Transformations of 2,4,5-trisubstituted pyrimidines. The syntheses and transformations of pyrimido[4,5-d]pyrimidine, 1,2,4-triazolo[4,3-a]˚pyrimidine, tetrazolo[1,5-a]pyrimidine, 1,2,4-triazolo[3,4-b]purine and tetrazolo[5,1-b]purine derivatives