5-Ethyl-4a-methoxy-1,3-dimethyl-4a,5-dihydrobenzo[g]pteridine-2,4(1H,3H)dione

The title compound, C15H18N4O3, was formed by the reaction of methanol with 5-ethyl-1,3-dimethylalloxazinium perchlorate. Its structure mimics those of possible flavin intermediates in flavoenzymes. The heterocyclic rings are substituted with methyl, ethyl and methoxy groups. The central tricyclic skeleton is bent due to the presence of an sp 3 C atom. There are weak intermolecular C—H⋯O interactions in the structure, forming a three-dimensional network.

The title compound, C 15 H 18 N 4 O 3 , was formed by the reaction of methanol with 5-ethyl-1,3-dimethylalloxazinium perchlorate. Its structure mimics those of possible flavin intermediates in flavoenzymes. The heterocyclic rings are substituted with methyl, ethyl and methoxy groups. The central tricyclic skeleton is bent due to the presence of an sp 3 C atom. There are weak intermolecular C-HÁ Á ÁO interactions in the structure, forming a three-dimensional network.
In this paper, the first crystal structure of the C4a-adduct of alloxazinium salt (Figs. 1 and 2) is reported. The adduct is formed by the reaction of methanol with 5-ethyl-1,3-dimethylalloxazinium perchlorate (Fig. 2). By this reaction, the hybridization of C20 atom (C4a atom in IUPAC numbering of alloxazine moiety) is changed from sp 2 to sp 3 (Fig. 2). This change of hybridization causes a folding of the tricyclic alloxazine skeleton. The value of the interplanar angle between the plane determined by the C2, N3, C5, and N7 atoms and the plane determined by the C9, N10, C11, C12, C13, C14, C15, C16, and N17 atoms is 15.69 (5)°. This angle is larger in comparison with that found in the case of the similar adducts of C-nucleophiles with isoalloxazine derivatives; e.g. the angle between the analogous planes in 4a,5-dihydro-4a-isopropyl-3,10-dimethylisoalloxazine (Bolognesi et al., 1978) is only 6.85 (9)°. The observed 'butterfly' arrangement of the tricyclic alloxazine subunit in the title compound corresponds to the structure of dihydroflavins already published by Werner & Rönnquist (1970) and Norrestam & Von Glehn (1972).
Due to the sp 3 hybridization, C20 atom is shifted out of the alloxazine plane by 0.313 (1)Å. On the other hand, the values of the bond angles around C20 are different from those expected for an sp 3 carbon atom, probably due to the rigidity of the dihydroalloxazine system. The conformation of the ring 1 (C2, N3, C5, N7, C9, C20) is between 5 H 6 and E 6 . The conformation of the ring 2 (C9, N10, C11, C16, N17, C20) is between 5 S 6 and E 6 , rather closer to E 6 . The distances, angles and puckering parameters (Cremer & Pople, 1975) were calculated using PARST97 (Nardelli, 1999).
Three weak intermolecular C-H···O interactions were found forming a three-dimensional network.

Experimental
The crystals of the title compound were obtained from a solution of 1,3-dimethyl-5-ethylalloxazinium perchlorate (20 mg, 0.054 mmol) and dry triethylamine (7.5 µl, 0.054 mmol) in dry methanol (1.8 ml). Single crystals suitable for analysis were grown overnight directly from the reaction mixture. M. p. 384 -386 K.

Refinement
The H atoms were found in the Δρ map and initially refined with the restraints on the bond lengths and angles to regularize their geometry (C methyl -H = 0.96 (2), C methylene -H = 0.97 (2), C aryl = 0.93 (2) Å. U iso (H) = 1.5 U eq C methyl or 1.2 U eq C methylene/aryl . After the convergement the geometrical restraints were substituted by the geometrical constraints.