Annulation reactions of allene-derived 1,3-dipole with 3-substituted-chromones:: Unusual recognition of 4π-component in 3-(N-aryliminomethyl)chromones through [4+3] annulation

GRAPHICS All-carbon dipole derived by the interaction of triphenylphosphine with allenic eater is able to locate the polarized 2 pi-component in 3-formylchromones through a regioselective [2 + 3] addition to the C2-C3 pi-bond, which is followed by deformylation leading to novel 3a,9a-dihydro-1-ethoxycarbonyl-1-cyclopenteno[5,4-b]benzopyran-4-ones. On the contrary, the dipole recognizes azadiene in 3 (N-aryliminomethyl) chromones through [4 + 3] annulation and initially formed adducts undergo tandem rearrangements to afford novel N-aryl-2,3-dihydro-4-ethoxycarbonylchromano[2,3-b]azepine-6-ones in good yield.