Aerobic oxidative tandem conversion of PEG-400 to acetyl radical via C–C and C–O bond cleavages followed by silver-catalyzed menisci-type addition to the C₃ position of quinoxalin-2(1H)-one is described. This reaction involves the in situ formation of the acetyl radical from PEG-400 via a sequence of acetaldehyde, 2-oxopropanal and, 2-oxopropanoic acid formation and successive oxidative cleavage to form the acetyl radical. This protocol uses cheap, readily available and environmentally-friendly PEG-400 as the acetyl source and solvent. The in situ generated acetyl radicals from PEG-400 have been coupled to a broad range of electron-deficient quinoxalin-2(1H)-one compounds in a menisci-type reaction.