DBU‐Promoted 6π‐Azaelectrocyclization and Hydrogen‐Migration to Prepare 6‐Alkyl Pyridine N ‐Oxides from N ‐Vinyl‐ α , β ‐Unsaturated Nitrones

We describe a DBU-controlled 6π-azaelectrocyclization of dibenzylideneacetone-derived N-vinyl-α,β-unsaturated nitrones to prepare 6-alkyl pyridine N-oxides in 24–82% yields. Mechanistic studies revealed that DBU had a great impact on the formation of 6-alkyl pyridine N-oxides and served as a base and the carrier of hydrogen sources to in situ reduce C=C bonds. The 6-alkyl pyridine N-oxide was prepared at gram-scales and converted to estrone-derived pyridine and pyrido[2,3-c]carbazole 4-oxide in 46% and 52% yields in two steps, respectively.