Demethoxycarbonylation and oxidation of 132(S/R)-hydroxy-chlorophyll a to 132-demethoxycarbonyl-132-oxo-chlorophyll a and Mg-purpurin-18 phytyl ester

The conversion of 132(S/R)-hydroxy-chlorophyll (Chl) a to 132-demethoxycarbonyl-132-oxo-Chl a in a yield of 40%, utilizing a simple pyrolysis technique, is described. About 10% of the phytyl ester of Mg-purpurin-18 was formed as a side product. The completely assigned 1H and 13C NMR spectra are presented for 132-demethoxycarbonyl-132-oxo-Chl a and a likely mechanism for its formation is proposed. A slight extension of this mechanism also explains the formation of the Mg-purpurin-18 side product. The proposed mechanism has several features comparable with those previously suggested for the allomerization of 132(R/S)-Chl a. Attempts to apply the same pyrolysis method to prepare 132-demethoxycarbonyl-132-oxo-Chl b from 132(S/R)-hydroxy-Chl b were unsuccessful.