Macrocycles encompassing four pyrrole rings are common motifs in biochemical and synthetic light harvesting and catalysis. Inaba et al. report a strained, smaller analog with three pyrroles. Their route cyclized a linear hexaketone precursor through condensation, restored the six carbonyls through an oxidation–reduction sequence, and then aminated it to produce the pyrroles. Crystallography highlighted the high strain inherent in the structure, which quickly underwent a ring-opening dimerization in acidic solution to form a calix[6] pyrrole. The results help to explain why the tetrapyrrole structure is the smallest of these macrocycles that is readily observed.
J. Am. Chem. Soc. 10.1021/jacs.1c06331 (2021).