Ugi-Post Functionalization, from a Single Set of Ugi-Adducts to Two Distinct Heterocycles by Microwave-Assisted Palladium-Catalyzed Cyclizations: Tuning the Reaction Pathways by Ligand Switch

Linear amides 4, prepared in one step by the Ugi four-component reaction, were converted to 3,4-dihydroquinoxalin-3-ones (5) or to 2-(2-oxoindolin-1-yl)acetamides (6) dependent on the catalytic conditions. While microwave irradiation was found to be determinant on the reaction efficiency, the choice of ligand diverged the reaction pathways. Heating a solution of 4 in dioxane/MeCN (v/v = 85/ 15) under microwave irradiation conditions in the presence of Pd(dba)(2) (0.05 equiv) and CS2CO3 (2 equiv), using XPhos as a supporting ligand, afforded the 3,4-dihydroquinoxalin-3-ones (5) via an intramolecular N-arylation of the secondary amide. On the other hand, using BINAP as ligand under otherwise identical conditions, intramolecular alpha-CH arylation of tertiary amide occurred to furnish the oxindoles (6).