Towards Novel Porphyrinoid Architectures: Synthesis of Porphyrin Analogues with Two Six-membered Exocyclic Rings.

Tripyrranes with terminal ester protective groups were prepared by reacting 7-acetoxy-4,5,6,7-tetrahydroindoles with 3,4-diethylpyrrole in refluxing acetic acid-ethanol. Dibenzyl esters were deprotected by hydrogenolysis over 10% Pd/C and the resulting dicarboxylic acids were condensed with a series of dialdehydes in the presence of trifluoroacetic acid, followed by oxidation with FeCl 3 or DDQ, to give porphyrinoid products with two six-membered exocyclic rings. Reactions with a pyrrole dialdehyde gave porphyrins in 27-30% yield and good yields were also obtained using 2,5-furandicarbaldehyde and 4-hydroxy-2,6-pyridinedicarbaldehyde to prepare oxaporphyrins and oxypyriporphyrins. However, poorer results were noted for thiaporphyrins, benzocarbaporphyrins and oxybenziporphyrins due to their poor stability in solution. The UV-vis spectra for these porphyrinoids showed significant bathochromic shifts compared to structures without 6-membered carbocyclic rings and in every case the proton NMR spectra showed that the diatropic ring currents associated with the macrocycles were reduced. These data give valuable insights into the properties of modified porphyrinoid systems and provide the foundations for investigations into the preparation of new porphyrinoid architectures. • Design of a synthetic strategy for preparing porphyrins and porphyrin analogues with two carbocyclic rings • Modification of the porphyrin chromophore by incorporating cycloalkano units • Diminishment of diatropicity in porphyrins, heteroporphyrins, carbaporphyrins and related systems due to the presence of carbocyclic rings • Development of the foundations to access novel porphyrinoid architectures