Synthesis of new methine class of dyes bearing maleimide ring system

Reaction of l-methyl-3-methylthiomaleimides (1a-c) with N,N-dialkylanilines (2) under refluxing in acetic acid condition gave the corresponding 3-(4-diakylamino)phenyl-1-methylmaleimides (3a-g). Treatment of these 1-methyl-3-phenylmaleimides (3a, c) with Lawesson's reagent under refluxing in toluene afforded new blue dyes, 4-(4-dialkylamino)phenyl-3-cyano-1-methyl-5-oxopyrrole-2-thiones (5a, b) which are brilliant blue dyes appearing at 606 and 615 nm (log ε: 4.59 and 4.50) in UV spectra. Reaction of 4-methoxycarbonyl-1-methyl-3-methylthiomaleimide (1b) with 3-dialkylamino-phenol under the same reaction conditions gave cyclizcd products, 2H, 4H-[1]-benzopyrano[3,4-c]pyrrole-1,3,4-triones (6a-c) in good yields.