Abstract The synthesis of the four possible 2-amino-and 2-guanidino-4-(2-amino-and 2- guanidinothiazolyl) pyrimi-dines is described and pKa. values are calculated. Guanidinopyrimidines are more basic than guanidinothiazoles. However, the reverse is true of the corresponding amino heterocycles; the amino thiazole is more basic than the amino pyrimidine.
