Paratropic 20π Macrocycles

Significance

In 1960 Woodward proposed a 20π-electron isophlorin as an intermediate in the synthesis of chlorophyll. Because no direct synthesis of isophlorins from simple precursors has been developed and because isophlorins are rapidly oxidized into 18π porphyrins under ambient conditions, it has been difficult to study their chemistry. Modified isophlorins 1 and 2, however, were synthesized under acid-catalyzed reaction conditions. UV-Vis absorption spectra indicate that 1 and 2 are unreactive with oxidizing agents (TFA, DDQ, I₂) and reducing agents (LAH, NaBH₄). The electron-withdrawing pentafluorophenyl meso-substituents of 1 and 2 serve to stabilize these molecules in their 20π electronic configuration.