some
aldol reactions in water provide good to excellent dia-
stereo- as well as enantio-selectivity, we selected an aldol
reaction as a model for estimation of the influence of a
chiral high dimensional environment on the stereoselectiv-
ity of chemical reactions. In this study, we examined the ef-
fects of a HA-constructed chiral mesh-like environment on
an organocatalyst-mediated aldol reaction in water.
RESULTS AND DISCUSSION
Since HA forms mesh-like structures that include wa-
ter molecules and expands to fill a space, the influence of
HA on stereoselectivity in an aldol reaction in aqueous me-
dia was examined. Lu et al. recently reported direct asym-
metric aldol reactions between various aldehydes and ke-
tones catalyzed by L-tryptophan in water, which resulted in
good anti- and enantioselectivity.10 Using their reaction
conditions we examined the aldol reaction between cyclo-
hexanone and p-nitrobenzaldehyde under a HA-constructed
mesh-like environment (Table 1). When L-tryptophan was
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
* Corresponding author. E-mail: ktotani@st.seikei.ac.jp
J. Chin. Chem. Soc. 2012, 59, 265-268
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
Ishikawa et al.
Table 1. Influence of hyaluronan on the stereoselectivity of an
Aldol reaction with a tryptophan catalysta
used as an organocatalyst in the absence of HA (entry 1), an
anti product (2R, 1’S) was mainly produced with good se-
lectivity (anti : syn = 5 : 1). Although no significant influ-
ence of the synthetic HA11-added conditions was observed
on enantioselectivity [entry 2; HAof 1000 kDa (HA-1000k),
entry 3; HA of 2000 kDa (HA-2000k)], the reaction under
these HA conditions showed a moderate tendency to pro-
duce more syn product (entry 2 and 3) than that observed
under the control conditions (entry 1). These results sug-
gested that the transition state of the aldol reaction might
associate with the HA-constructed mesh-like structure.
The fact that the HA-2000k caused a slightly decrease in
anti selectivity than the HA-1000k is consistent with the
likelihood that HAwith a high molecular weight may fill up
the higher dimensional environment inside a test tube more
effectively than a smaller HA. If HA can associate with the
transition state complex including L-tryptophan, the influ-
ence of HA on the stereoselectivity of a D-tryptophan cata-
lyzed aldol reaction will be independent from that when an
L-tryptophan is used. Indeed, the D-tryptophan catalyzed
aldol reactions under HA conditions showed a different
tendency of anti/syn selectivity than that of the L-trypto-
phan-catalyzed reactions, showing a slight increase in anti
selectivity (entry 4-6). These phenomena indicated that the
association state of the D-tryptophan mediated aldol reac-
tion under HAconditions is different from that catalyzed by
L-tryptophan. The combined results suggest that stereo-
selectivity of the aldol reaction might be affected by a
chiral high dimensional environment.
aReaction conditions: catalyst (0.05 mmol), additive (0.2 wt%), 1
(2.5 mmol), and 2 (0.5 mmol) in water (250 ìL). bIsolated yield.
cDetermined by chiral HPLC analysis. danti : syn = 5 : 1. eanti :
f
syn = 5 : 1. Major product (2R, 1’S). gMajor product (2S, 1’R).
that under control conditions, indicating that this brush-
like chiral environment of mucin does not affect the stereo-
selectivity of the aldol reaction. Thus, a HA-specific chiral
mesh-like structure appears to be essential for the observed
HA modulation of reaction stereoselectivity.
Since the size of the HA grid formed differs depend-
ing on its concentration,6 we examined the effect of various
concentrations of HA on the stereoselectivity of the reac-
tion (Table 3). Increasing the HA concentration did not sig-
nificantly alter stereoselectivity, although the relative vis-
cosity of the solutions did increase in a HA dose-dependent
We next examined the influence of other chiral addi-
tives on the stereoselectivity of the reaction, to estimate the
effect of the specific higher dimensional structure of HA
(Table 2). Since HA consists of GlcNAc and GlcA, we first
investigated the influence of GlcNAc, GlcA or combined
GlcNAc/GlcA as additives (entry 1-4). These additives ex-
erted no significant influence on the stereoselectivity of the
reactions, indicating that the chiral functional groups on
the sugar components of HA (GlcNAc and GlcA) by them-
selves do not affected the stereoselectivity of the aldol re-
action. These results suggest that a higher dimensional
structure is necessary for modulation of stereoselectivity.
We therefore next examined the influence of mucin from
jellyfish (Aurelia sp)12 on this reaction. Mucin is the major
component of the extracellular mucus blanket of the jelly-
fish and consists of glycans and a core protein that forms a
brush-like high dimension environment (entry 5).13 How-
ever, the resulting anti/syn selectivity was identical with
Table 2. Influence of other additives on the stereoselectivity of
an Aldol reaction with a L-tryptophan catalysta
aReaction conditions: L-tryptophan (0.05 mmol), additive (0.2
wt%), 1 (2.5 mmol), and 2 (0.5 mmol) in water (250 ìL). The
reaction mixture was incubated at 25 °C for 24 h. bIsolated yield.
cDetermined by chiral HPLC analysis. dGlcNAc (0.1 wt%),
GlcA(0.1 wt%) was added. eExtracted from Aurelia sp.
266
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Hyaluronan Effect on an Aldol Reaction
Table 3. Influence of various concentrations of HA-1000k on the
stereoselectivity of an Aldol Reaction with a L-
tryptophan catalysta
and a CO-2065 Plus column oven.
Typical procedure for the aldol reaction using a
D-/L-tryptophan catalyst in a HA solution
HA (1.0 mg) was added to a mixture of tryptophan
(10.2 mg, 0.0500 mmol) in water (250 mL) at 25 °C. The re-
action mixture was stirred for 10 min, and cyclohexanone
(1) (250 mL, 2.50 mmol) and p-nitrobenzaldehyde (2) (75.5
mg, 0.500 mmol) were then added. After stirring for 24 h,
the reaction mixture was extracted with ethyl acetate (3 ´
10 mL), and the organic extracts were dried over Na2SO4.
The crude aldol product was chromatographed on silica gel
using EtOAc/hexane (1 : 3) to give the aldol product (3).
Spectrometric characteristics of the aldol products were
identical with those previously reported.10 Diastereoselec-
tivity and enantiomeric excess (ee) were determined by 1H
NMR analysis and chiral HPLC analysis [DICEL Chiralpak
AD-H (4.6 mmf ´ 25 cm), mobile phase 2-PrOH/hexane
(10 : 90), flow rate 0.5 mLmin-1 at 40 °C. Detection: ab-
sorption at 254 nm]. 1H NMR (CDCl3, -CHOH, d) 5.45 (s,
syn-product), 4.88 (d, J = 8.5 Hz, anti-product). HPLC tR
syn-product [34.05 (minor), 42.45 (major)], anti-product
[46.17 (2S, 1’R), 60.67 (2R, 1’S)].
aReaction conditions: L-Tryptophan (0.05 mmol), HA-1000k (0
to 0.2 wt%), 1 (2.5 mmol), and 2 (0.5 mmol) in water (250 ìL).
bDetermined by 1H-NMR and chiral HPLC analysis. cViscosity
was measured by using a rotational viscometer at 4 ºC. dMeasured
value; 1.6 mPa•S.
manner. Since the mesh size of HA at concentrations below
1 wt% is known to be 10~50 nm,14 molecules that are
enough smaller than the size of the grid would not be af-
fected by changes in the size of the HA constructed mesh.
Based on these data we concluded that the local high di-
mensional structure of HA is necessary for the association
of HA with the transition state molecule of the aldol reac-
tion, although the macro structural change resulting from
changes in HA-concentration are not essential for changes
in the association state of transition state molecules.
In conclusion, we demonstrated that HA influences
the diastereoselectivity of aldol reactions in water. The re-
sults indicate that HA has the potential to function as a fac-
tor for stereocontrol by surrounding the target molecule
with its high dimension chiral environment. Although the
effect of HA in this study is moderate and HA is not a very
strong factor for controlling stereoselectivity, our findings
suggest the possibility of modulating the environment of a
chemical reaction using a molecule such as HA for im-
proving the stereoselectivity of chemical reactions,
which, if combined with current methods of asymmetric
synthesis, could complement the stereoselectivity of these
methods.
ACKNOWLEDGEMENTS
HAs and mucin were kind gifts from Prof. K. Ushida
(Kitasato University, Japan). This work was partially sup-
ported by a Grant-in-Aid for Scientific Research on Inno-
vative Areas (No. 21107524 and 23107727) from the Min-
istry of Education, Culture, Sports, Science and Technol-
ogy (Japan).
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J. Chin. Chem. Soc. 2012, 59, 265-268