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

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• Grind together in a porcelain mortar 4g of anhydrous sodium acetate and 5 g (0.028 mol) of dry a-D-glucose and place the powdered mixture in a 200-ml round-bottomed flask. 

• Add 27 g (25 ml, 0.26 mol) of acetic anhydride, attach a double surface condenser and heat on a boiling water bath until a clear solution is obtained (1), shaking the mixture from time to time. 

• Continue heating for a further 2 hours after a clear solution has been obtained and then pour the reaction mixture on to 250 ml of crushed ice. 

• Allow to stand for 1 hour, stirring occasionally to break up the solid lumps which separate. 

• Filter off the crystals, wash well with cold water and recrystallise from industrial spirit (or from methanol or ethanol) until the purified material has m.p. 131-132 °C, [a], 1 , 8 +4.0° (c4.5 in CHC1 3 ). The yield is 6.2 g (56%).

Conversion of 𝝱-into 𝝰-D-glucose penta-acetate

• Add 0.5 g of anhydrous zinc chloride rapidly to 25 ml of acetic anhydride in a 100-ml round-bottomed flask, fitted with a Liebig condenser, and heat on a boiling water bath to dissolve the solid. 

• Add 5 g of pure 𝝱-D-glucose penta-acetate, continue heating for 30 minutes, pour the mixture on to ice and purify the solid which separates as described above. 

• The effectiveness of the conversion may be monitored by t.l.c. on silica gel plates using cyclohexane/acetone (7:3) and locating the two closely running spots by immersing the developed and dried plate in a tank of iodine vapour.

Notes to keep in mind:

1.  It is dangerous to scale up this experiment without modifying the preparative procedure. If 50 g of glucose is to be acetylated, a 2-litre round-bottomed flask should be fitted with two wide-bore Liebig condensers in series, and a large vessel filled with ice-water should be readily available to plunge the reaction flask into, should the vigorous reaction which ensues on heating need controlling. With a scale using 100 g of glucose a procedure involving the addition of a-D-glucose to a pre-heated sodium acetate-acetic anhydric mixture at such a rate as to keep the mixture under reflux but without the reaction getting out of control has been described.