A novel, one-pot route for the synthesis of nonaromatic ring-fused 1,4-oxazepines and 1,4-oxazines has been developed. The reaction features a sequential rhodium(II)-catalyzed reaction of N-sulfonyl-1,2,3-triazoles with glycidols, followed by a regioselective Lewis acid Mg(OtBu)2-catalyzed intramolecular ring-opening reaction. It has been found that the regioselectivity in the epoxide ring-opening was largely determined by the substituents on the glycidols. Thus, substituted glycidols (R2 ≠ H) afforded seven-membered oxazepine derivatives selectively, while unsubstituted glycidols (R2 = H) afforded six-membered oxazine derivatives. Plausible reaction pathways are elucidated and supported by experiments with several glycidols bearing different substituents around the epoxide functionality.