2,4,6-Trihydroxybenzoic Acid-Catalyzed Oxidative Ugi Reactions with Molecular Oxygen via Homo- and Cross-Coupling of Amines.

Metal-free, oxidative four-component Ugi reactions (U-4CRs) were conducted to synthesize dipeptides from two different amines, isocyanides, and carboxylic acids using 2,4,6-trihydroxybenzoic acid catalyst in O atmosphere. The organocatalytic U-4CRs proceed via oxidative cross-coupling of benzylamines with other aliphatic or aromatic amines to form imines, followed by condensation with isocyanides and carboxylic acids. The U-4CRs via cross-coupling of amines are rare, and the simple, metal-free procedures are advantageous for further applications in drug and heterocycle syntheses.

Metal-Free Oxidative Coupling of Benzylamines to Imines under an Oxygen Atmosphere Promoted Using Salicylic Acid Derivatives as Organocatalysts.

The oxidative coupling of benzylamines proceeds efficiently using salicylic acid derivatives as organocatalysts under an oxygen atmosphere, affording the corresponding -benzylidenebenzylamines in high yields. Electron-rich salicylic acid derivatives such as 4,6-dimethoxysalicylic acid and 4,6-dihydroxysalicylic acid exhibit excellent catalytic activities for the oxidative coupling of benzylamines to give the corresponding imines. This amine oxidation can also be applied to the synthesis of nitrogen-containing heterocycles such as benzimidazole derivatives.