Facile transformation of the five-membered exocyclic E-ring in 132-demethoxycarbonyl chlorophyll derivatives by molecular oxygen with titanium oxide in the dark

Abstract

Major photosynthetic pigments chlorophyll(Chl)s have a cyclic tetrapyrrole with a five-membered exocyclic E-ring as photofunctional moieties. Its transformation is one promising methodology to develop functional pigments based on Chl chemistry. This paper report a facile conversion from Chl derivatives lacking the 132-methoxycarbonyl moiety to the corresponding chlorophyllous pigments, whose exocyclic E-ring had two oxo groups at the 131- and 132-positions, by contact with the TiO 2 particles in the presence of molecular oxygen in the dark. The conversion proceeded under mild conditions without hydrolysis of the ester group in the 17-propionate residue. This is in sharp contrast to the inevitable ester cleavage in the conventional synthesis of Chl derivatives that possess α-diketone in the E-ring. The reaction properties in the present method allowed us to perform a one-step conversion of Chl derivatives esterified with a natural isoprenoid alcohol phytol to the corresponding 132-oxo-pigments. No removal of central Zn from the chlorin macrocycle occurred in the present conversions using Zn pyropheophytin a as a starting material.