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

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.