Pteridines special

In many cases, addition or removal of water proceeds sufficiently slowly that some of the physical properties of unstable species (such as hydrated neutral quinazoline or anhydrous 2-hydroxypteridine) can be observed. In these cases, reaction kinetics can also be examined. Addition of water to pteridine is of special interest in relation to studies of the formation and hydrolysis of Schiflf bases. The reaction proceeds in two reversible stages, 3 4 5 ... 

This problem may, however, be a special case because the oxidation of pteridine alkenes in the presence of sensitive substituents such as alkylthio was shown to be possible using ligand-assisted catalysis with osmium tetraoxide or DMDO (Scheme 10). In the case of pteridine 65, a clean series of transformations to afford the pterin 66 was possible <20030BC664, 2005PTR53004/04>. However, if oxidation was carried out under moist conditions, the corresponding 2-oxopteridine (lumazine) was always obtained as illustrated by the conversion of 67 into the epoxide 68. 

More sophisticated mass spectrometric methods have been found in the electrospray (ES-MS) and plasma desorption (PD-MS) techniques which have successfully been applied directly to nonvolatile pteridines. A small peak can be detected with folic acid at m/z = 441 together with the mono-, di-, and trisodium species. The dihydro- and tetrahydro derivatives also give the expected results <83Mi 718-05). Fast atom bombardment (FAB) works also with folic acid in special matrices and is another tool for structural studies <83MI 718-08). Even in a molar mixture of 5-methyl-5,6,7,8-tetrahydropterin and tris(pentane-2,4-dionato)iron(III), the metal-pterin complex could be detected by ES-MS <92HCA1955>. 

Conformational studies in the pteridine series involve mainly the tetrahydro derivatives which adopt a half-chair orientation in the tetrahydropyrazine moiety, giving rise to some flexibility and special structural arrangements. 

Pteridine synthesis from pyrazine precursors are usually applied in such cases where the formation of special derivatives cannot be achieved easily by the pyrimidine approach. Methyl 3-amino-2-pyrazinecarboxylate (362) reacts with thiophosgene to give the corresponding 3-isothiocyanato derivative (363) which cyclizes with primary amines, arylalkylamines, aminoalcohols, amino acids,... 

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