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 degreesC for 6 h furnishes 6-bromo-1,3-dimethylhexahyd robenzofuro[3,2-d]pyrimidine-2,4-dione in 80% yield. The same heterocycle i s also obtained in 90% yield on treatment of the same substrate with pyridi ne hydrobromide perbromide for 30 min or with hexamethylenetetramine hydrot ribromide for 15 min in methylene chloride at 0-5 degreesC. 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-tetrahydr obenzofuro[3,2-d]pyrimidine-2,4-dione in 82% yield. The same substrate on t reatment with cold cone. sulfuric acid at 0-5 degreesC for 2 h gave 1,3-dim ethyl-5-oxabicyclo[3.3.1]nonano[3,2-d]pyrimidine-2,4-dione in 94% yield. (C ) 2001 Elsevier Science Ltd. All rights reserved.