Pyrazolopyrimidines tautomerism

Pyrazolo[3,4-d]pyrimidine, 4-amino-tautomerism, 5, 310 Pyrazolopyrimidinediones synthesis, 5, 324 Pyrazolopyrimidines acidity, 5, 309 anions... 

Pyrazolopyrimidines, amino-acidity, 5, 309 alkylation, 5, 310 N-oxide synthesis, 5, 324 synthesis, 4, 525 5, 328 Pyrazolopyrimidines, dimethyl-synthesis, 5, 316 Pyrazolo[ 1,5-a]pyrimidines electrophilic attack, 5,311 synthesis, 5, 271, 320, 331 Pyrazolo[ 1,5-c]pyrimidines electrophilic attack, 5, 312 Pyrazolo[3,4- d]pyrimidines nucleophilic attack, 5, 313 synthesis, 5, 161, 272, 323, 334 tautomerism, 5, 309 Pyrazolo[4,3-d]pyrimidines alkylation, 5, 310 synthesis, 5, 272... 

Of the four fundamental pyrazolopyrimidine systems two (1, 2) do not display tautomerism. The [3,4-d]-system exists as four NH-tautomers (3-6), and CH-tautomers can also be written (e.g., 7). We will normally write the [3,4-d]-system as in structure 3. The [4,3-d]-system possesses two uncharged (8, 9) and three zwitterionic NH- (e.g., 10) and CH-forms (e.g., 11). We will normally write this system as 8. 

Increasing the electron-donor properties of the R, substituent in the case of dihydro derivatives of 1,2,4-triazolo[l, 5-a]pyrimidines [X, Z are N, Y is C(R)], tetrazolo[l, 5-a]pyrimidines (X, Y, Z are N), and pyrimidofl, 2-a]benzimidazoles (X, Y are o-C() I4, Z is N) tends to stabilize the B form. This can be explained by the conjugation of Rj with the electron-withdrawing azomethine group and the azole moiety. Reduction of the electron-accepting properties of the azole fragment (imidazopyrimidines and pyrazolopyrimidines) decreases the influence of the Ri substituent on tautomeric composition. 

---