P. Kirsch, D. Maillard
FULL PAPER
solution of 1,1,1-trifluoroacetone N-cyclohexylimine (15b, 5.41 g,
28.0 mmol) in THF (30 mL). The resulting deep violet solution was
stirred between –60 °C and –70 °C for 15 min, and a solution of 3-
(trans-4-propylcyclohexyl)oxetane (11, 4.00 g, 21.9 mmol) in THF
(20 mL) was added, followed by BF3·OEt2 (6.28 mL, 50.0 mmol).
The solution was stirred at –60 °C for 2.5 h and was then allowed
to warm to room temperature. After 10 h of stirring at room tem-
perature, the reaction was quenched by addition of a saturated
aqueous NH4Cl solution (80 mL). The aqueous phase was sepa-
rated and extracted with tert-butyl methyl ether (2×50 mL). The
combined organic phases were washed with brine (40 mL), dried
with Na2SO4 and filtered, and the solvents were evaporated to dry-
ness. The residue was purified by chromatography (silica gel; n-
heptane/ethyl acetate, 7:1) to afford a yellow oil (2.66 g). In order
to remove residual traces of the cyclohexylimine, the crude product
was dissolved in a mixture of tert-butyl methyl ether (40 mL) and
hydrochloric acid (10%, 20 mL). The biphasic mixture was vigor-
ously stirred for 15 min, the layers were separated, the organic
phase was dried with Na2SO4 and filtered, and the solvents were
evaporated. The hemiketal 16b was obtained as colourless crystals
(1.58 g, 24%), which were used for the subsequent synthetic step
without further characterization.
mixture was stirred between –60 and –70 °C for 40 min before ad-
dition of a solution of 3-(trans-4-propylcyclohexyl)oxetane (11,
3.0 g, 16.5 mmol) in THF (20 mL) and subsequent slow addition
of BF3·OEt2 (5.10 mL, 40.60 mmol). The solution was stirred at
–60 °C for an additional 90 min, and was then allowed to warm
to room temperature. After the mixture had been stirred at room
temperature for 10 h, saturated aqueous NH4Cl (70 mL) was
added, and the aqueous phase was separated and extracted twice
with tert-butyl methyl ether (30 mL). The combined organic phases
were washed with brine (30 mL), dried with sodium sulfate, filtered
and concentrated in vacuo. The residue was chromatographed (sil-
ica gel; n-heptane/ethyl acetate, 8:1) to afford 16f (4.92 g, 81%) as
a colourless solid, which was used for the subsequent synthetic step
without further characterization.
The hemiketals 16a, 16e, 16g–j, 18, 20 and 22 were prepared ac-
cording to an analogous procedure.
Liquid Crystal 17f: A solution of the adduct 16f (3.0 g, 8.10 mmol),
dissolved in dichloromethane (100 mL), was treated dropwise at
–70 °C with triethylsilane (2.10 mL, 13.0 mmol) and then with
BF3·OEt2 (0.85 mL, 13.0 mmol). The mixture was stirred at the
same temperature for 3 h and was then allowed to warm to –10 °C.
The reaction was quenched and the mixture neutralized by careful
addition of aqueous sodium hydroxide (10%). The organic phase
was separated, and the aqueous phase was extracted with dichloro-
methane (2×40 mL). The combined organic layers were washed
with brine (30 mL), dried with sodium sulfate and concentrated in
vacuo to afford colourless crystals of 17f (2.64 g, 91%), containing
91% (HPLC) of the desired trans isomer. Repeated recrystallization
from n-heptane afforded trans,trans-17f with high purity (Ͼ99% by
HPLC). For mesophase sequence and electrooptical characteristics
The pentafluoroethyl and perfluoro-n-propyl hemiketals 16c and
16d were prepared according to an analogous procedure.
Liquid Crystal 17b: A solution of the hemiketal 16b (1.35 g,
4.57 mmol) in dichloromethane (50 mL) was treated dropwise at
–70 °C with triethylsilane (1.21 mL, 7.50 mmol), followed by
BF3·OEt2 (0.49 mL, 7.50 mmol). The reaction mixture was allowed
to warm to room temperature over 1 h and was then heated to
40 °C and stirred until the complete disappearance of the starting
material 16b was confirmed by TLC after ca. 6 h. After cooling in
an ice bath, the mixture was hydrolysed and neutralized by careful
addition of a solution of aqueous sodium hydroxide (10%). The
aqueous phase was extracted twice with dichloromethane (30 mL),
and the combined organic phases were washed with brine (30 mL),
dried with Na2SO4 and concentrated in vacuo to afford a colourless
oil (1.22 g). Purification by column chromatography (silica gel; n-
heptane/ethyl acetate, 8:1) afforded 17b (0.70 g, 55%), containing
80% (HPLC) trans isomer. Repeated recrystallization from n-hep-
tane afforded pure (96.5% by HPLC) trans-trans-17b. For melting
point and electrooptical characteristics see Table 3. 1H NMR
(500 MHz, CDCl3, 303 K): δ = 4.13 (ddd, J = 11.2 Hz, J = 4.0 Hz,
J = 2.0 Hz, 1 H), 3.68–3.54 (m, 1 H), 3.19 (t, J = 11.2 Hz, 1 H),
2.05–1.95 (m, 1 H), 1.88–1.37 (m, 7 H), 1.35–1.07 (m, 6 H), 1.05–
0.91 (m, 3 H), 0.87 (t, J = 7.3 Hz, 3 H), 0.84–0.76 (m, 2 H) ppm.
13C NMR (75 MHz, CDCl3, 303 K): δ = 124.0 (q, 1JCF = 278.8 Hz,
CF3), 75.2 (q, 2JCF = 31.1 Hz, CHCF3), 72.0, 40.3, 39.6, 39.5, 37.2,
33.0, 30.0, 29.7, 25.7, 24.3, 19.8, 14.2 ppm. 19F NMR (235 MHz,
1
see Table 3. H NMR (500 MHz, CDCl3, 303 K): δ = 7.58 (d, J =
8.4 Hz, 2 H, ar-H), 7.44 (d, J = 8.4 Hz, 2 H, ar-H), 4.29 (d, J =
11.0 Hz, 1 H), 4.19 (dq, J = 11.0 Hz, J = 3.5 Hz, 1 H), 3.33 (t, J
= 11.0 Hz, 1 H), 2.03–1.86 (m, 2 H), 1.81–1.69 (m, 3 H), 1.60–1.23
(m, 6 H), 1.19–1.11 (m, 3 H), 1.07–0.98 (m, 3 H), 0.88 (t, J =
7.3 Hz, 3 H, CH3), 0.89–0.81 (m, 2 H) ppm. 13C NMR (75 MHz,
2
CDCl3, 303 K): δ = 147.1, 129.3 (q, JCF = 32.3 Hz, ar-C-CF3),
3
1
125.8, 125.0 (d, JCF = 3.5 Hz, ar-C), 124.1 (q, JCF = 272.3 Hz,
CF3), 79.1, 72.3, 40.8, 40.0, 39.6, 37.3, 34.2, 33.2, 30.2, 29.8, 27.7,
19.8, 14.2 ppm. 19F NMR (235 MHz, CDCl3, 300 K): δ = –60.68
(s, 3 F, CF3). MS (EI, 70 eV): m/z (%) = 354 (55) [M]+, 335 (15),
323 (15), 210 (17), 198 (12), 185 (12), 172 (37), 164 (26), 159 (20),
123 (40), 109 (52), 95 (44), 83 (100), 79 (17), 69 (79), 55 (77).
The liquid crystals 17a, 17e, 17g–j, 19, 21 and 23 were prepared
according to an analogous procedure.
Hemiketal 20: The dioxanyl hemiketal 20 was prepared analogously
to 16f. The crude product was purified by chromatography (silica
gel; n-heptane/ethyl acetate, 3:1) to furnish 20 as a yellowish solid
in 48% yield. The product was used without further characteriza-
tion as starting material for 21.
3
CDCl3, 300 K): δ = –79.6 (d, JHF = 6.3 Hz, 3 F, CF3). MS (EI,
70 eV): m/z (%) = 278 (18) [M]+, 235 (24), 139 (16), 124 (58), 109
(19), 95 (25), 83 (78), 69 (100), 57 (15), 55 (61).
Liquid Crystal 21: The liquid crystal 21 was prepared from the
dioxanyl hemiketal 20 analogously to the synthesis of 17e. The
crude product was purified by chromatography (silica gel; n-hep-
tane/ethyl acetate, 5:1) to furnish 20 as a colourless solid (82%
yield) containing 88% (HPLC) of the desired trans isomer. Two
consecutive recrystallizations from ethanol afforded trans,trans-21
with high purity (100% purity by HPLC). For mesophase sequence
The liquid crystals 17c and 17d were prepared from the correspond-
ing methyl perfluoroalkyl N-cyclohexylketimines according to an
analogous procedure. Nevertheless, 17c and 17d could not be iso-
lated cleanly because they were accompanied by the corresponding
dihydropyranes 24 and 25, respectively.
Hemiketal 16f: A solution of TMP (3.90 mL, 22.90 mmol) in tetra-
hydrofuran (30 mL) was treated dropwise at –78 °C with nBuLi
(15% solution in hexane, 14.37 mL, 22.90 mmol). The mixture was
stirred at the same temperature for 20 min, and a solution of 4-
(trifluoromethyl)acetophenone N-cyclohexylimine (15f, 6.19 g,
23.00 mmol) in THF (30 mL) was then added slowly. The resulting
1
and electrooptical characteristics see Table 3. H NMR (500 MHz,
CDCl3, 303 K): δ = 4.18 (d, J = 5.3 Hz, 1 H), 4.08–3.99 (m, 3 H),
3.28–3.12 (m, 4 H), 2.03–1.76 (m, 3 H), 1.66–1.61 (m, 1 H), 1.52–
1.15 (m, 8 H), 1.02–0.95 (m, 2 H), 0.90 (t, J = 6.9 Hz, 3 H), 0.89
(t, J = 7.3 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3, 303 K): δ =
3330
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Eur. J. Org. Chem. 2006, 3326–3331