Synthesis of 4‐Oxo‐but‐2‐enals through tBuONO and TEMPO‐Promoted Cascade Reactions of Homoallylic Alcohols

An efficient and stereoselective synthesis of (E)‐4‐oxo‐but‐2‐enals through tert‐butyl nitrite (tBuONO)‐ and 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐promoted cascade reactions of homoallylic alcohols is presented. Mechanistically, the formation of the title compounds involves a cascade process including nitration of the carbon–carbon double bond, oxidation of the secondary alcohol unit, and Nef transformation of the in situ formed allylic nitro scaffold. In this process, tBuONO is an efficient nitration and oxidation reagent while TEMPO is believed to act as a mild oxidant and a novel reagent to promote the Nef reaction that is accomplished under mild and neutral conditions. As a further aspect, the (E)‐4‐oxo‐but‐2‐enals thus obtained were found to be suitable substrates to condense with hydrazine hydrate to give diversely substituted pyridazines with good efficiency.