Purine derivatives, naturally-occurring

Adenine (6-amino purine) and guanine (2-amino-6-oxy purine), the two common purines, are found in both DNA and RNA (Figure 11.4). Other naturally occurring purine derivatives include hypoxanthlne, xanthine, and uric acid (Figure 11.5). Flypoxanthine and xanthine are found only rarely as constituents of nucleic acids. Uric acid, the most oxidized state for a purine derivative, is never found in nucleic acids. 

FIGURE 11.5 Other naturally occurring purine derivatives—hypoxanthine, xanthine, and uric acid. 

The naturally occurring purines fall into 4 main groups. (1) Simple substituted derivatives of purine (1) such as adenine (2) and various 6-AT-substituted derivatives. (2) Monoxo-dihydropurines such as hypoxanthine (3), guanine (4), and isoguanine (5). f3) Dioxotetra-hydropurines such as xanthine (6) and methylated derivatives including the 3,7-dimethyl derivative theobromine (7), 1,3-dimethylxanthine or theophylline f8), and 1,3,7-trimethylxanthine or caffeine (9). (4) Trioxohexahydropurines such as uric acid (10). 

The Af-glycosylpurines or purine nucleosides include many naturally occurring compounds, especially 9-/3-D-ribofuranosyl and 9-/3-D-2-deoxyribofuranosyl derivatives of adenine and guanine which are of course major constituents of the various ribo (RNA) and deoxyribo (DNA) nucleic acids. The literature on purine nucleoside chemistry is very substantial and in this chapter it will only be possible to give a brief survey. However several reviews exist (see Table 2). 

Substituted purine nucleosides which include most of the naturally occurring compounds or their analogs are also produced by glycosylation of an appropriate purine anion derivative. 

Nucleoside synthesis.3 This reagent is superior to p-toluenesulfonic acid, ethyl polyphosphate, or zinc chloride for the preparation of nucleosides by fusion of purines with fully acetylated pentoses or hexoses. An example of the method4 is the fusion of purine (1) with tetra-O-acetyl-D-ribofuranose (2) at 174-180° (at the water pump vacuum) to give the two nucleosides (3) and (4). The main product (3) is the triacetyl derivative of a naturally occurring nucleoside antibiotic nebularine. 

In the past, the term nucleoside has had two connotations. It was first proposed by Levene and Jacobs for the carbohydrate derivatives of purines and pyrimidines obtained by the hydrolyas of nucleic acids, but since that time it has been more generally applied to aU naturally occurring glycosyl derivatives of purines and pyrimidines. The sugar moieties of nucleic acid nucleosides have so far proved to be either n-ribose or 2-deoxy-... 

Purine bases - Purine and its substitution derivatives, especially naturally occurring examples. [5]... 

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