Intermolecularly Hydrogen-Bonded Dimeric Helices. Tripyrrindiones

A rare and unusual class of tripyrrolic compounds, violet-colored tripyrrin-1,14-diones, can be prepared easily and in moderately high yields from base (piperidine)-catalyzed condensation of 3-pyrrolin-2-ones with 2,5-diformylpyrroles. Dipyrrinones adopt the all-syn-Z conformation leading to helical, lock-washer like structures, which form dimers that are held together by intermolecular hydrogen bonds in nonpolar solvents and in the crystal. Strong bathochromic spectral shifts of the tripyrrindione ∼480 nm long wavelength UV-visible absorption band are seen with added base: DBU, 615 nm; TFA, 573 nm; and Zn(OAc)₂, 586 nm.