- In a 250-ml conical flask fitted with a ground glass stopper place 40 ml of 1,2-dichloroethane and 20 g (0.029 mol) of πΌ-D-glucopyranose pentabenzoate (1).
- When all the solid has dissolved add 40 ml (0.225 mol) of a solution of hydrogen bromide in glacial acetic acid (45% w/v HBr), stopper the flask and allow the reaction mixture to stand in the refrigerator overnight or at room temperature for about 2 hours.
- Pour the mixture into ice-water, rinse the flask with 1,2-dichloroethane, separate the organic layer and shake it with several portions of a saturated aqueous solution of sodium hydrogen carbonate until no further effervescence occurs.
- Wash the organic layer with water, dry over magnesium sulphate, filter and remove the 1,2-dichloro-ethane under reduced pressure on a rotary evaporator.
- Dissolve the crystalline solid which remains in dry ether, heating to 35 °C, and slowly add with further heating light petroleum (b.p. 40-60 °C) until a slight persistent cloudiness develops; then add a little more ether to give a clear solution, which is left to cool slowly to room temperature and finally refrigerated.
- Filter off the purified product and allow it to dry in the air; the yield is 16.5 g (88%), m.p. 129-130 °C, [πΌ]α΄ ²ΒΊ + 125° (c2.0 in CHC1 3 ).
Notes to keep in mind:
1. A mixture of anomeric glucose pentabenzoates such
as might be obtained from a benzoylation reaction on glucose without careful
temperature control gives equally good results.
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