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M. FUJITA ET AL.
Preparation of (R)-4-methylcyclohexylidene-
methyl tri¯ate (6)
Product determinations
About 2 mg of a sample of 6 was dissolved in 4 ml of an
alcoholic or aqueous solvent in a Pyrex tube in the
absence of any salt or in the presence of tetrabutyl-
ammonium bromide (0.1 M). The sealed tube was left in a
silicone oil bath at 140 Æ 1°C for 4–28 days. Products
were extracted with diethyl ether or pentane containing
tetradecane as an internal standard for GC determination.
After removal of most of the solvent, the residual solution
was subjected to GC and GC–MS.
To a solution of (R)-817 of 85% ee (1.1 g) in diethyl ether
(14 ml) was added 1.56 M t-BuLi in pentane (9.0 ml)
dropwise at À78°C in 5 min. After stirring for an
additional 90 min at À78°C, Me2PhSiCl (1.05 ml) was
added to the mixture. The mixture was then allowed to
warm to room temperature, quenched by addition of
water and extracted with ether diethyl (Â3). The extracts
were purified by chromatography (SiO2, eluent hexane)
to give (R)-1-dimethylphenylsilylmethylene-4-methylcy-
clohexane (0.90 g, 63% yield) as a colorless oil.
To a solution of the silane (0.90 g) in CH2Cl2 (35 ml)
was added a solution of mCPBA (0.77 g) in CH2Cl2
(20 ml) dropwise in the presence of a small amount of
aqueous NaHCO3 in 20 min at 0°C. The mixture was
stirred for 90 min and extracted with CH2Cl2 (Â3) after
addition of aqueous NaHCO3. The extracts were purified
by chromatography (SiO2, eluent 6% diethyl ether in
hexane) to give a diastereomeric mixture of the epoxide
(0.91 g, 95% yield) as a colorless oil.
REFERENCES
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To a solution of the epoxide (50 mg) in CH2Cl2 (1 ml)
was added BF Et O (24 ml) at À78°C. The mixture was
3Á
2
stirred for 15 min, quenched with aqueous NaHCO3 at
À78°C, extracted with diethyl ether (Â3) and purified by
chromatography (SiO2, eluent 3% diethyl ether in
hexane) to give (R)-1-dimethylphenylsilyloxymethy-
lene-4-methylcyclohexane (31 mg, 62% yield) as a
colorless oil.
To a mixture of 0.83 M MeLi in diethyl ether (1.0 ml)
and 1,2-DME (1 ml) was added 1,2-DME solution
containing the silyl enol ether (105 mg) at À78°C. After
stirring at 0°C for 60 min, triflic anhydride (0.15 ml) was
added at À78°C. The mixture was allowed to warm to
room temperature and stirred overnight. After quenching
with aqueous NaHSO3, the mixture was extracted with
pentane and purified by chromatography (SiO2 eluent
hexane) to give (R)-4-methylcyclohexylidenemethyl
triflate (6) (54 mg, 52% yield) as a colorless oil. Chiral
GC analysis using Chirasil-DEX CB indicated that the
retention time of (R)-6 (17.2 min) is shorter than that of
(S)-6 (18.1 min) at a column temperature of 90°C. The
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ee of the
R
isomer was 73% in this run.
20
ꢀ À7:3(c = 1.15, CHCl3) (73% ee); 1H NMR
D
(400 MHz, CDCl3) ꢁ 6.34 (s, 1H), 2.77–2.70 (m, 1H),
2.20–2.12 (m, 1H), 2.00–1.93 (m, 1H), 1.83–1.78 (m,
3H), 1.59–1.50 (m, 1H), 1.05–0.93 (m, 2H), 0.90 ppm (d,
J = 6.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) ꢁ 133.3,
127.6, 118.6 (q, J = 318.5 Hz), 35.6, 34.6, 32.2, 29.3,
25.5, 21.8 ppm; MS (EI), m/z (relative intensity,%) 258
(6, M ), 107 (78), 79 (76), 69 (83), 55 (100); HRMS (EI),
calculated for C9H13SO3F3 (M) 258.0538, found
258.0557.
Another experiment gave (R)-6 of 68% ee. Racemic 6
was also prepared in the same way from the racemic
starting material.
Copyright 2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 550–555