Synthesis Of Oxyquinoliziniporphyrins And Pyridoquinoliziniporphyrins

Abstract

N-confused porphyrins are porphyrin analogues that introduce a nitrogen outside of the macrocyclic cavity. Two novel N-confused type porphyrins were targeted for this study. Both syntheses use the key intermediate 4-quinolizone. Previous studies of N-confused pyriporphyrin showed that the system is nonaromatic and resembles structurally analogous benziporphyrins. However, modifications to systems of this type can introduce aromatic characteristics. The first system targeted introduced a quinolizone subunit into a porphyrin system. Quinolizone contains a thermodynamically stable amide and this unit disrupts the aromatic pathway. However, dipolar resonance contributors can potentially reinstate some aromatic character. A quinolizone dialdehyde was condensed with tripyrranes under acidic conditions, and following an oxidation step, the first example of oxyquinoliziniporphyrins were isolated. These novel structures were very stable but exhibited only weakly-aromatic properties. Protonation and metalation studies were conducted and these modifications increased the diatropic ring current. In a related study, the introductions of an antiaromatic pyridoquinolizine unit has been investigated. A pyridoquinolizine diester was prepared from the 4-quinolizone intermediate used in the previous project. However, difficulties were encountered in converting the diester into the dialdehyde required to synthesize the macrocycle. If these problems can be overcome, it is anticipated that pyridoquinoliziniporphyrins will possess a significant degree of aromatic character.