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8 When H2NOTMS or H2NOTBDMS
was used, a mixture of the corresponding ω-hydroxy-O-silylated oxime and the desilylated
oxime was obtained. Use of azeotropic removal of water instead of
MgSO4, again gave a similar mixture.
9 In contrast, treatment of desilylated
congener of 2a with mesyl chloride (1 equiv)
and Et3N (2 equiv) gave an inseparable complex mixture.
12 The reaction required a prolonged
reaction time without MS 4Å.
13 Typical procedure. Preparation of 4a from 1a: A
mixture of 1a (500 mg, 3.12 mmol) and MgSO4 (1.5
g) in toluene (5 mL) was heated at reflux for 5 min. To this mixture
were added successively H2NOTBDPS (2.54 g, 9.36 mmol)
and PPTS (39.0 mg, 0.156 mmol) at the same temperature. After further
heating for 15 min, MgSO4 was filtered off, and the filtrate
was washed successively with an aqueous saturated solution of NaHCO3 and
brine, dried (MgSO4), and concentrated under reduced
pressure. The residue was purified by column chromatography on silica
gel with n-hexane-EtOAc (2:1)
to give 2a. Compound 2a was dissolved
in CH2Cl2 (8 mL). Mesyl chloride (0.73 mL,
9.26 mmol) and Et3N (0.60 mL, 9.27 mmol) were added to
the stirred solution at 0 °C. After stirring for 15 min,
water was added to the mixture, and the whole was extracted with CHCl3.
The organic phase was washed with brine, dried (MgSO4),
and concentrated under reduced pressure. The residue was purified
by column chromatography on silica gel with n-hexane-EtOAc
(3:1) to afford a 65:35 mixture of (E)-3a and (Z)-3a (1.51 g, 98% from 1a). (E)-3a: 1H NMR (500 MHz,
CDCl3) δ 1.09 (9 H, s), 1.36 (3 H, s), 1.53
(3 H, s), 2.97 (3 H, s), 4.16 (1 H, dd, J = 6.8,
11.2 Hz), 4.20 (1 H, dd, J = 4.5,
11.2 Hz), 4.46 (1 H, br dt, J = 4.5,
6.8 Hz), 4.79 (1 H, br t, J = 7.3
Hz), 7.39-7.41 (5 H, m), 7.64-7.69 (6 H, m). (Z)-3a: 1.11
(9 H, s), 1.31 (3 H, s), 1.49 (3 H, s), 2.86 (3 H, s), 4.08 (1 H,
dd, J = 5.5, 11.0 Hz), 4.27
(1 H, dd, J = 2.8, 11.0 Hz),
4.76 (1 H, br dt, J = 2.8, 7.3
Hz), 5.44 (1 H, br dd, J = 3.7,
7.3 Hz), 7.19 (1 H, d, J = 3.7
Hz), 7.36-7.43 (5 H, m), 7.61-7.68 (5 H, m). To
a boiling suspension of 3a obtained above
(603 mg, 1.23 mmol) and MS 4A (powder, 2.5 g) in THF (50 mL) was
added a solution of TBAT (682 mg, 1.23 mmol) in THF (3 mL), and
the mixture was further heated at the same temperature for 7 min.
After cooling, MS 4A was filtered off and the filtrate was concentrated
under reduced pressure. The residue was purified by column chromatography
on silica gel with EtOAc-MeOH (1:0 to 8:1) to give 4a (135 mg, 70%), mp 110-112 °C
(diisopropyl ether), [α]D
26 -26.3
(c 0.50, CH2Cl2) [lit.
[4]
mp 110-111 °C, [α]D
20 -28.0
(c 0.46, CH2Cl2)]. 1H
NMR (500 MHz, CDCl3) δ 1.38 (3 H, s), 1.47 (3
H, s), 4.05 (1 H, br d, J = 15.1
Hz), 4.14 (1 H, br dd, J = 4.4,
15.1 Hz), 4.92 (1 H, br t, J = 6.4 Hz),
5.31 (1 H, br d, J = 5.9 Hz),
6.89 (1 H, br s); 13C NMR (125 MHz,
CDCl3) δ 25.6, 27.1, 67.9, 73.5, 79.8, 112.1, 132.5.
The 1H NMR spectral data are identical with
those previously reported.
[4]