Linker-length dependent intra/intermolecular coordination of synthetic zinc chlorophyll-a derivatives bearing a pyridine terminal in the C132-substituent

Abstract

An ω-(m-pyridyl)alkyl group was introduced at the C132-methoxycarbonyl group of methyl pheophorbide-a as a chlorophyll-a derivative by exchange of the esterifying moiety. To avoid allomerization and epimerization of the C132-chiral center in the β-keto-ester moiety, a propyl group was substituted at the C132-position. The free bases were zinc-metalated to give zinc methyl 132-propyl-pheophorbides-a bearing an oligomethylene chain with a pyridyl terminal at the C132-position. Depending on the length of the alkyl chain between the terminal pyridine and C132-carboxylate moieties, the coordination of the m-pyridyl group with the central zinc atom in the stereochemically pure and fixed zinc chlorin–pyridine conjugates occurred in intra- or intermolecular fashions.