Carbohydrates, over-oxidation

For secondary alcohol groups in carbohydrates, although the use of Na(10 ) as co-oxidant with RuO or RuClj is cotmnon for generating RuO in these reactions, use of the sparingly soluble K(10 ) as co-oxidant is said to reduce over-oxidation [91, 171, 247, 280],... 

The reagent RuO /aq. Na(IO )/CCl oxidised a number of pyranoses and was the first catalytic Ru-based system for carbohydrates (Table 2.3) [2]. Although use of Na(10 ) as co-oxidant with RuO or RuClj is a common procedure for generating RuO in these reactions, it was noted that the use of the sparingly soluble K(IO ) in place of Na(10 ) as co-oxidant reduced over-oxidation [332] see also [330, 340, 341]. Conversion of pyranoses to lactones has been accomplished e.g. 2,4-di-O-benzyl-3-0-T)utyldiphenylsilyl-a,p-L-fucopyranose was oxidised by TPAP/NMO/ PMS/CHjCN to 2,4-di-0-benzyl-3-0- butyldiphenylsilyl-L-fuco-l,5-lactone (Fig. 2.17) [155]. 

The analytical phase generally involves the use of very dilute solutions and a relatively high ratio of oxidant to substrate. Solutions of a concentration of 0.01 M to 0.001 M (in periodate ion) should be employed in an excess of two to three hundred percent (of oxidant) over the expected consumption, in order to elicit a valid value for the selective oxidation. This value can best be determined by timed measurements of the oxidant consumption, followed by the construction of a rate curve as previously described. If extensive overoxidation occurs, measures should be taken to minimize it, in order that the break in the curve may be recognized, and, thence, the true consumption of oxidant. After the reaction has, as far as possible, been brought under control, the analytical determination of certain simple reaction-products (such as total acid, formaldehyde, carbon dioxide, and ammonia) often aids in revealing what the reacting structures actually were. When possible, these values should be determined at timed intervals and be plotted as a rate curve. A very useful tool in this type of investigation, particularly when applied to carbohydrates, has been the polarimeter. With such preliminary information at hand, a structure can often be proposed, or the best conditions for a synthetic operation can be outlined. 

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