Lactims, nomenclature

According to the triazine nomenclature, 5-azauracil is 2,4-dioxo-l,2,3,4-tetrahydro-l,3,5-triazine (2). The subject index of Chemical Abstracts prefers s-triazine-2,4(lH,3H)-dione. Furthermore, some authors use a name derived from the lactim structure, 2,4-dihydroxy-s-triazine (3). The numbering of the substituents is the same for all these types of nomenclature. 

According to the systematic nomenclature these substances were first named l-f-triazolo[d] pyrimidines in compliance with the general principles of the Ring Index/ More recent papers and Chemical Abstracts indexes use the term i -triazolo[4,5-d]pyrimidine (147) in accord with the lUPAC nomenclature. The numbering of substituents when using the last-mentioned name is different from that of the 8-aza analogs. For the formulas of oxygen and sulfur derivatives names derived from the lactim or thiolactim form are almost exclusively in use (in common with the purine derivatives). These derivatives are thus described as hydroxy and mercapto derivatives, respectively. The name 1,2,3,4,6-pentaazaindene is used only rarely for this system. 

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