27/-Pyrazolo pyrimidine-4,6-diones 5,7-dimethyl

Benzyl-4,6-dimethyl-2//-pyrazolo [3,4-d ] pyrimidine-2,4-( 1 f/,3ff)dione 1-oxide (273) affords the pyrimido[4,3-d]pyrimidine derivative 276 upon... 

One final interesting preparation of pyrimido[5,4-ring transformation of pyrazolo[4,3-d]pyrimidines. Reaction of the A-oxides (272) with sodium ethoxide in.refluxing ethanol leads to the isolation of l,3-dimethyl-6-substituted pyrimido[5,4-[Pg.365]

The UV spectra of isomeric pairs of pyrazolo[3,4-d]pyrimidine-4,6-diones and pyrazolo[4,3-<7]pyrimidine-5,7 diones show that the isomers with a [4,3-d] ring fusion absorb at longer wavelengths (red shift) compared to the [3,4-d] isomers <82IJC(B)585>. The />-quinonoid structure of the pyrimidine moiety in compounds (74) and (75) causes a bathochromic shift of 20-25 nm, relative to compounds (53) and (76). The close values of 2max for 7-methyl (77 R = Me) and 2,7-dimethyl allopurinol (74) suggest that the former is present in aqueous solutions as the A(2)-H tautomer <79JCS(Pl)2795>. 

Benzyl-4,6-dimethyl-2//-pyrazolo[3,4-(f pyrimidine-2,4-(l H,3H) dione-l-oxide (188) affords the pyrimidopyrimidine derivative (191), most likely via the intermediates (189) and (190) (Scheme 11)... 

Azido-6-formyl-l,3-dimethyluracil (473) is cyclized with hydrazines to form l-substituted-5,7-dimethyl-1 //-pyrazolo[3,4-. 

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