Purine nomenclature/numbering

More recently, Bayer disclosed potent, selective PDE2 inhibitors in a series of patents (Figure 9.34) [121]. A key feature of these inhibitors, which belong to the substrate-analog series, is a benzylic rather than a phenylic substitution at position 2 (using the common numbering system of purine nomenclature) of the heterocycle. Thus, inhibitors 145 and 146 demonstrated moderate potency toward PDE2 and... 

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. 

Bases, Nucleosides, and Nucleotides. The relationship of these components of a nucleic acid or polynucleotide is shown in Chart 10. The numbering of the pyrimidine (uracil) and the purine (adenine) shown is the IUPAC nomenclature used by Chemical Abstracts, and... 

CAS has fairly precise rules for denoting which structural systems are deemed to be tautomeric, and to which tautomer indexing is directed. In CAS nomenclature 1972-2006, tautomeric substances were often indexed under theoretical tautomers, e.g., the H- form of purines, which had no practical existence. CAS numbers for tautomeric substances therefore often referred to unknown tautomers. 

In this chapter, the authors complete their treatment of the biosyntheses of the major classes of macromolecular precursors by describing the synthesis of the purine and pyrimidine nucleotides. Besides being the precursors of RNA and DNA, these compounds serve a number of other important roles that are reviewed in the opening paragraph of the chapter. Nucleotide nomenclature is reviewed in the introduction to the chapter, as is an outline for the synthesis of nucleotides through de novo and salvage pathways. 

As was discussed for imidazole, tautomerism makes it difficult to predict the position of the proton in this component of the purine structure. In adenine, the proton is largely found at the 9-position, whereas in guanine a mixture of the two forms is present in solution. In nucleosides, it is the 9-position that is attached to the sugar moiety as noted in section 3.2.3. The numbering in purines is exceptional to the mles of nomenclature but is accepted by the International Union of Pure and Applied Chemistry (lUPAC). It is illustrated in Figure 9.2. 

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