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Cu-catalyzed cyclizations of (E)-O-propargylic oximes of α,β-unsaturated aldehydes were successfully carried out to afford the corresponding pyridine oxides in good to high yields. As an example, (E)-acrylaldehyde O-1,3-diphenylprop-2-ynyl oxime (1b) was reacted for 5 h in the presence of CuBr(PPh3)3 (10 mol %) and PPh3 (10 mol %) in DMSO at 120 °C to afford 3-benzyl-2-phenylpyridine-N-oxide (2b) in 84% yield. In this case, the reaction proceeded via Cu-catalyzed propargyl oxime-allenyl nitrone rearrangement followed by 6π-3-azatriene electrocyclization.
Nakamura et al. (Fri,) studied this question.