| Abstract: | The catalytic hydrogenation of isoalloxazines in aqueous solutions proceeds non stoichiometrically. This is explained by a reaction side path, which is competitive with the normal reversible 1,5-dihydrogenation and leads to the irreversible reduction of the ‘o-xyloquinoid’ (rather than benzenoid) subnucleus in the (iso)alloxazine system. In trifluoroacetic acid, this irreversible side reaction is found to be complete within several hours, while 8 H-equivalents are absorbed per flavin molecule. The products are shown to be cyclohexano-g]-pteridine-2,4-diones in the normal 5,6,7,8-tetrahydro state, which are oxidized to yield, subsequently, 7,8-dihydro- and ‘fully oxidized’ pteridines (= 6,7,8,9-tetrahydrofavins). The spectral characteristics, tautomeric structures and pK-values are discussed in context with recent data on 6,7,8-trialkylpteridine-2,4-diones 3]. |
