Reactivity of 10-thiaisoalloxazine. Ring-contraction by water (hydroxide ion) followed by redox reaction

Abstract

Heating of 3-methyl-10-thiaisoalloxazine (1) in aqueous acetonitrile results in the formation of 1,5-dihydro-3-methyl-10-thiaisoalloxazine(3),1′-methylspiro[2,3-dihydrobenzothiazole-2,5′-pyrimidine]-2′,4′,6′-trione (4), and 1-(benzothiazol-2-yl)carbonyl-3-methylurea (5). This type of reaction is significantly accelerated by the addition of a base such as sodium bicarbonate. In sharp contrast, 3,10-dimethylisoalloxazine (2) is very stable under these conditions. Our studies suggest that the formation of (3) and (5) arises from a redox reaction between the ring-contracted product (4), initially produced by hydrolysis of (1), and unreacted (1). These reactions can be attributed to the increased susceptibility of (1) to nucleophilic addition and its higher oxidation capacity compared to the isoalloxazine (2).