Novel macrocycles by modification of the beta,beta\u27-position of porphyrins

Nearly all living things contain porphyrins and chlorins. They play key roles in metabolic processes. Synthetic porphyrins are also intensely studied, as they have found use in a number of applications, such as photodynamic therapy, photodiagnosis, and light harvesting devices. ^ It was earlier shown that the β,β′-position of porphyrins could undergo cleavage and ring-closure reactions to generate novel porphyrinic macrocycles. We have concentrated on the further elucidation of the reactivity of the β-positions of chlorins using meso-tetraaryldiolchlorins as starting materials. These derivatives offer the possibility to design rational syntheses for a wide variety of pyrrole-modified porphyrins. The results of these syntheses are described, in particular: (1) Porphyrins in which one of the β,β′-double bonds has been formally replaced by a lactone moiety are called porpholactones. We show that these porpholactones can be synthesized from diol chlorins. Novel lactol and oxazole containing chromophores can also be formed by reduction of the porpholactones. (2) If one of the pyrrolic subunits in porphyrins is replaced with a morpholine ring, morpholinochlorins are formed. The metal-templated synthesis of these chromophores by reaction of the Ni(II) secochlorin bisaldehyde with an alcohol is known. However, since it proved impossible to remove the central metal ion without concomitant destruction of the macrocycle, the free base analogues could not be formed. Thus, a removable metal template was sought to access free base morpholinochlorins. We found that Ag(II) can act as a suitable template. (3) Free base morpholinochlorins could also be prepared directly from diol chlorin in the absence of a templating metal. Generation and in situ reaction of the free base bisaldehyde with alcohols generates free base morpholinochlorins in high yields. (4) The synthesis of a novel class of secochlorin-based chromophores containing direct o-aryl-to-β-position linkages is described. ^ The spectroscopic properties of all novel chromophores are discussed. These studies further illustrate the reactivity of the β,β ′-position of porphyrins and the photophysical effects of a manipulations of these positions.