Synthetic studies related to diketopyrrolopyrrole (DPP) pigments. Part 1: The search for alkenyl-DPPs. Unsaturated nitriles in standard DPP syntheses: a novel cyclopenta[c]pyrrolone chromophore

Reactions of the anion of ethyl 4,5-dihydro-5-oxo-2-phenylpyrrole-3-carboxylate with the Diels–Alder adducts of acrylonitrile and various dienes rarely yield the expected DPP derivatives. The reaction with cyclohex-3-enecarbonitrile provides a noteworthy exception: thermolysis of the resulting cyclohexenyl-DPP gives butadiene and impure 3-ethenyl-6-phenyl-DPP, the latter being thermally unstable. Michael additions predominate when the above anion reacts with α,β-unsaturated nitriles: acrylonitrile and methacrylonitrile give 4,4-bis(cyanoethyl) and 4,4-bis(2-cyanopropyl) derivatives, and cinnamonitrile, substituted cinnamonitriles and 3-(2-thienyl)acrylonitrile give deep red 3-aryl-5-cyano-4-hydroxy-2H-cyclopenta[c]pyrrol-1-ones. These ambident nucleophiles may undergo N- and either O- or C-alkylation according to the alkylating agent used.