Towards room temperature biaxial nematics

Article abstract of DOI:10.1080/15421400903053826

A series of new liquid crystalline compounds based on the 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) core was synthesized in an effort to access the biaxial nematic phase at low temperatures. Derivatives with secondary terminal alkoxy groups and de

Full text of DOI:10.1080/15421400903053826

Mol. Cryst. Liq. Cryst., Vol. 511, pp. 203=[1673]–217=[1687], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903053826
Towards Room Temperature Biaxial Nematics
Lori L. Cooper1, Edward T. Samulski1,
and Eric Scharrer2
1Department of Chemistry, University of North Carolina,
Chapel Hill, NC, USA
2Department of Chemistry, University of Puget Sound, Tacoma,
WA, USA
A
series of new liquid crystalline compounds based on the 2,5-bis-
(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) core was synthesized in an effort to
access the biaxial nematic phase at low temperatures. Derivatives with secondary
terminal alkoxy groups and derivatives with either two or four lateral methyl
groups were prepared. Secondary alkoxy terminal groups either suppressed meso-
phase formation altogether or increased the nematic onset temperature relative to
primary groups. The dimethylated derivatives showed significant reductions in
the nematic onset temperatures compared to the previously reported unmethylated
compounds. The tetramethylated analogs generally possessed low clearing
temperatures, but exhibited only monotropic mesomorphism.
Keywords: biaxial nematic phase; lateral methylation; oxadiazole containing liquid
crystals
INTRODUCTION
Freiser’s biaxial nematic phase [1], posited for monomeric calamitic
liquid crystals—molecules with a single, biaxially-shaped mesogenic
core—has, for the most part, eluded liquid crystallographers. Initially
chemists tried to induce the Nb phase by exaggerating the core’s shape
biaxiality with, in retrospect, no apparent success [2]. Sidechain
We thank Drs. Dingemans, Madsen, and Zafiropoulos for helpful conversations. This
work was supported by NSF grant DMR-9971143. Eric Scharrer also wishes to acknowl-
edge the University of Puget Sound for a Lantz Sabbatical Award that allowed him to
spend the 2006–07 academic year at the University of North Carolina.
Address correspondence to Edward T. Samulski, Department of Chemistry,
University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA. E-mail:
et@unc.edu
203=[1673]

204=[1674]
L. L. Cooper et al.
polymer-based liquid crystals, on the other hand, readily exhibited Nb
phases [3]. Previously, we reported a family of nonlinear, symmetric
alkyl and alkoxy benzoate ester mesogens based on the disubstituted
2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) core of which
OC12-Ph(ODBP), 1b, and C7-Ph(ODBP), 1c, were shown to exhibit a
high temperature (>170ꢀC) biaxial nematic phase (Fig. 1) [4].
The nonlinear oxadiazole heterocycle has an exocyclic bond angle of
134ꢀ and additionally possesses a large electric dipole moment (ꢁ5
Debye) bisecting the mesogenic core’s boomerang-like shape. The
resulting negative dielectric anisotropy of the ODBP core makes these
compounds putative candidates for fast switching phases [5]. Since the
initial report confirming the presence of a biaxial nematic phase, we,
and others, have investigated a variety of structural modifications
(for oxadiazole and other liquid crystals) in an attempt to access this
unique phase in a more useful temperature range [6]; achieving this
goal should allow for more in-depth studies of the biaxial nematic
phase. Our initial attempts focused on the incorporation of terminal
FIGURE 1 Previously prepared oxadiazole liquid crystals. 1b and 1c were
shown to exhibit a biaxial nematic phase with conoscopy and deuterium
NMR spectroscopy; SmX, SmY, and SmZ represent smectic phases that have
not been fully characterized.

Towards Room Temperature Biaxial Nematics
205/[1675]
alkyl chains possessing tertiary groups, but this led to elimination of
all liquid crystallinity [6h].
It is well known that modest substitutions (e.g., halogens, short
alkyl groups, etc.) on the mesogenic core can change the mesophase
transition temperatures [7]. Generally, if small changes in the core’s
shape are not overcompensated for by directional electrostatic interac-
tions, the transition temperature can be lowered without loss of the
mesophase. To that end, we have explored the introduction of methyl
groups on the aromatic rings of the ODBP mesogens as well as the
incorporation of secondary terminal alkyl chains, and we report the
effects of such changes herein.
RESULTS AND DISCUSSION
Synthesis
The preparation of the target compounds with lateral methyl groups
requires access to various methylated alkoxybenzoic acids. Such com-
pounds are not commercially available and were prepared as shown in
Scheme 1. The appropriate 4-hydroxybenzoic acid was esterified using
ethanol, and the phenol was then alkylated in the presence of KOH
and butyl iodide or dodecyl iodide. Hydrolysis of the ester under basic
conditions followed by acidification led to the substituted benzoic acid
derivatives, 2.
The synthesis of the unsubstituted bisphenol oxadiazole, 2,5-bis-
(4-hydroxyphenyl)-1,3,4-oxadiazole, 3a, has been described elsewhere
[8], but the synthesis of 2,5-bis-(4-hydroxy-2-methylphenyl)-1,3,4-
oxadiazole, 3b, was carried out as shown in Scheme 2. This method
represents an adaptation of a procedure to prepare a related com-
pound [6e]. 2-methyl-4-methoxy benzoic acid was converted to the acid
SCHEME 1 Synthesis of methylated 4-alkoxybenzoic acids.

206/[1676]
L. L. Cooper et al.
SCHEME 2 Synthesis of the oxadiazole bisphenol core.
chloride and then reacted with 0.45 equivalents of hydrazine which led
to the formation of the amide linkage. Ring closure to give the oxadia-
zole was effected by using a large excess of thionyl chloride. Demethy-
lation to give the bisphenol was carried out with BBr3.
The general synthesis of the target oxadiazole derivatives, 4–9,
involved the reaction of the appropriately substituted benzoic acid,
2, with the appropriate oxadiazole bisphenol core, 3, in the presence
of ethyl dimethylaminopropylcarbodiimide hydrochloride (EDC) and
catalytic 4-(dimethylamino)pyridine (DMAP) (Scheme 3). In addition
to assigning compound numbers, we have also provided a simplified
SCHEME 3 Synthesis of liquid crystalline oxadiazole derivatives.

Towards Room Temperature Biaxial Nematics
207/[1677]
nomenclature system to facilitate discussion. For example OC4-
2MePh(ODBP) refers to a compound that has butoxy chains and
possesses methyl groups at the 2-position of both outer benzene rings
and no methyl groups on the interior benzene rings. OC12-
Ph(2MeODBP) refers to a compound with dodecyloxy groups, no
methyl groups on the outer benzene rings and methyl groups at the
2-position of both inner benzene rings.
Phase Behavior
Effects of Modifying the Terminal Alkyl Groups
As described earlier, ODBP derivatives with tertiary alkyl groups did
not exhibit mesomorphism. This led us to investigate the effects of
incorporating alkyl groups with secondary carbons. The symmetric
derivative with (S) 2-butoxy groups, 4a, was prepared as shown in
Scheme 3 [9]. However, this compound melted directly to the isotropic
state at 193ꢀC (Fig. 2). Clearly, the presence of the branch point so
close to the aromatic core has a strongly detrimental effect on the for-
mation of a mesophase. Therefore, an isomeric derivative, 4b, posses-
sing isobutyloxy groups was prepared. The separation of the branch
point from the core via a methylene spacer restored the liquid crystal-
linity. However, the nematic onset occurred at 225ꢀC (0.81 kJ=mol)
with a range of only 5 degrees (Fig. 2). It is evident that the mesophase
behavior of the ODBP derivatives is extremely sensitive to any
changes in the nature of the terminal aliphatic groups. Based on the
relatively high clearing points of the latter two derivatives, it appears
that the transition temperatures are controlled by the typical electro-
static and van der Waals attractions in combination with a presumed
unique packing arrangement in the crystal. It is remarkable that var-
iations in the nature of the aliphatic chains for the isomeric C4H9 deri-
vatives would have such a strong impact on the very stable nematic
phase (87ꢀC range) exhibited by the n-butyl parent compound, 1a.
FIGURE 2 Phase behavior of ODBP derivatives with secondary alkoxy
groups.

208/[1678]
L. L. Cooper et al.
Effects of Lateral Methyl Substitution
The incorporation of lateral methyl groups on benzene rings has
been demonstrated to lower phase transition temperatures in rod-like
liquid crystals [7,10]. This structural modification also proved to be
successful for the bent-core oxadiazole derivatives, 5–7; the onset tem-
peratures were significantly reduced relative to the non-methylated
analogs, and good mesophase ranges were maintained. The phase
transition temperatures and enthalpies of the dimethylated deriva-
tives are given in Table 1.
With the derivatives in which the methyl groups are on the outer
benzene rings, the butoxy 2-methyl derivative, 6a, OC4-
2MePh(ODBP), exhibits a nematic onset at 130ꢀC and a nematic range
of 56ꢀC. The corresponding 3-methyl derivative, 5a, OC4-
3MePh(ODBP), has a nematic range of 55ꢀC. In both cases, the clear-
ing temperatures are comparable with the nematic onset temperature
(193ꢀC) of the unsubsituted OC4 derivative, 1a. However, the nematic
range is compressed by about 30 degrees in the case of the dimethyl-
ated derivatives.
For the corresponding dodecyloxy derivatives, the nematic onset
temperature is lowered by 56ꢀC for the 3-methyl derivative, 5b,
OC12-3MePh(ODBP), compared to the unsubstituted compound, 1b,
while the nematic range of 10ꢀC is nearly identical to that of the
unsubstituted version. For the 2-methyl derivative, 6b, OC12-
2MePh(ODBP), the nematic onset is lowered by a remarkable 87ꢀC
and the nematic range of 28ꢀC is significantly larger than that of
TABLE 1 Phase Transition Temperatures and Enthalpies of Oxadiazole
Derivatives, 5–9
Transition temperature to indicated phase T [ꢀC]a
Cr2
(Enthalpy
[kJ=mol])
Smectic Xb
(Enthalpy
[kJ=mol])
Nematic
(Enthalpy
[kJ=mol])
Isotropic
(Enthalpy
[kJ=mol])
Compound
5a, OC4-3MePh(ODBP)
5b, OC12-3MePh(ODBP)
6a, OC4-2MePh(ODBP)
6b, OC12-2MePh(ODBP)
7a, OC4-Ph(2MeODBP)
7b, OC12-Ph(2MeODBP)
—
—
—
123.1 (85.0)
—
150.0 (50.7) 205.1 (0.96)
137.3 (6.3) 147.7 (0.79)
130.6 (38.1) 186.1 (0.95)
106.6 (1.9) 134.3 (0.79)
129.3 (52.5) 180.1 (1.35)
—
92.3 (7.9)
112.8 (9.2)
99.5 (60.3)
—
70.2 (23.9) [111.6 (3.18)]c 117.3 (67.1) 130.5 (1.41)
aAll temperatures and enthalpies are based on the second DSC heating run.
bThis represents an uncharacterized smectic phase.
cTemperatures in brackets indicate a monotropic transition.

Towards Room Temperature Biaxial Nematics
209/[1679]
the unsubstituted compound. This onset temperature of 106ꢀC repre-
sents the lowest nematic onset temperature of any of our dimethylated
ODBP compounds.
The derivatives in which the methyl groups are located on the inter-
ior benzene rings do not exhibit phase behavior which deviates much
from that of the derivatives described above. The butoxy derivative,
7a, OC4-Ph(2MeODBP), has a nematic range of 51ꢀC with a nematic
onset temperature only 1 degree lower than that of 6a, OC4-
2MePh(ODBP). The dodecyloxy derivative, 7b, OC12-Ph(2MeODBP),
gives a nematic range of 13 degrees with an onset temperature in
between that observed for 6b, OC12-2MePh(ODBP), and 5b, OC12-
3MePh(ODBP). The clearing temperatures of 7a and 7b are slightly
lower than the clearing temperatures for the derivatives with the
methyl groups on the outer rings. A direct comparison of the phase
transition temperatures of all dimethylated derivatives with those of
the unmethylated analogs can be seen in Figure 3.
To summarize the impact of lateral methylation for the dimethy-
lated derivatives described above, the lowest onset temperature was
observed for 6b, OC12-2MePh(ODBP) at 106ꢀC. All dodecyloxy deriva-
tives showed a nematic range at least as large as that of the unmethy-
lated analog with compound 6b possessing a nematic range almost
three times as large as this. In contrast, all the butoxy derivatives
FIGURE 3 A comparision of the phase transition temperatures for all
dimethylated ODBP derivatives with those of the unmethylated analogs.

210/[1680]
L. L. Cooper et al.
demonstrated compressed nematic ranges (by about 30 degrees) com-
pared to the unmethylated derivative.
When the compounds described here are examined under polarized
light, all show a preponderance of 2 brush disclinations. An example of
this can be seen in a micrograph for 7a, OC4-Ph(2MeODBP) (Fig. 4).
Moreover, when attempts were made to generate homeotropically
aligned samples, the material remained birefringent. However,
neither of these observations is sufficient evidence of biaxiality, and
2
detailed conoscopic studies and H NMR spectroscopy are in progress
in an effort to quantitate the degree of biaxiality that might be
present.
While the incorporation of two lateral methyl groups was successful
in terms of lowering onset temperatures, the nematic onset tem-
peratures remained above 100ꢀC. In an effort to further lower the
phase transition temperatures, the related series of tetramethylated
compounds, with methyl groups on both the inner and outer benzene
rings, was prepared: 8a OC4-3MePh(2MeODBP), 8b OC12-3MePh(2-
MeODBP), 9a OC4-2MePh(2MeODBP), and 9b OC12-2MePh(2-
MeODBP). While the clearing temperatures are reduced significantly
relative to the dimethylated derivatives (Fig. 5), none of these com-
pounds exhibit enantiotropic liquid crystalline behavior.
However, all of these compounds do exhibit complex monotropic
mesomorphism. In the case of the OC12 derivatives, crystal growth
into the liquid crystal phase occurs almost immediately after meso-
phase formation. With the OC4 derivatives, rapid cooling appears to
FIGURE 4 Nematic texture of OC4-Ph(2MeODBP), 179ꢀC, crossed polars,
20X.

Towards Room Temperature Biaxial Nematics
211/[1681]
FIGURE 5 Phase transition temperatures of tetramethylated oxadiazole
derivatives (All temperatures are from the initial DSC heating run).
lock the mesophase into a glassy state which persists to room tempera-
ture. The complex phase behavior of the tetramethylated derivatives is
illustrated with the DSC thermograms for OC4-3MePh(2MeODBP), 8a
(Fig. 6). On the initial heating, the material clears directly to a liquid
at 153ꢀC. Upon cooling, a monotropic nematic phase forms at 89ꢀC
FIGURE 6 DSC thermograms of OC4-3MePh(2MeODBP), 8a.

212/[1682]
L. L. Cooper et al.
(0.86 kJ=mol). With a cooling rate of 10ꢀ=min., a slow transition to a
viscous, presumably smectic phase occurs at 58ꢀC. When a sample is
examined by polarized light microscopy with slower cooling rates,
the formation of crystallites interspersed in the smectic phase is
evident. This crystallization is kinetically controlled and tends to be
incomplete (crystallization slows as the sample is cooled further).
Evidence of this partial crystallization can be seen in the form of a
broad exotherm at 19ꢀC during the first DSC cooling trace. If any of
the crystallite regions do form on cooling, then during the subsequent
heating, the crystallization continues as shown by an exothermic peak
at 41ꢀC (24.5 kJ=mol) (2nd trace). This is followed by a second exo-
thermic transition at 127ꢀC (4.65 kJ=mol) to a highly birefringent crys-
talline state. This new crystal phase persists until the clearing point
and is always observed on subsequent heatings. However, if the mate-
rial is cooled quickly only to room temperature (2nd cooling trace), a
glassy smectic phase persists indefinitely. No crystallization occurs,
and as can be observed in the 3rd heating trace, the exothermic crys-
tallization peak at 41ꢀC is absent.
SUMMARY
In conclusion, the incorporation of terminal secondary alkoxy chains
in the oxadiazole derivatives led to the elimination of liquid crystalli-
nity or dramatic reduction of the nematic phase range. However,
inclusion of two lateral methyl groups on the benzene rings in the
target compounds led to a substantial reduction in nematic onset
temperatures and these derivatives also showed reasonably robust
nematic ranges. Attempts to further lower the nematic onset tempera-
ture by incorporating four lateral methyl groups were unsuccessful;
the tetramethyl compounds melted directly to the isotropic state.
Research on the related, asymmetrically methylated derivatives is
currently underway.
EXPERIMENTAL
General
All materials were purchased from Sigma-Aldrich or Fisher and used
as received. Column chromatography was carried out using Fisher
brand Silica gel (230–400 mesh) and fractions were analyzed using
Aluminum backed TLC plates with fluorescent indicator purchased
from Fluka. The syntheses of 2,5-bis-(4-hydroxyphenyl)-1,3,4-
oxadiazole [8], 4-(2-butoxy)benzoic acid [11], and 4-isobutyloxybenzoic

Towards Room Temperature Biaxial Nematics
213/[1683]
acid [12] have been described elsewhere. Nuclear Magnetic Resonance
Spectroscopy was carried out using either a Bruker AMX 300 MHz
NMR Spectrometer or a Bruker AV 400 MHz Widebore NMR Spectro-
meter. Chemical Shifts are given relative to TMS and coupling con-
stants are reported in Hz. Thermal analysis was carried out using a
Seiko DSC 220 C and a Nikon Microphot-FX polarizing microscope
equipped with a Mettler FP82HT Hot Stage and a Mettler Toledo
FP90 Central Processor. Video of the microscopy was acquired using
a Sony CCD-IRIS color video camera and the video was processed
using Roxio Dell Movie Studio software.
4-Butoxy-3-Methylbenzoic Acid, OC4-2b
4-hydroxy-3-methylbenzoic acid (2.97 g, 19.5 mmol) was dissolved in
absolute ethanol (30 mL) and ca. 1 mL sulfuric acid was added. The
reaction flask was equipped with a water condenser and heated under
reflux for 17 hours. After the reaction flask was cooled, ethanol was
removed by rotary evaporation. Water (100 mL) and diethyl ether
(50 mL) were added to the residue and this was transferred to a
separatory funnel. The organic layer was washed a total of three times
with water and the combined aqueous washings were backextracted
once with fresh diethyl ether. The combined ether extracts were dried
over MgSO4. After filtration and removal of solvent by rotary evapora-
tion, ethyl 4-dodecyloxy-3-methylbenzoate (3.10 g, 88%) was obtained
as a white crystalline material and used without further purification.
A slurry of ethyl 4-hydroxy-3-methylbenzoate (1.0 g, 5.55 mmol),
iodobutane (1.45 g, 7.88 mmol), and pulverized KOH (0.74 g, 13.1 mmol)
was stirred in DMSO (40 mL) for 42 hours. To this mixture was added
water (80 mL) and diethyl ether (40 mL). After stirring for 15 minutes,
the solution was transferred to a separatory funnel and the organic
layer was washed a total of three times with water. The combined aqu-
eous washes were backextracted once with fresh diethyl ether. The
combined ether extracts were dried over MgSO4. After filtration and
removal of solvent by rotary evaporation, an oily product was
obtained. This was hydrolyzed directly by stirring for 18 hours in
the presence of 20 mL of 2 M KOH (in ethanol). The solution was
poured into water (80 mL) and then acidified with 6 M HCl which
led to the formation of a white precipitate. This powder was collected
via Buchner filtration and it was then recrystallized using 2:1 etha-
nol:water to give the acid (0.66 g, 57%) as a fluffy white solid. (Found:
C, 69.18; H, 7.82. C12H16O3 requires C, 69.21; H 7.74); dH (399.73 MHz;
CDCl3; Me4Si) 7.96 (d, 1H, J ¼ 8.5 Hz, Ar), 7.91 (s, 1H, Ar), 6.86 (d, 1H,
J ¼ 8.6 Hz, Ar), 4.06 (t, 2H, J ¼ 6.4 Hz, OCH2CH2), 2.27 (s, 3H, CH3),
1.83 (pentet, 2H, J ¼ 6.5 Hz, OCH2CH2), 1.55 (sextet, 2H, J ¼ 7.5 Hz,

214/[1684]
L. L. Cooper et al.
TABLE 2 Yields and Elemental Analyses for Compound 2
Compound
Percent yield
C% calc. (found)
H% calc. (found)
2a-OC4
2a-OC12
2b-OC4
2b-OC12
48
58
57
31
69.21 (69.35)
74.96 (75.00)
69.21 (69.18)
74.96 (74.83)
7.74 (7.78)
10.06 (9.90)
7.74 (7.82)
10.06 (9.96)
CH2CH2CH3), 1.01 (t, 3H, J ¼ 7.4 Hz, CH2CH3); dC (100.55 MHz;
CDCl3; Me4Si) 172.2, 161.8, 132.5, 130.1, 126.8, 120.6, 109.9, 67.8,
31.2, 19.2, 16.1, 13.8.
All other alkoxybenzoic acids were prepared in the same manner
and the yields (from the phenol) and elemental analysis data are
compiled in Table 2.
2,5-Bis-(4-Hydroxy-2-Methylphenyl)-1,3,4-Oxadiazole, 3b
To a 100 mL flask equipped with a Claisen adapter was added
4-methoxy-2-methylbenzoic acid (3.00 g, 18.0 mmol). This was placed
under an Argon atmosphere and dissolved in 15 mL dry dichloro-
methane. The flask was placed in an ice bath and 18.1 mL of 2.0 M oxa-
lyl chloride (36.1 mmol) was slowly added. After several minutes, a
few drops of dry DMF were added which led to vigorous gas evolution.
The reaction mixture was allowed to stir overnight at which point the
solvent and excess oxalyl chloride were removed under vacuum. The
acid chloride was isolated as a white solid (2.38 g, 71%) and used
without further purification.
The acid chloride (2.38 g, 12.9 mmol) was dissolved in 15 mL dry
THF and cooled to 0ꢀC. In a different flask, hydrazine hydrate
(0.28 mL, 5.81 mmol) and triethylamine (1.62 mL, 11.6 mmol) were dis-
solved in 15 mL dry THF. The acid chloride solution was slowly added
to the hydrazine solution which led to the formation of large amounts
of precipitate. The reaction mixture was allowed to stir for 12 hours.
Water (100 mL) was added to the reaction flask and the solid product
was isolated using Buchner filtration. The filter cake was washed with
additional water and then dried under vacuum. The solid was recrys-
tallized from ethanol=acetone to give a bright white powder (1.54 g,
80.6%) dH (399.73 MHz; DMSO; Me4Si) 9.95 (s, 2H), 7.44 (d, 2H),
6.86–6.78 (mult., 4H), 3.76 (s, 6H), 2.40 (s, 6H).
The amide (1.54 g, 4.69 mmol) was placed in a 100 mL round bottom
flask equipped with a reflux condenser. Thionyl chloride (17.1 ml,
235 mmol) was added and the reaction was heated at reflux for 3
hours. The hot reaction mixture was poured onto 75 g ice which led

Towards Room Temperature Biaxial Nematics
215/[1685]
to the formation of an off white precipitate. After the ice had
completely melted, the granular solid was isolated by Buchner
filtration and washed with 100 mL water. The product (2,5-bis-(4-
methoxy-2-methylphenyl)-1,3,4-oxadiazole) was recrystallized from
ethanol to give fine, colorless needles (1.24 g, 86%). dH (399.73 MHz;
DMSO; Me4Si) 7.96 (d, 2H), 7.04–6.92 (mult., 4H), 3.83 (s, 6H), 2.66
(s, 6H).
2,5-bis-(4-methoxy-2-methylphenyl)-1,3,4-oxadiazole (1.07 g, 3.45
mmol) was added to a 100 mL round bottom flask equipped with a
Claisen adapter and the reaction vessel was placed under an Argon
atmosphere. Dichloromethane (15 mL) was added and the reaction
mixture was cooled to 0ꢀC. BBr3 (3.02 g, 12.1 mmol) was added drop-
wise over a period of 15 minutes and the mixture was allowed to stir
for 15 hours. The reaction mixture was then poured over 100 g of ice
which gave a white precipitate. This solid was collected by Buchner
filtration and then washed with additional water. The product was
recrystallized from ethanol=acetone to give colorless plates, and used
without further purification (0.76 g, 79%). dH (399.73 MHz; DMSO;
Me4Si) 10.14 (br. singlet, 2H, OH), 7.84 (d, 2H, J ¼ 9.1 Hz, Ar),
6.80–6.74 (mult., 4H, Ar), 2.57 (s, 6H, CH3); dC (100.55 MHz;CDCl3;
Me4Si) 163.84, 160.36, 157.03, 140.05, 131.11, 118.71, 114.05, 22.24.
2,5-Bis-(4-(4-Butoxy-3-Methylbenzoyloxy)-Phenyl)-
1,3,4-Oxadiazole, 5a
A 250 mL two neck round bottom flask was charged with
4-butoxy-3-methylbenzoic acid (0.382 g, 1.84 mmol) and 2,5-bis-(4-
hydroxyphenyl)-1,3,4-oxadiazole (0.262 g, 1.03 mmol). The flask was
placed under an Argon atmosphere and 35 mL dry dichloromethane
was added. Then, 4-(dimethylamino)pyridine (0.063 g, 0.52 mmol)
and ethyl dimethylaminopropylcarbodiimide HCl (0.406 g, 2.12 mmol)
were added and the reaction was allowed to stir for 2 days at which
point, TLC analysis (2:1:1 hexanes:dichloromethane:acetone) showed
that the reaction had gone to completion. Additional dichloromethane
and 50 mL water were added to the reaction flask. The mixture was
stirred vigorously for 10 minutes and then transferred to a separatory
funnel. The organic layer was washed twice with water and once with
1 M HCl. The combined aqueous washes were backextracted once with
fresh dichloromethane and the combined organic extracts were then
dried over MgSO4, filtered, and concentrated by rotary evaporation
to give a white solid. The crude product was purified using flash chro-
matography (3:2:1 hexanes:dichloromethane:ethyl acetate) to give a
white solid which was recrystallized from ethanol and a minimal

216/[1686]
L. L. Cooper et al.
TABLE 3 Yields and Elemental Analysis Data for the ODBP Derivatives, 4–9
Compound
Percent yield
C% calc. (found)
H% calc. (found)
N% calc. (found)
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
77
39
57
89
71
78
76
63
65
60
55
41
71.27 (71.38)
71.27 (71.11)
71.91 (71.78)
75.49 (75.26)
71.91 (71.77)
75.49 (75.14)
71.91 (71.58)
75.49 (75.12)
72.49 (72.11)
75.81 (75.43)
70.56 (70.57)
75.81 (75.54)
5.65 (5.58)
5.65 (5.57)
6.03 (6.03)
8.21 (7.87)
6.03 (5.89)
8.21 (8.24)
6.03 (5.80)
8.21 (8.13)
6.39 (6.28)
8.41 (8.16)
7.02 (6.63)
8.41 (8.64)
4.62 (4.63)
4.62 (4.56)
4.41 (4.44)
3.26 (3.33)
4.41 (4.43)
3.26 (3.16)
4.41 (4.36)
3.26 (3.18)
4.23 (4.17)
3.16 (3.08)
3.83 (3.88)
3.16 (3.08)
amount of chloroform to give the product (0.34 g, 57%). Found (C,
71.78; H, 6.03; N, 4.44. C38H38N2O7 requires C, 71.91; H, 6.03; N,
4.41); dH (399.73 MHz; CDCl3; Me4Si) 8.24 (d, 4H, J ¼ 8.8 Hz, Ar),
8.07 (d 2H, J ¼ 8.6 Hz, Ar), 8.02 (s, 2H, Ar), 7.43 (d, 4H, J ¼ 8.8 Hz,
Ar), 6.92 (d, 2H, J ¼ 8.7 Hz, Ar), 4.10 (t, 4H, J ¼ 6.4 Hz, OCH2CH2),
2.31 (s, 6H, CH3), 1.87 (pentet, 4H, J ¼ 6.9 Hz, OCH2CH2), 1.58 (sextet,
4H, J ¼ 7.5 Hz, CH2CH2CH3), 1.04 (t, 6H, J ¼ 7.3 Hz, CH2CH3). dC
(100.55 MHz; CDCl3; Me4Si) 164.2, 164.1, 161.9, 153,8, 132.5, 130.2,
128.3, 127.1, 122.7, 121.2, 120.2, 110.1, 67.9, 31.1, 19.2, 16.2, 13.8.
All other target compounds (4–9) were prepared in an analogous
manner. Yields and elemental analysis data are shown in Table 3.
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