Design, synthesis and gelation studies of 4,6-O-butylidene-α,β- unsaturated-β-C-glycosidic ketones: Application to plant tissue culture

Article abstract of DOI:10.1039/b902064k

A series of α,β-unsaturated-β-C-glycosidic ketones was synthesized starting from d-glucose in three steps. 4,6-O-Butylidene-β-C- glycosidic ketones on aldol condensation with aromatic and heteroaromatic aldehydes in the presence of a suitable organocataly

Full text of DOI:10.1039/b902064k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design, synthesis and gelation studies of 4,6-O-butylidene-a,b-unsaturated-b-
C-glycosidic ketones: application to plant tissue culture†
a
a
Subbiah Nagarajan, Thangamuthu Mohan Das, Pandian Arjunb and Nanjian Raamanb
*
Received 30th January 2009, Accepted 7th April 2009
First published as an Advance Article on the web 19th May 2009
DOI: 10.1039/b902064k
A series of a,b-unsaturated-b-C-glycosidic ketones was synthesized starting from D-glucose in three
steps. 4,6-O-Butylidene-b-C-glycosidic ketones on aldol condensation with aromatic and
heteroaromatic aldehydes in the presence of a suitable organocatalyst led to the stereoselective
formation of the products in good yield. Hydrogen bonding and p–p stacking of these derivatives were
established by single-crystal XRD. The soft material derived from this method had a diameter of
10–200 nm with a three-dimensional network, as determined by SEM and HR-TEM. In the present
study, we discuss the mechanism of formation of these self-assembled nanostructures, the morphology
of the soft material and its thermal stability. We also demonstrate the utility of these soft materials in
the field of plant tissue culture.
H-bond forming segments are prone to form gels even at low
concentration.9
Introduction
b-C-Glycosides are becoming useful building blocks for the
synthesis of 1-O-cinnamoylglucose derivatives, which can act as
potential b-C-glycosidase inhibitors and have been reported to
possess plant growth inhibitory activity.1 In general, stereo-
selective synthesis of C-glycosides has attracted considerable
interest, and the methodology has advanced significantly in the
past decade.2 Though some routes are available to synthesise
b-C-glycosides, in practice their use is very limited due to their
low yields, poor selectivity, or lack of general applicability
leading to complicated procedures involved in synthetic steps.
Typically, b-C-glycosides can be synthesised by addition of
a nucleophilic reagent to a sugar lactone, giving the corre-
sponding hemiacetal or exoglycal, which is then reduced by
various reagents3 or by Wittig or Grignard reaction at the
Molecular self-assembly is becoming a popular tool to
construct different types of micro- and nanostructured mate-
rials.10 Low molecular weight organogelators are known to self-
assemble into various types of fibrils, strands, and tapes in
organic solvents via weak intermolecular interactions.11 In an
organogel medium, one dimensional (1-D) supramolecular fibers
are bundled up together and entangled at nodes, the so-called
‘‘junction point’’, to form three-dimensional (3-D) networks, in
which the solvent molecules are trapped. The nanostructured
functional molecular assembly thus created is a promising
candidate for organic devices with intriguing photo- and
electrochemical functions.12 In the present study, we have
synthesised and characterised a series of a,b-unsaturated-b-C-
glycosidic ketones and found these derivatives to be excellent
candidates for forming gels.
anomeric
position.1d,2c,2p,4
a,b-Unsaturated-b-C-glycosidic
ketones of D-glucose have served as the core moiety in natural
ligands for making covalent bonds with many receptors, such as
the peroxisome proliferator-activated receptor g (PPARg).5
Moreover, b-C-glycosides can also act as non-hydrolyzable
mimics of N- and O-glycosides and are useful in designing new
chemotherapeutics and biological tools.6 Saccharides and their
derivatives have a wide range of applications6,7 in the fields of
materials, biology, drug design and natural product chemistry. In
recent years, low molecular weight organogelators have played
an important role in materials chemistry.8 In particular, certain
4,6-O-protected-D-glucoses and D-mannoses possessing strong
Plant tissue culture is the aseptic culture of cells, tissues, organs
and their components under defined physical and chemical
conditions in vitro. It is an important tool in both basic and
applied studies, as well as in commercial applications,13 and agar
is currently commonly used.14 Our studies aim to develop our
new gelators as a substitute for agar, and with this in mind, we
tested Tribulus terrestris, an annual plant of the Zygophylla-
ceae,15 for plant tissue culture studies.
Experimental
Materials and methods
aDepartment of Organic Chemistry, University of Madras, Guindy
Campus, Chennai, 600 025, India. E-mail: tmdas_72@yahoo.com; Fax:
+91-44-22352494; Tel: +91 44 22202814
bNatural Products and Tissue Culture Laboratory, Centre for Advanced
Studies in Botany, University of Madras, Guindy Campus, Chennai, 600
025, India
D-Glucose and various aromatic aldehydes used for the synthesis
of soft materials were obtained from Sigma-Aldrich Chemicals
Pvt. Ltd, USA, and were of high purity. The organocatalyst
(pyrrolidine) was obtained from SRL, India. Other reagents,
hydrochloric acid, sodium bicarbonate and solvents (AR Grade)
were obtained in high purity from Sd-fine, India, and were used
without any further purification. Column chromatography was
performed on Silica Gel (100–200 mesh). NMR spectra were
† Electronic supplementary information (ESI) available: Copies of NMR
and mass spectra. CCDC reference numbers 702166–702168. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b902064k
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 4587–4596 | 4587

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recorded on a Bruker DRX 300 MHz instrument, and differen-
tial scanning calorimetry (DSC) was performed on a Netzsch
DSC 204 instrument. Scanning electron microscopy (SEM)
images were recorded using a Hitachi-S-3400W microscope, and
high-resolution transmission electron microscopy (HRTEM)
was carried out on a JEOL JEM 3010 model (LaB6 filament).
Mass spectra were recorded using a Thermo Finnigan-ESI mass
spectrometer, and optical rotation was performed using
a Rudolph-Autopol II digital polarimeter. Elemental analyses
were carried out on a Thermoquest microanalyser.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(2-hydroxyphen-
yl)but-3-en-2-one 4b. Compound 4b was obtained by the reaction
of b-C-glycosidic ketone (2) (1 mmol, 0.274 g) and 2-hydrox-
ybenzaldehyde (3b) (1.2 mmol, 0.13 ml) as a colorless solid: mp
ꢀ
192–194 C; Yield: 0.28 g (74%); Rf 0.74 (0.5 : 9.5 hexane–ethyl
acetate); 1H NMR (300 MHz, TMS, ppm): 7.82 (d, J ¼ 16.2 Hz,
1H), 7.39 (d, J ¼ 7.8 Hz, 1H), 7.10–7.15 (t, J ¼ 7.8 Hz, 1H), 6.86
(d, J ¼ 8.1 Hz, 1H), 6.74–6.80 (m, 2H), 4.44–4.47 (t, J ¼ 4.8 Hz,
1H), 3.97–4.01 (dd, J ¼ 3.6 Hz, J ¼ 9.9 Hz, 1H), 3.79–3.84 (t, J ¼
8.7 Hz, J ¼ 7.8 Hz, 1H), 3.50–3.56 (t, J ¼ 8.4 Hz, 1H), 3.29–3.35 (t,
J ¼ 9.9 Hz, 1H), 3.12–3.23 (m, 3H), 3.03–3.08 (m, 1H), 2.73–2.81
(dd, J ¼ 9.0 Hz, J ¼ 7.2 Hz, 1H), 1.52–1.58 (m, 2H), 1.31–1.41 (m,
2H), 0.81–0.86 (t, J ¼ 7.5 Hz, 3H); 13C NMR (75 MHz, TMS,
ppm): 203, 162, 144, 136, 133, 131, 126, 124, 121, 107, 85, 81, 80
(2C), 75, 73, 48, 41, 22, 19; [a]3D0 ꢁ25.5 (c 1.0 in DMSO); Elemental
Anal. Found: C, 63.41; H, 6.82. Calc. for C20H26O7: C, 63.48;
H, 6.93%.
(4,6-O-Butylidene-b-D-glucopyranosyl)propan-2-one (2)
To a solution of 4,6-O-butylidene-D-glucopyranose (1) 2.34 g (10
mmol) in H2O-THF (1 : 9) were added NaHCO3 (4 equiv.) and
2,4-pentanedione (2 equiv.). The reaction mixture was stirred at
reflux temperature for 36 h, then cooled to room temperature,
allowed to stand for 2 h and the solution was filtered to remove
the suspended NaHCO3, followed by washing the residue well
with DCM. Evaporation of the solvents followed by column
chromatography purification (4 :ꢀ6 hexane–ethyl acetate) resulted
in colourless solid. mp 102–104 C; Yield: 2.32 g (85%) Rf 0.64
(0.5 : 9.5 hexane–ethyl acetate) 1H NMR (300 MHz, TMS, ppm):
4.44–4.47 (t, J ¼ 5.1 Hz, 1H), 4.03–4.08 (m, J ¼ 9.9 Hz, J ¼ 4.2
Hz, 1H), 3.72–3.79 (m, 1H), 3.58–3.64 (t, J ¼ 8.7 Hz, 1H), 3.11–
3.38 (m, 4H), 2.96–2.99 (m, 2H), 2.79–2.86 (dd, J ¼ 3.9 Hz, J ¼
16.5 Hz, 1H), 2.54–2.62 (q, J ¼ 8.0 Hz, 1H), 2.13 (s, 3H), 1.52–
1.59 (m, 2H), 1.31–1.39 (m, 2H), 0.83–0.88 (t, J ¼ 7.5 Hz, 3H);
13C NMR (75 MHz, TMS, ppm): 207, 102, 80, 76, 75, 74, 70, 68,
46, 36, 31, 17, 14; Elemental Anal. Found: C, 56.63; H, 8.26.
Calc. for C13H22O6: C, 56.92; H, 8.08%
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(2-hydroxy-3-
methoxy-phenyl)but-3-en-2-one (4c). Compound 4c was
obtained by the reaction of b-C-glycosidic ketone (2) (1 mmol,
0.274 g) and O-vanillin (3c) (1.2 mmol, 0.182 g) as a colorless
solid: mp 148–150 ꢀC; Yield: 0.38 g (93%); Rf 0.8 (0.5 : 9.5
1
hexane–ethyl acetate); H NMR (300 MHz, TMS, ppm): 7.85
(d, J ¼ 16.2 Hz, 1H), 7.03 (d, J ¼ 7.5 Hz, 1H), 6.72–6.83 (m,
3H), 4.44–4.47 (t, J ¼ 4.5 Hz, 1H), 3.98–4.03 (dd, J ¼ 3.3 Hz,
J ¼ 6.3 Hz, 1H), 3.82 (s, 3H), 3.53–3.59 (t, J ¼ 8.4 Hz, 1H),
3.04–3.36 (m, 5H), 2.76–2.84 (q, J ¼ 8.4 Hz, J ¼ 7.5 Hz, 1H),
1.51–1.59 (m, 2H), 1.29–1.41 (m, 2H), 0.81–0.85 (t, J ¼ 7.5 Hz,
3H); 13C NMR (75 MHz, TMS, ppm): 199, 147, 146, 138, 127,
121, 120, 119, 112, 102, 81, 75(2C), 71, 68, 56, 43, 36, 17, 14; [a]D30
ꢁ20.1 (c 1.0 in DMSO); Elemental Anal. Found: C, 62.02; H,
6.83. Calc. for C21H28O8: C, 61.75; H, 6.91%.
General procedure for (E)-1-(4,6-O-butylidene-b-D-
glucopyranosyl)-4-phenylbut-3-en-2-one (4)
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(2-hydroxy-5-
bromophenyl)but-3-en-2-one (4d). Compound 4d was obtained
by the reaction of b-C-glycosidic ketone (2), (1 mmol, 0.274 g)
and 5-bromo-2-hydroxybenzaldehyde (3d) (1.2 mmol, 0.241 g)
as a colorless solid: mp 178–180 ꢀC; Yield: 0.37 g (81%); Rf 0.83
(0.5 : 9.5 hexane–ethyl acetate); 1H NMR (300 MHz, TMS,
ppm): 7.71 (d, J ¼ 18.0 Hz, 1H), 7.51 (d, J ¼ 7.8 Hz, 1H), 7.17–
7.20 (m, 1H), 6.72–6.81 (m, 2H), 4.44–4.46 (m, 1H), 3.97–4.00
(m, 2H), 3.02–3.79 (m, 6H), 2.71–2.79 (m, 1H), 1.52–1.54 (m,
2H), 1.31–1.36 (m, 2H), 0.81–0.83 (t, J ¼ 7.5 Hz, 3H); 13C NMR
(75 MHz, TMS, ppm): 198, 156, 137, 134, 131, 127, 123, 118,
111, 102, 80, 76, 75, 74, 70, 68, 43, 36, 17, 14; Elemental Anal.
Found: C, 52.18; H, 5.45. Calc. for C20H25BrO7: C, 52.53; H,
5.51%.
To a solution of b-C-glycosidic ketone, 2 (1 mmol) in dry DCM
were added pyrrolidine (30% mol) and substituted benzaldehyde
(3a–q) (1.2 mmol). After stirring at room temperature for given
period of time, the reaction mixture was evaporated under
reduced pressure and extracted with EtOAc–water. The ethyl
acetate layer was dried over anhyd. Na2SO4 and concentrated to
dryness. The product was further purified by flash column
chromatography.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(phenyl)but-3-
en-2-one 4a. Compound 4a was obtained by the reaction of b-C-
glycosidic ketone (2) (1 mmol, 0.274 g) and benzaldehyde (3a)
ꢀ
(1.2 mmol, 0.12 ml) as a colorless solid: mp 192–194 C; Yield:
1
0.31 g (86%); Rf 0.74 (0.5 : 9.5 hexane–ethyl acetate); H NMR
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(3,4-dioxanephen-
yl)but-3-en-2-one (4e). Compound 4e was obtained by the reaction
of b-C-glycosidic ketone (2) (1 mmol, 0.274 g) and piperonal (3e)
(1.2 mmol, 0.18 g) as a colorless solid: mp 199–201 ꢀC; Yield: 0.35 g
(86%); Rf 0.87 (0.5 : 9.5 hexane–ethyl acetate); 1H NMR (300 MHz,
TMS, ppm): 7.40 (dd, J ¼ 1.8 Hz, J ¼ 16.2 Hz, 1H), 6.97 (d, 2H),
6.73–6.76 (dd, J ¼ 1.8 Hz, J ¼ 7.8 Hz, 1H), 6.50–6.56 (dd, J ¼ 1.8
Hz, J ¼ 15.9 Hz, 1H), 5.95 (d, J ¼ 2.1 Hz, 2H), 4.45–4.47 (m, 1H),
4.00–4.08 (m, 1H) 3.80–3.89 (m, 1H), 3.55–3.62 (m, 1H), 3.00–3.40
(m, 7H), 2.70–2.85 (m, 1H), 1.50–1.60 (m, 2H), 1.30–1.40 (m, 2H),
(300 MHz, TMS, ppm): 7.55–7.59 (m, 3H, Ar–H), 7.45–7.49 (m,
3H, Ar–H), 6.78 (d, J ¼ 16.8 Hz, 1H), 4.79 (d, J ¼ 10.2 Hz, 1H),
4.55 (t, J ¼ 4.7 Hz, 1H), 4.06–4.11 (dd, J ¼ 3.6 Hz, J ¼ 3.9 Hz, J
¼ 10.1 Hz, 1H), 3.87–3.94 (m, 1H), 3.61–3.68 (m, 1H), 3.13–3.44
(m, 5H), 2.81–2.89 (m, 1H), 1.65–1.68 (m, 2H), 1.55–1.60 (m,
2H), 0.91 (t, J ¼ 7.4 Hz, 3H); 13C NMR (75 MHz, TMS, ppm):
198, 143, 134, 131, 129, 128, 126, 102, 81, 77, 75 (2C), 71, 68, 43,
36, 17, 14. Elemental Anal. Found: C, 66.38; H, 7.12. Calc. for
C20H26O6: C, 66.28; H, 7.23%.
4588 | J. Mater. Chem., 2009, 19, 4587–4596
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1
0.81–0.86 (m, 3H); 13C NMR (75 MHz, TMS, ppm): 198, 150, 148,
143, 129, 125 (2C), 109, 107, 102 (2C), 81, 77 (2C), 75 (2C), 71, 68,
43, 36, 17, 14; ESI-MS; Calc. for C21H26O8 406.16; m/z found,
407.13 (M + H+); [a]3D0 ꢁ17.2 (c 0.16 in DMSO).
(9 : 1 chloroform–methanol); H NMR (300 MHz, TMS, ppm):
8.68 (d, J ¼ 35.1 Hz, 2H), 8.25 (s, 1H), 7.48 (d, J ¼ 16.5 Hz, 1H),
6.97 (d, J ¼ 16.2 Hz, 1H), 4.43–4.46 (t, J ¼ 4.8 Hz, 1H), 3.90–3.95
(dd, J ¼ 3.9 Hz, J ¼ 4.5 Hz, 1H), 3.74–3.79 (t, J ¼ 9.0 Hz, 1H),
3.00–3.58 (m, 8H), 2.70–2.79 (dd, J ¼ 8.7 Hz, J ¼ 7.2 Hz, 1H),
1.47–1.51 (m, 2H), 1.30–1.37 (m, 2H), 0.81–0.86 (t, J ¼ 7.2 Hz,
3H); 13C NMR (75 MHz, TMS, ppm): 197, 151, 148, 137 (2C),
129, 101, 81, 76, 74 (2C), 70, 68, 44, 36, 17, 14; ESI-MS: Calc. for
C19H24BrNO6 441.07, m/z found 444.07 (M + H+); [a]D30 ꢁ13.8 (c
1.0 in DMSO).
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(2-nitrophenyl)-
but-3-en-2-one (4f). Compound 4f was obtained by the reaction of
b-C-glycosidic ketone (2) (1 mmol, 0.274g) and 2-nitro-
benzaldehyde (3f) (1.2 mmol, 0.18 g) as a pale brown solid: mp
98–100 ꢀC; Yield: 0.3 g (73%); Rf 0.83 (0.5 : 9.5 hexane–ethyl
acetate); 1H NMR (300 MHz, TMS, ppm): 7.91–8.00 (t, J ¼ 13.5
Hz, 2H), 7.58 (s, 2H), 7.50 (d, J ¼ 6.3 Hz, 1H), 6.55 (d, J ¼ 16.2
Hz, 1H), 4.46 (s, 1H), 4.04–4.07 (t, 1H), 3.87 (s, 1H), 3.62–3.68 (t,
J ¼ 9.0 Hz, 1H), 3.09–3.39 (m, 5H), 2.86–2.93 (dd, J ¼ 8.7 Hz,
J ¼ 7.2 Hz, 1H), 1.55 (d, J ¼ 4.2 Hz, 2H), 1.31–1.38 (m, 2H),
0.82–0.87 (t, J ¼ 6.0 Hz, 3H); 13C NMR (75 MHz, TMS, ppm):
198, 139, 134, 131 (2C), 130, 129, 125, 102, 80, 76, 75, 74, 71, 68,
43, 36, 17, 14; Elemental Anal. Found: C, 59.05; H, 6.26, N, 3.26.
Calc. for C20H25NO8: C, 58.96; H, 6.18; N, 3.44%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(1,4-dioxane-
phenyl)but-3-en-2-one (4j). Compound 4j was obtained by the
reaction of b-C-glycosidic ketone (2) (1 mmol, 0.274 g), 1,4-
benzodioxan-6-carboxaldehyde (3j) (1.2 mmol, 0.197 g) as
a colorless solid: mp 150–152 ꢀC; Yield: 0.32 g (76%); Rf 0.87
(0.5 : 9.5 hexane–ethyl acetate); 1H NMR (300 MHz, TMS,
ppm): 7.40 (d, J ¼ 16.5 Hz, 1H), 6.99 (d, J ¼ 10.8 Hz, 2H), 6.80
(d, J ¼ 8.4 Hz, 1H), 6.55 (d, J ¼ 16.2 Hz, 1H), 4.44–4.47 (t, J ¼
5.1 Hz, 1H), 4.18–4.23 (m, 2H), 4.03–4.07 (dd, J ¼ 3.9 Hz, J ¼
9.6 Hz, 1H), 3.80–3.87 (m, 1H), 3.62–3.68 (t, J ¼ 3.3 Hz, 1H),
3.14–3.37 (m, 4H), 2.99–3.06 (dd, J ¼ 3.6 Hz, J ¼ 12.1 Hz, 1H),
2.84 (m, 3H), 1.52–1.59 (m, 2H), 1.28–1.41 (m, 2H), 0.82–0.85
(t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz, TMS, ppm): 198, 146,
143 (2C), 128, 125, 123, 118, 117, 102, 80, 77, 76, 75 (2C), 71, 68,
65, 64, 43, 36, 17, 14; Elemental Anal. Found: C, 63.04; H, 6.62.
Calc. for C22H28O8: C, 62.85; H, 6.71%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(5-thiophene-
50-thiophenyl)but-3-en-2-one (4g). Compound 4g was obtained
by the reaction of b-C-glycosidic ketone (2) (1 mmol, 0.274 g)
and 2,20-bithiophene-5-carbaldehyde (3g) (1.2 mmol, 0.23 g) as
a pale brown solid: mp 156–160 ꢀC; Yield: 0.34 g (76%); Rf 0.87
1
(0.5 : 9.5 hexane–ethyl acetate); H NMR (300 MHz, TMS,
ppm): 7.54 (d, J ¼ 15.6 Hz, 1H), 7.26 (d, J ¼ 4.8 Hz, 1H), 7.19
(d, J ¼ 3.6 Hz, 1H), 7.08 (d, J ¼ 3.9 Hz, 1H), 6.96–6.99 (t, J ¼
4.2 Hz, 1H), 6.39–6.44 (d, J ¼ 15.6 Hz, 1H), 4.42–4.45 (t, J ¼
5.7 Hz, 1H), 3.94–3.99 (dd, J ¼ 3.9 Hz, J ¼ 4.2 Hz, 1H), 3.73–
3.81 (m, 2H), 3.46–3.52 (t, J ¼ 8.7 Hz, 1H), 2.98–3.34 (m, 6H),
2.64–2.72 (dd, J ¼ 9.0 Hz, J ¼ 6.6 Hz, 1H), 1.49–1.55 (m, 2H),
1.30–1.40 (m, 2H), 0.79–0.84 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75
MHz, TMS, ppm): 196, 140, 138, 136, 135, 133, 128, 127, 126,
124 (3C), 101, 80, 78 (2C), 77, 74 (2C), 70, 68, 43, 36, 17, 14;
Elemental Anal. Found: C, 58.48; H, 5.94. Calc. for
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(4-nitrophenyl)-
but-3-en-2-one (4k). Compound 4k was obtained by the reaction
of b-C-glycosidic ketone (2) (1 mmol, 0.274 g) and 4-nitro-
benzaldehyde (3k) (1.2 mmol, 0.181 g) as a pale brown solid: mp
170–172 ꢀC; Yield: 0.32 g (78.6%); Rf 0.85 (0.5 : 9.5 hexane–
ethyl acetate); 1H NMR (300 MHz, TMS, ppm): 8.15 (d, J ¼ 8.7
Hz, 2H), 7.66 (d, J ¼ 8.4 Hz, 2H), 7.49 (d, J ¼ 16.2 Hz, 1H),
6.80 (d, J ¼ 16.2 Hz, 1H), 4.22–4.45 (t, J ¼ 4.8 Hz, 1H), 3.94–
3.99 (m, 3H), 3.76–3.82 (m, 1H), 3.50–3.56 (t, J ¼ 8.4 Hz, 1H),
3.27–3.33 (t, J ¼ 10.8 Hz, 1H), 3.06–3.23 (m, 4H), 2.72–2.80 (dd,
J ¼ 8.7 Hz, J ¼ 15.9 Hz, 1H), 1.45–1.52 (m, 2H), 1.26–1.36 (m,
2H), 0.78–0.83 (t, J ¼ 7.5 Hz, 3H); 13C NMR (75 MHz, TMS,
ppm): 197, 148, 139, 130, 128 (2C), 124 (2C), 102, 80, 77, 74, 70,
68, 43, 36, 17, 13; Elemental Anal. Found: C, 58.77; H, 5.98; N,
3.56. Calc. for C20H25NO8: C, 58.96; H, 6.18; N, 3.44%.
C
22H26O6S2: C, 58.64; H, 5.82%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(40-phenyl)phen-
ylbut-3-en-2-one (4h). Compound 4h was obtained by the reaction
of b-C-glycosidic ketone (2) (1 mmol, 0.274 g) and 4-biphenyl
carboxaldehyde (3h) (1.2 mmol, 0.218 g) as a colorless solid: mp
200–202 ꢀC; Yield: 0.4 g (92.6%); Rf 0.6 (9 : 1 chloroform–
methanol); 1H NMR (300 MHz, TMS, ppm): 7.51–7.67 (m, 7H),
7.38–7.43 (t, J ¼ 7.2 Hz, 2H), 7.29–7.34 (t, J ¼ 7.2 Hz, 1H), 6.80
(d, J ¼ 16.2 Hz, 1H), 4.42–4.47 (t, J ¼ 5.7 Hz, 1H), 3.92–3.97 (dd,
J ¼ 3.9 Hz, J ¼ 4.2 Hz, 2H), 3.41–3.46 (t, J ¼ 8.4 Hz, 1H), 3.27–
3.33 (t, J ¼ 9.6 Hz, 1H), 3.01–3.21 (m, 5H), 2.73–2.81 (dd, J ¼ 6.9
Hz, J ¼ 9 Hz, 1H), 1.48–1.52 (m, 2H), 1.31–1.38 (m, 2H), 0.82–
0.87 (t, J ¼ 7.5 Hz, 3H); 13C NMR (75 MHz, TMS, ppm): 197, 142
(2C), 139, 133, 126–128 (10C), 101, 81, 77, 74, 70, 68, 43, 36, 17,
14; ESI-MS: Calc. for C26H30O6 438.20, m/z found, 439.20 (M +
H+); [a]3D0 ꢁ87.6 (c 0.07 in DMSO).
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(9-anthranyl)-
but-3-en-2-one (4l). Compound 4l was obtained by the reaction
of b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and 9-anthral-
dehyde (3l) (1.2 mmol, 0.247 g) as a yellow solid: mp 136–138
ꢀC; Yield: 0.42 g (90.9%); Rf 0.83 (0.5 : 9.5 hexane–ethyl
1
acetate); H NMR (300 MHz, TMS, ppm): 8.40–8.47 (dd, J ¼
3.9 Hz, J ¼ 17.7 Hz, 1H), 8.35 (d, J ¼ 4.2 Hz, 1H), 8.09 (d, J ¼
4.5 Hz, 2H), 7.91 (d, J ¼ 3.0 Hz, 2H), 7.40 (d, J ¼ 4.2 Hz, 4H),
6.62–6.69 (dd, J ¼ 4.2 Hz, J ¼ 12.3 Hz, 1H), 4.47 (d, J ¼ 3.9 Hz,
1H), 4.11–4.13 (t, J ¼ 3.9 Hz, 1H), 4.05 (d, J ¼ 7.5 Hz, 2H),
3.19–3.74 (m, 7H), 2.95–3.00 (t, J ¼ 7.8 Hz, 1H), 1.55 (s, 2H),
1.34 (s, 2H), 0.83–0.86 (t, J ¼ 6.0 Hz, 3H); 13C NMR (75 MHz,
TMS, ppm): 198, 140, 135, 125–131 (14C), 102, 80, 77, 75 (2C),
71, 68, 43, 36, 17, 14; Elemental Anal. Found: C, 72.56; H, 6.31.
Calc. for C28H30O6: C, 72.71; H, 6.54%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(5-bromo-3-pyr-
idyl)but-3-en-2-one (4i). Compound 4i was obtained by the
reaction of b-C-glycosidic ketone (2) (1 mmol, 0.274 g) and 5-
bromo-3-pyridine-carboxaldehꢀyde (3i) (1.2 mmol, 0.223 g) as
a colorless solid: mp 188–190 C; Yield: 0.37 g (83.9%); Rf 0.43
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(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(4,5-dimethoxy-
phenyl)but-3-en-2-one (4m). Compound 4m was obtained by the
reaction of b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and 3,4-
dimethoxybenzaldehyde (3m) (1.2 mmol, 0.199 g) as a colorless
solid: mp 162–164 ꢀC; Yield: 0.35 g (83%); Rf 0.76 (0.5 : 9.5
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(3-pyridyl)but-
3-en-2-one (4q). Compound 4q was obtained by the reaction of
b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and 3-pyr-
idinecarboxaldehyde (3q) (1.2 mmol, 0.128 g) as a colorless
solid: mp 147–149 ꢀC; Yield: 0.20 g, (55%); Rf 0.34 (0.5 : 9.5
1
1
hexane–ethyl acetate); H NMR (300 MHz, TMS, ppm): 7.25–
hexane–ethyl acetate); H NMR (300 MHz, TMS, ppm): 8.70
7.30 (m, 1H), 6.86–6.92 (m, 1H), 6.67 (d, J ¼ 8.1 Hz, 1H), 6.43 (d,
J ¼ 15.9 Hz, 1H), 4.3 (s, 1H), 3.81–3.86 (m, 2H), 3.7 (m, 6H),
3.37–3.42 (t, J ¼ 8.4 Hz, 1H), 2.88–3.20 (m, 6H), 2.55–2.63 (m,
1H), 1.38 (d, J ¼ 6.8 Hz, 2H), 1.18–1.22 (m, 2H), 0.65–0.69 (t, J ¼
6.9 Hz, 3H); 13C NMR (75 MHz, TMS, ppm): 198, 149, 143, 127,
124, 123, 111, 110, 102, 86, 77, 75 (2C), 71, 68, 56 (2C), 43, 36, 17,
14; [a]3D0 ꢁ11.7 (c 1.0 in DMSO); Elemental Anal. Found: C,
62.72; H, 7.31. Calc. for C22H30O8: C, 62.55; H, 7.16%.
(br, 1H), 8.55 (br, 1H), 7.33–7.93 (m, 3H), 6.79 (d, J ¼ 16.2 Hz,
1H), 4.47 (d, J ¼ 4.8 Hz, 1H), 3.99–4.09 (m, 1H), 3.79–3.85 (t,
J ¼ 7.8 Hz, 1H), 3.08–3.56 (m, 9H), 2.74–2.83 (m, 1H), 1.54 (d, J
¼ 4.5 Hz, 2H), 1.31–1.38 (m, 2H), 0.80–0.85 (t, J ¼ 7.2 Hz, 3H);
13C NMR (75 MHz, TMS, ppm): 197, 150, 149, 138, 134, 128,
102, 80, 76, 74 (2C), 70, 68, 43, 36, 17, 13; [a]3D0 ꢁ33.7 (c 1.0 in
DMSO); Elemental Anal. Found: C, 63.11; H, 6.83; N, 3.76.
Calc. for C19H25NO6: C, 62.80; H, 6.93; N, 3.85%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(ferrocyl)but-3-
en-2-one (4n). Compound 4n was obtained by the reaction of
b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and ferrocenecarbox-
aldehyde (3n) (1 mmol, 0.214 g) as a dark red solid was obtained
with flash column chromatography. mp 150–152 ꢀC; Yield: 0.38 g
(81%) Rf 0.89 (0.5 : 9.5 hexane–ethyl acetate); 1H NMR (300 MHz,
TMS, ppm): 7.46 (d, J ¼ 13.8 Hz, 1H), 6.32 (d, J ¼ 14.7 Hz, 1H),
4.21–4.54 (m, 5H), 4.09 (m, 5H), 3.66–3.82 (m, 3H), 3.23–3.36 (m,
Gelation test
Gelation was tested for 8 solvents or mixtures of solvents. The
gelation test was carried out as follows: the gelator was mixed in
a capped test tube with appropriate amount of solvent, and the
mixture was heated until the solid was dissolved. By this
procedure the solvent boiling point becomes higher than that
under standard atmospheric pressure. The sample vial was
ꢀ
5H), 2.84–2.95 (m, 2H), 1.56 (s, 2H), 1.36 (s, 2H), 0.85 (s, 3H); 13
C
allowed to cool in air to 25 C, left for 1 h at this temperature,
and the tube was then turned upside-down. When the result at
this stage was a clear or slightly opaque gel, it was denoted
by ‘‘G’’.
NMR (75 MHz, TMS, ppm): 198, 146, 124, 102, 81, 78, 75 (2C), 68–
72 (12C), 43, 36, 17, 14; Elemental Anal. Found: C, 61.43; H, 6.23.
Calc. for C24H30FeO6: C, 61.29; H, 6.43%.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(cinnamoyl)but-
3-en-2-one (4o). Compound 4o was obtained by the reaction of
b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and cinnamaldehyde
(3o) (1.2 mmol, 0.15 ml) as a colorless solid: mp 208–210 ꢀC;
Yield: 0.24 g, (62%); Rf 0.70 (0.5 : 9.5 hexane–ethyl acetate); 1H
NMR (300 MHz, TMS, ppm): 7.43 (d, J ¼ 6.9 Hz, 2H), 7.21–7.32
(m, 4H), 6.81–6.96 (m, 2H), 6.24 (d, J ¼ 15.3 Hz, 1H), 4.20–4.45
(t, J ¼ 5.0 Hz, 1H), 3.92–3.97 (m, 1H), 3.59–3.75 (m, 3H), 3.42–
3.48 (t, J ¼ 8.4 Hz, 1H), 3.26–3.32 (t, J ¼ 9.8 Hz, 1H), 3.10–3.16
(m, 3H), 2.94–3.00 (m, 1H), 2.62–2.70 (dd, J ¼ 9.0 Hz, J ¼ 15.9
Hz, 1H), 1.47–1.54 (m, 2H), 1.29–1.37 (m, 2H), 0.80–0.85 (t, J ¼
7.4 Hz, 3H); 13C NMR (75 MHz, TMS, ppm): 197, 142, 141, 136,
130, 129, 128, 127, 126, 102, 80, 76, 74 (2C), 70, 68, 43, 36, 17, 14;
ESI-MS: Calc. for C22H28O6, 388.19; m/z found, 389.40 (M +
H+); [a]3D0 ꢁ12.7 (c 1.0 in DMSO).
Plant tissue culture method
Leaves were collected from well-grown plants, and washed
thoroughly under running tap water without damage to the
tissues. The leaves were surface-sterilized with (1%) sodium
hypochlorite for 20 min followed by (0.5%) mercuric chloride for
3 min and finally 3 to 5 times in sterilized distilled water under
aseptic conditions. The leaves were cut into small pieces and
inoculated into Murashige Skoog (MS) medium with different
concentrations of a-naphthaleneacetic acid (NAA) with addition
of 0.8% (w/v) agar and 3% (w/v) sucrose. The culture tubes were
ꢀ
capped with cotton plugs and autoclaved at 121 C for 15 min.
The cultures were incubated under a 16 h light/8 h dark cycle, and
were inspected regularly for the induction of callus. The callus,
once formed on the MS medium (with agar), was sub-cultured on
MS medium containing 0.75% 4i, which was used instead of agar.
All experiments were carried out in triplicate.
(E)-1-(4,6-O-Butylidene-b-D-glucopyranosyl)-4-(2-pyridyl)but-
3-en-2-one (4p). Compound 4p was obtained by the reaction of
b-C-glycosidic ketone (2), (1 mmol, 0.274 g) and 2-pyr-
idinecarboxaldehyde (3p) (1.2 mmol, 0.128 g) as a colorless
solid: mp 138–140 ꢀC; Yield: 0.19 g, (52%); Rf 0.61 (0.5 : 9.5
hexane–ethyl acetate); 1H NMR (300 MHz, TMS, ppm): 8.61 (d,
J ¼ 4.2 Hz, 1H), 7.70–7.75 (t, J ¼ 7.2 Hz, 1H), 7.45–7.55 (m,
2H), 7.25–7.29 (m, 1H), 7.14–7.20 (m, 1H), 4.43–4.47 (t, J ¼ 4.8
Hz, 1H), 4.01–4.06 (dd, J ¼ 3.9 Hz, J ¼ 9.6 Hz, 1H), 3.84–3.91
(m, 1H), 3.63–3.69 (t, J ¼ 8.7 Hz, 1H), 3.10–3.39 (m, 9H), 2.81–
2.89 (m, 1H), 1.51–1.55 (m, 2H), 1.27–1.39 (m, 2H), 0.80–0.85 (t,
J ¼ 7.4 Hz, 3H); 13C NMR (75 MHz, TMS, ppm): 198, 153, 150,
141, 138, 130, 125 (2C), 102, 81, 76, 75 (2C), 71, 68, 44, 36, 17, 14;
[a]3D0 ꢁ9.0 (c 1.0 in DMSO); Elemental Anal. Found: C, 62.72; H,
6.74; N, 3.60. Calc. for C19H25NO6: C, 62.80; H, 6.93; N, 3.85%.
Results and discussion
1. Synthesis and characterization of a,b-unsaturated-b-C-
glycosidic ketones
4,6-O-Butylidene-D-glucopyranose (1) was synthesized from
D-glucose.16 1-(4,6-O-Butylidene-b-D-glucopyranosyl)propan-2-
one (2) was synthesised from compound 1 in good yield by
following a procedure similar to that in the literature.17 Aldol
condensation of b-C-glycoside 2 with various aromatic and
heteroaromatic aldehydes (3a–q) was carried out at ambient
temperature in the presence of a catalytic amount of pyrrolidine
(30 mol%) (Table 1). To optimize the reaction conditions,
condensation was carried out using three organic solvents
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Table 1 Synthesis of a,b-unsaturated-b-C-glycosidic ketones 4a–q.
NMR data for products 4a–q
R
Time/h
4
Yield (%)
86
d(Ha)/ppm
d(Hb)/ppm
3J(Ha,Hb)/Hz a
b
—
a
b
6.78
16.8, —b
6
6
74
93
6.77
6.78
7.82
7.85
—b, 16.2
—b, 16.2
c
d
e
8
4
81
86
6.77
6.53
7.71
7.40
—b, 18.0
15.9, 16.2
f
12
10
73
76
6.55
6.42
7.96
7.54
16.2, —b
g
15.6, 15.6
h
i
5
7
93
84
6.79
6.96
7.51–7.67
7.47
16.2, —b
16.2, 16.5
j
5
76
79
91
6.55
6.80
6.65
7.40
7.49
8.43
16.2, 15.9
16.2, 16.2
12.3, 17.7
k
l
10
15
b
m
n
7
83
81
6.42
6.31
—
15.9, —b
12
7.46
14.7, 13.8
b
—
o
8
62
6.23
15.3, —b
b
b
p
q
8
8
52
55
—
—
—b, —b
b
—
6.79
—b, 16.2
a
Coupling constants as determined from the Ha and Hb peaks are given separately. b Peaks overlapped with aromatic protons.
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(chloroform, DCM and CH3CN) and also three different bases
(pyrrolidine, NaOH and triethylamine). Moderate to good yields
(70–90%) of a,b-unsaturated-b-C-glycosidic ketones (4a–q) was
obtained by using DCM in the presence of pyrrolidine as an
1
organocatalyst. H NMR spectrum of the a,b-unsaturated-b-C-
glycosides (4a–q) showed a signal around 6.5–7.0 ppm for Ha
(nearest the aromatic moiety) and 7.3–7.8 ppm for Hb (nearest
the carbonyl group) (Table 1). A similar observation has been
reported for ‘‘E’’-isomeric phenylpropenoid-b-D-glucopyrano-
side [7.83 (d, J ¼ 15.6 Hz), 6.57 (dt, J ¼ 15.6, J ¼ 5.6 Hz)].18
Moreover, the coupling constant (3JHa,Hb) observed shows the
existence of the ‘‘E’’-isomer (Table 1). Furthermore, the presence
of unsaturated carbonyl group is confirmed from the 13C NMR
spectrum [d(C]O) ꢂ197 ppm]. In these a,b-unsaturated-b-C-
glycosidic ketones (4a–q), a signal at around 6.5–8.0 ppm
corresponds to the aromatic protons and those for the protecting
group were observed around 2.5–4.0 ppm.
Fig. 2 ORTEP view of 4q.
2. Single-crystal XRD studies
In general, gelator molecules tend to pile up in a one-dimensional
direction, resulting in needle-like crystals or an amorphous solid,
and this makes it difficult to obtain a single crystal suitable for
XRD studies from organogelators, 2, 4e, 4i and 4p. However, we
were able to obtain single crystals suitable for XRD analysis
from the compounds 4b and 4q. In the absence of XRD data for
4e, 4i and 4p, characterization of 4b and 4q helps us to under-
stand the self-assembly, hydrogen bonding and p–p stacking in
organic solvents.
ORTEP views of compounds 4b and 4q are shown in Fig. 1
and 2 respectively. In molecule 4b, strong hydrogen bonding
interactions O(1)–H(1)/O(2), O(3)–H(3A)/O(1) and O(4)–
H(4A)/O5 result in the formation of chains of molecules with
overlapping phenyl moieties, as shown in Fig 3. The existence of
a b-anomeric form and an ‘‘E’’-isomeric form of 4b and 4q is
identified from the corresponding torsion angles [4b: H(11)–
C(11)–C(12)–H(12) ¼ 177.86ꢀ and C(1)–C(7)–C(8)–C(9) ¼
179.3(2)ꢀ] and [4q: H(1)–C(1)–C(2)–H(2) ¼ 179.08ꢀ and C(12)–
C(13)–C(14)–C(15) ¼ 178.53(18)ꢀ]. In the case of 4q, there are
strong H-bonding interactions between the free hydroxyl group
(C(2)–OH) and the pyridyl nitrogen (N) [C(2)–O(5)/N ¼
Fig. 3 H-bonding interactions and p–p stacking of 4b.
˚
(2.917(3) A] and the free hydroxyl group (C(3)–OH) and the
˚
(2.917(2) A)]. The
carbonyl (C]O) [C(3)–O(4)/O(6)
¼
hydrogen bonding and p–p stacking arrangement of 4q is shown
in Fig. 4. Crystallographic data for both 4b and 4q are given in
Table 2.
Fig. 4 H-bonding interactions and p–p stacking of 4q.
3. Gelation studies
From the test tube inversion method, we conclude that
compounds 4e, 4i and 4p, which bear the partially protected
sugar moiety, gelate three, four and two solvent molecules
respectively, of the eight solvents tried (Table 3). However, b-C-
glycoside 2, the precursor of 4a–q, also forms a gel. Of the
different derivatives, 4i (bearing the pyridyl group with a 5-
bromo substituent) is an excellent gelator [critical gelation
concentration (CGC): 4e (1.2%), 4i (0.75%) and 4q (1.0%)].
Compound 4i is insoluble in water, but when it is solubilized
Fig. 1 ORTEP view of 4b.
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Table 2 Crystallographic data for compounds 4b and 4q.
Parameters
4b
4q
Formula
C20H26O7
378.41
Monoclinic
P21
4.9409(7)
33.746(5)
5.7154(9)
90
101.967(9)
90
932.2(2)
1
1.348
0.102
293(2)
2268
9350
0.0376, 0.0962
1.080
Colorless prisms
C21H28N2O6
404.45
Triclinic
P1
4.8023(2)
9.3324(3)
12.6403(4)
70.868(2)
80.910(2)
77.056(2)
519.41(3)
2
1.293
0.095
293(2)
4287
11660
0.0430, 0.1160
1.098
Colorless needles
Formula weight
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
Z
Calculated density (Mg/m3)
m (mmꢁ1
T (K)
)
Fig. 5 1H NMR spectra (aromatic region) of 5i recorded using different
No. of unique reflections
No. of observed reflections
R1, Rw
GOF
Crystal habit
ratios of CDCl3–DMSO-d6: (a) 4 : 1; (b) 3 : 2; (c) 2 : 3; (d) 1 : 4.
5. NMR studies of gelator 4i
Of the b-C-glycosidic ketones, only 4i shows broad peaks in the
aromatic region, which may be due to the organogelating nature
of this compound influencing the broadening of the aromatic
protons. Broadening of NMR signals can be induced by various
processes, such as aggregation and ligand exchange. However,
definitive interpretation for such behavior requires careful anal-
ysis. In our case, further evidence for the positive influence of the
association of molecules was obtained from the 1H NMR studies
of 4i using different ratios of solvent in the mixture (Fig. 5).
From the 2D NMR studies (Fig. 6), the appearance of cross-
peaks at 8.3, 8.6 and 8.75 ppm in 1 : 4 CDCl3–DMSO-d6 (Fig. 6a)
indicates the coupling of Ha, Hb and Hc respectively, attributed
to the absence of aggregation. However, in 4 : 1 CDCl3–DMSO-
d6 Fig. 6b), the cross-peaks are absent, due to molecular aggre-
gation which results in the upfield shift of Ha. These results show
that the pyridyl moiety is engaged with molecular aggregation in
the gelation process.
Table 3 Solvents tested for gelation with 3, 4e, 4i and 4p.a
Solvent
2
4e
4i
4p
Water
Methanol
Ethanol
DMSO + H2O
DMF + H2O
Acetonitrile
Dichloroethane
Chloroform
S
I
I
S
G
G
S
S
S
G
G
S
S
G
S
G
G
G
G
P
I
G
G
S
S
S
G
G
P
S
I
I
a
G, S, P and I denote gelation, solvation, precipitation and insoluble
respectively.
using the minimum amount of DMF/DMSO, further addition of
H2O leads to the formation of a gel.
6. Microscopy studies
The self-assembled aggregates of gelators, 2, 4e and 4i were
studied by SEM (Fig. 7). Two types of aggregation mode were
observed: fibrous and lamellar.
4. Structural relationship
Of the 17 a,b-unsaturated-b-C-glycosidic ketones tested, 4e, 4i
and 4p form a gel. In order to understand why this is, the role of
the substituents on the aromatic moiety was examined.
Compound 4e (possessing 1,3-dioxane) was found to be a gela-
tor, whereas the presence of additional methylene group in the
dioxane (1,4-dioxane, 4j) was not. In addition to this, compounds
4i and 4p (possessing 5-bromo-3-pyridyl and 2-pyridyl groups,
respectively) were gelators, 4i being better than 4p. These
observations show that the presence of bromine in these
heterocyclic derivatives plays a vital role in gelation. However,
gelation was not observed with the 3-pyridyl derivative (4q).
Thus, even a small modification of molecule leads to a drastic
change in the gelation properties. From these results, we believe
that in addition to the hydrogen bonding and p-stacking, weak
interactions play a vital role.
Compounds 4e, 4i and 4p exhibit a fibrous aggregated
morphology, whereas the corresponding b-C-glycoside precursor
2 has a lamellar structure. Fibrous structures can incorporate
more solvent than the lamellar structures because of their greater
void volumes (300–500 nm in diameter) as is shown by SEM and
HRTEM. This seems to explain why 4e, 4i and 4p are better
organogelators than the precursor 2; indeed, similar observations
were reported with a sugar-appended azonaphthol gelator.19 In
the SEM image of gelator 2, the larger flat tubules are composed
of fibrous network as shown in Fig. 7a. The bulk structure of this
organogelator 2 is lamellar, and the high-magnification image
shows a dense fibrous network (Fig 7a,b). Compounds 4e, 4i and
4p did not show a lamellar structure, but they did have three-
dimensional fibrous networks with a breadth of less than 100 nm
(Fig. 8).
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Fig. 7 SEM images (with different magnification) of: 2 in CHCl3 [(a) 10
mm; (b) 500 mm]; 4i in MeOH [(c) 5 mm; (d) 1 mm]; and 4e in MeOH [(e) 3
mm; (f) 3 mm].
Fig. 6 1H–1H COSY spectrum (aromatic region) of 4i recorded using
different ratios of CDCl3–DMSO-d6: (a) 1 : 4; (b) 4 : 1.
7. Thermal studies
Fig. 8 HRTEM images of: (a) 4e in EtOH; (b) 4i in DMSO–H2O; (c) 4p
in EtOH (1 mm); (d) 4p in EtOH (500 nm).
To study the thermal properties of the organogelators 4e and 4i,
we obtained data from DSC (Fig. 9). The melting peak of 4e is
199.4 ꢀC in the solid phase with DH ¼ 95.12 J/g, and 197.4 ꢀC in
the gel phase with DH ¼ 9.67 J/g. In comparison, the melting
peak of 4i is 186.8 ꢀC in the solid phase with DH ¼ 13.94 J/g, and
92.5 ꢀC in the gel phase with DH ¼ 284.3 J/g. In general, the DH
value for the gel is dependent on the nature of the solvent
(molecular weight, hydrophilicity and density) – a similar type of
observation has been reported in the literature.20
The peak at 77.6 ꢀC in the case of 4e and 4i in gel phase is
due to the liberation of solvent (i.e., ethanol). The peaks
observed at 93.9 ꢀC in solid phase and at 74.3 ꢀC in gel phase of
4i are due to liberation of water (moisture) and solvent (ethanol)
respectively. The boiling point (transition temperature) of
ethanol is 78.5 ꢀC, whereas the transition temperature of
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Fig. 10 Callus formation observed from leaf explants: (A) MS medium
with agar; (B) sterilized MS medium with 4i; (C) unsterilized MS medium
with 4i; (D) sterilized MS medium with 4i (34th day).
Conclusions
A series of D-glucose-based soft materials were obtained in the
form of self-assembled nanofibers using facile synthetic proce-
dures, and characterised by 1D and 2D NMR experiments. Even
small modifications of the molecules led to the drastic changes in
the gelation properties. Gels result even at low concentrations
(critical gelation concentration 0.75%), and structural charac-
terization clearly shows the presence of a three-dimensional
fibrous network. DSC studies of these soft materials show that
they are thermally stable. Of the various gelators synthesised,
(E)-1-(4,6-O-butylidene-b-D-glucopyranosyl)-4-(5-bromo-3-pyr-
idyl)but-3-en-2-one (4i) was found to be an ideal alternative
gelling agent for plant tissue culture. Further studies in this area
are in progress.
Fig. 9 Differential scanning calorimetry (DSC) of (a) 4e and (b) 4i.
the solvent present in the gelator is lower, due to a gelator–
solvent interaction rather than a solvent–solvent interaction.
Thus our study indicates that organogelators 4e and 4i have
good thermal stability and can form solid-like gels in organic
solvents.
8. Demonstration as a soft material for plant tissue culture
En masse, undifferentiated parenchymatous cells are known as
a callus. Their induction from leaf explants was observed on MS
medium with 0.8% agar and different concentrations of NAA
(0.2–0.5 mg/L). Callus initiation was observed from the 9th day
onwards, and maximum callus was obtained on MS medium
supplemented with 0.5 mg/L of NAA. Subculture was performed
on the 28th day after initial culture, with the medium having the
same composition as MS medium except for the absence of agar
(Fig. 10). The subculture was split into two groups; first group
used sterilized medium and second used unsterilized medium. It
is interesting to note that neither sterilized nor unsterilized media
were contaminated by microorganisms, indicating that 4i has
antimicrobial activity. The gelator used in the media did not
harm the growth of the callus. From these results, we conclude
that (E)-1-(4,6-O-butylidene-b-D-glucopyranosyl)-4-(5-bromo-3-
pyridyl)but-3-en-2-one (4i) can be used as a antimicrobial gel for
solidifying media.
Acknowledgements
The authors acknowledge DST (New Delhi, India) for financial
support. The authors also acknowledge Prof. C. P. Rao
(Department of Chemistry, IIT Bombay, Mumbai) for valuable
discussions. T. M. thanks DST for allowing use of the FIST
facility at the Department of Organic Chemistry, University of
Madras, Chennai, India.
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