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Structures of (A) pyrimidine and triazine derivatives (X=C/N), model I; (B) pyrazolo[3,4-d]pyrimidines, pyrrolo[2,3- d]pyrimidines, triazolo[4,5-d]pyrimidines and 6-arylpurines derivatives (X=C/N and Y=C/N), model II and (C) thieno[3,2-d]pyrimidines derivatives, model III.
