Dihydroazines Based on α,β-Unsaturated Ketones Reactions

The present review is devoted to heterocyclizations of unsaturated carbonyl compounds which result in the formation of six-membered partially hydrogenated heterocycles. Three main pathways of such reactions are discussed: syntheses of dihydropyridines by preliminary transformation of unsaturated ketone, cyclocondensations of unsaturated carbonyl compounds with nitrogen containing 1,3-binucleophiles and heterocyclizations with participation of additional functional groups of enones. In the first part of review reactions of cyclic and acyclic enamines with unsaturated ketones are observed. The participation of cyclic, noncyclic enamines and enaminonitriles in formation of various dihydroazoles is described. The significant part of the review is concerned with reactions of unsaturated ketones with urea and its analogs such as thiourea, guanidine and amidines leading to dihydropyrimidine-2-thiones, dihydrothiazines, dihydropyrimidin-2-ones, dihydropyrimidine-2-ilamines and 2-alkyl(aryl) substituted 1,4- or 1,6-dihydropyrimidines respectively. The mechanisms of the interaction of urea and its analogs with aromatic unsaturated ketones are also discussed. The reactions of malonic acid derivatives, e.g. malononitrile, malonamide, thiomalonamide, cyanoacetamide, thiocyanacetamide and cyanoacetic ester are described in the next part of the review. Some features of these reactions and their possible mechanisms observed in details. The last part is concerned with cyclocondensation of aminoazoles and α,β-unsaturated carbonyl compounds or Mannich bases as the most common method of synthesis of dihydroazolopyrimidines with a nodal nitrogen atom. The reactions of synthe tic precursors of chalcones - substituted benzaldehydes and acetophenones as well their mechanisms are also observed.