Regioselective Oxidative Ring Cleavage of a Phlorin Analogue: An Approach for Synthesizing Linear Tetrapyrroles.

Oxidation of neo-N-confused phlorin 1 with excessive FeCl3 leads to regioselective ring opening, generating three linear tetrapyrroles, i.e., (10 Z,15 Z)-biladienone 2, (10 Z,15 E)-biladienone 3, and 19-methoxy (10 Z,15 E)-biladiene 4 with an overall yield of 88%. The coordination of these compounds with Zn(II) is affected by the inverted terminal pyrrolic unit, and the presence of the electron-withdrawing keto moiety. The structural identities were also clearly elucidated by the crystal structures of 3 and 4-Zn and were rationalized by DFT calculations.