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T. T. Upadhya et al. / Tetrahedron: Asymmetry 10 (1999) 2899–2904
repeatedly slurried in fresh benzene, which was then filtered off. This process was repeated two more
times and after the last solvent removal the flask containing the white solid was attached to vacuum line
1
and dried for 10 h to give methoxymethylenetriphenyl phosphonium chloride. Yield: 18 g (90%); H
NMR (200 MHz, CDCl ): δ 3.7 (s, 3H), 5.9 (d, J=4 Hz, 2H), 7.75–7.9 (m, 15H).
3
Potassium tert-butoxide (1.23 g, 0.011 mol) was added in three portions to vigorously stirred slurry
of methoxymethylenetriphenylphosphonium chloride (3.76 g, 0.011 mol) in 25 ml diethyl ether. Within
5
0
min a red solution resulted and this was allowed to stir for another 1.5 h. Hydroxy aldehyde 3 (1 g,
.0098 mol) was diluted to 5 ml with dry diethyl ether and this solution was added over 10 min to the
reaction mixture. The reaction was then allowed to stir at room temperature for 10 h, at which time 10
ml of water was added followed by 10 ml of pet. ether. The aqueous layer was separated and the organic
layer was washed with water and brine, dried over Na SO and concentrated to give impure olefin, which,
2
4
on column chromatography purification (10% ethyl acetate in pet. ether), gave 2:3 (E) and (Z) isomers
1
−1
[
3
determined by H NMR (200 MHz)] of hydroxy enol ether 4. Yield: 1.02 g (80%); IR (neat, cm ):
1
400–3300, 1610, 1100; H NMR (200 MHz, CDCl ): δ 1.1 (s, 6H), 3.5 (s, 2H), 3.6 (s, 3H), 4.15 (d,
3
J=6.5 Hz, 1H), 5.8 (d, J=6.5 Hz, 1H), 4.7 (d, J=13 Hz, 1H), 6.35 (d, J=13 Hz, 1H) {mixture of cis and
trans isomers}.
2.3. Preparation of (R)-pantolactone 2
To a well-stirred mixture of (DHQD) PHAL (0.018 g, 0.023 mmol), K FeCN (2.27 g, 6.9 mmol),
2
3
6
K CO (0.94 g, 6.9 mmol), OsO (0.0012 g, 0.0046 mmol) and methane sulfonamide (0.22 g, 2.3 mmol)
2
3
4
in tert-butanol–water (5 ml each) at 0°C, the hydoxy enol ether 4 (0.3 g; 2.3 mmol) was added. The
reaction mixture was stirred for 16 h at 0°C. Solid sodium sulphite (1 g) was added and the reaction
mixture was stirred for an additional hour. CH Cl (20 ml) was added to the reaction mixture and, after
2
2
the separation of the layers, the aqueous phase was continuously extracted with CH Cl . The combined
2
2
organic extracts were dried over Na SO and concentrated to give impure 2, which was further purified
2
4
by column chromatography (20% ethyl acetate in pet. ether) to give 2 as solid (for characterization of 2
vide infra under experimental). Yield: 0.024 g (8%); mp 88°C; [α]D25 −22.8 (c, 2 in H O); lit. [α]D
25
2
−
50.7 (c, 2.05 in H O).
2
2.4. Preparation of bromo ether 5 using NBS
Ethyl vinyl ether (2 g, 0.0278 mol) has been added dropwise over a 15 min period to a mixture of
methallyl alcohol (4 g, 0.0556 mol) and N-bromosuccinimide (4.95 g, 0.0278 mol) in 20 ml CH Cl at
2
2
−48°C. The reaction mixture was stirred for 3 h at −48°C, then brought to room temperature and extracted
with CH Cl (2×25 ml). The organic layer was separated and washed with NaOH (10%), H O and brine,
2
2
2
dried over Na SO and concentrated to give crude 5 which, after column chromatography purification
2
4
−1
(
1
2
5% ethyl acetate in pet. ether), gave pure 5. Yield: 5.24 g (85%); IR (neat, cm ): 1655, 1446, 1120,
061, 901, 619; H NMR (CDCl , 200 MHz): δ 1.25 (t, J=5.5 Hz, 3H), 1.8 (s, 3H), 3.35 (d, J=5 Hz,
H), 3.7 (q, J=4 Hz, 2H), 4.0 (d, J=7 Hz, 2H), 4.7 (t, J=5 Hz, 1H), 4.95 (d, J=18 Hz, 2H); C NMR (50.3
1
3
13
MHz, CDCl ): δ 15.41, 19.73, 31.93, 62.70, 70.84, 101.40, 112.76, 141.90; MS m/z (% rel. intensity):
3
+
2
1
4
22.5 (M , 1), 179 (2), 154 (28), 153 (23), 151 (28), 150 (30), 129 (63), 126 (40), 125 (60), 123 (100),
00 (35), 99 (27), 85 (20), 72 (23), 71 (21), 55 (25), 54 (22); elemental analysis: C H O Br requires C,
8
15
2
3.24; H, 6.76; Br, 36.03%. Found C, 43.25; H, 6.80; Br, 36%.