Selective Oxidation of Hydroxymethyl Groups

3 Selective Oxidation of Hydroxymethyl Groups. Platinum on charcoal in the presence of air is the conventional method for selectively oxidising hydroxymethyl groups to carboxylic acids, and thus hexoses to uronic acids anomeric protection is required, however, and the mechanism of this heterogeneous reaction is unknown. The reaction fails with polymers, probably for 

A widely used mediator of the oxidations of carbohydrates, particularly polysaccharides, is the stable nitroxide radical TEMPO (2,2,6,6-tetramethyl-piperidine-l-oxyl) and its 4-substituted derivatives. Removal of a single 

Although the generation of the oxoammonium ion from TEMPO involves one-electron chemistry, the actual oxidation of the substrate is heterolytic, as is shown by the oxidation of cyclopropylcarbinol to cyclopropylaldehyde without ring opening. Substituent effects of X on P-XC6H4CH2OH oxidation are modest, suggesting little charge build-up on the oxidised carbon. 

In accord with its crowded cyclic transition state, the basic pathway has a very high selectivity for oxidation of primary OH groups, whereas the slower acid pathway, with the more open transition state, has lower selectivity, oxidising secondary alcohols only modestly slower than primary ones. 

If the anomeric centre is not protected, TEMPO-mediated oxidations of monosaccharides with CI2 or Br2 yield aldaric acids, i.e. co-diacids in which the all secondary hydroxyl groups are preserved the use of the 4-acetamido derivative of TEMPO is to be preferred because of its greater stability and lower volatility. This method of making aldaric (saccharic) acids supersedes the use of nitric acid, used in Emil Fischer s classical work to make both ends of 

---