Allopurinol, tautomers

The UV spectra of isomeric pairs of pyrazolo[3,4-d]pyrimidine-4,6-diones and pyrazolo[4,3-<7]pyrimidine-5,7 diones show that the isomers with a [4,3-d] ring fusion absorb at longer wavelengths (red shift) compared to the [3,4-d] isomers <82IJC(B)585>. The />-quinonoid structure of the pyrimidine moiety in compounds (74) and (75) causes a bathochromic shift of 20-25 nm, relative to compounds (53) and (76). The close values of 2max for 7-methyl (77 R = Me) and 2,7-dimethyl allopurinol (74) suggest that the former is present in aqueous solutions as the A(2)-H tautomer <79JCS(Pl)2795>. 

Disconnection of hydrazine 30a from allopurinol leaves a diamide with an extra aldehyde group 33. Disconnection of that aldehyde leaves a simple and available symmetrical heterocycle that exists in the dihydroxy tautomer 34 and will require a one-carbon electrophile corresponding to the synthon 35. We shall meet reagents for this synthon soon. 

An important effort is being presently done to study, both experimentally and theoretically, the excited states of compounds that exist in several tautomeric forms for instance, the singlet and triplet excited-state dynamics of the keto and enol tautomers of cytosine [27], the ultrafast excited-state decay of allopurinol keto-N9H tautomer from gas phase to aqueous solution [28], the reduced aromaticity in lysine-tryptophan dipeptide (lys-trp) cations, and the fact that the high pH tautomer correlates with lower quantum yield and shorter hfetimes [29]. The structure of the compounds appear in bold to call the attention to their biological and pharmaceutical nature. 

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