Determination of N‐centered stereochemistry in N22‐methylated chlorophyll‐a derivatives and their epimer‐dependent optical spectra

Abstract

An N‐centered epimeric mixture of chlorophyll‐a derivatives methylated at the inner nitrogen atom was separated by reverse‐phase high‐performance liquid chromatography. Circular dichroism (CD) spectroscopic analyses of the epimerically pure N22‐methyl‐chlorins revealed that the minor first‐eluted and major second‐eluted stereoisomers were (22S)‐ and (22R)‐configurations, respectively. Their visible absorption and CD spectra in solution were dependent on the N22‐stereochemistry. The epimer‐dependent spectral changes were independent of the substituents at the peripheral 3‐position of the core chlorin chromophore.

Extra-Annulated Chlorophyll Derivatives by Scholl Reaction

“…The CD spectral simulation is thus useful for the stereochemical determination. It is noteworthy that the present helicity assignment using the CD Cotton effects is analogous to the N -center chirality characterization in distorted Chl derivatives reported recently …”