Page 91 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

epoxidation of allylic alcohols 77

. 50 mL Two-necked flask with a magnetic stirrer bar
. Magnetic stirrer
. Acetone/Dry ice cooling bath equipped with contact thermometer, ÿ35 8C
. Syringes
. Bu Èchner funnel with glass frite, 30 mL
. Glass wool
. Separating funnel, 250 mL
. Rotary evaporator

Procedure

1. A 50 mL two-necked flask equipped with a stirrer bar was placed in an oven
at 120 8C overnight, cooled under vacuum and flushed with nitrogen.
2. The flask was filled with dry dichloromethane (50 mL) and l-(‡)-diisopro-
pyl tartrate (350 mg). The mixture was cooled to ÿ35 8C using the cooling
Ê
bath, then activated powdered 4 A molecular sieves (1.2 g), titanium isoprop-
oxide (297 mL) and a solution of tert-butyl hydroperoxide (5.5 M in isooc-
tane, 7.2 mL) were added sequentially. The mixture was stirred at ÿ35 8C for
1 hour.
3. The (E)-2-methyl-3-phenyl-2-propenol (2.9 mL) was added dropwise via a
syringe over 30 minutes.
4. The reaction was monitored by TLC (eluent: petroleum ether±diethyl ether,
6:4). 2-Methyl-3-phenyl-2-propenol (UV active) visualized with a p-anisal-
dehyde dip stained blue, R f 0.50 and the epoxide stained brown, R f 0.33.
5. The mixture was stirred for 2.5 hours at ÿ35 8C, then the bath was warmed
to 0 8C and the reaction quenched by the addition of water (6 mL). The
resulting mixture was stirred for 30 minutes.
6. The solution was warmed to room temperature. Hydrolysis of the tartrate
was then effected by adding an aqueous solution of sodium hydroxide (30 %)
saturated with sodium chloride (6 mL) and stirring vigorously for 1 hour.
7. The mixture was transferred into a separating funnel. The aqueous phase
was extracted with dichloromethane (2 30 mL). Then the combined or-
ganic layer phase were dried over sodium sulfate, filtered and concentrated
under reduced pressure to give a yellow oil.
If the phase separation does not occur, the reaction mixture is transferred
into a separating funnel. A small amount of methanol is added to the mixture
(5 mL) followed by a very brief shaking. Immediate phase separation often
occurs, allowing for the simple removal of the lower organic phase. If the
emulsion is still a problem, then the mixture is filtered once or twice through a
small plug of glass wool washed with dichloromethane.
8. The crude material was purified by flash chromatography over silica gel
(100 g) using petroleum ether±diethyl ether (8:2) as eluent to give (2S,3S)-
2-methyl-3-phenyloxiranemethanol as a white solid (3 g, 18.6 mmol, 93 %).
The ee (94.5 %) was determined by HPLC (Chiralpak 1 OD column, flow
1 mL/min, ethanol±n-hexane; 1:99); (2S,3S)-enantiomer: R t 16.0 min,