Synthesis and structural studies of flavin and alloxazine adducts with O-nucleophiles

Five flavin (isoalloxazine) and alloxazine adducts with O-nucleophiles, 5-ethyl-4a-hydroxy-3,7,8,10-tetramethyl-4a,5-dihydroisoalloxazine (1a-OH), 5-ethyl-4a-hydroxy-3,10-dimethyl-4a,5-dihydroisoalloxazine (1b-OH), 5-ethyl-4a-methoxy-3,10-dimethyl-4a,5-dihydroisoalloxazine (1b-OMe), 5-ethyl-4a-hydroxy-1,3-dimethyl-4a,5-dihydroalloxazine (2a-OH) and 5-ethyl-4a-methoxy-1,3-dimethyl-4a,5-dihydroalloxazine (2a-OMe) were prepared from the corresponding salts, 5-ethyl-3,7,8,10-tetramethylisoalloxazinium (1a), 5-ethyl-3,10-dimethylisoalloxazinium (1b) and 5-ethyl-1,3-dimethylalloxazinium (2a) perchlorates by the addition of a nucleophile (water or methanol) and triethylamine as a base. The prepared adducts represent artificial analogs of flavin cofactor derivatives which are essential for the functioning of flavoenzymes. They were characterized by 1H and 13C NMR, HR-MS and UV–VIS spectra. In the cases of 1a-OH, 1b-OH, and 2a-OMe, the crystal structures were determined by X-ray diffraction. Flavinium and alloxazinium salts are in rapid equilibria with their adducts in water or methanolic solutions without the presence of a base. It was found that the equilibrium constants for flavin adduct formation is higher by six orders of magnitude than those for alloxazine derivatives. The presence of the sp3 hybridized C4a atom in the molecule of the adducts causes deviation from planarity. The interplanar angles between benzene and the pyrimidine ring were found to be 31.5°, 23.64° and 15.62° for 1a-OH, 1b-OH and 2a-OMe, respectively, which are much higher than those of previously published adducts of C-nucleophiles. In isoalloxazine adducts, delocalization of π electrons between the N10–C10a and C10a–N1 bonds was detected while the length of the N10–C10a and C10a–N1 bonds in the alloxazine adducts corresponds to a double and single bond, respectively.

► Synthesis of adducts of flavinium and alloxazinium salts with water and methanol.
► Flavins are substantially more reactive towards nucleophiles than alloxazines.
► Structure of adducts was solved by NMR, HR-MS and X-ray diffraction.
► The sp3 hybridized C4a atom in the adducts causes deviation from planarity.