Chemical Biology 3OA3, September 2009 Page 29 One of you will use zinc chloride as a catalyst, giving the α-anomer of glucose penta-acetate, while your partner will prepare the β-anomer using sodium acetate. Why does the choice of catalyst alter the stereochemical outcome of the reaction? It has been shown that BOTH catalysts promote interconversion of the two anomers of glucose. The β-anomer reacts rapidly with acetic anhydride, but the α-anomer only slowly. As the β-anomer is consumed, the α-and β-anomers keep re-equilibrating and so α-D-glucose is converted to the β-anomer in preference to reacting. The result is that BOTH catalysts initially form the β-D-glucose penta-acetate. This is the fastest formed product, and we say that it is formed under kinetic control . However, the α-D-glucose penta-acetate is in fact the most stable of the two products: it will be formed under thermodynamic control , i.e. if the two product species can equilibrate. Zinc chloride catalyses
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