Disproportionation in hydrolysis of pyrimido[4,5-b]quinoline-2(3H),4(10H)-diones (5-deazaflavins)

Abstract

Treatment of 5-deazaflavins with concentrated aqueous potassium hydroxide led to the exclusive formation of 1,5-dihydro-5-deazaflavins and 1,5-dihydro-5-deazaflavin-5-ones via intermolecular oxidation–reduction between initially formed 5-hydroxy-1,5-dihydro-5-deazaflavins and unchanged 5-deazaflavins; under dilute alkaline conditions the reverse oxidation–reduction between 1,5-dihydro-5-deazaflavins and 1,5-dihydro-5-deazaflavin-5-ones occurred to form the original 5-deazaflavins and 5-hydroxy-1,5-dihydro-5-deazaflavins, which were oxidized to 1,5-dihydro-5-deazaflavin-5-ones by air. When hydrolysis was carried out with dilute alkaline solution, the corresponding 2-oxoquinoline-3-carboxylic acids were obtained besides the disproportionation products 1,5-dihydro-5-deazaflavins and 1,5-dihydro-5-deazaflavin-5-ones. This disproportionation and hydrolytic scission at the 2-position compete with each other. Higher concentrations of hydroxide ion favoured the formation of the reduced 5-deazaflavins and 5-ketones by disproportionation and reduced the proportion of 2-quinolones formed by hydrolytic scission.