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C. V. Yelamaggad et al. / Tetrahedron Letters 46 (2005) 2623–2626
N
F
F
O
8-FBS : R = n-Octyloxy (58%)
N
N
R'
O
10-FBS : R = n-Decyloxy (60%)
R
N
(S)8-FBS : R = (1S)-1-Methylheptyloxy (57%)
(R)8-FBS : R = (1R)-1-Methylheptyloxy (61%)
O
O
1a : R' = H; 1b : R' = Br
n-FBS
F
F
F
F
F
F
(i ) or (ii)
(iv )
(iii)
HO
R
B(OH)2
R
3a : R = n-Octyloxy (88%)
4a : R = n-Octyloxy (93%)
4b : R = n-Decyloxy (83%)
4c : R = (1S)-1-Methylheptyloxy (78%)
4d : R = (1R)-1-Methylheptyloxy (76%)
3b : R = n-Decyloxy (66%)
3c : R = (1S)-1-Methylheptyloxy (68%)
3d : R = (1R)-1-Methylheptyloxy (64%)
Scheme 1. Reagents and conditions: (i) 1-bromooctane/decane, anhyd K2CO3, acetone, reflux, 12 h; (ii) (R)/(S)-2-octanol, Ph3P, DEAD, THF, 10–
15 ꢁC to rt, 12 h; (iii) n-BuLi, THF, [(CH3)2CHO]3B, ꢀ70 ꢁC, aq HCl; (iv) 1b, [(C6H5)3P]4Pd, Na2CO3, DME, reflux, 12 h.
produce structures with properties between those of
covalent and ionic LCs.
The target compounds thus prepared were characterized
by spectroscopic and elemental analyses. The liquid
crystalline properties of the target molecules were inves-
tigated primarily with optical polarizing microscopy
(OPM) and differential scanning calorimetry (DSC).
The results are summarized in Table 1. Compounds 8-
FBS and 10-FBS displayed an enantiotropic SmA phase
as shown by the OPM observation of the characteristic
focal-conic texture in slides treated for planar orienta-
tion and a psuedoisotropic texture when treated for
homeotropic geometry. This observation supports our
Our first targets were both conventional and non-con-
ventional optically active mesoionic LCs. It would be
advantageous if the molecular design directed the
self-assembly of the molecules into smectic structures.
In view of this, for conventional systems a biphenyl
core substituted with a ÔfloppyÕ tail and a polar sydnone
moiety as terminal substituent appeared to be an ideal
target.8a Furthermore, earlier investigations have re-
vealed that lateral fluoro-substituents in rigid (e.g.,
biphenyl or terphenyl) cores help in reducing the melt-
ing point of the parent system.9 It is also well establi-
shed that the transverse polarity and hence dielectric
biaxiality of smectic LCs can be increased by the incor-
poration of lateral fluoro-substituents in the core.
Thus, a laterally fluoro-substituted biphenyl core with
a chiral flexible tail and a sydnone ring as terminal enti-
ties appeared to be favourable systems. Initially, achi-
ral systems were prepared and their LC behaviour
was found to agree with the molecular design men-
tioned above. Both achiral (8-FBS and 10-FBS) and
chiral [(S)8-FBS and (R)8-FBS] target fluorobiphenyl-
sydnones (n-FBS), were prepared by Suzuki coupling8c
of 3-(4-bromophenylsydnone) 1b with 2,3-difluoro-4-
alkoxyphenylboronic acids8a,b (4a–d) as shown in
Scheme 1.
Molecular structural characterization data for selected compounds:
(S)8-FBS: A brownish solid; mp: 50–51 ꢁC; Rf: 0.48 in 50% EtOAc–
23
hexane; ½aꢁD ꢀ22 (c 1, CHCl3); UV–vis (CHCl3): kmax = 296 nm,
e = 2.53 · 103 L molꢀ1 cmꢀ1; IR (KBr pellet): mmax cmꢀ1 3116, 2930,
2858, 1759, 1631, 1056 and 850; 1H NMR (200 MHz, CDCl3): d 7.78
(s, 4H, Ar), 7.13 (dt, J = 8 and 2.2 Hz, 1H, Ar), 6.86 (dt, J = 7.2 and
2.2 Hz, 1H, Ar), 6.75 (s, 1H, sydnone-H), 4.46 (m, 1H, 1 · –CH),
1.82–1.46 (m, 10H, 5 · CH2),1.38 (d, J = 6 Hz, 3H, 1 · CH3) and 0.89
(t, J = 6.6 Hz, 3H, 1 · CH3); 13C NMR (100 MHz, CDCl3): d 168.94,
149.39 (dd, 1JC-F = 206 Hz, 2JC-F = 13 Hz), 147.90 (d, 2JC-F = 12 Hz),
1
2
142.7 (dd, JC-F = 246 Hz, JC-F = 14 Hz), 139.58, 133.85, 130.44,
2
123.52, 121.46, 120.43 (d, JC-F = 10.8 Hz), 112.08, 93.50, 76.99,
36.42, 31.78, 29.22, 25.40, 22.60, 19.83 and 14.06; MS (FAB+): m/z
for C22H25F2N2O2 (M+1), calcd: 403; found: 403; Elemental analysis:
calculated (found): C, 65.66 (65.73); H, 6.03 (6.01); N, 6.5 (6.96). DS-
5: A yellow solid; mp: 215 ꢁC (decomposes); Rf: 0.40 in 20% EtOAc–
23
hexane; ½aꢁD 8.3 (c 1, CHCl3); IR (KBr pellet): mmax in cmꢀ1 2948,
Recently chiral dimers possessing a cholesteryl ester unit
as the chiral entity joined to other aromatic mesogens
through a polymethylene spacer have attracted attention
as they show remarkable mesomorphic properties.5b–d
In particular, a dimer formed by joining a cholesteryl
ester moiety to a SchiffÕs base or salicylaldimine entity
through a paraffinic spacer supports the formation of
smectic structures.5d Therefore we aimed to prepare chi-
ral mesoionic dimers by covalently connecting a chol-
esteryl ester entity to mesoionic salicylaldimines
through either an odd or an even paraffinic spacer. Four
dimers DS-3, DS-4, DS-5 and DS-7 were synthesized by
condensing cholesteryl n-(3-hydroxy-4-formylphenoxy)-
2867, 2364, 1732, 1624, 1593, 1119 and 835; 1H NMR (400 MHz,
CDCl3): d 13.04 (s, 1H, 1 · –OH), 8.55 (s, 1H, 1 · –CH@N), 7.76 (d,
J = 8.8 Hz, 2H, Ar), 7.44 (d, J = 8.8 Hz, 2H, Ar), 7.31 (d, J = 8.3 Hz,
1H, Ar), 6.71 (s, 1H, 1 · –CH), 6.53 (m, 2H, Ar), 5.37 (br d,
J = 4.2 Hz, 1H, 1 · olefinic), 4.63 (m, 1H, 1 · –CHOCO), 4.03 (t,
J = 6.4 Hz, 2H, 1 · –OCH2), 2.33 (m, 4H, 2 · allylic methylene),
2.02–1.10 (m, 32H, 13 · –CH2, 6 · –CH), 1.02 (s, 3H, 1 · –CH3), 0.91
(d, J = 6.5 Hz, 3H, 1 · CH3), 0.87 (d, J = 1.7 Hz, 3H, 1 · CH3), 0.85
(d, J = 1.7 Hz, 3H, 1 · CH3) and 0.68 (s, 3H, 1 · CH3); 13C NMR
(100 MHz, CDCl3): d 172.92, 164.58, 163.97, 163.84, 152.65, 139.76,
134.28, 122.78, 122.66, 122.45, 112.76, 108.30, 101.74, 93.35, 73.92,
68.13, 56.77, 56.27, 50.16, 42.39, 39.83, 39.57, 38.24, 37.07, 36.66,
36.25, 35.81, 34.57, 31.96, 28.76, 28.23, 28.03, 27.90, 25.58, 24.76,
24.32, 23.88, 22.55, 21.10, 19.33, 18.76 and 11.89; MS (FAB+): m/z
for C48H66N3O6 (M+1), calcd: 780.5; found: 781; Elemental analysis:
calculated (found): C, 73.91 (73.65); H, 8.40 (8.62); N, 5.39 (5.55).
alkanoates5d (5a–d) with 3-(4-aminophenyl)sydnone6j
2
as depicted in Scheme 2.