MsOH‐Mediated Cascade Cyclization of <i>O</i> ‐Homopropargyl Hydroxylamines with Aldehydes to Access 4‐Arylidene‐1,2‐Oxazines

Brønsted acid‐mediated synthesis of 4‐arylidene oxazines from O ‐homopropargyl hydroxylamines is developed. A series of control experiments have been carried out, revealing that the formation of 4‐arylidene oxazines proceeds through an alkyne–oximium cyclization, ring‐opening reaction, and subsequent condensation. Synthetic versatility of the developed methodology was highlighted in the synthesis of 1,4‐aminoalcohol as well as γ ‐hydroxy ketone by the reduction of 4‐arylidene oxazine.