Organogels of lignin-derived stable metabolic intermediate, 2-pyrone-4,6-dicarboxylic acid (PDC), Bearing cholesteryl groups

Article abstract of DOI:10.1246/cl.2010.400

The organogelation behavior of a lignin-derived stable metabolic intermediate bearing two cholesteryl groups was examined for the first time, and it was found that the carbamate spacers possessing a hydrogen-bonding ability are essential for gelation of aliphatic or aromatic organic solvents.

Full text of DOI:10.1246/cl.2010.400

doi:10.1246/cl.2010.400
400
Published on the web March 13, 2010
Organogels of Lignin-derived Stable Metabolic Intermediate,
2-Pyrone-4,6-dicarboxylic Acid (PDC), Bearing Cholesteryl Groups
Tsuyoshi Michinobu,*1,2 Kenta Hiraki,1 Nozomu Fujii,1 Kazuhiro Shikinaka,1 Yoshihiro Katayama,3
Eiji Masai,4 Masaya Nakamura,5 Yuichiro Otsuka,5 Seiji Ohara,5 and Kiyotaka Shigehara*1
1Graduate School of Engineering and Institute of Symbiotic Science and Technology,
Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
2Global Edge Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550
3Graduate School of Bio-Applications and Systems Engineering,
Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
4Department of Bioengineering, Nagaoka University of Technology, Nagaoka 940-2188
5Forestry and Forest Products Research Institute, Tsukuba 305-8687
(Received February 3, 2010; CL-100112; E-mail: jun@cc.tuat.ac.jp, michinobu.t.aa@m.titech.ac.jp)
The organogelation behavior of a lignin-derived stable
metabolic intermediate bearing two cholesteryl groups was
examined for the first time, and it was found that the carbamate
spacers possessing a hydrogen-bonding ability are essential for
gelation of aliphatic or aromatic organic solvents.
O
(a)
O
O
O
O
O
O
O
O
O
O
HO
O
O
1
OH
O
O
O
O
Low molecular weight organogalators have attracted much
attention because of their potential applications as new soft
materials, gel electrolytes, photochemical templates, etc.1,2 For
such practical or industrial applications, it is important to
consider the repeated circulation of carbon resources. One of the
most promising approaches is the utilization of biomass, and
there are many reports on the preparation of biomass-based
functional materials.3 In the past, we successfully established a
massive production protocol of a small pseudo-aromatic ring
molecule, 2-pyrone-4,6-dicarboxylic acid (PDC), from lignin
biometabolic intermediates by transformed bacterium4 and
synthetic methods for a series of biodegradable PDC molecules
and polymers.5
PDC
(b)
O
O
O
N
H
O
O
O
O
N
H
O
O
2
Scheme 1. Synthesis of cholesteryl PDC derivatives. (a)
Cholest-5-en-3-yl 2-hydroxyethyl carbonate, DIC, DMAP,
THF, 20 °C, 1 h, 40%; (b) Cholest-5-en-3-yl (2-hydroxyethyl)-
carbamate, DIC, DMAP, THF, 20 °C, 1 h, 46%.
broadening effect originating from partial self-assembly was
detected in CDCl3 (10 mM) at 20 °C, whereas 1 did not show
any such broadening.
It is well known that organogels are formed through the 1D
alignment of gelator molecules in terms of van der Waals,
dipole-dipole, ³-³, and hydrogen-bonding interactions.6
Steroid and sugar derivatives are often employed as a source
of van der Waals interactions. On the other hand, functional
moieties, such as fluorescent dyes and redox active ³ chromo-
phores, tend to self-assemble through dipole-dipole or ³-³
interactions, and most of them are usually synthetic substances.
Our previous studies demonstrated the interesting fluorescent
behavior of the PDC molecule, and X-ray crystallography of
single crystals revealed the unusually short intermolecular
contact between the 2-pyrone rings, indicating strong dipole-
dipole interaction.7 Therefore, to increase the content of natural
carbon resources in functional organogelator molecules, we
designed new PDC derivatives bearing two cholesteryl groups
through either carbonate or carbamate spacers. We now report
the synthesis and gelation ability of PDC-based organogelators.
Cholesterol-appended PDC derivatives, bis(2-{[(cholest-5-
en-3-yloxy)carbonyl]oxy}ethyl) 2-oxo-2H-pyran-4,6-dicarbox-
ylate (1) and bis(2-{[(cholest-5-en-3-yloxy)carbonyl]amino}eth-
yl) 2-oxo-2H-pyran-4,6-dicarboxylate (2), were synthesized
from PDC by condensation, and unambiguously characterized
Thermal analyses of both PDC derivatives were carried out
by thermogravimetric and differential thermal analysis (TG-
DTA) and differential scanning calorimetry (DSC). Neither
derivative showed any decomposition at least up to 200 °C
(Figure 1SI).8 DSC measurements revealed a transition from a
glass state to a liquid crystalline (LC) phase at 77.0 °C for 1 and
86.7 °C for 2 (Table 1SI), and the LC phase was determined to
be smectic A from the polarized optical microscopy (POM)
images (Figure 2SI).8 The X-ray diffraction (XRD) patterns
of the LC phases provided the layer distance of 43.63 ¡ for 1
and 44.62 ¡ for 2, corresponding to single molecular sizes
(Figure 2SI).8
The gelation properties of 1 and 2 were assessed in various
organic solvents. It was found that 2 possesses an excellent
gelation ability for hexane, cyclohexane, ligroin, and benzene at
a minimum concentration of 10, 4.1, 3.7, and 7.4 wt %,
respectively, whereas 1 did not show any gelation in the
examined organic solvents (Figure 1a). This result implies that
the carbamate spacers between PDC and the cholesteryl group
play an important role in gelation in terms of the hydrogen
bonds. The prepared organogels did not exhibit gel-sol
transitions up to the boiling points of the employed solvents,
indicating significantly high stability as a physical gel.
1
by H and 13C NMR, FT-IR, MALDI-TOF-MS, and elemental
analysis (Scheme 1).8 In the 1H NMR spectrum of 2, a line-
Chem. Lett. 2010, 39, 400-401
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

401
is most likely associated with the dipolar interaction of the 2-
pyrone ring.7
In conclusion, PDC-based organogels were described for the
first time. Gelation of some aliphatic hydrocarbons and benzene
was realized by the van der Waals interaction of the cholesteryl
groups, dipolar interaction of the 2-pyrone ring, and hydrogen
bonds of the carbamate moieties of 2. Despite a similar thermal
phase behavior, 1 with a carbonate spacer did not show any
gelation in organic solvents. This result suggests the importance
of the molecular design and control of the weak intermolecular
interactions. Further diversification of this biomass-based novel
organogelator is now underway.
This research was supported in part by a Grant-in-Aid for
Scientific Research and the Special Coordination Funds for
Promoting Science and Technology from MEXT, Japan and
General Sekiyu Research Promotion. We thank Prof. J.
Watanabe and Prof. M. Tokita (Tokyo Inst. Technol.) for the
use of XRD instrumentation and Prof. S. Kajita (Tokyo Univ.
Agr. Technol.) for useful discussion.
Figure 1. (a) Photograph of cyclohexane gel of 2 (4.1 wt %) in
an NMR tube. (b) XRD diffraction pattern and (c) SEM image of
the xerogel.
References and Notes
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Figure 2. FT-IR spectra of 2 in a CH2Cl2 solution (solid line)
and in the cyclohexane gel (4.1 wt %) state (dotted line).
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value was independent of the gelator concentration at
>4.1 wt %, the gelation always occurs in the same process.
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To elucidate the hydrogen-bonding interactions, the FT-IR
spectra of 2 were measured in a CH2Cl2 solution and a
cyclohexane gel. We noted the characteristic peaks of the NH
and C=O groups of the carbamate and PDC moieties. In a
CH2Cl2 solution, where no gelation was observed, 2 exhibited
¹1
the NH stretching band at 3446 cm (Figure 2a). However, the
¹1
band shifted to 3377 cm in the cyclohexane gel, suggesting a
hydrogen-bond formation. Similarly, the C=O stretching band
¹1
of the carbamate moieties observed at 1716 cm in CH2Cl2
6
7
shifted to the lower energy of 1698 cm¹1 in the cyclohexane gel
(Figure 2b). This result again supports the hydrogen-bond
formation, which stabilizes the self-assembled structure of 2 in
a gel state. Moreover, the C=O stretching bands of the 2-pyrone
8
9
¹1
ring and ester moieties are overlapped at around 1740 cm in
CH2Cl2 (Figure 2b). These bands were definitely split into two
¹1
bands in the cyclohexane gel. The low energy shift to 1729 cm
Chem. Lett. 2010, 39, 400-401
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/