Facile Chemodivergent Synthesis of Fused Indolo[2,3‑<i>b</i>]quinoxalines and Benzo[4,5]imidazo[1,2‑<i>c</i>]quinazolin-6(5<i>H</i>)‑ones: An Organocatalytic Trifluoromethanesulfonic Acid-Controlled Mechanistic Switch between Condensation and Ring Expansion of the Isatin Core

A mild metal-free chemodivergent synthesis leading to products of two different chemotypes, such as indolo[2,3-b]quinoxalines and benzo[4,5]imidazo[1,2-c]quinazolin-6(5H)-ones, has been described from common starting materials, isatin and 1,2-diaminobenzene, at room temperature in an environmentally benign EtOH solvent. The EtOH mediated condensation between isatin and 1,2-diaminobenzene delivers exclusively fused indolo[2,3-b]quinoxalines in good/excellent yields (65–89%) in 8 h, and catalytic protic acid (CF3SO3H) in EtOH furnishes N-fused benzo[4,5]imidazo[1,2-c]quinazolin-6(5H)-ones selectively in good/excellent yields (66–88%) in 2 h via rearrangement of the isatin core following C2–C3 bond cleavage and N-atom insertion with 1,2-diaminobenzene. Thus, the switch selectivity between benzo[4,5]imidazo[1,2-c]quinazolin-6(5H)-ones and indolo[2,3-b]quinoxalines could be finely programmed with the presence/absence of catalytic CF3SO3H induced by an ionic vs non-ionic pathway. A wide array of starting materials housing structural and electronic variations (e.g., electronically neutral, moderate/strong electron-donating, moderate electron-attracting functionalities, and heteroarene) allows the scope of diversity creation and provides facile access to molecular libraries with elaborate, well-decorated structures. The operational simplicity, ease of product purification (in the case of indolo[2,3-b]quinoxalines) by precipitation/filtration, recyclability of catalytic CF3SO3H, and a high degree of chemoselectivity through decisive cleavage of the C2–C3 bond over other labile bonds under protic acid catalysis signify distinct advantages of developed protocols. The late-stage synthetic manipulation at the free-NH site of final products and a “gram-scale” preparation further demonstrate the promising usefulness of chemodivergent synthesis.