Hydrogel behavior of a sugar-based gelator by introduction of an unsaturated moiety as a hydrophobic group

Article abstract of DOI:10.1039/b602279k

The new sugar-based gelators 1 and 2 were synthesized, and their gelation abilities were evaluated in organic solvents and in water. Compound 1 gelates both water and organic solvents whereas 2 gelates only organic solvents. Superstructural difference between hydrogel 1 and organogel 2 was investigated by CD, TEM, AFM, ¹H NMR and XRD. Hydrogel 1 displays a well-developed helical ribbon structure with 20-150 nm diameter and a length of several hundred m whereas organogel 2 shows a twisted fiber structure of diameter 20 nm. CD measurements of hydrogel 1 and organogel 2 indicate that hydrogel 1 maintains a well-ordered chiral structure whereas organogel 2 maintains a relatively disordered chiral structure. The ¹H NMR and XRD results suggest that the hydrophobic interaction in hydrogel 1 are relatively weak, with a relatively small region interdigitated between lipophilic alkyl groups. In addition, upon irradiation at 254 nm wavelength, hydrogel 1 reveals a red coloration at 540 nm. These results indicate that the self-assembled hydrogel 1 was polymerized by UV-irradiation. The intensity of the CD spectrum of the polymerized hydrogel markedly decreased. This result indicates that upon polymerization the highly ordered chiral structure of hydrogel 1 changes to a disordered molecular packing structure. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602279k

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Hydrogel behavior of a sugar-based gelator by introduction of an unsaturated
moiety as a hydrophobic group†
Jong Hwa Jung,*a Jeong Ah Rim,b Won Seok Han,b Soo Jin Lee,a Young Joo Lee,b Eun Jin Cho,b
Jong Seung Kim,c Qingmin Jid and Toshimi Shimizud
Received 15th February 2006, Accepted 20th March 2006
First published as an Advance Article on the web 18th April 2006
DOI: 10.1039/b602279k
The new sugar-based gelators 1 and 2 were synthesized, and their gelation abilities were evaluated in
organic solvents and in water. Compound 1 gelates both water and organic solvents whereas 2 gelates
only organic solvents. Superstructural difference between hydrogel 1 and organogel 2 was investigated
by CD, TEM, AFM, 1H NMR and XRD. Hydrogel 1 displays a well-developed helical ribbon structure
with 20–150 nm diameter and a length of several hundred lm whereas organogel 2 shows a twisted fiber
structure of diameter 20 nm. CD measurements of hydrogel 1 and organogel 2 indicate that hydrogel 1
maintains a well-ordered chiral structure whereas organogel 2 maintains a relatively disordered chiral
structure. The 1H NMR and XRD results suggest that the hydrophobic interaction in hydrogel 1 are
relatively weak, with a relatively small region interdigitated between lipophilic alkyl groups. In addition,
upon irradiation at 254 nm wavelength, hydrogel 1 reveals a red coloration at 540 nm. These results
indicate that the self-assembled hydrogel 1 was polymerized by UV-irradiation. The intensity of the
CD spectrum of the polymerized hydrogel markedly decreased. This result indicates that upon
polymerization the highly ordered chiral structure of hydrogel 1 changes to a disordered molecular
packing structure.
Since most organogelators are rarely soluble in water due
Introduction
to their hydrophobic property, development of a water-soluble
A recent issue in supramolecular chemistry is the focus on the
hydrogelator based on a sugar-based organogelator applicable for
organization of monomeric species into desired superstructures.
drug delivery systems and medical implants has been our major
There has been an intense interest in the development of efficient
challenge in this research. Recently, we found that aldopyranose
and tunable small molecule gelators for industrial purposes (e.g., in
foods, deodorants, cosmetics, athletic shoes and chromatography),
amphiphilic gelators can gelate water in the presence of a small
amount of polar organic solvent or of several other organic
solvents.7a,b
as a consequence of versatile gel functions on both microscopic
and macroscopic scales.1–6 Those materials are characterized by
more than one length scale, through noncovalent interactions
(hydrogen bonding, solvophobic effects, charge transfer and
van der Waals interactions). Many organogelators that form
hierarchical networks of superstructures in organic fluids have
been synthesized. To date, however, only a limited number of
“hydrogels” composed of such aggregates have been reported.2c,7
In this context, most researchers have made hydrogelators by
adding various types of hydrophilic group (e.g., peptide, amino
acid, ammonium ion or sugar, etc.) to gelators.2c,7 However,
corresponding studies of the hydrogelators with unsaturated
hydrophobic groups have never been explored to date.
In a continuation of our work on the application of organogela-
tors as hydrogelators, we designed and synthesized 1 having a
sugar moiety as the hydrophilic group and an unsaturated diacety-
lene unit as the hydrophobic group. Particularly, the unsaturated
alkyl chain group would result in a loose molecular packing
structure of the hydrogel formed from 1. In order to investigate
the influence of the unsaturated diacetylene unit on the gelation
we also prepared 2, having saturated hydrocarbons as a reference.
aDepartment of Chemistry, Research Institute of Natural Science,
Gyeongsang National University, Chinju 660-701, S. Korea. E-mail:
s_jielee@gaechuk.gsnu.ac.kr; Fax: +82-55-761-0244; Tel: +82-55-751-
6021
bKorea Basic Science Institute (KBSI), Daejeon 305-333, Korea
cDepartment of Chemistry, Institute of Nanosensor & Biotechnology,
Dankook University, Seoul, 140-714, Korea
Results and discussion
dNanoarchitectonics Research Center (NARC), National Institute of Ad-
vanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8562, Japan
The gelators 1 and 2 were synthesized as shown in Scheme 1
1
and their structures were identified by H NMR, FT-IR, Mass
† Electronic supplementary information (ESI) available: Figures S1 and
S2. See DOI: 10.1039/b602279k
spectroscopy and elemental analyses (see Experimental Section).
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Scheme 1 Synthetic routes for gelators 1 and 2.
The gelation ability of sugar-based 1 and 2 toward various
organic solvents and water was examined by dissolving approxi-
mately 0.1–10 mg of compound in 1.0–2.0 mL of the testing solvent
followed by heating. Upon cooling to room temperature, a gel, a
precipitate, or a clear solution was obtained, depending on the
solvents used. The results are summarized in Table 1. Compound
1 showed only 3 ‘G’ with 1 ‘PG’, 1 ‘I’ and 8 ‘S’ whereas 2 gelated
9 among 13 solvents. This observation seems to be much related
to solubility of the compounds in the solvents tested. The better
solubility in the solvent, the poorer is the gelation ability of the
compound.
Of particular interest is that in the absence of organic solvents
water gelation by gelator 1 was successful, but that by 2 was not. It
is noteworthy in this study that the triple bond of 1 plays an impor-
tant role in the gelation of water. This feature is due to the relatively
stronger intermolecular hydrogen-bonding interaction between
the sugar moieties of 1 by disordered molecular packing structure
between diacetylene moieties, in comparison with the gelator 2.
Upon irradiation by 254 nm wavelength, gelator 1 reveals a red
coloration at 540 nm (Fig. 1A). The intensity of the shifted band
increased with time and was maximized upon 32 min irradiation,
as shown in Fig. 1A. These results indicate that the self-assembled
hydrogel 1 was polymerized by UV-irradiation.
To obtain an insight into the chiral orientation of the gelator
in the hydrogel system, we took CD spectra of hydrogel 1 and
organogel 2 (Fig. 1B). The kmax values of 1 and 2 in the UV
absorption spectra appear at around 225 nm (Electronic Supple-
mentary Information: Fig. S1†) which corresponds to kh = 0 in the
CD spectra. The CD spectra of both hydrogel 1 and organogel
2 exhibit a negative first Cotton effect, implying that their dipole
moments orient to a clockwise direction in the aggregate of the
gels (Fig. 1B). In contrast, no CD Cotton effect was observed
Table 1 Gelation abilitya of 1 and 2 in organic solvents and water
Solvent
1
2
Methanol
Ethanol
S
S
S
S
n-Butanol
S
S
S
G
G
G
G
G
I
G
G
G
I
tert-Butanol
Tetrahydrofuran
Chloroform
Dichloromethane
n-Hexane
Ethyl acetate
Dimethylformamide (DMF)
Dimethyl sulfoxide (DMSO)
Water
G
G
I
PG
S
S
G
S
Water–Methanol (1 : 1 v/v)
G
a Gelator = 0.05–5.0 wt%. G: stable gel formed at room temperature. S:
soluble. I: insoluble. PG: partially gelatinized.
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Fig. 1 (A) UV-vis absorption spectra for hydrogel 1 in water [inset, photo
images of the self-assembled nanofibers derived from 1 in water (left) and
in the polymerized nanofibers with UV irradiation (right)]. (B) CD spectra
of hydrogel 1 (a) before and (b) after polymerization, and (c) methanol
solution 1 (0.1 wt%). (d) the mixed water–methanol gel 2.
Fig. 2 AFM images of hydrogel 1 (a) before and (b) after polymerization.
(c) TEM image of the mixed water–methanol gel 2.
for 1 in methanol, indicating that the hydrogel forms a highly
ordered, chiral structure in comparison to the solution state. In
addition, before polymerization, intensity of the CD signal of 1
is much higher than that of hydrogel 2, suggesting that hydrogel
1 has a highly ordered chiral structure. On the other hand, we
observed a markedly decreased CD spectrum of hydrogel 1 when
it was polymerized. With this result, we can imagine that upon
the polymerization a highly ordered chiral structure of hydrogel 1
changes to a disordered molecular packing structure (vide-infra).
To obtain visual insights into the aggregation mode of hydrogel
1 and organogel 2, we took AFM images of the self-assembled gel 1
before and after polymerization. Before polymerization, hydrogel 1
displays a well-developed helical ribbon structure of diameter 20–
150 nm and a length of several hundred lm (Fig. 2a). On the other
hand, upon polymerization, 1 revealed a typical fiber structure
with a diameter of 20 nm (Fig. 2b), but not a well-defined helical
structure, which is in good accordance with our CD observations
(vide supra). After polymerization, the macroscopic helicity of
hydrogel 1 decreased in comparison to that before polymerization.
This finding indicates that upon polymerization, the helical
molecular packing structure of 1 becomes disordered, which also
decreases the macroscopic and microscopic helicities of hydrogel
1. In contrast, the self-assembled morphology of organogel 2 ob-
served by TEM in the presence of methanol displays a twisted fiber
structure with a 30 nm diameter, showing that the macroscopic
helicity of organogel 2 is weak in comparison to hydrogel 1.
In general, NMR techniques can provide a great deal of
information on the self-assembly process in the gel state. 1H NMR
experiments, especially, may give an insight into how molecules
are oriented in the self-assembled state. Aromatic proton signals
of hydrogel 1 appeared at 7.12 and 6.90 ppm (Fig. 3a), whereas
aromatic proton signals of 1 in CD3OD solution appeared at 7.43
and 7.06 ppm (Fig. 3d). Upon heating, the aromatic proton signals
Fig. 3 1H NMR spectra of hydrogel 1 at (a) 30 ◦C, (b) 50 ◦C, (c) 70 ◦
and (d) CD3OD solution of 1 at 30 ◦C.
C
of the hydrogel 1 are gradually shifted downfield, suggesting
that the self-assembled hydrogel 1 forms strong p–p stacking
interaction between phenyl groups.
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We also observed peak changes in the infra-red spectrum of
the gelator 1. The declining C≡C stretching band at 2254 cm−1
by UV-irradiation supports the proposed polymerization of 1
within the nanofibers (See graphical abstract, and ESI: Fig. S2†).
The molecular weight of the polymerized nanofiber could not be
determined because of its poor solubility in organic solvents such
as DMSO, DMF, THF, CHCl3, CH3OH and toluene.
CD spectrum of the hydrogel was markedly decreased following
polymerization. This finding indicates that upon polymerization
the highly ordered chiral structure of hydrogel 1 changes to
a disordered molecular packing structure. Consequently, the
hydrogelator could be an innovative tool in drug delivery systems
and medical implants.
From X-ray diffraction patterns we also obtained important
information for the molecular packing mode of the gelator
molecules, as shown in Fig. 4. The X-ray diffraction diagram
of hydrogel 1 before polymerization reveals a single sharp peak
at d = 4.05 nm in the small-angle region (Fig. 4A). This length
of 4.05 nm is shorter than twice that of the extended molecular
length of 1 (2.9 nm, by CPK molecular modeling), but longer
than the length of one molecule. Thus, the hydrogel 1 should have
an interdigitated bilayer structure with a thickness of 4.05 nm
(Fig. 4A-a and Fig. 4B-a). On the other hand, the obtained long
spacing (d) of hydrogel 1 upon the polymerization was 4.20 nm,
slightly larger than that before polymerization (Fig. 4A-b and
Fig. 4B-b). This might be due to a loose chiral packing structure
between sugar moieties resulting from the polymerization.
The obtained long spacing (d) of organogel 2 was 3.70 nm (one
molecular length = 3.2 nm). This value is also compatible with
a bilayer structure with a relative large region interdigitated by
hydrophobic interactions, forming a relatively stronger hydropho-
bic interaction. Thus one can suggest that the hydrogel 1 forms
a bilayer structure with a relatively small region interdigitated
through hydrophobic interactions, unlike organogel 2.
Experimental
Spectroscopy measurements
1H and 13C NMR spectra were measured on a Bruker ARX
300 apparatus. IR spectra were obtained in KBr pellets using
a Shimadzu FT-IR 8100 spectrometer, and MS spectra were
obtained with a Hitachi M-250 mass spectrometer. Circular
dichroism (CD) spectra were measured on a JASCO J-820KS
spectrophotometer (cell diameter 10 mm).
XRD measurements
The XRD of a freeze-dried sample was measured with a Rigaku
diffractometer (Type 4037) using graded d-space elliptical side-by-
side multilayer optics, monochromated Cu Ka radiation (40 kV,
30 mA), and an imaging plate (R-Axis IV). The typical exposure
time was 10 min with a 150 mm camera length. Freeze-dried
samples from 2–9 were vacuum-dried to constant weight and then
put into capillary tubes, without being powdered.
TEM observations
The aqueous dispersions of the nanostructures (0.1 mg mL−1)
were dripped onto an amorphous carbon grid, and excess water
was blotted with filter paper. TEM was done with a Carl-Zeiss
LEO912 instrument operated at 50 keV. Images were recorded on
an imaging plate (Fuji Photo Film Co. Ltd. FDL5000 system)
with 20 eV energy windows at 3000–250 000 × and were digitally
enlarged.
Conclusions
We demonstrated that the sugar-based gelator 1 containing a
lipophilic diacetylene group can gelate water efficiently in the
absence of organic solvent even at extremely low concentration
(0.05 wt%). The hydrogel 1 forms a well-ordered bilayer structure
in water by self-assembly through intermolecular hydrogen bond-
ing, p–p stacking and interdigitated hydrophobic interactions,
in contrast to the sugar-based gelator 2. In addition, upon
irradiation at a wavelength of 254 nm, the hydrogel 1 reveals a red
coloration at 540 nm. These results indicate that the self-assembled
hydorgel 1 was polymerized by UV-irradiation. The intensity of the
Gelation test of organic fluids
In typical gelation experiment, a weighed amount of gelator and
0.1–1 mL of the solvent were put in a sample bottle, after which the
Fig. 4 (A) Power-XRD patterns of hydrogel 1 (a) before and (b) after polymerization and (c) mixed water-methanol gel 2. (B) Proposed molecular
packing modes of hydrogel 1 (a) before and (b) after polymerization.
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sample bottle was tightly sealed with a screw cap. The bottle was
then heated with shaking until all the solid material had dissolved.
The solution was set aside and allowed to cool to 25 ◦C. Gelation
was stable to inversion when the sample bottle was turned upside
down.
MS (FAB): 470 (M + H)+ (calcd MW = 469.1); elemental analysis
calcd (%) for C20H23NO12: C 51.18, H 4.94, N 2.98; found: C 51.01,
H 5.24, N 2.86.
(2R,3R,4S,5R,6S)-2-(Acetoxymethyl)-6-(4-aminophenoxy)-
tetrahydro-2H-pyran-3,4,5-triyl triacetate (9). Compound
8
(1.0 g) was dissolved with 25 mL of MeOH. After 5 min of N2
purging, Pd on activated carbon (10 wt%, 100 mg) was added.
Under 2 atm of H2, the reaction was allowed to proceed for 3 h.
After filtration and evaporation, pure 9 was obtained as a pale
yellow solid (2.48 g, quantitative). (1/1 v/v, Rf = 0.5). Yield 75%.
Synthesis
Compounds 3, 4, 6 and 7 are commercially available.
N -(4-((2S,3S,4S,5S )-3,4,5-Trihydroxy-6-(hydroxymethyl)-
tetrahydro-2H-pyran-2-yloxy)phenyl) heptadeca-10,12-diynamide
(1). A mixture of 9 (0.15 g, 0.33 mmol) and NaOH (0.52 g,
1.3 mmol) in MeOH (16 mL) and H2O (4 mL) was stirred for 3 h
at room temperature. The solution was concentrated in vacuo, and
acidified with 0.1 M HCl solution. The pr◦ecipitate was filtered and
dried in vacuo. White solid. Mp: 123–126 C. Yield 90%. 1H NMR
(300 MHz, DMSO-d6): d 9.70 (s, 1H, NH), 7.49 (d, 2H, J = 9,
Ar–H), 6.97 (d, 2H, J = 9, Ar–H), 5.24 (d, 1H), 5.24 (d, 1H), 5.02
(d, 1H), 4.96 (d, 1H), 4.77(d, 1H), 4.52(t, 1H), 3.72 (m, 1H), 3.48
(m, 1H), 3.26(s, 5H), 2.30 (m, 6H), 1.57 (m, 2H), 1.45–1.28 (m,
14H), 0.89 (t, 3H, CH3); 13C NMR (75 MHz, CDCl3): 181.3, 148.4,
125.1, 118.8, 115.3, 85.3, 74.5, 70.6, 65.8, 64.6, 63.1, 25–28, 20.5,
13.1 ppm; IR (KBr, cm−1): 3378, 3288, 2929, 2850, 1654, 1604,
1533, 1509, 1407, 1234, 1074; MS (FAB): 544 (M + H)+ (calcd
MW = 543.3); elemental analysis: calcd (%) for C31H45NO7: C
68.10, H 8.44, N 2.54; found: C 68.48, H 8.34, N 2.58.
◦
1H NMR (300 MHz, CDCl3, 25 C): d 6.85 (d, 2H, J = 9, Ar–
H), 6.64 (d, 2H, J = 9, Ar–H), 5.24 (m, 3H), 4.96 (d, 1H, J =
9), 4.3–4.1 (m, 2H), 3.81 (m, 1H), 3.48 (s, 2H), 2.05 (s, 12H); 13
C
NMR (75 MHz, CDCl3): 170.3, 158.9, 130.2, 121.0, 119.3, 97.1,
73.4, 69.1, 64.6, 20.5, 19.1 ppm; MS (FAB): 438 (M + H)+ (calcd
MW = 439.1); elemental analysis: calcd (%) for C20H25NO10: C
54.89, H 6.10, N 3.40; found: C 54.67, H 5.73, N 3.19.
(3R,4S,5S,6S)-2-(Acetoxymethyl)-6-(4-heptadeca-10,12-diyn-
amidophenoxy)-tetrahydro-2H-pyran-3,4,5-triyl triacetate (10).
A mixture of 5 (0.15 g, 0.34 mmol), 9 (0.1 g, 0.34 mmol), and
triethylamine (0.17 g, 1.7 mmol) in dry THF (20 mL) was refluxed
for 3 h under N2 atmosphere. The solution was filtered after cooling
to room temperature, and concentrated in vacuo. The residue was
purified by column chromatography on silica gel with EA/Hx (2/1
1
v/v, Rf = 0.6). Yield 75%. H NMR (300 MHz, CDCl3): d 9.70
(s, 1H, NH), 7.49 (d, 2H, J = 9, Ar–H), 6.97 (d, 2H, J = 9, Ar–
H), 5.21 (d, 1H), 5.16 (d, 1H), 4.98 (d, 1H), 4.91 (d, 1H), 4.75(d,
1H), 4.58(t, 1H), 3.78 (m, 1H), 3.35 (m, 1H), 3.21(s, 5H), 2.33
(m, 6H), 2.08 (m, 12H), 1.53 (m, 2H), 1.45–1.28 (m, 14H), 0.83
(s, 3H, CH3); 13C NMR (75 MHz, CDCl3): 180.2, 169.3, 155.4,
129.1, 119.9, 119.1, 89.1, 77.4, 71.3, 69.1, 64.6, 61.1, 25–28, 20.5,
19.1, 12.8 ppm; MS (FAB): 713 (M + H)+ (calcd MW = 711.4);
elemental analysis calcd (%) for C39H53NO11: C 65.14, H 8.21, N
2.01; found: C 65.80, H 7.50, N 1.97.
N -(4-((2S,3S,4S,5S )-3,4,5-Trihydroxy-6-(hydroxymethyl)-
tetrahydro-2H-pyran-2-yloxy)phenyl)heptadecanamide (2). Syn-
thetic procedures are same as for 1 starting from 11. H NMR
1
(300 MHz, DMSO-D6): d 9.60 (s, 1H, NH), 7.50 (d, 2H, J = 9,
Ar–H), 6.88 (d, 2H, J = 9, Ar–H), 5.13 (d, 1H), 5.20 (d, 1H),
4.99 (d, 1H), 4.88 (d, 1H), 4.70(d, 1H), 4.61(t, 1H), 3.80 (m, 1H),
3.18(s, 5H), 1.50 (m, 2H), 1.45–1.28 (m, 30H), 0.82 (m, 3H, CH3);
13C NMR (75 MHz, CDCl3): 171, 131.0, 121.5, 116.2, 102.2, 71.1,
66.0, 65.7, 65.2, 64.2, 31–27, 21.5, 12.5 ppm; IR (KBr, cm−1): 3375,
3288, 2929, 2850, 1654, 1600, 1535, 1512, 1407, 1234, 1072; MS
(FAB): 538.5 (M + H)+ (calcd MW = 537.73); elemental analysis:
calcd (%) for C30H51NO7; C 67.01, H 9.56, N 2.60. found: C 65.07,
H 9.11, N 2.50.
( 3R,4S,5S,6S ) - 2 - ( Acetoxymethyl ) - 6 - ( 4 - heptadecanamido
phenoxy)-tetrahydro-2H-pyran-3,4,5-triyl triacetate (11). The
same reaction procedures as for 10 were used starting from stearoyl
chloride and 9. 1H NMR (300 MHz, CDCl): d 9.70 (s, 1H, NH),
7.49 (d, 2H, J = 9, Ar–H), 6.97 (d, 2H, J = 9, Ar–H), 5.21 (d, 1H),
5.16 (d, 1H), 4.98 (d, 1H), 4.91 (d, 1H), 4.75(d, 1H), 4.58(t, 1H),
3.78 (m, 1H), 2.08 (m, 12H), 1.53 (m, 2H), 1.45–1.28 (m, 30H),
0.82 (m, 3H, CH3); 13C NMR (75 MHz, CDCl3): 178, 171.2, 156.2,
128.6, 121.4, 110.6, 97.1, 75.0, 73.1, 66.1, 65.1, 64.1, 33–27, 20.8,
16.1; MS (FAB): 706.5 (M + H)+ (calcd MW = 705.41); elemental
analysis: calcd (%) for C38H59NO11: C 64.66, H 8.42, N 1.98.; found:
C 64.35, H 8.22, N 2.00.
10, 12-Heptadecadiynoyl chloride (5). A mixture of 10,12-
heptadecadiynoic acid 3 (0.10 g, 0.37 mmol), 4 (0.12 mL,
3.96 mmol) and DMF (1 ∼ 2 drops) was dissolved in CH2Cl2
(2.0 mL), and the reaction mixture was then stirred for 10 h at
room temperature. The residual oxalyl chloride and solvent were
removed in vacuo. The product was directly used for the coupling
reaction without further purification.
(2R,3R,4S,5R,6S )-2-(Acetoxymethyl)-6-(4-nitrophenoxy)-
tetrahydro-2H-pyran-3,4,5-triyl triacetate (8). Nitrophenyl-b-D-
glucopyranoside 6 (1.0 g, 2.54 mmol) was dissolved in 1.0 mL of
pyridine and 1.0 mL of 7. The reaction mixture was allowed to
stand overnight and was then evaporated in vacuo to dryness with
dry toluene. The residue was purified by column chromatography
on silica gel with EA/Hx (1/1 v/v, Rf = 0.5). Yield 75%. 1H NMR
(300 MHz, CDCl3): d 7.23 (d, 2H, J = 9, Ar–H), 7.08 (d, 2H,
J = 9, Ar–H), 5.24 (m, 3H), 5.04 (d, 1H, J = 9), 4.3–4.1 (m, 2H),
3.87 (m, 1H), 1.97 (s, 12H); 13C NMR (75 MHz, CDCl3): 170.5,
158.6, 130.5, 123.1, 115.3, 98.1, 71.4, 68.3, 65.4, 20.8, 19.6 ppm;
Acknowledgements
This work was supported by the KOSEF (F01-2004-10061-0 and
R01-2005-000-10229-0).
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2038 | Org. Biomol. Chem., 2006, 4, 2033–2038
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