The use of immobilized enzymes in organic synthesis. Part 8. Oxidation of 1-aryl-3-carbamoylpyridinium chlorides by rabbit liver aldehyde oxidase and bovine milk xanthine oxidase

Oxidation of 1-aryl-3-carbamoylpyridinium chlorides with immobilized rabbit liver aldehyde oxidase gave predominantly 1-aryl-1,6-dihydro-6-oxo-3-pyridinecarboxamides, together with the corresponding 1-aryl-1,4-dihydro-4-oxo-3-pyridinecarboxamides. In general, the site of oxidation by this enzyme is determined by steric factors. The rate of oxidation by free aldehyde oxidase is very sensitive to electronic effects. A more electron-withdrawing aryl substituent increases the reaction rate for oxidation to 6-oxo product. Consequently, a positive ρ value of about 3.6 was calculated for free aldehyde oxidase. Oxidation of these compounds by bovine milk xanthine oxidase yielded mainly 4-oxo products. Preparative-scale reactions with both free and immobilized xanthine oxidase gave low product yields. The oxidation rate of free xanthine oxidase is only slightly affected by substituents in the aryl group.