Opening of the three-membered ring in heterocyclic systems incorporating a dichloroaziridine ring fused to eight-membered O,N-or S,N-heterocycles is accompanied by transannular reactions with participation of the endocyclic oxygen and sulfur atoms. Depending on the conditions, the products are 1,4-benzoxazine (1,4-benzothiazine) or 1,3-benzoxazole (1,3-benzothiazole) derivatives. The discovered transformations were used as a basis of methods for the preparation of new heterocyclic systems, 2,3,4,4a-tetrahydro-1H-pyrido-[3,2- b][1,4]benzoxa(thia)zine derivatives, in domino or consecutive modes, as well as of pyrrolidinyl-substituted 1,3-benzoxa(thia)zoles.