Pteridines pterins

Subsequent studies by the same group (W7) have shown that the increased endogenous fluorescence in patients with chronic renal failure is due to the unconjugated pteridine, xanthopterin (2-amino-4,6-pteridinedione, 179 Da see Table 4 and Figs. 10 and 11) (B23, Dll, L15, Pll). Unlike the conjugated pteridines (folates), the function of many of the unconjugated pteridines (pterins) has yet to be elucidated (U2, Z3). So far only biopterin has been shown to have a defined role, being a cofactor in the hydroxylation of several aromatic amino acids involved in the formation of neuronal hormones such as catecholamines and serotonin. 

Scheme 1 Structures of pteridine, pterin, tetrahydrobiopterin, molybdopterin, and tetrahydrofolate.

AT-Oxidation is very sensitive to steric effects, since 1-substituted lumazines and pterins give only 5-oxides and the presence of bulky substituents at position 7 also directs oxidation to N-5. The pteridine 5-oxide (52) and 8-oxide (53) and the 5,8-dioxide (55) contain the AT-oxide groups as such, even when the possibility of AT-hydroxy tautomers exists, as in (53) i(54). 

The action of sulfur nucleophiles like sodium bisulfite and thiophenols causes even pteridines that are unreactive towards water or alcohols to undergo covalent addition reactions. Thus, pteridin-7-one smoothly adds the named S-nucleophiles in a 1 1 ratio to C-6 (65JCS6930). Similarly, pteridin-4-one (73) yields adducts (74) in a 2 1 ratio at C-6 and C-7 exclusively (equation 14), as do 4-aminopteridine and lumazine with sodium bisulfite. Xanthopterin forms a 7,8-adduct and 7,8-dihydropterin can easily be converted to sodium 5,6,7,8-tetrahydropterin-6-sulfonate (66JCS(C)285), which leads to pterin-6-sulfonic acid on oxidation (59HCA1854). 

Aminopteridine is the most sensitive to acid hydrolysis, and 6-amino- and 6-dimethyl-amino-pteridine are also hydrolyzed, even by cold 0.0IN hydrochloric acid, too rapidly for accurate determination of the cation form (52JCS1620). 2-Amino- and 4-amino-pteridine are not readily attacked by IN HCl at 20 °C but at 100 °C the former compound is destroyed and the latter converted into pteridin-4-one (5UCS474). 2,4-Diaminopteridine can be hydrolyzed by refluxing in 6N HCl for 30 minutes to 2-aminopteridin-4-one (pterin 2) and after... 

Various 6- and 7-methyl- and 6,7-dimethyl-pteridines bearing either oxo or amino groups in the 2- and 4-positions can be oxidized to the corresponding carboxylic acids by alkaline potassium permanganate on heating. Various lumazine and pterin mono- and di-carboxylic acids have been prepared in this way (48JA3026, 78CB3790). 

A new, versatile and selective synthesis of 6- and 7-substituted pteridines was reported by Rosowsky (73JOC2073). /3-Keto sulfoxides, which can be viewed as latent a keto aldehydes, react with (251) to give 6-substituted pterins, and the use of a-keto aldehyde hemithioacetals leads in a regiospecific synthesis to the isomeric 7-substituted pterins (equation 85). 

Pterins — These are pigments derived from pteridine skeletons. All natural pterins are 2-amino-4-hydroxypteridines bearing various substituents at Cg and C7 and having different oxidation states of N5 and Ng. 

Rutowski, R.L. et al.. Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly, CoUa eurytheme, Proc. R. Soc. B, 272, 2329, 2005. Oliphant, L.W., Pteridines and purines as major pigments of the avian iris. Pigment Cell Res., 1, 129, 1987. 

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