1- 4-hydroxy-5- pyrimidine-2,4-dione

Isoxazolo[5,4-d]pyrimidine, 4-hydroxy-3-methyl-UV, 6, 620 <64JOC2116) Isoxazolo[5,4-d]pyrimidine, 3-methyl-4-(methylamino)-UV, 6, 620 <64JOC2116) Isoxazolo[5,4-d]pyrimidine, 3-phenyl- H NMR, 6, 619 <77H(7)51) Isoxazolo[3,4-d]pyrimidine-4,6(5//,7W)-dione, 5,7-dimethyl-... 

Pyrimidine-annulated methanooxepene 35a is formed via regioselective heterocyclization of 6-cyclopent-2-enyl-5-hydroxy-l,3-dimethylpyrimidine-2,4(1/7,3//)-dione in the presence of concentrated sulfuric acid. The above-mentioned pyrimidinedione and 5-cyclopent-2-enyl-6-hydroxy-l,3-dimethylpyrimidine-2,4(1/7,3/7)-dione give similar annulated bromomethanooxepenes 35b and 36, respectively, when cyclization is performed with pyridine hydrotribromide or hexamethylenetetramine hydrotribromide <2003M1137>. 

The hydroxy group of l-(2-hydroxyethyl)thieno[3,4-d]pyrimidine-2,4(3//)-dione 344 was chlorinated with thionyl chloride in a mixture of pyridine and chloroform, or mesylated with methanesulfonyl chloride in pyridine. No ring chlorination was observed under these conditions. The resulting l-(2-chloro or 2-methanesulfonylethyl) derivative 365 was cyclized to l,2-dihydrooxazolo[2,3-6]thieno[3,4-d]pyrimidin-5-one 366 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (89H985). A similar transformation of 3-(2-hydroxyethyl)thieno[3,4-d]pyrimidine-2(l//),4-diones 333a,b into the 3-(2-chloroethyl) derivatives 367 with thionyl chloride occurred in chlo-... 

The four most important tautomers of pyrazolo[4,3-<7]pyrimidine-5,7(477,677)-dione (52a-d) were studied using ab initio LCAO-MO methods <91CPL(181)123>. Calculation indicates that structure (52a) is the predominant form both in nonpolar and polar environments. The second most stable species, the oxo hydroxy tautomers (52b) and (52c), have relative energies of 38.7 kJ mol . For the dioxo to oxo-hydroxy transition, the calculated energy gap corresponds to an equilibrium constant... 

Chlorination of 5-hydroxy- or 5,7-dihydroxypyrido[2,3-<5 ]pyrimidines using SOCl2 in DMF depends upon reaction conditions. Chlorination of 5-hydroxy-l,3-dimethylpyrido[2,3-d]pyrimidine-2,4-diones gave the 5,6-dichloro derivative, whereas the 6-chloro-5,7-dihydroxy derivative gave the corresponding 6,7-dichloro derivative <92KGS1230>. 

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