Regiospecific ring closure reactions of 1, 3-diphenylthiobarbituric acid and dimedone: Formation of spiro vs fused heterocycles

The one pot reaction of 1, 3-diphenylthiobarbituric acid with benzylidene acetophenone, benzaldehyde and ammonium acetate in presence of catalytic amount of acetic acid furnishes regiospecifically an azaspiro compound. The azaspiro compound has also been synthesized in a two step reaction in which the intermediate, obtained by the reaction of 1,3-diphenyl thiobarbituric acid and benzylidene acetophenone, undergoes reaction with benzaldehyde and ammonium acetate. When dimedone is used in place of 1,3-diphenylthiobarbituric acid in the above reactions surprisingly a hydroquinolinone derivative is formed instead of the expected azaspiro compound. When these reactions are carried out in the absence of arylaldehyde both 1,3-diphenylthiobarbituric acid and dimedone yield the corresponding fused heterocyclic products. The probable reaction mechanism leading to these products are discussed.