Brønsted Acid Triggers [6/7 + 1] Cascade Cyclization by <i>N</i>-Alkyl Amine C(sp³)–N Cleavage: Mild Synthesis of Benzo[1,4]oxazepane and Dihydrobenzo[1,5]oxazocine

A catalyst-free mild synthesis was reported to produce medium-ring oxazepane and oxazocine derivatives from aminomaleimides and <i>N</i>-alkyl amines. The substrate and acidic additives were employed to cleave the C(sp³)-N bond as a one-carbon synthon for C-C and C-O coupling, thus facilitating the [<i>n</i> + 1] cascade cyclization reaction, which enabled the construction of seven- and eight-membered <i>N</i>,<i>O</i>-heterocycles at room temperature. The method exhibits abroad substrate scope and remarkable tolerance toward various functional groups (seven-membered 28 examples, eight-membered 8 examples, and activated <i>N</i>-alkyl amine 12 examples) and utilization of natural products (2 examples).