Synthesis and thermal properties of rigid oxa-bridged-containing dimers and tetramers

Article abstract of DOI:10.1016/j.tet.2010.09.003

Several symmetrical, linear dimers and some star-like tetramers were prepared using easily accessible oxa-bridged derivatives as central rigid cores. In order to produce new liquid crystalline materials various mesogenic/promesogenic groups, containing bi

Full text of DOI:10.1016/j.tet.2010.09.003

Tetrahedron 66 (2010) 8745e8755
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Synthesis and thermal properties of rigid oxa-bridged-containing dimers
and tetramers
, ,
,
Faiz Ahmed Khan a y, Karuppasamy Parasuraman a, Bertrand Donnio b
*
*
a Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
b Institut de Physique et Chimie des Matériaux de StrasbourgdIPCMS, CNRS-Université de Strasbourg (UMR 7504), 23 rue du Loess BP 43, 67034 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 April 2010
Received in revised form 1 September 2010
Accepted 1 September 2010
Available online 8 September 2010
Several symmetrical, linear dimers and some star-like tetramers were prepared using easily accessible
oxa-bridged derivatives as central rigid cores. In order to produce new liquid crystalline materials various
mesogenic/promesogenic groups, containing biphenyl-4,40-diol, 4,40-(diazene-1,2-diyl)diphenol and
cholesterol sub-moieties, were connected through oxy-ethylene linkers via an ester linkage, in the end-
on fashion. Their thermal behaviour and mesomorphic properties were investigated by differential
scanning calorimetry (DSC), polarized optical microscope (POM), and variable temperature small-angle
X-ray diffraction (XRD) methods. While essentially all the biphenyl-4,40-diol- and 4,40-(diazene-1,2-diyl)-
based dimeric derivatives exhibited smectic liquid crystalline mesophases, the nature of which are
mesogenic groups dependent, none of the cholesterol-containing compounds and tetramers were
mesomorphic.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
(Fig. 1, 2),6 pseudo-rigid molecular framework like bicyclo [2.2.2]
octane (Fig. 1, 3)7 and cubane (Fig. 1, 4),8 or even carborane de-
Liquid-crystalline linear oligomers (dimers, trimers, etc.), as low
molecular-weight model compounds of linear main-chain liquid
crystal polymers, discovered during the 1980s, still remain the focus
of recent research.1 Liquid crystal dimers are molecules composed of
two conventional mesogenic groups linked together via spacers of
variable chemical natures. The dimers can be made symmetrical or
unsymmetrical based on whether the two mesogenic end-units are
identical or different.1,2 Moreover, depending on the topology of
attachment of the lateral mesogenic entities to the spacer (end-on or
side-on types), dimers can approximate I-shape (two end-on
mesogens), T-shape (an end-on and a side-on mesogens) or even H-
shape (two side-on mesogens), leading to versatile molecular
structures and a wide variety of supramolecular arrangements
whose mesomorphic properties can be subtly modulated.3 Several
families of liquid-crystalline dimers have been prepared until now
with a great choice of spacers, the most commonly used being
a simple flexible alkyl chain (Fig. 1, 1).1,3,4 With the aim to modulate
and control thermal and physico-chemical properties, mesogenic
dimers incorporating siloxane fragments,5 oxy-ethyelene and per-
fluorinated groups,3 cyclic non rigid moieties, such as cyclohexane
rivatives (Fig. 1, 5)9 have also been synthesized. Functional liquid
crystal dimers successfully integrating active moieties, such as
Mesogenic Unit
Mesogenic Unit
Spacer
* Corresponding authors. E-mail addresses: faiz@iitk.ac.in (F.A. Khan), bdonnio@
ipcms.u-strasbg.fr (B. Donnio).
y
Present address: Department of Chemistry, Indian Institute of Technology Hy-
Fig. 1. Schematic representation of a selection of liquid-crystalline dimers with flexible
derabad, Ordnance Factory Estate, Yeddumailaram 502205, Andhra Pradesh, India.
and rigid spacers.
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.09.003

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F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
imidazolium cation,10 metallic fragments,11 ferrocene (Fig. 1, 6),12 or
fullerene (Fig.1, 7)13 have also been reported. Incorporation of a rigid
core as a spacer is believed to provide the necessary anisotropic
interactions for the occurrence of the liquid crystalline phase and to
a large extent, to dictate the properties of the bulk materials.9b
Recently, we have demonstrated an easy access to sterically
constrained oxa-bridged derivatives.14 We thought of utilizing oxa-
bridged derivatives as the rigid central spacers to construct novel
type of liquid-crystalline dimeric compounds and rationalize and
analyze accordingly their thermal behaviours and optical proper-
ties. Our strategy consisted of incorporating the oxa-bridged de-
rivatives 8, 9 and 10 (Fig. 2) into liquid-crystalline dimers and
tetramers by direct esterification methods with suitable mesogens.
MeO
E
H
OMe
E
H
MeO OMe
DIBAL-H
CH2Cl2
O
O
O
HO
OH
H
H
H
H
-78 0 oC - rt, 3 h
H
E
H
E
OH
HO
O
MeO OMe
MeO OMe
10
11
E = COOMe
py, Ac2O, CH2Cl2
0 oC - rt, 9 h, 72%
(2-steps)
MeO OMe
AcO
O
OAc
H
H
H
H
OMe
MeO
HO
MeO OMe
O
OAc
O
AcO
O
O
OH
HO
O
OH
H
H
H
H
MeO OMe
12
8
9
Scheme 1. Synthesis of tetrol 10 and its acetyl homologue 12.
OH HO
H
H
MeO
MeO
OMe
OMe
techniques (DCC/DMAP4c,16 or via acid chloride8,9b) proved to be
inefficient. When the coupling reaction between oxa-bridged diol 8
with 4-dodecyloxybenzoic acid was performed in presence of the
DCC/DMAP (room temperature, 37 h), a mixture of mono-esterified
product and the desired disubstituted product was obtained in 38%
yield each. Surprisinglyhowever, underthesamereaction condition,
coupling reaction between 8 and 2(40-(dodecyloxy)biphenyl-4-
yloxy)acetic acid afforded only the corresponding mono-esterified
product in 30% yield. Finally, when the esterification reaction
was performed between 2(40-(dodecyloxy)biphenyl-4-yloxy)acetyl
chloride and oxa-bridged diol 8 in presence of DMAP, only 5% of the
desired product 14 and 36% of the mono-esterified product 13 were
recovered (Scheme 2).
The mixed anhydride of the mesogenic acid, obtained by the
treatment of 2(40-(dodecyloxy)biphenyl-4-yloxy)acetic acid with 2-
methylpropyl chlorocarbonate in triethylamine (room tempera-
ture, 5 h) in the presence of the diol 8, at room temperature first for
6 h, and then at reflux temperature (THF) for 10 h, did not react to
yield the corresponding dimer 14.
O
O
H
H
OH HO
10
Fig. 2. Oxa-bridged core-derivatives.
2. Results and discussion
2.1. Synthesis of oxa-bridged compounds
The mono-oxa-bridged diol 8 was obtained by reducing di-
methyl ester derivative of oxa-bridged compound 914 with sodium
borohydride in THF under reflux condition, a convenient method
reported from our laboratory for related system.15 A similar ap-
proach for the preparation of bis-oxa-bridged tetrol 10, starting
from the corresponding tetramethyl ester 11,14 furnished an iso-
meric mixture.14 The tetrol 10 was finally obtained by treating 11
with DIBAL-H in dichloromethane (Scheme 1). For characterization
purpose, the highly polar tetrol 10 was converted into its corre-
sponding tetraacetate 12 in 72% overall yield.
2.2.2. Esterification of oxa-bridged diacid
9
using 2,4,6-tri-
chlorobenzoyl chloride. The poor efficiency of the esterification of 8
led us to consider the oxa-bridged diacid 9 instead. Following
a literature precedent from cubane chemistry,8 we first attempted
esterification via the acid chloride route. The acid chloride of 9,
prepared in situ from (COCl)2 in Et3N/CH2Cl2 at 0 ꢀC and then at
room temperature for 5 h, reacted at reflux in pyridine/CH2Cl2 for
2.2. Synthesis of dimers
2.2.1. Esterification of diol 8. Initial efforts to prepare ester de-
rivatives of 8e10 with known mesogenic groups using conventional
Scheme 2. Synthesis of dimer 14.

F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
8747
2 h with 1-octanol to give the corresponding diester in moderate
yield (45%), whilst the same reaction with 40-(dodecyloxy)bi-
phenyl-4-ol failed.
The dimers with rigid oxa-bridged spacer were finally
obtained by treating the mixed anhydride formed from diacid 9
and 2,4,6-trichlorobenzoyl chloride (Yamaguchi reagent,17 widely
used for the macrocyclisation in multistep synthesis) with the
appropriate alcohol counterpart. As depicted in Scheme 3, the
Liquid crystals derived from naturally occurring cholesterol
represent an exemplary and emerging class of chiral liquid crystals,
which may find practical and useful utility in various applications.18
From literature, cholesterol moiety containing molecules generally
show chiral nematic (or cholesteric) mesophase.3,19 We synthesized
cholesterol-based symmetric dimers with the rigid oxa-bridged
spacer 9. As illustrated in Scheme 5, mixed anhydride 15 treated
with cholesterol, or derivatives having two or three oxy-ethylene
units, afforded good to excellent yields (22, 23 and 24).
oxa-bridged diacid
9 was first treated with the 2,4,6-tri-
chlorobenzoyl chloride in presence of triethylamine in THF for
2 h at room temperature and the resulting crude mixed anhy-
dride 15 was exposed to 2-(40-dodecyloxy-biphenyl-4-yloxy)-
ethanol in presence of DMAP to obtain the desired product 16 in
excellent yield. Increasing the number of oxy-ethylene units
(eCH2CH2eOe) did not affect the yield of the products (17 and
18, Scheme 3). Similarly, reaction of mixed anhydride 15 with
various 2[4-(4-dodecyloxy-phenylazo)-phenoxy]-ethanol oxy-
ethylene homologues afforded the desired materials, 19, 20 and
21, in good yields (Scheme 4).
2.3. Synthesis of tetramers
Using the same procedure, three tetramers derived from bis-
oxa-bridged tetrol 10 were successfully prepared. The mixed an-
hydrides of 2-(40-(dodecyloxy)biphenyl-4-yloxy)acetic acid, 10-
(biphenyl-4-yloxy)decanoic acid and cholesterol-derived acid with
2,4,6-trichlorobenzoyl chloride were treated with bis-oxa-bridged
tetrol 10 in presence of DMAP to afford the tetramers 25, 26 and 27
(Fig. 3), respectively, in good yields.
Scheme 3. Synthesis of dimers 16e18 biphenylene-based mesogens.
MeO OMe
HO
O
O
O
N
n
N
OR
O
O
O
N
O
O
N
n
n
RO
N
15
N
OR
DMAP, THF, rt, 3 h
n=1: ; 77%
19
20
R = C12H25
n=2: ; 87%
n=3: 21; 85%
Scheme 4. Synthesis of dimers 19e21 azobenzene-based mesogens.
Scheme 5. Synthesis of dimers 22e24 cholesterol-based mesogens.

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F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
3. Mesomorphic behaviour
Having successfully synthesized various symmetrical oligomers
bearing liquid crystalline side-groups, their physico-chemical
properties were investigated. The thermal behaviour was analyzed
by differential scanning calorimetry (DSC) with heating/cooling
rate of 5 ꢀC/min, optical textures were observed using polarized
optical microscope (POM) at variable temperatures and the struc-
tures of the mesophases analyzed by temperature-dependent
small-angle X-ray diffraction analysis. The transition temperatures
and the phase behaviour of the nine symmetrical dimers and three
tetramers are collected in Table 1.
In the case of the dimers three different mesogenic units were
linked via an oxy-ethylene spacer with increasing number of seg-
ments. Clearly, it became quickly apparent that the mesophase
behaviour of the dimers was strongly dependent on the mesogenic
group nature as well as on the length of the connecting group.
3.1. Dimers-based 40-(dodecyloxy)biphenyl-4-ol (16, 17, 18)
The three dimeric systems having the oxa-bridged core linked
on both side with 40-(dodecyloxy)biphenyl-4-ol were investigated
as a function of the spacer length. Compound 16, with one oxy-
ethylene segment, was obtained as a colourless floppy solid at room
temperature. DSC analysis revealed (Fig. 4a) a complex thermal
behaviour. On heating, an endothermic peak centred at 57 ꢀC, fol-
lowed by small events (corresponding to successive crystallizations
and meltings between approximately 75e80 ꢀC and 70e100 ꢀC,
Fig. 3. Tetramers synthesized from oxa-bridged spacer 10.
Table 1
Thermal properties of oxa-bridged dimers 16e21 and tetramers 25e27a
Entry
Compound
Heating
Cooling
1
2
3
4
5
6
7
8
16
17
18
19
20
21
22
23
24
25
26
27
Cr 57 (50.6) M w70 (ꢁ)b Cr0 w80 (ꢁ)b Cr00 w85 (ꢁ)b Cr000 100 (31.3) I
I 74 (ꢁ10.7) Cr0 37 (ꢁ3.9) Cr
I 82 ꢁ9.5) SmA 59 (ꢁ24.3) Cr
I 74 (ꢁ8.6) SmA 62 (ꢁ26.6) Cr
I 119 (ꢁ)d SmC 78 (ꢁ4.1) Cr0 58 (ꢁ1.3) Cr
I 90 (ꢁ)d SmC 74 (ꢁ5.5) Cr0 56 (ꢁ15.5) Cr
I 70 (ꢁ)d SmC 64 (ꢁ)c SmF/I 60(ꢁ22.1)e Cr
I 189 (ꢁ39.4) Cr
Cr 66 (26.1) SmA 84 (10.1) I
Cr 64 (24.5)c SmA 75 (8.4) I
Cr 109 (38.9) SmC 129 (1.9) I
Cr 71 (ꢁ)c Cr0 76 (29.6)e SmC 95 (3.6) I
Cr 60 (5.4) SmF/I 72 (ꢁ)c SmC 73 (20.9)e
I
Cr 258 (76.3) I
Cr 169 (36.3) I
I
I 143 (ꢁ18.2) Cr
9
I
10
11b
12
Cr 177 (18.6) I
Cr 90 (22.3) I
Cr 192 (21.9) I
I 125 (ꢁ24.5) Cr
I
I 141 (ꢁ7.1) Cr
a
Phase transition peak temperatures (ꢀC) and associated enthalpy changes (kJ molꢁ1 in parentheses). Cr, Cr0, Cr00, Cr000¼crystalline phases, SmA/C/F/I¼smectic A/C/F/I phases,
M¼unidentified phase, I¼isotropic.
b
Not measured.
c
Unresolved peaks.
d
Transition was observed under microscope but too weak to be detected by DSC on cooling.
Cumulated enthalpies: CreCr0 for 20 and SmFeSmC for 21.
e
Fig. 4. DSC traces of (a) compound 16 (b) compound 17 and (c) compound 18.

F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
8749
respectively) and a broad peak at 100 ꢀC were observed. On cooling
from the isotropic liquid phase, only two exothermic peaks were
seen at 74 ꢀC and at 37 ꢀC, respectively. POM carried out during
heating, despite the observation of remnant birefringence when
light pressure was applied onto the glass cover slip, was not suffi-
ciently relevant to properly identify this complex phase behaviour
(Fig. 5a). The high value for the melting enthalpy transition at ca.
100 ꢀC (Table 1), suggesting a transition between a high-ordered
structure into the isotropic liquid, and the important supercooling
effects were however strong indications of the absence of
mesomorphism.
of the mesogenic arms within the sub-layers, likely with a hexago-
nal symmetry (vide infra). Considering a density close to 1 for par-
tially molten 16, the molecular area (obtained from the ratio of the
molecular volume20 by the smectic periodicity, A¼V/d¼1740 Å3/
62.1 ż28 Å2) is fairly compatible with the transverse area of one
biphenylene unit calculated in an orthogonal-like lamellar phase
with a hexagonal in-layer ordering (hexagonal cross-section area:
4.6ꢂ4.6ꢂ2/O3¼24.4 Å2), and suggests that the dimer adopts a fully
stretched conformation with the mesogen units on each side of the
oxabridge, parallel to the layer normal. At ca. 70 ꢀC, the sample melts
and immediately recrystallizes (T¼75 ꢀC) before melting again at ca.
Fig. 5. POM texture of (a) compound 16 at 68 ꢀC (b) compound 17 at 79 ꢀC and (c) compound 18 at 72 ꢀC.
Compounds 17 and 18 (with two and three oxy-ethylene frag-
ments spacers, respectively, between the oxabridge and the meso-
genic units) were also obtained as colourless solids. DSC traces
(Fig. 4b and c) showed exothermic transitions with melting peaks
temperatures at 66 and at 64 ꢀC and clearing peaks temperatures at
84 ꢀC and 75 ꢀC for 17 and 18, respectively. These transitions were
fully reversible on cooling, with little supercooling effects. POM
textures observed on cooling were consistent with the presence of
a fluid mesophase (Fig. 5b and c), and the observation of needle-like
defects and large homeotropic zones were reminiscent of orthogo-
nal smectic phase (e.g., SmA or SmB type). Continuing cooling fur-
ther, the mesophases reversibly transformed into crystalline solids.
The complex thermal behaviour of 16 was analyzed by X-ray
diffraction at various temperatures and was found to be consistent
with DSC measurements. Between room temperature and up to
a temperature <60 ꢀC, a poorly crystallized state was deduced
(likely corresponding to locally entangled amorphous and semi-
organized domains), as evidenced by the detection of broad diffu-
sions in the small-angle part and sharp reflections in the wide- and
mid-angle parts (a few intense, not-too-sharp diffraction peaks at
11.0, 4.7, 4.3 and 3.7 Å were measured, Fig. 6, T¼50 ꢀC). When
heating was continued above 60 ꢀC, the X-ray pattern was modified,
indicating the transformation of the structure into a more organized
intermediate phase. The diffractogram recorded at T¼60 ꢀC (Fig. 6)
exhibited two sharp and intense small-angle peaks, with the re-
ciprocal spacings in the ratio 1:2 (d001¼62.2 and d002¼31.0 Å),
characterizing a lamellar morphology with an average interlayer
periodicity d¼62.1 Å, of the same magnitude of the length of the
molecule in its stretched conformation (ca. 60e65 Å). In addition,
two broad wide-angle diffusions (8.0e9.0 and 4.6 Å) were also
detected, corresponding, respectively, to weak molecular fluctua-
tions and lateral short-range ordering of the molten chains and
biphenylene units. The deconvolution of the peak in the wide-angle
at 4.6 Å revealed that it consisted in fact of the superimposition of
two signals, namely a broad one that characterizing the molten
chains, and a weaker but sharper peak (h0¼4.6 Å, see peak decon-
volution in Fig. 6), reflecting a supplementary short-range ordering
85e90 ꢀC into another metastable phase, immediately transforming
into the isotropic liquid at ca. 100 ꢀC (Fig. 6). On cooling from the
isotropic liquid, no sign of such a behaviour was detected, nor was
a mesophase evidenced by XRD, and only crystalline phases formed
below 74 ꢀC. This complicated thermal behaviour is reversible on
subsequent heat-cool cycles (after annealing at room temperature
for several hours), and therefore the mesophase detected for 16 can
be qualified as re-entrant or transient. Such behaviour is quite rare,
although it has been observed sometimes with few compounds
possessing complicated molecular structures or for compounds
whose molecular structures change reversibly with temperature.
Fig. 6. Diffractograms of compound 16 recorded at various temperatures (T¼50, 60,
70 ꢀC); deconvolution of the wide-angle peak into the diffuse Gaussian scattering halo
(short-range order of the chains in their melted conformation) and sharp peak cor-
responding to some local in-layer ordering.

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F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
A possible explanation may be rather the high viscosity of the sys-
tem combined with the enhancement of the segregation at the
molecular scale: the dimers likely first self-organize into a smectic
mesophase, and then due to this specific lamellar organization,
crystallization is favoured. As shown in the diffractogram at T¼70 ꢀC
(Fig. 6), a lamellar crystal phase can be detected with sharp peaks at
44.4, 22.1, 14.8 Å in the ratio 1:2:3 (and several sharp peak in the
wide-angle part, corresponding to chains and mesogens crystalli-
zation). The reduction of the layer periodicity in this case is attrib-
uted to interdigitation and/or tilt of the mesogenic groups.
The smectic behaviour was confirmed for both compounds 17
and 18 by XRD (Figs. 7 and 8): two sharp small-angle peaks, with
the reciprocal spacings in the ratio 1:2 (d001¼57.7 and d002¼28.94 Å
for 17 and d001¼65.57 and d002¼32.57 Å for 18), characterizing
a lamellar morphology with interlayer periodicities d¼57.8 and
65.35 Å, respectively, and two broad wide-angle diffusions (8.0e9.0
and 4.6 Å), corresponding, respectively, to weak molecular fluctu-
ations (oxa-bridged core) and lateral short-range ordering of the
molten chains and biphenylene units. Again, the molecules are
likely in a fully stretched conformation and lie quasi parallel to the
smectic layer normal, as supported by the molecular areas values
(A¼32.6 and 31.1 Å2, respectively), in agreement with a disordered
smectic phase with orthogonal symmetry (i.e., SmA).
Fig. 8. Diffractogram of compound 18 (at T¼68 ꢀC on cooling, SmA phase); the diffuse
scattering halos are fitted by a Gaussian function in agreement with the short-range
order of the chains in their melted conformation.
still mesomorphic, its mesophase stability temperature range has
been substantially reduced by almost 10 ꢀC, when compared to 17.
These results emphasize the strong mesomorphism sensitivity to
the extended coupling between both the central core and meso-
gens. The mesophases are formed by the lateral two-dimensional
registry of the dimeric molecules in cylindrical conformation, with
the mesogenic groups oriented quasi parallelly to the layer normal.
Alternatively, they may also adopt the shape of a hairpin, and in this
case would form a head-to-head pair to generate the cylindrical
shape; note that X-ray experiment does not allow distinction
between both conformations.
3.2. Dimers-based 4{[4-(dodecyloxy)phenyl]diazenyl}phenol
(19, 20, 21)
All the compounds of this series, 19e21, having the oxa-bridged
core linked at both sides with 4{[4-(dodecyloxy)phenyl]diazenyl}
phenol with one (19), two (20) or three (21) oxy-ethylene chain
spacers, were obtained as a yellowish floppy solid at room tem-
perature. For compound 19, the DSC analysis revealed (Fig. 9a) two
endothermic peaks at 109 ꢀC and at 129 ꢀC. On cooling from the
isotropic phase, two exothermic transitions were observed at 78 ꢀC
(for the crystallization) and at 58 ꢀC (for a transition from one
crystal to another crystal phase); the isotropic-mesophase transi-
tion was not detected by this technique. Again POM analysis alone
was not relevant for phase identification, although, a birefringent
fluid mesophase was observed. DSC traces of 20 showed three
endothermic transitions at 71, 76 (although both peaks are fused
and not resolvable) and at 95 ꢀC (Fig. 9b). On cooling from the
Fig. 7. X-ray diffraction patterns of compound 17 (at T¼65 ꢀC on cooling, SmA phase);
the diffuse scattering halos are fitted by a Gaussian function in agreement with the
short-range order of the chains in their melted conformation.
Thus, to conclude, the lengthening of the pegylated segment
from one to two units contributes to a substantial enhancement of
the mesophase stability in this series of oxa-bridged dimers. Whilst
16 shows a complicated thermal behaviour, 17 in contrast exhibits
an enantiotropic mesophase between 66 and 84 ꢀC. Whereas 18 is
Fig. 9. DSC traces of (a) compound 19, (b) compound 20 and (c) compound 21.

F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
8751
isotropic phase, exothermic peaks at 74 ꢀC and at 56 ꢀC were ob-
served; again isotropization at 90 ꢀC was not detected. By POM
analysis, a texture reminiscent to that of the SmC-type phase was
recognized (at around 79 ꢀC on heating): locally some schlieren-
like as well as broken focal-conic fan textures were observed si-
multaneously on cooling from the isotropic phase between clean
glass substrates.21 Finally, DSC analysis of 21 (Fig. 9c) showed two
transitions, a sharp endothermic peak centred at 60 ꢀC and a broad
peak at ca. 73 ꢀC. On cooling, two major exothermic peaks could be
recorded, at ca. 70 ꢀC and around 64 ꢀC. A birefringent and fluid
texture was observed by POM, although totally irrelevant for
mesophase assignment.
XRD experiments confirmed in all cases the presence of smectic
phases, expected on the basis of the molecular shape of the terms of
this series. Diffractogram of compound 19 (Fig. 10a) recorded on
cooling at 104 ꢀC, exhibits a single sharp reflection in the small-
angle (d¼d001¼30.9 Å), corresponding to the smectic periodicity,
were detected by XRD for compound 21. The high-temperature
phase (recorded on cooling at 68 ꢀC, Fig. 10d) corresponds to a dis-
ordered tilted smectic phase, SmC (d001¼54.7 and d002¼27.26,
hch¼4.55 Å, A¼38.9 Å2). Whereas the low-temperature phase,
whilst still smectic (d001¼55.2 and d002¼27.8, d003¼18.3, h0¼4.2,
hch¼4.55 Å, A¼38.9 Å2), possesses additional local in-layer order-
ing: the presence of the intense reflection h0 suggests hexatic type
of organization within the layer, and since the molecules are still
tilted within the layer, the mesophase can be assigned to as SmF or
SmI phase (tilted hexatic phase), not yet distinguishable.19,22
From the above experiments, the three samples exhibit liquid
crystalline smectic phases, all assigned as SmC phase, with an ad-
ditional SmI/F phase in one case. On increasing the oxy-ethylene
chain length from 1 to 3 segments (19/21), the transition tem-
peratures were considerably affected, with a net decrease of both
the melting and the clearing temperatures from 109 to 60 ꢀC and
from 129 ꢀC to 73 ꢀC, respectively, and thus leading concomitantly
Fig. 10. X-ray diffraction patterns of (a) compound 19 (at T¼104 ꢀC on cooling, SmC phase), (b) compound 20 (at T¼70 ꢀC on cooling, SmC phase), and (c) compound 21 (at T¼70 ꢀ
on heating, SmF/I phase) and (d) compound 21 (at T¼68 ꢀC on cooling, SmC phase); Gaussian fits for the molten chains signals, and peak deconvolution.
C
and the broad scattering at ca. 4.6 Å corresponding to the molten
aliphatic chains. The periodicity being much smaller than the mo-
lecular length (ca. 60e65 Å), the mesogenic arms ought to be highly
tilted with respect to the layer normal. Indeed, a rough estimation of
the molecular volume considering a density of 1 g cm3, gives a large
value of the cross-section area of 59 Å2, implying a substantial tilt of
the rigid cores in order to be able to compensate the rather large
expansion of the transverse chain area, in agreement with a SmC-
type mesophase. Either, the entire molecule, in its fully stretched
conformation, is tilted within the layer, or the dimer adopts a bow-
shape.1 As for compound 20, two sharp small-angle reflections in
the ratio 1:2 (d001¼46.58 and d002¼23.27 Å) of a lamellar structure
along the broad diffusion at ca. 4.6 Å were detected in the dif-
fractogram (Fig. 10b), confirming again the smectic nature of the
mesophase: the periodicity, 46.6 Å, smaller than the molecular
length, conveys to a SmC phase. As above, the cross-section area is
large, with a value of 42.5 Å2, which suggests important tilt of the
anisotropic end-groups within the layers. Finally, two mesophases
to a substantial diminution of the mesophase temperature range. In
all cases, a substantial tilt of the rigid cores, necessary to com-
pensate the rather large expansion of the transverse chain area, has
been deduced from XRD experiments. Such compensation may be
achieved in two ways: either, the entire molecule, in its fully
stretched conformation (or head-to-head pairs), is tilted within the
layer, or it adopts the shape of a bow. As above, the mesophases are
formed by the lateral two-dimensional registry of the dimers but
with the mesogenic groups tilted with the layer normal.
3.3. Dimers-based on cholesterol (22, 23, 24)
The oxa-bridged compounds derived from cholesterol only
showed one transition peak by DSC, at 258 ꢀC for 22, and 169 ꢀC for
23; 24 was obtained at a room temperature oil. The unexpected
absence of mesomorphic behaviour was confirmed by POM anal-
ysis. Moreover, on increasing the spacer length, the crystalline
phase stability is drastically reduced. A possible explanation for the

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F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
lack of liquid-crystalline behaviour may be connected to the bulk-
iness of the cholesterol substituents, which hinders the ideal
packing of the molecules into layers.
over potassium hydroxide pellets. Methanol and ethanol were
refluxed and distilled over magnesium turnings and stored over 4 Å
molecular sieves. Distilled water was used for aqueous work-up.
Differential scanning calorimeter (DSC) was recorded in Per-
kineElmer DSC 6. The identification of the mesophases and the
transition temperatures of the compounds were determined using
a polarizing microscope (Leica DM2500P) in conjunction with
a programmable hot stage (Linkam THMS600). The XRD patterns
were obtained with two different experimental set-ups. In all cases,
3.4. Tetramers (25, 26, 27)
DSC analysis of the three tetramers (25e27) showed a single
broad peak at 177 ꢀC (25), 90 ꢀC (26) and 192 ꢀC (27), with strong
supercooling for 25 (125 ꢀC) and 27 (141 ꢀC). The absence of mes-
ophase was corroborated by POM, for which no specific texture,
although reminiscent of crystalline or highly ordered arrangements
(sharp defects), below the clearing temperatures were revealed.
Here the low flexibility of the core, the mismatch between trans-
verse cross-sections of the various molecular parts and in-
appropriate spacer lengths between the mesogenic side-arms and
central multivalent cores may be responsible for the loss of liquid
crystalline properties.
a linear, monochromatic Cu K
a
1 beam (
l
¼1.5405 Å) was obtained
using a sealed-tube generator (900 W) equipped with a bent quartz
monochromator. In the first set, the transmission Guinier geometry
was used, whereas a DebyeeScherrer-like geometry was used in
the second experimental set-up. The crude powder was filled in
Lindemann capillaries of 1 mm diameter and 10 mm wall thickness.
5.1.1. Compounds 13e14. To a stirred suspension of 2-(40-(dodecyl)
biphenyl-4-yloxy)acetic acid (41 mg, 0.099 mmol) in benzene
(2 mL) were added oxalyl chloride (0.5 mL) and a drop of DMF. The
reaction mixture was stirred at room temperature for 12 h. The
volatiles were removed under vacuum and crude acid chloride was
dissolved in dry dichloromethane (3 mL) and added to the mixture
of diol 8 (10 mg, 0.038 mmol), triethylamine (0.15 mL) and DMAP
(0.5 mg) in dichloromethane (2 mL). The reaction mixture was
stirred for 42 h at room temperature under argon atmosphere and
then quenched by adding distilled water (5 mL). The solution was
extracted with dichloromethane (2ꢂ4 mL) and the combined or-
ganic layers were washed with distilled water (1ꢂ2 mL), brine so-
lution (2 mL) and then dried over anhydrous sodium sulfate,
filtered and concentrated afforded the crude product, which on
purification by silica gel column chromatography using 10e40%
ethyl acetate/hexane afforded compound 13 (9 mg, 36%) and
compound 14 (2 mg, 5%). Compound 13: viscous solid; Rf (25% ethyl
4. Conclusion
In conclusion, several dimers and tetramers built up around
original and rigid oxa-bridged cores have been prepared for the
first time. We demonstrated that such a bulky central unit was not
detrimental to the formation of liquid-crystalline mesophases.
Indeed, induction of various low-temperature smectic phases
(SmA/C/F/I) was confirmed for most dimeric compounds and their
structure and stability were found to depend strongly on the end-
mesogen type and spacer length. By XRD, it was shown that di-
mers with 40-(dodecyloxy)biphenyl-4-ol as mesogenic end-unit
exhibit an orthogonal smectic (SmA) liquid-crystalline phase,
whilst the 4-((4-(alkyloxy)phenyl)diazenyl)phenol-containing
dimers show a SmC mesophase (and for the longer chain homo-
logue, an additional low-temperature SmF/I). In contrast, dimers
derived from the cholesterol moieties as well as the tetramers
have failed to exhibit liquid crystalline property. Within each
mesomorphic series, the maximum stability temperature range of
mesophases was obtained for mid-spacer chain-lengths, and azo-
containing compounds are more prone to phase induction than
biphenyl-based derivatives.
acetate/hexane) 0.30; 1H NMR (400 MHz, CDCl3):
d 7.40e7.36 (m,
4H), 6.88e6.85 (m, 4H), 4.63 (s, 2H), 4.35 (dd, J¼12.2, 17.0 Hz, 2H),
3.91 (t, J¼6.6 Hz, 2H), 3.79 (dd, J¼12.9, 27 Hz, 2H), 3.45 (s, 3H), 3.18
(s, 3H), 2.22e2.21 (m, 2H), 1.76e1.63 (m, 8H), 1.39e1.19 (m, 20H),
0.81 (t, J¼6.6 Hz, 3H). Compound 14: colourless solid; mp: 78 ꢀC; Rf
(25% ethyl acetate/hexane) 0.60; 1H NMR (400 MHz, CDCl3):
d
7.47e7.43 (m, 8H), 6.95e6.91 (m, 8H), 4.69 (s, 4H), 4.41e4.4 (m,
5. Experimental
5.1. General
4H), 3.98 (t, J¼6.6 Hz, 4H), 3.35 (s, 3H), 3.17 (s, 3H), 2.26 (m, 2H),
1.83e1.76 (m, 4H), 1.46e1.44 (m, 4H), 1.27 (m, 40H), 0.88 (t,
J¼6.6 Hz, 6H).
All the reactions were performed in oven dried apparatus and
the reaction mixtures were magnetically stirred. Reactions were
monitored using thin layer chromatography (TLC), performed on
silica gel coated on microscopic slides and visualized under UV light
or exposure to iodine. Column chromatography was performed
using silica gel (100e200 mesh) and ethyl acetate/hexane was used
as an eluent unless otherwise mentioned. Evaporation of solvent
was performed at reduced pressure, using a rotary evaporator.
Melting points recorded are uncorrected. IR spectra were recorded
as KBr pellets (solids) or thin films (liquids). 1H NMR and proton-
decoupled 13C NMR spectra were recorded at 400 and 100 MHz,
respectively, on JEOL spectrophotometer. Data are reported as fol-
lows: (s¼singlet, d¼doublet, t¼triplet, q¼quartet, m¼multiplet;
coupling constant(s) in hertz). Samples for NMR were made in
CDCl3; tetramethylsilane was used as the internal standard. Mass
spectra were recorded on WATERS-HAB213 mass spectrometer.
Commercial grade solvents were distilled before use. Ethyl acetate
was distilled over anhydrous sodium carbonate. Dichloromethane
was distilled over phosphorous pentoxide and stored over 4 Å
molecular sieves. Tetrahydrofuran (THF) was distilled from sodium
benzophenone ketyl under nitrogen. Benzene was refluxed and
dried over sodium. Triethylamine, pyridine was distilled and stored
5.2. General procedure for the preparation of compounds
16e18
To a mixture of oxa-bridged diacid 9 (200 mg, 0.7 mmol), trie-
thylamine (177 mg, 1.75 mmol) in THF (4 mL) was added
2,4,6-trichlorobenzoyl chloride (350 mg, 1.43 mmol). The reaction
mixture was stirred at room temperature for 2 h and resultant
solution was then added to a mixture of 2-(40-(dodecyloxy)bi-
phenyl-4-yloxy)ethanol (641 mg, 1.61 mmol) and DMAP (197 mg,
1.61 mmol) in THF (6 mL). The reaction mixture was stirred at room
temperature for 3 h under argon atmosphere. After reaction was
completed, sodium bicarbonate solution (3 mL) was added fol-
lowed by distilled water (15 mL). The solution was extracted with
ethyl acetate (4ꢂ4 mL) and the combined organic layers were
washed with distilled water (3ꢂ3 mL), brine solution (3 mL) and
dried over anhydrous sodium sulfate, filtered and concentrated
afforded the crude product, which on purification on silica gel
column chromatography using 20% ethyl acetate/hexane afforded
16 (621 mg, 85%).
5.2.1. Compound 16. Colourless solid; mp: 92 ꢀC; Rf (30% ethyl ac-
etate/hexane) 0.40; 1H NMR (400 MHz, CDCl3):
d 7.38e7.35 (m, 8H),

F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
8753
6.87e6.81 (m, 8H), 4.61e4.57 (m, 2H), 4.44e4.40 (m, 2H), 4.16e4.14
(m, 4H), 3.91e3.88 (m, 4H), 3.37 (s, 3H), 3.25 (s, 3H), 2.64e2.63 (m,
2H), 1.73e1.56 (m, 6H), 138e1.19 (m, 42H), 0.86e0.78 (m, 6H); 13C
(2C), 17.6 (2C), 14.1 (2C); IR (KBr): 2850, 1720, 1600, 1500,
1240 cmꢁ1; HRMS (ESI): m/z (MþH)þ calculated for C65H90N4O11
:
1103.6684; found: 1103.6681.
NMR (100 MHz, CDCl3):
d 166.2 (2C), 158.3 (2C), 157.3 (2C), 133.99
(2C), 132.97 (2C), 127.7 (4C), 127.6 (4C), 114.72 (4C), 114.69 (4C),
110.2, 92 (2C), 68 (2C), 65.5 (2C), 63.3 (2C), 52.1, 51.6, 41.9 (2C), 31.9
(2C), 29.62 (2C), 29.6 (2C), 29.57 (4C), 29.4 (2C), 29.3 (2C), 29.2 (2C),
26 (2C), 22.6 (2C), 17.8 (2C), 17.5 (2C), 14.1 (2C); IR (KBr): 2850, 1720,
1600, 1500, 1460, 1220 cmꢁ1; HRMS (ESI): m/z (MþH)þ calculated
for C65H90O11: 1047.6561; found: 1047.6564.
5.3.2. Compound 20. Yield: 87%; yellowish glassy solid; mp: 62 ꢀC;
1H NMR (400 MHz, CDCl3):
8H), 4.44e4.39 (m, 2H), 4.29e4.24 (m, 2H), 4.08 (t, J¼4.8 Hz, 4H),
3.94 (t, J¼6.6 Hz, 4H), 3.79e3.72 (m, 8H), 3.37 (s, 3H), 3.26 (s, 3H),
2.63e2.61 (m, 2H), 1.75e1.69 (m, 8H), 1.39e1.19 (m, 40H), 0.81 (t,
d
7.79 (d, J¼8.7 Hz, 8H), 7.19e6.89 (m,
J¼6.4 Hz, 6H); 13C NMR (100 MHz, CDCl3):
d 166.3 (2C), 161.3 (2C),
160.6 (2C), 147.1 (2C), 146.7 (2C), 124.4 (4C), 124.3 (4C), 114.7 (4C),
114.6 (4C), 110.1, 92 (2C), 69.5 (2C), 68.9 (2C), 68.3 (2C), 67.7 (2C),
63.9 (2C), 52.2, 51.6, 41.9 (2C), 31.9 (2C), 29.6 (8C), 29.3 (4C), 29.2
(2C), 25.9 (2C), 22.7 (2C), 17.9 (2C), 17.5 (2C), 14.1 (2C); IR (KBr):
2850, 1720, 1580, 1440, 1240 cmꢁ1; HRMS (ESI): m/z (MþH)þ cal-
culated for C69H98N4O13: 1191.7209; found: 1191.7202.
5.2.2. Compound 17. Yield: 95%; colourless solid; mp: 54 ꢀC; 1H
NMR (400 MHz, CDCl3):
d
7.37 (d, J¼8.6 Hz, 8H), 6.86 (d, J¼8.6 Hz,
8H), 4.4e4.38 (m, 2H), 4.28e4.2 (m, 2H), 4.0 (t, J¼4.4 Hz, 4H), 3.9 (t,
J¼6.6 Hz, 4H), 3.77e3.71 (m, 8H), 3.37 (s, 3H), 3.26 (s, 3H),
2.64e2.61 (m, 2H), 1.75e1.54 (m, 6H), 1.4e1.19 (m, 42H), 0.82e0.79
(m, 6H); 13C NMR (100 MHz, CDCl3):
d 166.3 (2C), 158.3 (2C), 157.7
(2C), 133.8 (2C), 133.1 (2C), 127.6 (8C), 114.8 (4C), 114.7 (4C), 110.2,
92 (2C), 69.6 (2C), 68.9 (2C), 68.1 (2C), 67.5 (2C), 63.9 (2C), 52.1, 51.6,
41.9 (2C), 31.9 (2C), 29.5 (8C), 29.3 (6C), 26 (2C), 22.6 (2C), 17.9 (2C),
5.3.3. Compound 21. Yield: 85%; yellowish glassy solid; mp: 64 ꢀC;
1H NMR (400 MHz, CDCl3):
d
7.78 (d, J¼8.8 Hz, 8H), 6.92 (dd, J¼11.5,
8.8 Hz, 8H), 4.38e4.33 (s, 2H), 4.26e4.21 (s, 2H), 4.11 (t, J¼4.6 Hz,
4H), 3.93 (t, J¼6.4 Hz, 4H), 3.79 (t, J¼4.6 Hz, 4H), 3.65e3.57 (m,
12H), 3.37 (s, 3H), 3.27 (s, 3H), 2.64e2.61 (m, 2 H), 1.76e1.69 (m,
8H), 1.39e1.19 (m, 40H), 0.80 (t, J¼6.6 Hz, 6H); 13C NMR (100 MHz,
17.6 (2C), 14.1 (2C); IR (KBr): 2850, 1720, 1600, 1490, 1220 cmꢁ1
;
HRMS (ESI): m/z (MþH)þ calculated for C69H98O13: 1135.7085;
found: 1135.7085.
CDCl3): d 166.3 (2C),161.2 (2C),160.7 (2C),147 (2C),146.7 (2C),124.3
5.2.3. Compound 18. Yield: 90%; colourless solid, mp: 61 ꢀC; 1H
(4C),124.2 (4C), 114.7 (4C),114.6 (4C), 110.1, 91.9 (2C), 70.7 (2C), 70.5
(2C), 69.6 (2C), 68.7 (2C), 68.2 (2C), 67.6 (2C), 63.9 (2C), 52.1, 51.6,
41.8 (2C), 31.8 (2C), 29.6 (4C), 29.5 (4C), 29.33 (2C), 29.29 (2C), 29.1
(2C), 25.9 (2C), 22.6 (2C), 17.9 (2C), 17.5 (2C), 14.0 (2C); IR (KBr):
2850, 1720, 1600, 1500, 1240 cmꢁ1; HRMS (ESI): m/z (MþH)þ cal-
culated for C73H106N4O15: 1279.7734; found: 1279.7736.
NMR (400 MHz, CDCl3):
d
; 7.37 (d, J¼8.5 Hz, 8H), 6.89e6.85 (m, 8H),
4.38e4.33 (m, 2H), 4.25e4.19 (m, 2H), 4.07 (t, J¼5.8 Hz, 4H), 3.89 (t,
J¼6.6 Hz, 4H), 3.77 (t, J¼4.8 Hz, 4H), 3.64e3.57 (m, 12H), 3.37 (s,
3H), 3.26 (s, 3H), 2.64e2.61 (m, 2H), 1.73e1.69 (m, 8H), 1.29e1.19
(m, 40H), 0.8 (t, J¼6.8 Hz, 6H); 13C NMR (100 MHz, CDCl3):
d 166.3
(2C), 158.3 (2C), 157.9 (2C), 133.7 (2C), 133.2 (2C), 127.6 (8C), 114.9
(4C), 114.8 (4C), 110.2, 92 (2C), 70.8 (2C), 70.6 (2C), 69.8 (2C), 68.8
(2C), 68.1 (2C), 67.6 (2C), 63.9 (2C), 52.1, 51.6, 41.9 (2C), 31.9 (2C),
29.6 (6C), 29.4 (4C), 29.3 (4C), 26 (2C), 22.6 (2C), 17.9 (2C), 17.6 (2C),
14 (2C); IR (KBr): 2850, 1700, 1600, 1220 cmꢁ1; HRMS (ESI): m/z
(MþH)þ calculated for C73H106O15: 1223.7611; found: 1223.7614.
5.4. General procedure for the preparation of compounds
22e24
To a mixture of oxa-bridged diacid 9 (200 mg, 0.7 mmol), trie-
thylamine (177 mg, 1.75 mmol) in THF (4 mL) was added 2,4,6-tri-
chlorobenzoyl chloride (350 mg, 1.43 mmol). The reaction mixture
was stirred at room temperature for 2 h and resultant solution was
then added to a mixture of cholesterol (622 mg, 1.61 mmol) and
DMAP (197 mg, 1.61 mmol) in THF (8 mL). The reaction mixture was
stirred at room temperature for 3 h under argon atmosphere. After
reaction was completed sodium bicarbonate solution (3 mL) was
added followed by distilled water (15 mL). The solution was
extracted with ethyl acetate (4ꢂ4 mL) and the combined organic
layers were washed with distilled water (3ꢂ3 mL), brine solution
(3 mL) and dried over anhydrous sodium sulfate, filtered and con-
centrated afforded the crude product, which on purification on
silica gel column chromatography using 8% ethyl acetate/hexane
afforded 22 (450 mg, 63%).
5.3. General procedure for the preparation of compounds
19e21
To a mixture of oxa-bridged diacid 9 (200 mg, 0.7 mmol), trie-
thylamine (177 mg, 1.75 mmol) in THF (4 mL) was added
2,4,6-trichlorobenzoyl chloride (350 mg, 1.43 mmol). The reaction
mixture was stirred at room temperature for 2 h and resultant
solution was then added to a mixture of 2-(4-((4-(dodecyloxy)
phenyl)diazenyl)phenoxy)ethanol (685 mg, 1.61 mmol) and DMAP
(197 mg,1.61 mmol) in THF (6 mL). The reaction mixture was stirred
at room temperature for 3 h under argon atmosphere. After re-
action was completed sodium bicarbonate solution (3 mL) was
added followed by distilled water (15 mL). The solution was
extracted with ethyl acetate (4ꢂ4 mL) and the combined organic
layers were washed with distilled water (3ꢂ3 mL), brine solution
(3 mL) and dried over anhydrous sodium sulfate, filtered and
concentrated afforded the crude product, which on purification on
silica gel column chromatography using 50% ethyl acetate/hexane
afforded 19 (593 mg, 77%).
5.4.1. Compound 22. Colourless solid; mp: 236 ꢀC; Rf (10% ethyl
acetate/hexane) 0.50; 1H NMR (400 MHz, CDCl3):
d 5.38e5.35 (m,
2H), 4.82e4.73 (m, 2H), 3.46 (s, 3H), 3.36 (s, 3H), 2.68e2.66 (m, 2H),
2.43e2.28 (m, 4H), 2.02e1.94 (m, 4H), 1.88e1.04 (m, 56H), 1.0 (s,
6H), 0.91 (d, J¼6.6 Hz, 6H), 0.86 (dd, J¼6.6, 1.7 Hz, 12H), 0.67 (s, 6H);
13C NMR (100 MHz, CDCl3):
d 165.9 (2C),139.6 (2C),122.9 (2C),110.1,
91.9 (2C), 75.1 (2C), 56.7 (2C), 56.2 (2C), 52.2, 51.5, 50 (2C), 42.3 (2C),
41.9 (2C), 39.7 (2C), 39.5 (2C), 37.9 (2C), 36.9 (2C), 36.6 (2C), 36.2
(2C), 35.8 (2C), 31.9 (4C), 28.2 (2C), 27.9 (2C), 27.6 (2C), 24.3 (2C),
23.8 (2C), 22.8 (2C), 22.5 (2C), 21 (2C), 19.3 (2C), 18.7 (2C), 18.1 (2C),
17.6 (2C), 11.8 (2C); IR (KBr): 2800, 1740, 1440, 1260 cmꢁ1; HRMS
(ESI): m/z (MþH)þ calculated for C67H106O7: 1023.8018; found:
1023.8017.
5.3.1. Compound 19. Yellowish glassy solid; mp: 92 ꢀC; Rf (40%
ethyl acetate/hexane) 0.40; 1H NMR (400 MHz, CDCl3):
d 7.82e7.8
(m, 8H), 6.92e6.87 (m, 8H), 4.65e4.6 (m, 2H), 4.47e4.43 (m, 2H),
4.21 (m, 4H), 3.96 (t, J¼6.6 Hz, 4H), 3.37 (s, 3H), 3.25 (s, 3H),
2.66e2.64 (m, 2H), 1.77e1.71 (m, 8H), 1.4e1.19 (m, 40H), 0.81 (t,
J¼6.6 Hz, 6H); 13C NMR (100 MHz, CDCl3):
d 166.2 (2C), 161.3 (2C),
160.1 (2C), 147.5 (2C), 146.9 (2C), 124.3 (8C), 114.7 (8C), 110.3, 92.1
(2C), 68.4 (2C), 65.8 (2C), 63.2 (2C), 52.2, 51.5, 41.9 (2C), 31.9 (2C),
29.61 (2C), 29.6 (6C), 29.4 (4C), 29.2 (2C), 26 (2C), 22.6 (2C), 17.9
5.4.2. Compound 23. Yield: 90%; colourless solid; mp: 166 ꢀC; Rf
(30% ethyl acetate/hexane) 0.40; 1H NMR (400 MHz, CDCl3):

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F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
d
5.34e5.33 (m, 2H), 4.47e4.42 (m, 2H), 4.32e4.27 (m, 2H),
acetate (4ꢂ4 mL) and the combined organics were washed with
distilled water (3ꢂ3 mL), brine solution (3 mL) and dried over an-
hydrous sodium sulfate, filtered and concentrated afforded the
crude product, which on purification on silica gel column chro-
matography using 25% ethyl acetate/hexane afforded the com-
pound 25 (46 mg, 37%). Colourless solid; mp: 165 ꢀC; Rf (30% ethyl
3.74e3.71 (m, 4H), 3.59 (s, 8H), 3.45 (s, 3H), 3.53 (s, 3H), 3.18e3.12
(m, 2H), 2.72e2.69 (m, 2H), 2.37e2.33 (m, 2H), 2.22e2.16 (m, 2H),
2.02e1.02 (m, 60H), 0.99 (s, 6H), 0.91 (d, J¼6.6 Hz, 6H), 0.86 (dd,
J¼6.6,1.72 Hz,12H), 0.67 (s, 6H); 13C NMR (100 MHz, CDCl3):
d 166.2
(2C), 140.9 (2C), 121.5 (2C), 110.1, 92 (2C), 79.5 (2C), 70.8 (2C), 68.7
(2C), 67.3 (2C), 63.9 (2C), 56.8 (2C), 56.2 (2C), 52.1, 51.5, 50.2 (2C),
42.3 (2C), 41.9 (2C), 39.8 (2C), 39.5 (2C), 39.1 (2C), 37.2 (2C), 39.8
(2C), 36.2 (2C), 35.7 (2C), 31.9 (4C), 28.4 (2C), 28.2 (2C), 27.9 (2C),
24.3 (2C), 23.8 (2C), 22.7 (2C), 22.5 (2C), 21.1 (2C), 19.3 (2C), 18.7
(2C), 18 (2C), 17.6 (2C), 11.8 (2C); IR (Neat): 2800, 1720, 1440,
acetate/hexane) 0.40; 1H NMR (400 MHz, CDCl3):
d 7.38e7.34 (m,
16H), 6.88e6.82 (m, 16H), 4.58 (s, 8H), 4.29 (s, 8H), 3.88 (t, J¼6.6 Hz,
8H), 3.24 (s, 6H), 3.04 (s, 6H), 2.17e2.14 (m, 4H), 1.74e1.67 (m, 4H),
1.39e1.16 (m, 80H), 0.81 (t, J¼6.8 Hz, 12H);13C NMR (100 MHz,
CDCl3):
d 168.3 (4C), 158.4 (4C), 156.6 (4C), 134.6 (4C), 132.7 (4C),
1240 cmꢁ1; HRMS (ESI): m/z (MþH)þ calculated for C75H122O11
:
127.8 (8C), 127.7 (8C), 114.9 (8C), 114.8 (8C), 105.8 (2C), 92.5 (4C), 68
(4C), 65.1 (4C), 58.8 (4C), 51.8 (2C), 51.4 (2C), 37.9 (4C), 31.9 (4C),
29.65 (4C), 29.62 (8C), 29.59 (4C), 29.57 (4C), 29.4 (4C), 29.3 (4C),
29.27 (4C), 26 (4C), 22.7 (2C), 14.1 (4C); IR (KBr): 2800, 1720, 1600,
1199.9066; found: 1199.9064.
5.4.3. Compound 24. Yield: 91%. Thick liquid, Rf (30% ethyl acetate/
hexane) 0.40; 1H NMR (400 MHz, CDCl3):
5.31e5.30 (m, 2H),
d
1440,1220 cmꢁ1
C124H168O22: 2032.193; found: 2032.086.
;
MALDI-TOF: m/z (MþNa)þ calculated for
4.34e4.39 (m, 2H), 4.29e4.24 (m, 2H), 3.70e3.66 (m, 4H), 3.59 (s,
16H), 3.42 (s, 3H), 3.31 (s, 3H), 3.17e3.1 (m, 2H), 2.68e2.66 (m, 2H),
2.36e2.31 (m, 2H), 2.21e2.17 (m, 2H), 1.99e0.98 (m, 60H), 0.96 (s,
6H), 0.88 (d, J¼6.6 Hz, 6H), 0.83 (dd, J¼6.6, 1.7 Hz, 12H), 0.64 (s, 6H);
5.4.6. Compound 26. To a mixture of 10-(biphenyl-4-yloxy)decanoic
acid (53 mg, 0.156 mmol), triethylamine (24 mg, 0.24 mmol) in THF
(2 mL) was added 2,4,6-trichlorobenzoyl chloride (42 mg,
0.172 mmol). The reaction mixture was stirred at room temperature
for 2 h and resultant solution was then added to a mixture of tetrol
10 (15 mg, 0.035 mmol) and DMAP (19 mg, 0.155 mmol) in THF
(3 mL). The reaction mixture was stirred at room temperature for 8 h
under argon atmosphere. After reaction was completed, sodium bi-
carbonate solution (3 mL) was added followed by distilled water
(10 mL). The solution was extracted with ethyl acetate (4ꢂ4 mL) and
the combined organic layers were washed with distilled water
(3ꢂ3 mL), brine solution (3 mL) and dried over anhydrous sodium
sulfate, filtered and concentrated afforded the crude product, which
on purification on silica gel column chromatography using 30% ethyl
acetate/hexane afforded the compound 26 (46 mg, 77%). Colourless
solid; mp: 78 ꢀC; Rf (25% ethyl acetate/hexane) 0.60; 1H NMR
13C NMR (100 MHz, CDCl3):
d 166.3 (2C), 140.9 (2C), 121.5 (2C), 110.2,
92 (2C), 79.5 (2C), 70.9 (2C), 70.6 (4C), 68.7 (2C), 67.3 (2C), 63.9 (2C),
56.8 (2C), 56.2 (2C), 52.1, 51.6, 50.2 (2C), 42.3 (2C), 41.9 (2C), 39.8 (2C),
39.5 (2C), 39.1 (2C), 37.3 (2C), 36.9 (2C), 36.2 (2C), 35.8 (2C), 31.9 (4C),
28.4 (2C), 28.2 (2C), 27.9 (2C), 24.3 (2C), 23.8 (2C), 22.8 (2C), 22.5
(2C), 21.1 (2C), 19.4 (2C), 18.7 (2C), 18 (2C), 17.6 (2C), 11.9 (2C); IR
(KBr): 2850, 1720, 1440, 1220 cmꢁ1; HRMS (ESI): m/z (MþH)þ cal-
culated for C79H130O13: 1287.9590; found: 1287.9594.
5.4.4. Compound 12. Bis-oxa-bridged tetra-ester (300 mg,
0.552 mmol) in dry dichloromethane (5 mL) was added DIBAL-H
(0.88 g, 4.4 mL, 20 wt % in toluene) at ꢁ78 ꢀC under argon at-
mosphere. Reaction mixture was warmed to room temperature
and stirred for 3 h. Reaction was quenched using triethylamine/
water (1:1) at ꢁ78 ꢀC and slowly brought to room temperature. A
white solid separated, was filtered through small pad of Celite,
washed with 10% MeOH/CHCl3. The filtrate was concentrated
under reduced pressure afforded 235 mg of oxa-bridged tetrol.
The crude tetrol in dry dichloromethane (3 mL) was added pyr-
idine (1 mL) followed by acetic anhydride (1.2 mL) at 0e5 ꢀC.
Reaction mixture was stirred at room temperature for 9 h and
quenched with water (5 mL) and the solution was extracted with
dichloromethane (4ꢂ3 mL), combined organic layers were
washed with 1% HCl (2ꢂ2 mL), water (2ꢂ2 mL), brine solution
(2 mL) and dried over anhydrous sodium sulfate, filtered and
concentrated afforded the crude product, which on purification
on silica gel column chromatography using 40e50% ethyl acetate/
hexane afforded the compound 12 (237 mg, 72%). Colourless
solid; mp: 208e210 ꢀC; Rf (50% ethyl acetate/hexane) 0.50; 1H
(400 MHz, CDCl3): d 7.48e7.41 (m, 16H), 7.34e7.3 (m, 8H), 7.24e7.19
(m, 4H), 6.89e6.86 (8H), 4.28 (d, J¼12.4 Hz, 4H), 4.18 (d, J¼12.4 Hz,
4H), 3.9 (t, J¼6.6 Hz, 8H), 3.31 (s, 6H), 3.12 (s, 6H), 2.35e2.28 (m, 4H),
2.24 (t, J¼7.5 Hz, 8H),1.75e1.68 (m, 8H),1.55e1.52 (m, 8H),1.42e1.35
(m, 8H), 1.23e1.18 (m, 36H); 13C NMR (100 MHz, CDCl3):
d 173 (4C),
158.8 (4C), 140.9 (4C), 133.6, (4C), 128.8 (8C), 128.2 (8C), 126.8 (8C),
126.7 (8C), 114.8 (8C), 105.9 (2C), 92.8 (4C), 68.1 (4C), 58.3 (4C), 51.9
(2C), 51.6 (2C), 38.3 (4C), 34.1 (4C), 29.53 (4C), 29.47 (4C), 29.4 (4C),
29.3 (2C), 29.2 (2C), 26.2 (4C), 24.9 (4C), 17.2 (2C); IR (KBr): 2800,
1720, 1600, 1440 cmꢁ1; MALDI-TOF: m/z (MþNa)þ calculated for
C108H136O18: 1743.963; found: 1743.891.
5.4.7. Compound 27. To a mixture of cholesterol-derived acid
(77 mg, 0.158 mmol), triethylamine (24 mg, 0.24 mmol) in THF
(2 mL) was added 2,4,6-trichlorobenzoyl chloride (42 mg,
0.172 mmol). The reaction mixture was stirred at room temperature
for 2 h and resultant solution was then added to a mixture of tetrol
10 (15 mg, 0.035 mmol) and DMAP (19 mg, 0.155 mmol) in THF
(3 mL). The reaction mixture was stirred at room temperature for
8 h under argon atmosphere. After reaction was completed sodium
bicarbonate solution (3 mL) was added followed by distilled water
(15 mL). The solution was extracted with ethyl acetate (4ꢂ4 mL)
and the combined organic layers were washed with distilled water
(3ꢂ3 mL), brine solution (3 mL) and dried over anhydrous sodium
sulfate, filtered and concentrated afforded the crude product,
which on purification on silica gel column chromatography using
20e30% ethyl acetate/hexane afforded the compound 27 (61 mg,
76%). Colourless solid; mp: 180 ꢀC; Rf (40% ethyl acetate/hexane)
NMR (400 MHz, CDCl3):
d
4.3 (d, J¼12.4 Hz, 4H), 4.19 (d,
J¼12.4 Hz, 4H), 3.37 (s, 6H), 3.2 (s, 6H), 2.37 (dd, J¼8.0, 4.7 Hz,
4H), 2.04 (s, 12H), 1.45e1.28 (m, 4H); 13C NMR (100 MHz, CDCl3):
d
170.2 (4C), 105.8 (2C), 92.6 (4C), 58.3 (4C), 51.8 (2C), 51.4 (2C),
38.0 (4C), 20.7 (4C), 17.1 (2C); HRMS (ESI): m/z (MꢁH)ꢁ calculated
for C28H40O14: 599.2339; found: 599.2345.
5.4.5. Compound 25. To a mixture of 2-(40-(dodecyloxy)biphenyl-
4-yloxy)acetic acid (107 mg, 0.26 mmol), triethylamine (39 mg,
0.39 mmol) in THF (2 mL) was added 2,4,6-trichlorobenzoyl chlo-
ride (70 mg, 0.29 mmol). The reaction mixture was stirred at room
temperature for 2 h and resultant solution was then added to
a mixture of tetrol 10 (25 mg, 0.058 mmol) and DMAP (32 mg,
0.26 mmol) in THF (3 mL). The reaction mixture was stirred at room
temperature for 8 h under argon atmosphere. After reaction was
completed sodium bicarbonate solution (3 mL) was added followed
by distilled water (10 mL). The organics were extracted with ethyl
0.60; 1H NMR (400 MHz, CDCl3):
d 5.37e5.36 (m, 4H), 4.61e4.58 (m,
4H), 4.37 (d, J¼12.2 Hz, 4H), 4.23 (d, J¼12.2 Hz, 4H), 3.39 (s, 6H),
3.23 (s, 6H), 2.63e2.59 (m, 16H), 2.41e2.29 (m, 8H), 2.02e1.04 (m,
112H), 1.01 (s, 12H), 0.91 (d, J¼6.6 Hz, 12H), 0.86 (dd, J¼6.6, 1.7 Hz,

F.A. Khan et al. / Tetrahedron 66 (2010) 8745e8755
8755
24H), 0.68 (s, 12H); 13C NMR (100 MHz, CDCl3):
d 171.5 (4C), 171.4
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(4C), 139.6 (4C), 122.7 (4C),105.9 (2C), 92.7 (4C), 74.4 (4C), 58.5 (4C),
56.8 (4C), 56.3 (4C), 51.8 (2C), 51.5 (2C), 50.1 (4C), 42.4 (4C), 39.8
(4C),39.5 (4C), 38.1 (4C), 37.0 (4C), 36.6 (4C), 36.2 (4C), 35.8 (4C),
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2328.492.
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We thank the Department of Science and Technology (DST),
New Delhi for financial assistance. F.A.K. acknowledges the DST for
a Swarnajayanti Fellowship. K.P. thank CSIR, New Delhi for fellow-
ship. B.D. thank the CNRS and the University of Styrasbourg for
support.
Supplementary data
15. Khan, F. A.; Dash, J. J. Org. Chem. 2003, 68, 4556.
16. Joo, S.-H.; Yun, Y.-K.; Jin, J.-I.; Kim, D.-C.; Zin, W.-C. Macromolecules 2000, 33,
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.09.003.
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