Crystal structure of 6-amino-3-ethyl-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one.

interpret the detailed mechanisms of reactions of tautomeric heterocycles properly. The correct dominant tautomeric structure also makes it possible to interpret the biological activity and functions of potential tautomeric heterocycles.1 Many of these aza/deaza analogues of purine and their nuclosides have drawn considerable interest in biological activity. Compound 6-amino-1,2,4-triazolo[3,4-f ][1,2,4]triazin8-one (4,8-diaza-9-deazaguanine, 1), the isosteric isomer of guanine, may retain the Watson–Crick type hydrogen bonding sites of the aglycon moiety. As a part of our ongoing program we studied the most contributing prototropic tautomerism of this bicycloheterocyclic moiety of compound 1. We considered three tautomers: the amino-oxo forms 5H-tautomer (1a), 7Htautomer (1b), and the amino-hydroxy form (1c), plus a rotamer (1d) (not considering improbable imino forms) (Fig. 1). To our knowledge, this issue has never been discussed in the literature. A clear-cut answer is the use of X-ray crystallography analysis, which provides precise information about the most stable structure among tautomers. In the present paper, we report on the crystal structure of the title compound based on a singlecrystal X-ray analysis. A chunky colorless crystal suitable for single-crystal X-ray diffraction measurements of the title compound 6-amino-3ethyl-1,2,4-triazolo[3,4-f ][1,2,4]triazin-8(7H)-one (2) (Fig. 1) was obtained by recrystallization from H2O solution. The compound 2 prepared by the method of Lovelette2 was one of the 3-substituted derivative compounds of 1 that we had synthesized. The results of the X-ray structure determination are given in Tables 1 and 2. The ORTEP drawing for the title compound is shown in Fig.2. From this X-ray analysis of the title compound, the structural framework is constructed from the 1,2,4-triazole five-membered ring fused at C(1)–N(5) with a distorted 1,2,4-triazinone sixmembered ring, which bears an H2O molecule by a intermolecular hydrogen bond. Meanwhile, it has been clarified 853 ANALYTICAL SCIENCES JULY 2002, VOL. 18 2002 © The Japan Society for Analytical Chemistry