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

The reaction of β-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 β-carbon with its alkyl substituent was excised and replaced by an oxygen atom, and the neighboring β-carbon was hydroxylated. This work clarifies the nature of the products first described by Fischer and Deželić, in 1933, and verifies the work by Shulg'a and coworkers, 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.