Synthesis of polysaccharide derivatives bearing pyridine N-oxide groups and their use as asymmetric organocatalysts

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.07.010

Novel amylose and cellulose derivatives bearing pyridine N-oxide groups were synthesized, and their performance as asymmetric organocatalysts was investigated. The amylose derivatives bearing 3-pyridyl N-oxide groups enantioselectively catalyzed the allyl

Full text of DOI:10.1016/j.reactfunctpolym.2011.07.010

Reactive & Functional Polymers 71 (2011) 1055–1058
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis of polysaccharide derivatives bearing pyridine N-oxide groups
and their use as asymmetric organocatalysts
⇑
Tomoyuki Ikai , Munetsugu Moro, Katsuhiro Maeda, Shigeyoshi Kanoh
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel amylose and cellulose derivatives bearing pyridine N-oxide groups were synthesized, and their
performance as asymmetric organocatalysts was investigated. The amylose derivatives bearing 3-pyridyl
N-oxide groups enantioselectively catalyzed the allylation of benzaldehyde by allyltrichlorosilane with
different enantiomeric excesses (ee; 13–32% ee), depending on the degree of substitution of the pyridine
N-oxide. In contrast, the corresponding cellulose derivatives showed the opposite enantioselectivity with
much lower ee (2–11% ee). These results suggest that the higher-order structures of polysaccharides play
an important role in the enantioselective allylation of benzaldehyde.
Received 30 April 2011
Received in revised form 19 July 2011
Accepted 20 July 2011
Available online 26 July 2011
Keywords:
Amylose
Asymmetric catalysis
Organocatalysis
Polysaccharide
Pyridine N-oxides
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
lysts with Lewis-base properties [20–25]. To date, a wide variety
of chiral amine N-oxides have been successfully used as chiral
Materials development using environmentally friendly and sus-
tainable resources has been attracting considerable attention in
many fields. Polysaccharides such as amylose and cellulose are
considered strong candidates in this regard because they are the
most abundant and renewable resources on the Earth. In addition,
native polysaccharides are optically active and can discriminate
among enantiomers, although their recognition abilities are not
sufficient for practical purposes [1,2]. Polysaccharides have hydro-
xy groups and can be easily derivatized by reactions with suitable
reagents to afford benzoate and phenylcarbamate derivatives
[3–5]. These polysaccharide derivatives are known to show high
chiral recognition abilities for many racemates when they are used
as chiral stationary phases (CSPs) for high-performance liquid
chromatography (HPLC) [6–10]. At present, nearly 90% of chiral
compounds can be successfully separated using polysaccharide-
based CSPs [11]. In contrast to the successful use of polysaccharide
derivatives as CSPs for enantioseparation, their use as chiral cata-
lysts for asymmetric reactions has been limited to reports from a
few research groups [12–16].
organocatalysts in asymmetric reactions such as the allylation of
aldehydes [26–30], cyanosilylation of carbonyl and imine com-
pounds [31], aldol-type reactions [32,33], and desymmetrization
of meso-epoxides [34,35]. However, to the best of our knowledge,
amylose and cellulose have never been used as chiral scaffolds
for asymmetric organocatalysts [13,36]. In the present study, we
synthesized novel amylose and cellulose derivatives bearing pyri-
dine N-oxide groups (1–3 in Fig. 1) and investigated their abilities
as asymmetric organocatalysts for the allylation of benzaldehyde 4
using allyltrichlorosilane 5.
2. Experimental
2.1. Materials
Amylose (DPꢀ300) and Chiralcel OD-H (25 Â 0.46 cm ID) were
kindly supplied by Daicel Chemical Industries (Tokyo, Japan).
Cellulose (Avicel, DPꢀ200) was purchased from Merck (Darmstadt,
Germany). p-Toluoyl chloride and isonicotinic acid N-oxide were
from Tokyo Kasei (Tokyo, Japan). Allyltrichlorosilane was obtained
from Aldrich (Milwaukee, WI, USA). Benzaldehyde, nicotinic acid
N-oxide, tetrabutylammonium iodide, thionyl chloride, and
N-ethyldiisopropylamine were purchased from Wako (Osaka,
Japan). Pyridine and dichloromethane were purchased from Kanto
(Tokyo, Japan) as anhydrous solvents. Acid chlorides of nicotinic acid
N-oxide and isonicotinic acid N-oxide were prepared from the
Interest in asymmetric organocatalysts is growing because of
their advantages over metal-based catalysts, including their cost
effectiveness, stability in air, and low environmental toxicity
[17–19]. Amine N-oxides are known to be promising organocata-
⇑
Corresponding author. Tel./fax: +81 76 234 4781.
E-mail address: ikai@t.kanazawa-u.ac.jp (T. Ikai).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.07.010

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T. Ikai et al. / Reactive & Functional Polymers 71 (2011) 1055–1058
Fig. 1. Synthesis of amylose (1) derivatives bearing 3-pyridyl N-oxides (a) and structures of amylose (2), cellulose (3), and glucose (Glu–NO) derivatives bearing pyridine N-
oxide groups (b).
corresponding carboxylic acid through a reaction with thionyl chlo-
2.2.5. Synthesis of 3a
ride at 80 °C.
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (DMSO-d6, 80 °C, 500 MHz) d 1.8–2.7 (9.8H, br,
Ph-CH3), 2.8–5.7 (7H, br, glucose protons), 6.5–9.0 (12H, br, aro-
2.2. Synthesis of polysaccharide derivatives 1–3 and glucose derivative
Glu–NO
matic) ppm. IR (KBr): 1727 cmÀ1
(mC@O). Elemental analysis calcd.
(%) for C28.69H26.04N0.65O8.65Á0.2H2O: C, 65.98; H, 5.10; N, 1.77.
Found: C, 66.00; H, 5.09; N, 1.76.
2.2.1. Synthesis of 1b
Amylose (DPꢀ300, 0.50 g, 3.09 mmol) was first dispersed in
pyridine (15 ml) and was allowed to react with p-toluoyl chloride
(0.93 ml, 7.04 mmol) for 20 h at 80 °C. Then, the remaining hydro-
xy groups were treated with an excess of the acid chloride of nic-
otinic acid N-oxide (1.11 g, 7.05 mmol) for 28 h at 80 °C. After the
reaction, the target amylose derivative 1b (1.14 g, 71%) was
obtained by the usual work-up. 1H NMR (DMSO-d6, 80 °C,
500 MHz) d 1.8–2.4 (6.9H, br, Ph-CH3), 3.7–6.2 (7H, br, glucose pro-
2.2.6. Synthesis of 3b
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (CDCl3, 55 °C, 500 MHz) d 1.9–2.6 (8.3H, br, Ph-
CH3), 2.8–5.6 (7H, br, glucose protons), 6.5–8.9 (12H, aromatic)
ppm. IR (KBr): 1729 cmÀ1
(mC@O). Elemental analysis calcd. (%) for
C28.40H25.60N0.80O8.80Á0.3H2O: C, 65.01; H, 5.03; N, 2.16. Found: C,
64.97; H, 5.14; N, 2.15.
tons), 6.7–9.6 (12H, br, aromatic) ppm. IR (KBr): 1719 cmÀ1
(mC@O).
Elemental analysis calcd. (%) for C28.62H25.93N0.69O8.69Á0.7H2O: C,
2.2.7. Synthesis of 3c
64.71; H, 5.19; N, 1.82. Found: C, 64.78; H, 5.11; N, 1.88.
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (DMSO-d6, 80 °C, 500 MHz) d 1.8–2.5 (7.5H, br, Ph-
CH3), 3.5–6.1 (7H, br, glucose protons), 6.6–9.9 (12H, aromatic)
2.2.2. Synthesis of 1a
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (CDCl3, 55 °C, 500 MHz) d 1.8–3.2 (8.3H, br, Ph-CH3),
3.5–6.5 (7H, br, glucose protons), 6.8–8.9 (12H, aromatic) ppm. IR
ppm. IR (KBr): 1731 cmÀ1
(mC@O). Elemental analysis calcd. (%) for
C
27.75H24.62N1.13O9.13Á1.1H2O: C, 61.83; H, 5.02; N, 2.88. Found: C,
61.75; H, 4.97; N, 2.99.
(KBr): 1725 cmÀ1
(mC@O). Elemental analysis calcd. (%) for
C
28.85H26.27N0.58O8.58Á0.9H2O: C, 64.86; H, 5.30; N, 1.49. Found: C,
2.2.8. Synthesis of Glu–NO
64.72; H, 5.16; N, 1.53.
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (CDCl3, 55 °C, 500 MHz) d 2.1–2.5 (11.3H, br,
Ph-CH3), 4.3–4.8 and 5.6–6.4 (7H, br, glucose protons), 7.0–8.7
2.2.3. Synthesis of 1c
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (DMSO-d6, 80 °C, 500 MHz) d 1.8–2.5 (6.0H, br,
Ph-CH3), 3.5–5.8 (7H, br, glucose protons), 6.8–8.9 (12H, aromatic)
(20H, aromatic) ppm. IR (KBr): 1719 cmÀ1
(mC@O). Elemental analy-
sis calcd. (%) for C43.47H38.21N1.27O12.27Á0.3H2O: C, 66.93; H, 5.01; N,
2.28. Found: C, 66.94; H, 5.07; N, 2.29.
ppm. IR (KBr): 1719 cmÀ1
(mC@O). Elemental analysis calcd. (%) for
C28.02H25.03N0.99O8.99Á0.6H2O: C: 63.46; H: 4.99; N: 2.61. Found: C,
2.3. Allylation of benzaldehyde using allyltrichlorosilane catalyzed by
polysaccharide derivatives bearing pyridine N-oxide groups
63.39; H, 5.07; N, 2.65.
2.2.4. Synthesis of 2
Tetrabutylammonium iodide (443 mg, 1.2 mmol), catalyst (0.1
mmol based on pyridine N-oxide), allyltrichlorosilane (0.17 ml,
1.2 mmol), and N-ethyldiisopropylamine (0.83 ml, 5.0 mmol) were
dissolved in dichloromethane (2.0 ml) and cooled to 0 °C. Benzalde-
hyde (0.10 ml, 1.0 mmol) was added, and the resulting mixture was
stirred for 48 h at 0 °C. The reaction was quenched by the addition of
aqueous saturated NaHCO3, and the reaction system was poured
This compound was synthesized by a procedure similar to that
of 1b. 1H NMR (DMSO-d6, 80 °C, 500 MHz) d 1.8–2.4 (6.9H, br,
Ph-CH3), 3.8–6.0 (7H, br, glucose protons), 6.7–9.5 (12H, br, aro-
matic) ppm. IR (KBr): 1723 cmÀ1
(mC@O). Elemental analysis calcd.
(%) for C27.93H24.90N1.03O9.03Á0.8H2O: C, 62.89; H, 5.01; N, 2.68.
Found: C, 62.81; H, 5.03; N, 2.76.

T. Ikai et al. / Reactive & Functional Polymers 71 (2011) 1055–1058
1057
into methanol to remove the catalyst. The supernatant was concen-
trated and extracted with ethyl acetate (3 Â 10 ml). The combined
organic layer was washed with brine (10 ml), dried over Na2SO4,
and evaporated under reduced pressure. The crude product was
purified by silica gel chromatography with hexane–ethyl acetate
(20/1, v/v) as the eluent. The enantiomeric excess (ee) of the isolated
product 6 was determined by chiral HPLC analysis on Daicel Chiral-
cel OD-H (hexane-2-propanol (99/1, v/v); flow rate 0.8 ml/min;
temperature 20 °C; k 220 nm): tR = 27.2 min, tS = 34.4 min. 1H NMR
(CDCl3, rt, 500 MHz) d 2.10 (s, 1H), 2.5–2.6 (m, 2H), 4.7–4.8 (m,
1H), 5.1–5.2 (m, 2H), 5.7–5.9 (m, 1H), 7.2–7.4 (m, 5H) ppm. 13C
NMR (125 MHz, CDCl3, rt): d 43.8, 73.4, 118.3, 125.9, 127.6, 128.4,
134.6, 144.0 ppm. Elemental analysis calcd. (%) for C10H12O: C,
81.04; H, 8.16. Found: C, 79.99; H, 8.34.
2.4. Instruments
The 1H NMR (500 MHz) spectra were taken in CDCl3 at 55 °C
and DMSO-d6 at 80 °C using a JEOL ECA-500 spectrometer (JEOL,
Tokyo, Japan). IR spectra were obtained using a JASCO FT-IR-620
spectrometer as a KBr pellet. Ultraviolet (UV) and circular dichro-
ism (CD) spectra were measured in chloroform in a 0.1 mm quartz
cell using a JASCO Ubset-55 and a JASCO J-720 spectrometer,
respectively. Chromatographic experiments were performed using
a JASCO PU-980 Intelligent HPLC pump equipped with UV (JASCO
970-UV) and polarimetric (JASCO OR-990) detectors (JASCO, Tokyo,
Japan) at 20 °C. A solution of a chiral compound was injected into
the chromatographic system by a Rheodyne Medel 7125 injector
(Rheodyne, Rohnert Park, CA, USA).
Fig. 2. CD (upper) and UV (lower) spectra of amylose (1a and 1b) and cellulose (3a
and 3b) derivatives in chloroform at room temperature. (Cell length: 0.1 mm,
concentration: 2.0 Â 10À3 M based on glucose units).
3.3. Enantioselective allylation catalyzed by amylose derivatives 1
bearing pyridine N-oxide groups
The pyridine N-oxide-bound amylose derivatives 1a–c were
used as organocatalysts for the asymmetric addition of allyltrichlo-
rosilane 5 to benzaldehyde 4; this allylation is often used as a test
reaction for new chiral nucleophilic catalysts. As described in the
literature [26], 4 (1.0 mmol) was reacted with 5 (1.2 mmol) in
the presence of the amylose derivatives 1a–c (0.1 mmol based on
pyridine N-oxide), Bu4N+IÀ (1.2 mmol), and N-ethyldiisopropyl-
amine (iPr2NEt) (5.0 mmol) in dichloromethane (CH2Cl2) (2.0 mL)
for 48 h at 0 °C (Fig. 3). After purification of the crude product by
silica-gel chromatography with hexane/ethyl acetate (20/1, v/v)
as the eluent, the ee of the isolated 1-phenyl-3-buten-1-ol 6 was
determined by chiral HPLC analysis on Daicel Chiralcel OD-H.
Table 1 summarizes the results of the enantioselective allyla-
tion of 4 with 5 using the amylose derivatives 1a–c as catalysts.
The amylose derivatives 1a–c (entries 1–3) bearing 3-pyridyl
N-oxide groups catalyzed the reaction to give (R)-6 in moderate
yields (47–62%) with different ee values (13–32% ee), depending
on the degree of substitution of the pyridine N-oxide. Conversely,
1d (entry 4), without pyridine N-oxide groups, showed no catalytic
activity in this reaction, as expected. When the amount of pyridine
N-oxide was increased from 19% (1a, entry 1) to 23% (1b, entry 2),
both the yield and enantioselectivity improved from 47% and 13%
ee to 62% and 32% ee, respectively. However, a further increase
in the amount of 3-pyridyl N-oxide caused the enantioselectivity
to deteriorate (24% ee; 1c, entry 3). On the other hand, when the
D-glucose-based catalyst (Glu–NO) was used for the allylation
reaction, instead of the amylose derivatives (entry 9), a poor and
opposite enantioselectivity (8% ee, S configuration) was observed
for 6. These results indicate that the enantioselectivity obtained
from the amylose-based catalysts (1) was attributable to higher-
order structures, such as one-handed helical structures, and that
a slight difference in their higher-order structures could influence
the enantioselectivity in this allylation reaction.
3. Results and discussion
3.1. Synthesis of polysaccharide derivatives 1–3
The amylose derivatives 1a–c bearing 3-pyridyl N-oxide groups
at the 2, 3, and 6-positions of the glucose units were synthesized
by the sequential additions of p-toluoyl chloride and the acid chlo-
ride of nicotinic acid N-oxide, as shown in Fig. 1a. The content of
the pyridine N-oxide groups could be controlled by the amount
of p-toluoyl chloride that was used in the initial step. Derivative
1d, without pyridine N-oxide groups, was also prepared for com-
parison [37]. The introduction of the pyridine N-oxide groups
was confirmed by 1H NMR and elemental analysis of the products.
In addition, the corresponding cellulose derivatives 3a–c and an
amylose derivative 2 bearing 4-pyridyl N-oxide groups were
synthesized in a similar way (Fig. 1b). The D-glucose derivative
Glu–NO was also prepared as a model compound consisting of a
monosaccharide.
3.2. Circular dichroism spectra of polysaccharide derivatives 1 and 3
The CD and UV spectra of amylose (1a and 1b) and cellulose
(3a and 3b) derivatives bearing 3-pyridyl N-oxide groups are
shown in Fig. 2. Because 1c and 3c were insoluble in chloroform,
their spectra could not be measured. The CD spectral patterns of
the amylose derivatives were similar to each other, although the
wavelengths and the peak intensities depend on the degree of
substitution. The cellulose derivatives showed opposite Cotton
effects to those of the amylose derivatives in the absorption re-
gion because of aromatic chromophores. This result indicates that
the pendant 3-pyridyl N-oxide as well as 4-methylbenzoate of
both 1 and 3 seems to be arranged in opposite chiral environ-
ments, probably because of the higher-order structures of the
polysaccharides.
Fig. 3. Enantioselective allylation of benzaldehyde (4) using allyltrichlorosilane (5)
catalyzed by polysaccharide (1–3) and glucose (Glu–NO) derivatives.

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T. Ikai et al. / Reactive & Functional Polymers 71 (2011) 1055–1058
Table 1
efficient polysaccharide-based asymmetric organocatalyst may be
created by an appropriate combination of polysaccharides and cat-
alytically active groups as a result of the synergistic effects of the
chiralities of the constituent units and higher-order structures,
such as one-handed helical structures.
Enantioselective allylation catalyzed by polysaccharide (1–3) and glucose (Glu–NO)
derivatives.a
Entry
Catalyst
DS (%)b
Yield (%)c
ee (%)d
Configuratione
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
2
3a
3b
3c
19
23
33
0
34
22
27
37
25
47
62
59
0
37
70
64
59
55
13
32
24
–
<1
11
2
R
R
R
–
n.d.
S
S
S
S
Acknowledgements
This work was partly supported by the Foundation for the
Promotion of Ion Engineering and a Grant-in-Aid (No. 21750120)
from the Ministry of Education, Culture, Sports, Science, and Tech-
nology of Japan.
8
8
Glu–NO
a
Reactions were carried out at 1 mmol scale in CH2C12 (2 mL) with allyltri-
chlorosilane (1.2 equiv.), N-oxide (10 mol%), Bu4N+IÀ (1.2 equiv.), and Pr2NEt (5.0
i
References
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b
Degree of substitution of pyridine N-oxide determined by elemental analysis.
Yield of the isolated product.
Determined by chiral HPLC on Chiralcel OD-H (hexane-2-propanol (99/1, v/v)).
Absolute configuration of the predominant isomer assigned by the comparison
c
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We present a facile synthesis of amylose and cellulose deriva-
tives bearing various amounts of pyridine N-oxide groups, and
their use as organocatalysts for the enantioselective allylation of
benzaldehyde with allyltrichlorosilane. The present results indi-
cate that the higher-order structures that are inherent in each
polysaccharide derivative significantly affect the efficiency of chi-
ral induction in asymmetric reactions. We anticipate that a more