Regioselective Internal Carbonylation of the 2-Aza-21-carbaporphyrin: Access to Configurationally Stable Chiral Porphyrinoids

Liu, B.; Li, X.; Maciołek, J.; Stępień, M.; Chmielewski, P. J.
J. Org. Chem., 2014, 79, 3129–3139 | view at publisher's site

Reaction of paraformaldehyde with meso-tetraaryl-2-aza-21-carbaporphyrins (NCP) in the presence of a basic catalyst afforded fused lactam derivatives comprising a >C═O bridge linking the internal carbon C21 with one of the internal nitrogens. The isomer 2 with C21–C(O)–N24 bridge is formed with about 9-fold molar excess over that with C21–C(O)–N22 bridge (3). The 1H NMR and UV–vis spectral characteristics indicate aromatic character of the derivatives. For meso-tetrakis(3′,5′-dimethoxy)- and meso-tetrakis(3′,4′,5′-trimethoxy)-21,24-carbonyl-NCP an efficient external ring fusion by linking C3, i.e., the external carbon of the confused pyrrole, with an ortho carbon of the adjacent aryl was observed under acidic conditions, yielding derivatives 4c and 4d, comprising a linear system of five fused rings. The chirality and configurational stability of these carbonylated systems were established by a chiral stationary phase HPLC and circular dichroism. The interaction of 2 with chiral acids and alcohols leading to the formation of diastereomers was observed by 1H NMR. Slow racemization of 2 under acidic conditions was established by HPLC and 1H NMR and a mechanism for this process was proposed.