β-Alkyloxazolochlorins: Revisiting the Ozonation of Octaalkylporphyrins, and Beyond

The reaction of beta-octaalkylporphyrins (octaethylporphyrin and etioporphyrin I) with ozone generated the corresponding heptaalkyloxazolochlorinhemiacetals in which a pyrrolic subunit of the porphyrins was replaced by an oxazoline moiety. Thus, a pyrrolic beta-carbon with its alkyl substituent was excised and replaced by an oxygen atom, and the neighboring beta-carbon was hydroxylated. This work clarifies the nature of the products first described by Fischer and Dezelic, in 1933, and verifies the work by Shulg'a and co-workers, from 1977. Furthermore, the chemistry of the oxazolochlorin hemiacetals was studied: They could be dehydroxylated or converted to alkyl acetals and gem-dialkyl derivatives, all possessing chlorin-type optical spectra. Their oxidative conversions generated a unique tetrahydrofuran-linked oxazolochlorin dimer and a hexaethylporpholactone. The work expands on the knowledge of converting porphyrins to porphyrinoids of potential utility containing nonpyrrolic building heterocycles.