Synthesis of methyl (132R/S)-alkyl-pyropheophorbide a and a non-epimerized chlorophyll a mimic

Abstract

The (132R/S)-methoxycarbonyl group of methyl pheophorbides a/a′ (chlorophyll a/a′ derivatives) was converted to methyl, ethyl, propyl, and isopropyl groups through the C132-alkylation under basic conditions followed by pyrolysis in 2,4,6-collidine with lithium iodide. All the resulting products, methyl 132-alkyl-pyropheophorbides a, predominantly gave the (132R)-stereoisomers with about one tenth of the (132S)-epimers. Their stereochemistry was determined by 1D/2D NMR and their optical properties were characterized by visible absorption and circular dichroism spectroscopy. Methyl (132R)-propyl-pyropheophorbide a was converted to (132R)-propyl-pyrochlorophyll a by ester exchanging and magnesium chelating reactions. The synthetic chlorophyll a analogue showed non-epimerization at the 132-position in pyridine-d5 at 40°C, while naturally occurring chlorophyll a was easily epimerized under the same conditions to give its epimeric mixture.