A formal (4 + 1) cycloaddition of oxene occurs in the reaction of 2-azabuta-1,3-dienes with m-chloroperoxybenzoic acid under mild conditions to give 3-oxazolines. Mechanistically, the reaction is an extended Prilezhaev reaction involving the formation of previously unknown (1-azavinyl)oxiranes, followed by the acid-catalyzed ring expansion. This cycloaddition was used for the one-pot synthesis of densely substituted 3-oxazolines, inaccessible by other protocols, starting from the Cu(II)- or Rh(II)-catalyzed reaction of 2H-azirines with diazo compounds.