An efficient strategy for the synthesis of chiral liquid crystals using Evans' methodology

Article abstract of DOI:10.1081/SCC-120017193

The synthesis of liquid crystal compound 2 was achieved using Evans' methodology. The strategy was based on three key synthetic reactions: alkylation of chiral amide enolates, Mitsunobu and, finally, esterification. The final compound presents a stable smectic A phase.

Full text of DOI:10.1081/SCC-120017193

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An Efficient Strategy for the Synthesis of Chiral Liquid
Crystals Using Evans' Methodology
Aloir A. Merlo a & Mirele S. Fernandes a
a Chemistry Institute, Federal University of Rio Grande do Sul, Campus do Vale, Porto
Alegre, RS, Brazil
Published online: 21 Aug 2006.
To cite this article: Aloir A. Merlo & Mirele S. Fernandes (2003) An Efficient Strategy for the Synthesis of Chiral Liquid Crystals
Using Evans' Methodology, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 33:7, 1167-1178, DOI: 10.1081/SCC-120017193
To link to this article: http://dx.doi.org/10.1081/SCC-120017193
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 7, pp. 1167–1178, 2003
An Efficient Strategy for the Synthesis
of Chiral Liquid Crystals Using
Evans’ Methodology
*
Aloir A. Merlo and Mirele S. Fernandes
Chemistry Institute, Federal University of Rio Grande
do Sul, Campusdo Vale, Porto Alegre,
RS, Brazil
ABSTRACT
The synthesis of liquid crystal compound 2 wasachieved uisng
Evans’ methodology. The strategy was based on three key synthetic
reactions: alkylation of chiral amide enolates, Mitsunobu and,
finally, esterification. The final compound presents a stable smectic
A phase.
*Correspondence: Aloir A. Merlo, Chemistry Institute, Federal University of
Rio Grande do Sul, Bento GoncalvesAvenue, 9500 Campusdo Vale, 91501
970-Porto Alegre, RS, Brazil; E-mail: aloir@ig.ufrgs.br.
1167
DOI: 10.1081/SCC-120017193
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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1168
Merlo and Fernandes
The design and synthesis of new materials with liquid-crystalline
propertiesand technological application derived from chiral tsarting
reagents constitute a permanent challenge to the synthetic organic
chemist. In the literature, there are several synthetic methodologies,
which have been reported that are related to the synthesis of chiral
liquid crystals (CLCs). Most of them start from chiral reagents with
adequate functionalities. (S)-(ꢀ)-Ethyl lactate,[1] (S)-(ꢀ)-2-methyl-
1-butanol,[2] (R)-(ꢀ)-2-octanol[3] and a-aminoacids[4] are the most
relevant reagents used in the field of the synthesis of chiral liquid crystals.
The reagentsabove are extensively used due to their availability in high
optical purity, low cost and commercial accessibility.
Few examples are found in the literature using stereogenic synthesis
methodology in the preparation of chiral materialswith liquid-crystalline
properties.[5] Walba and co-workers[6] reported the first synthesis of chiral
aryl cyanohydrin ether 1 from chiral oxazolidinone template. The key
step in their synthesis work was the diastereoselective hydroxylation of
chiral imide enolateswith oxaziridine oxidants(Fig. 1).
The enantiomerically pure 2-oxazolidinone system is an excellent
chiral auxiliary and is one of the most popular tools for the synthesis
of chiral a-alkylcarbonyl derivatives. Evans and co-workers[7] were the
pioneers in the development of that synthesis methodology. This chiral
agent—known asEvan’s oxazolidinone—revealsto be effective in the
aspect of the stereochemical control of new C–C bonds, which are
made in the alkylation reaction of chiral amide enolates.[8]
Based upon the considerations discussed above, we report here
preliminary results using chiral oxazolidinone as powerful tool in chiral
enolate alkylation reaction and the synthesis of chiral liquid crystals
using the methodology of Evans. We used this approach to construct
either alcohol b-methyl-8 and phenol 12. The general chemical
structure of the chiral liquid crystal 2 prepared in thiswork ishsown
in the Fig. 2.
Figure 1. Chiral aryl cyanohydrin 1 from Walba’sApproach.

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Chiral Liquid Crystals
1169
Figure 2. Chemical structure of chiral liquid crystal 2
Scheme 1.
Synthesis of the chiral auxiliaries (S)-4-benzyl-2-oxazolidinone (5).
The synthesis of the final compound 2 wascarried out asdecsribed in
Schs. 1–4. The chiral auxiliary 5 (Sch. 1) wasobtained from L-phenyla-
lanine (3) in three steps: esterification reaction (SOCl2, EtOH) followed
by in situ reduction (NaBH4, EtOH/NaOH) to afford the intermediate
(S)-2-amino-3-phenylpropan-1-ol (4) in 67% yield and [a]D ¼ ꢀ24 (1.03;
EtOH).[9] Condensation reaction with diethylcarbonate gives the desired
chiral auxiliary (S)-4-benzyl-2-oxazolidinone (5) in 62% yield {[a]D ¼ þ5
(1.10; EtOH)}.[10]
Alkylation reaction of the chiral amide enolate from 6. The (S)-N-
[11]
acyloxazolidinone 6 wasreadily acceisbsle
by reaction of caprylic
acid with the auxiliary 5 for 48 h at room temperature with 1,3-dicyclo-
hexylcarbodiimide (DCC) asdehydrating agent and catalytic quantities
of 4-(N,N-dimethylamino)pyridine (DMAP) in 65% yield and
[a]D ¼ þ 95 (1.2; EtOH). The key step in our synthetic planning is the
diastereoselective alkylation reaction of amide enolate[12] under kinetic
conditionsfrom ( S)-N-acyl-4-benzyl-2-oxazolidinone (6). The alkylation
reaction of the lithium enolate of 6, prepared with LDA (1.5 Eq.) at
ꢀ78ꢁC for 1 h, followed by addition of alkylating agent (CH3I, 5 Eq.)
for 12 h (ꢀ78ꢁ ! 25ꢁC) givesthe alkylated product 7 asa colorlesoil in
68% yield {[a]D ¼ þ 92ꢁ(1,55, EtOH)} after column chromatography on
silica gel with hexane-ethylacetate as eluent (9/1) (Sch. 2, Fig. 3) and with
[13]
excellent levelsof asymmetric induction.
The reductive removal of the chiral auxiliary (NaBH4, THF:water) [14]
furnishes the chiral 2-methylalkanol 8 in 65% yield and [a]D ¼ ꢀ21
(0.875; CH2Cl2). The chiral auxiliary 5 wasrecovered tsereochemically

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1170
Merlo and Fernandes
Scheme 2.
Figure 3. 1H-NMR spectrum (200 MHz, CDCl3) of the compound 7 obtained
after column chromatography.
unchanged {[a] D¼ þ 5 (1.10; EtOH)}.[10] The diastereomeric excess of the
1
crude mixture could not be estimated by H-NMR and was assumed
to be >95%.
Synthesis of the chiral phenol 12 by Mitsunobu reaction. The synthesis
of the key intermediate 12 outlined in Sch. 3 wasachieved by Mitsunobu
reaction[15] starting from hydroquinone monobenzoate[16] 10 and the
chiral alcohol 8. Compound 10 wassynthesized in 63% yield by conver-
sion of acid 9 to the acyl chloride followed by reaction with hydroqui-
none in pyridine. The correspondent diester-PhCO2C6H4O2CPh was
obtained in 16% yieldsasby-product. We have optimized the reaction
conditionsin the Mitsunobu reaction and obtained compound 11 in 57%
yield. The choice of the solvent is critical due to low solubility of the
compound 10. In CH2Cl2, the reaction yieldsthe deisred product 11,
but all attemptsto repeat that reaction, however, gave only poor yields
(15–25% yield) or failed.[17]

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Chiral Liquid Crystals
1171
Scheme 3.
Scheme 4.
The best reaction conditions use THF as solvent and addition
of a THF solution of 8 and diethyl azodicarboxylate within 1 h to the
solution of 10 and PPh3 at 0ꢁC ! 25ꢁC for 48 h. Alkaline hydrolysis
afforded (S)-(þ)-4-[1-(2-methyl)octyloxy] phenol (12) in 73% yield and
[a]D ¼ þ 47 (1.44; CH2Cl2).
Convergent synthesis of the chiral liquid crystal 2. With compound 12
available, we started to prepare the acid portion 14 outlined in Sch. 4.
Alkylation of methyl p-hydroxybenzoate (13) with alkyl bromidesfol-
lowed by hydrolysis afforded the p-n-alkoxybenzoic acid (14) in 80%
yield.[18] Our convergent synthesis finishes with esterification of phenol
12 using dicyclohexylcarbodiimide (DCC) as dehydrating agent and
DMAP ascatalyst to give the target molecule 2 in 63% yield.
The thermal transitions and the mesomorphic textures were deter-
mined using a Leitz Ortholux polarizing microscope in conjunction with a

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1172
Merlo and Fernandes
Mettler FP-52 heating stage. The texture of the mesophase[19] wasiden-
tified by microscopy studies. The thermal transitions for 2 are: K 39.5 SA
55.5 I. When the sample is cooled from its isotropic phase, the smectic
A phase appears, which exhibited focal-conic texture. The stable enantio-
tropic smectic A phase was found. On heating, the sample enters into the
smectic A phase at 39.5ꢁC and finally meltsto an iostropic liquid
at 55.5ꢁC. The range of temperature for the mesophase is 16ꢁC.
The two peaksobesrved at 39.5
ꢁC and 55.5ꢁC were associated
with K ! SA and SA ! N transitions, respectively, during the microscopy
studies.
In conclusion, we have presented an efficient and practical synthesis
of the chiral liquid crystal 2 using Evans’ methodology.
EXPERIMENTAL SECTION
1H NMR and 13C NMR spectra in CDCl3 were obtained using
Varian-200 and 300 MHz spectrometers using TMS as the internal stan-
dard. IR Spectra were recorded in Nujol on a 3000 Galaxy Seriesspectro-
meter. Optical rotationswere recorded on a Perkin-Elmer 341 polarimeter
at the sodium D line. The thermal transitions and the mesomorphic
textureswere determined using a Leitz Ortholux polarizing microcsope
in conjunction with a Mettler FP-52 heating stage. Mass spectra were
recorded on a GC–MS Shimadzu QP-5050 (EI, 70 eV). Purification by
column chromatography was carried out on 70–230 mesh Merck silica gel
60. L-Phenylalanine, p-hydroxy methylbenzoate, 4-(N,N-dimethylamino)-
pyridine (DMAP), 1,3-dicyclohexylcarbodiimide (DCC), diethylazodi-
carboxilate (DEAD), butyllithium (2.1 M), sodiumborohydride,
diisopropylamine and triphenyl phosphine (PPh3) were purchased from
Aldrich, and used as received unless otherwise specified. Chemicals were
used without further purification. THF was distilled over sodium-benzo-
phenone. Dichloromethane (CH2Cl2) wasdistilled over calcium hydride
(CaH2) under argon immediately before use. Analytical thin-layer chro-
matography (TLC) wasconducted on Merck aluminum plateswith
0.2 mm of silica gel 60F-254. Anhydrous sodium sulfate was used to dry
all organic extracts.
(S )-2-Amino-3-phenylpropan-1-ol (4). The compound
4
was
synthesized according to Ref.[9]. White solid; M.p.: 89–91ꢁC;
Yield: 67%; [a]D ¼ ꢀ24 (1.03; EtOH). 1H NMR (CDCl3, 200 MHz):
2
ꢀ 1.90 (broad, 3H, NH2, OH), 2.52 (dd, 1H, CHCHPh, Jgem ¼ 13.5,
2
3
3Jtrans ¼ 8.4 Hz), 2.80 (dd, 1H, CHCHPh, Jgem ¼ 13.6, Jcis ¼ 4.9 Hz),
3.10 (m, 1H, CHN), 3.39 (dd, 1H, CHHO, 3Jtrans ¼ 7.2,

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Chiral Liquid Crystals
1173
3
2
2Jgem ¼ 10.5 Hz), 3.64 (dd, 1H, CHHO, Jcis ¼ 3.8, Jgem ¼ 10.7 Hz), 7.20
(m, 5H Ar). 13C NMR (CDCl3, 50 MHz): ꢀ 40.5, 54.5, 63.2, 127.1, 128.5,
129.3, 139.0.
(S )-4-Benzyl-2-oxazolidinone (5). The compound 5 wasprepared
according to Ref.[10] White solid; M.p.: 89ꢁC; Yield: 62%; [a]D ¼ þ5
(1.10; EtOH); 1H NMR (CDCl3, 200 MHz): ꢀ 2.90 (m, 2H, CH2Ph),
4.10 (m, 2H, CH2O), 4.40 (m, 1H, CHN), 6.10 (broad, 1H, NH) 7.30
(m, 5H, Ar). 13C NMR (CDCl3, 50 MHz): ꢀ 40.9, 52.0, 69.8, 127.0, 128.8,
128.9, 135.8, 159.5.
(S )-N-Octanoyl-4-benzyl-2-oxazolidinone (6). (S)-4-Benzyl-2-oxazoli-
dinone (5) (7.15 g, 40.0 mmol) and caprylic acid (5.0 g, 34.0 mmol), were
added in dry CH2Cl2 (30 mL) under argon and the solution was stirred at
room temperature for 10 min. DCC (7.6 g, 37 mmol) and DMAP (0.410 g,
0.37 mmol) were then added. The solution was stirred for 48 h at room
temperature. The solution was filtered off and the solvent evaporated.
The crude product waspurified by column chromatography (islica gel,
diethyl ether/hexane ¼ 1:9) to yield 6.7 g (65%) of 6 ascolorlessoil.
1
[a]D ¼ þ45 (1.12; EtOH). H NMR (CDCl3, 200 MHz): ꢀ 0.90 (t, 3H,
CH3), 1.40 (m, 8H, CH2), 1.70 (m, 2H, CH2CH2CO), 2.70–3.10 (m,
3H, CHPh, CH2CO), 3.30 (dd, 1H, CHPh), 4.20 (m, 2H, CH2O), 4.60
(m, 1H, CHN), 7.30 (m, 5H, Ar).
(S )-N-[(S )-(2-Methyl)octanoyl]-4-benzyl-2-oxazolidinone (7). To a
solution of 15.4 mmol of LDA at ꢀ78ꢁC (prepared from diisopropyla-
mine (1.55 g) and n-BuLi 1.3 M (12.0 mL) in 16 mL of anhydrousTHF
was added dropwise a solution of 10.2 mmol (3.10 g) of (S )-N-octanoyl-4-
benzyl-2-oxazolidinone (6) in 20 mL of THF. After the reaction mixture
wastsirred at ꢀ78ꢁC for 1.5 h, 7.25 g (51.0 mmol, 3.17 mL) of MeI in
7 mL of THF at ꢀ78ꢁC was added slowly. The solution was stirred for
4 h at ꢀ78ꢁC, left at room temperature for 8 h and then quenched by the
addition of 20 mL of aqueous saturated ammonium chloride solution.
The solvent and the volatile components were removed by rotary
evaporation, and the resultant yellow slurry was extracted with
dichloromethane (3 Â 75 mL). The organic layerswere combined and
washed with 5% NaHCO3 solution (2 Â 25 mL) and water (2 Â 25 mL),
and dried over anhydrous sodium sulfate, and concentrated to give a
yellow oil. The alkylated product 7 wasobtained asa colorlesoil in
68% yield {[a]D ¼ þ 92ꢁ(1.55, EtOH)} after column chromatography
on silica gel with hexane-ethylacetate as eluent (9/1). Analysis by CG
(SPB-5, 23 psi, 30 m  0.25 m) showed a single peak with Rf 19.33. The
1
minor isomer was not detected in this experiment. H NMR (CDCl3,
200 MHz): ꢀ 0.80 (t, 3H, CH3CH2), 1.15 (d, 3H, CH3CH, J ¼ 6.9 Hz),
1.20 (m, 9H, CH2, CH), 1.70 (m, 1H, CHCO), 2.50 (dd, 1H, CHHPh,

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1174
Merlo and Fernandes
3
2
2Jgem ¼ 13.3, Jtrans ¼ 9.40 Hz), 3.18 (dd, 1H, CHHPh, Jgem ¼ 13.4,
3Jcis ¼ 3.20 Hz), 3.63 (sext., 1H, CHCH3, J ¼ 6.7 Hz), 4.10 (m, 2H,
CH2O), 4.60 (m, 1H, CH–N), 7.20 (m, 5H, Ar). 13C NMR
(CDCl3, 50 MHz): ꢀ 14.6, 17.9, 23.1, 27.7, 29.8, 32.2, 33.9, 38.2, 38.4,
55.7, 66.5, 127.7, 129.3, 129.9, 135.8, 152.5, 177.7.
(S )-2-Methyloctanol (8). Thiscompound wasprepared according to
Ref.[14] To a mixture of the 7 (6.50 mmol) in THF (12.5 mL) wasadded a
solution of sodium borohydride (26.0 mmol, 0.97 g) in water (4 mL)
at 20–25ꢁC. The mixture wasstirred at room temperature for overnight.
ꢁ
To the reaction mixture wasadded 2 N HCl (24 mL) at 20–25 C. The
reaction mixture wasextracted with ethyl acetate (3 Â 50 mL).
The organic phase were washed with brine (2 Â 50 mL) and concentrated.
The solid residue was washed with cold hexane and the chiral auxiliary
was easily separated from the solution by simple filtration. The yellow
oil residue was purified by silica gel chromatography with hexane-ethyl
ether aseluent (9/1) to give compound ( 8) in 65% yield. [a]D ¼ ꢀ21
1
(0.875; CH2Cl2). H NMR (CDCl3, 300 MHz): ꢀ 0.85 (t, 3H, CH3CH2),
0.90 (d, 3H, CH3CH, J ¼ 6.7 Hz), 1.15 (m, 9H, CH2, CH), 1.60 (m, 2H,
2
CH2), 1.80 (broad, 1H, OH), 3.40 (dd, 1H, CHHO, Jgem ¼ 10.3,
2
3
3Jtrans ¼ 6.60 Hz), 3.50 (dd, 1H, CHHO, Jgem ¼ 13.4, Jcis ¼ 5.80 Hz).
13C NMR (CDCl3, 50 MHz): ꢀ 14.0, 16.5, 22.6, 26.9, 29.6, 31.8, 33.2,
35.7, 68.3. IR ꢁmax/cmꢀ1 3360, 2910, 2856, 1640, 1460, 1370, 1034 (film).
4-Hydroxyphenyl benzoate (10). The compound 10 wasysntheiszed
following the procedure reported by Helgee and co-workers.[16]
Data: yield 63%; M.p.: 172ꢁC; 1H NMR (CDCl3/DMSO, 200 MHz):
ꢀ 6.90 (dd, 4H, Ar), 7.52 (m, 3H, Ar), 8.10 (d, 2H, Ar). I.R. ꢁmax/cmꢀ1
:
3368, 2928, 1515, 1463, 1377, 1242, 1105, 770, 722.
(S )-(Q)-4-[1-(2-methyl)octyloxy]phenyl benzoate (11). The solution
of diethylazodicarboxylate (DEAD, 0.87 g, 5 mmol) and (S )-2-methyloc-
tanol (8) (0.490 g, 3.4 mmol) in THF (5 mL) wasadded dropwise to a cold
stirred solution of 4-hydroxyphenyl benzoate (0.73 g, 3.4 mmol),
triphenylphosphine (1.31 g, 5 mmol) and THF (30 mL) for 1 h.
The mixture was stirred at room temperature for 48 h and the solvent
was evaporated from the reaction mixture. The resulting oil was dissolved
in ethylacetate and on addition four timesthe volume of light petroleum.
Triphenylphosphine oxide was precipitated. The solid was filtered
off and the solution was evaporated to dryness. The material
obtained waspurified by column chromatography on ilsica gel
with hexane-ethylacetate aseluent (9/1) in 57% (0.66 g) yield of
yellow oil. 1H NMR (CDCl3, 200 MHz): ꢀ 0.90 (m, 3H, CH3), 1.0
(d, 3H, CH3, J ¼ 6.65 Hz), 1.30 (m, 11H, CH2, CH), 3.70
2
3
(dd, 1H, CHHO, Jgem ¼ 8.90, Jtrans ¼ 6.70 Hz), 3.80 (dd, 1H, CHHO,

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Chiral Liquid Crystals
1175
3
2Jgem ¼ 8.90, Jcis ¼ 5.80 Hz), 6.90 (d, 2H, Ar, J ¼ 9.20 Hz), 7.10
(d, 2H, Ar, J ¼ 9.10 Hz), 7.40–7.70 (m, 3H, Ar), 8.20 (d, 2H,
Ar, J ¼ 7.50 Hz).
(S )-(Q)-4-[1-(2-methyl)octyloxy]phenol (12). To 0.66 g (1.94 mmol)
of (S)-(þ)-4-[1-(2-methyl)octyloxy]phenyl benzoate (11) in the mixture
ethanol/water (20 mL), 0.26 g (4.8 mmol) of potassium hydroxide was
added. The mixture wastsirred overnight at room temperature. The
ethanol wasevaporated and the product wasextracted from diethyl
ether and washed with water (2 Â 25 mL) and dried over Na2SO4 and
purified on silica gel using ethyl acetate/hexane (1:9) to yield 0.34 g
(73%) aspale oil. [ a]D ¼ þ 47 (1.44; CH2Cl2); 1H NMR (CDCl3,
300 MHz): ꢀ 0.90 (m, 3H, CH3), 0.95 (d, 3H, CH3, J ¼ 6.60 Hz), 1.15
(m, 11H, CH2, CH), 3.60–3.90 (m, 2H, CH2O), 6.80 (m, 4H, Ar).
13C NMR (CDCl3, 50 MHz): ꢀ 14.0, 17.0, 22.6, 26.8, 29.5, 31.8,
33.2, 33.5, 74.1, 115.6, 115.9, 149.4, 153.3. IR ꢁmax/cmꢀ1 3360, 2920,
2855, 1600, 1510, 1228, 1100, 1030, 824, 770 (nujol).
(S )-40-[1-(2-methyl)octyloxy]phenyl 4-n-decyloxybenzoate (2). (S)-
(þ)-4-[1-(2-methyl)octyloxy]phenol (12) (70.8 mg, 0.30 mmol) and 4-n-
decyloxybenzoic acid (14) (83.4 mg, 0.30 mmol), were added in dry
CH2Cl2 (10 mL) under argon and the solution was stirred at room tem-
perature for 10 min. 1,3-Dicyclohexylcarbodiimide (DCC) (74.0 mg,
0.36 mmol) and 4-(N,N-dimethylamino)pyridine (DMAP) (0.036 mmol)
were then added. The solution was stirred for 24 h at room temperature.
The solution was filtered and the solvent evaporated. The crude product
waspurified by column chromatography (silica gel, acetate/hexane ¼ 1:9)
to yield 93.0 mg (63%) of 2 aswhite solid. Thermal transitionsfor
2 are:
K 39.5 SA 55.5 I. 1H NMR (CDCl3, 300 MHz): ꢀ 0.90 (m, 6H, CH3), 1.00
(d, 3H, CH3), 1.20–1.90 (m, 27H, CH2, CH), 3.80 (m, 2H, OCH2CH),
4.00 (t, 2H, CH2O), 6.90 (m, 4H, Ar), 7.10 (d, 2H, Ar, J ¼ 8.6 Hz), 8.10
(d, 2H, Ar, J ¼ 9.0 Hz). 13C NMR (CDCl3, 50 MHz): ꢀ 14.09, 17.02, 22.65,
25.95, 26.88, 29.08, 29.19, 29.23, 29.30, 29.34, 29.53, 29.55, 31.80, 31.84,
31.87, 33.13, 33.48, 68.26, 73.60, 114.20, 115.03, 121.66, 122.41, 132.17,
144, 30, 156.93, 163.40, 165.32.
ACKNOWLEDGMENT
The authorsacknowledge PADCT-CNPq, FAPERGS for financial
support. Mirele S. Fernandes is a undergraduate student and acknowl-
edgesPADCT-III and PIBIC-UFRGS for a fellowship.

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1176
Merlo and Fernandes
REFERENCES
1. (a) Merlo, A.A.; Gallardo, H. (S)-(À)-Ethyl lactate asa convenient
precursor for synthesis of chiral liquid crystals. Synt. Comm. 1993,
23, 2159; (b) Merlo, A.A.; Livotto, P.R.; Gallardo, H.; Taylor, T.R.
Synthesis and phase behavior of ferroelectric chiral imines from (S)-
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Received in the USA May 15, 2002