Synthesis of fused dihydropyrimidinones from cyclic-1,3-dicarbonyl compounds:: Modified Biginelli synthesis of 1,2,3,4,5,6,7,8-octahydroquinazolinediones and 3,4-dihydro-1H-indeno[1,2-d]pyrimidine-2,5-diones

Syntheses of bicyclo-3,4-dihydropyrimidinediones from cyclic 1,3-diketones such as 5,5-dimethyl-1,3-cyclohexanedione (dimedone) and 1,3-indanedione have been attempted. During the multicomponent coupling of dimedone with (thio)urea and aldehydes, the unprecedented products of 9-aryl xanthene derivatives are obtained instead of the expected 4-aryl-1,2,3,4,5,6,7,8-octahydroquinazolinediones. Also, this reaction is sensitive to solvent and the mole ratio of the ingredients used. High yields of bicyclic and tricyclic DHPMs with good selectivity are obtained using excess of urea and 1,3-dicarbonyl compounds. Interestingly, DHPMs are also formed in good to excellent yields by neat stirring of 1,3-dicarbonyl compounds, aldehydes and urea, keeping oil-bath temperature at 110degreesC. The mechanistic aspects of the reaction are also mentioned.