Facile, regioselective [4+2] cycloaddition involving 1-aryl-4-phenyl-1-azadienes and allenic esters: An efficient route to novel substituted 1-aryl-4-phenyl-1,4-dihydropyridines

[GRAPHICS] 1-Aryl-4-phenyl-1-azadienes undergo facile, regioselective [4 + 2] cycloaddition to the C2,C3 pi -bond of allenic esters in refluxing benzene, and the formed adducts undergo a 1,3-H shift to afford novel 5 alkyl-1-aryl-3-ethoxycarbonyl-4-phenyl-1,4-dihydropyridines (78-97%), However, when the reaction is carried at room temperature, besides the [4 + 2] addition, the [2 + 2] mode of addition involving C=N of azadiene and C3,C4 pi -bond of allenic esters also intervenes, The resulting N-aryl-2-ethoxy-carbonyl-methylidene-4-styrylazetidines (17-28%) undergo reorganization on silica gel to afford 5 cyclohexen-1-ones.