Regioselective synthesis of pyrimidine annelated heterocycles from 6-(cyclohex-2-enyl)-1,3-dimethyl-5-hydroxyuracil

6-(Cyclohex-2-enyl)-1,3-dimethyl-5-hydroxyuracil when treated with bromine in chloroform at 0–5°C for 6 h furnishes 6-bromo-1,3-dimethylhexahydrobenzofuro[3,2-d]pyrimidine-2,4-dione in 80% yield. The same heterocycle is also obtained in 90% yield on treatment of the same substrate with pyridine hydrobromide perbromide for 30 min or with hexamethylenetetramine hydrotribromide for 15 min in methylene chloride at 0–5°C. Cyclization of the same substrate with bis-benzonitrile palladium(II) chloride in benzene in the presence of sodium methoxide afforded 1,3-dimethyl-6,7,8,9-tetrahydrobenzofuro[3,2-d]pyrimidine-2,4-dione in 82% yield. The same substrate on treatment with cold conc. sulfuric acid at 0–5°C for 2 h gave 1,3-dimethyl-5-oxabicyclo[3.3.1]nonano[3,2-d]pyrimidine-2,4-dione in 94% yield.