The hypervalent iodine reagent PhI(OAc)2 (PIDA) mediated the formal oxidative C=Cbond cleavage and subsequent cyclization of internal olefins, that is, α‐oxo‐ketene N,N‐acetals, which afforded substituted oxazolines. Isothiazoline derivatives were obtained from the reactions of α‐thioxo‐ketene N,N‐acetals with PIDA under the same conditions. Hydrolysis of the resultant oxazoline derivatives led to