SYNTHESIS OF 1,2,3,4,6-PENTA-O-ACETYL-𝛂-D-GLUCOPYRANOSE (𝛂-D-Glucopyranose penta-acetate)

Method A

• Into a 100-ml round-bottomed flask place 0.5 g of anhydrous zinc chloride (1) and 13.5 g (12.5 ml, 0.13 mol) of acetic anhydride; attach a Leibig reflux condenser and heat the mixture on a boiling water bath for 5-10 minutes with occasional shaking until the zinc chloride has largely dissolved. 

• Add slowly 2.5 g (0.014 mol) of powdered a-glucose, shaking the mixture gently during the addition to control the vigorous reaction which ensues. 

• Finally heat the flask for 1 hour on a boiling water bath (2). 

• Pour the contents of the flask into 125 ml of ice-water and stir vigorously to assist the hydrolysis of unreacted acetic anhydride. 

• After about 30 minutes the oil which first separates will gradually solidify. 

• Filter, wash well with cold water and recrystallise several times from industrial spirit until the m.p. is constant. The pure product melts at 110-111 °C; the yield is 3.5 g (63%).

Method B

• To a mixture of 16.25g (15ml, 0.16mol) of acetic anhydride and 25 ml of glacial acetic acid in a conical flask add 5 g (0.028 mol) of powdered glucose. 

• Add dropwise (Pasteur pipette) and with shaking 1 ml of perchloric acid-acetic anhydride catalyst (3), at such a rate that the temperature of the mixture does not exceed 35 °C. 

• Leave at room temperature for 30 minutes and then pour the liquid into a mixture of ice and water. 

• Filter off the crystalline solid which separates on vigorous stirring and wash it thoroughly with cold water. 

• Recrystallise from industrial spirit until the m.p. is constant; the pure product has m.p. 110-111 °C, [a]f> 8 + 101.6° (cO.28 in CHC1 3 ). The yield is 8g(72%).

Notes to keep in mind:

1.  Zinc chloride is extremely deliquescent and it must therefore be introduced into the flask as rapidly as possible. Place a small stick of zinc chloride in a glass mortar, powder rapidly, and weigh out the required amount.

2.  Although the time of heating may be reduced to 30 minutes by heating in an air bath the product is somewhat discoloured and requires at least one recrystallisation involving the use of decolourising charcoal.

3.  The perchloric acid-acetic anhydride catalyst may be prepared by adding 1 .0 g of 60 per cent perchloric acid to 2.3 g of acetic anhydride maintained at °C.