A-Glucose penta-acetate

Conversion of (3- into a-glucose penta-acetate. Add 0-5 g. of anhydrous zinc chloride rapidly to 25 ml. of acetic anhydride in a 200 ml. round-bottomed flask, attach a reflux condenser, and heat on a boiling water bath for 5-10 minutes to dissolve the solid. Then add 5 g. of the pure P glucose penta-acetate, and heat on a water bath for 30 minutes. Pour the hot solution into 250 ml. of ice water, and stir vigorously in order to induce crystaUisation of the oily drops. Filter the solid at the pump, wash with cold water, and recrystaUise from methylated spirit or from methyl alcohol. Pure a-glucose penta-acetate, m.p. 110-111°, will be obtained. Confirm its identity by a mixed m.p. determination. 

Acetates. Complete acetylation of all the hydroxyl groups is desirable in order to avoid mixtures. In some cases, the completely acetylated sugars may be obtained in the a- and p-forms depending upon the catalyst, e.g., zinc chloride or sodium acetate, that is employed in the acetylation. The experimental details for acetylation may be easily adapted from those already given for a- and p-glucose penta-acetates (Section 111,137). 

Conversion of /1-into a-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... 

P-Glucose penta-acetate. Grind together in a dry porcelain or glass mortar 4 g. of anhydrous sodium acetate (Section 11,50,d) and 5 g. of dry glucose and place the powdered mixture in a 200 ml. round-bottomed flask. Add 25 ml. of acetic anhydride, attach a reflux condenser and heat on a water bath until a clear solution is obtained (about 30 minutes shake the mixture from time to time). Continue the heating for a further 2 hours. Pour the reaction mixture into 250 ml. of ice-cold water in a beaker. Break up the solid lumps and allow to stand with occasional stirring for about 1 hour. Filter the crystals at the pump, wash well with water, and recrystallise from methylated spirit (or from methyl or ethyl alcohol) until the m.p. is 131-132° two recrystallisations are usually necessary. The yield is 7 g. 

The Acid-Catalyzed Anomerization of the o-Glucose Penta Acetates. A Kinetic Thermodynamic and Mechanistic Study." /. Am. Chem. Soc. 73,2659 (1951). Anderson, C. B., and Sepp, D. T. "Conformation and the Anomeric Effect in 2-Halotetrahydropyrans."/. Org. Chem. 32,607 (1967). Eliel, E. L., and Giza, C. A. "Conformational Analysis. XVll1.2-Alkoxy- and 2-Alkylthiotetrahydropyrans and 2-Alkoxy-l,3-Dioxanes. Anomeric Effect." /. Org. Chem. 33,3754 (1968). Bottom Another example of a strong conformational bias introduced by the anomeric effect. 

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