C132-Methylation of methyl pheophorbide a and stereoselective preparation of methyl (132R)-methylpyropheophorbide a

The (13(2)R)-methoxycarbonyl group of methyl pheophorbide a, one of the chlorophyll-a derivatives, was converted to a methyl group through methylation at the CI3(2) position followed by removal of the methoxycarbonyl group. The methylation of the C13(2) carboanion gave a 4:1 mixture of methyl 132-methyl-pheophorbide a and its 132-epimer. The successive pyrolysis of the major methylated product afforded methyl (132R)-methyl-pyropheophorbide a with a small amount of its (132S)-epimer. The substitution effects at the C13(2) position including stereochemistry were discussed on the basis of ID/2D NMR, UV-vis absorption, and circular dichroism spectroscopic analyses as well as molecular modeling simulation. (C) 2014 Elsevier Ltd. All rights reserved.