Macrocycle contraction and expansion of a dihydrosapphyrin isomer

Cyclization of a pentapyrrane with two terminal β-linked pyrroles afforded a dihydrosapphyrin isomer (1) with the pyrroles linked in a unique β,α-α,β mode, which was rather reactive, and thus it readily underwent a ring-contracted rearrangement to a pyrrolyl norrole (2), and succeeding ring expansion to a terpyrrole-containing isosmaragdyrin analogue (4). 1, 2, and 4 contain the internal ring pathways with a minimum of 17, 15, and 16 atoms, respectively. 1, 2, and 4 are almost nonfluorescent, whereas the complex of 2 with Zn²⁺ shows a distinct NIR emission peak at 741 nm. The unprecedented pyrrole transformation chemistry by confusion approach is illustrated.