7-Aminopyrazolo pyrimidines tautomerism

Neutral 4-aminopyrazolo[3,4-d]pyrimidines exists in water in two tautomeric forms 1//-4-amino (1H4APP) and 2//-4-amino (2H4APP) isomers (X = 2H4APP/1H4APP = O.I at lOO C and OH tautomerization = 9.0kcal mol ). Interconversion of the two forms is catalyzed by and OH through either an intermediate cation common to both tautomers or through an intermediate anion. 

The tautomerism of 4-aminopyrazolo[3,4-J]pyrimidines as well as aminopyrazolo[4,3-d]py-rimidines has been investigated. 4-Aminopyrazolo[3,4-J]pyrimidines exist in water in two tautomeric forms l//-4-amino (85) and 2//-4-amino (86) [K (86/85) = 0.1 at 100°C AH = 9.0 kcal, mol ]. Interconversion of these two forms is catalyzed by acid or base <77JA7257, 80ZN(C)878, 82MI 712-02). Together with these two predominant species there are smaller proportions of the 7//-4-amino (87) (1 x 10-3) and the 5//-4-amino isomer (2 x 10-4). The species (87) exists only as an amino tautomer whereas the 5//-4-amino species in water has a partial imino structure (amino/imine =10) <77JA7257>. 

Several papers have appeared on tautomerism in pyrimidines, including the parent molecule, pyrimidyl-2- and -4-cyanoacetic esters, and AT-substituted 4-aminopyrazolo[3,4-< ]pyrimidines. The last-mentioned system (5) is in equilibrium with (6) in aqueous solution. Interconversion is catalysed by acid and base, and proceeds via either an intermediate cation that is common to both neutral tautomers or through the anion. Other studies of condensed systems include... 

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