Efficient Synthesis of Long-wavelength Absorbing Cyanochlorophyll a Derivatives via Stereoselective Horner-Wadsworth-Emmons Reaction

In this article, we report the stereoselective Horner-Wadsworth-Emmons reaction of reactive carbonyl-substituted chlorophyll a derivatives with diethyl cyanomethylphosphonate. We found that the formyl or keto group at the chlorin periphery can react stereoselectively with phosphonate carbanions to produce a series of cyanomethylene-substituted chlorins with predominantly E geometry. Considering that no consistent records are available in the literature on the results of the allomerization of methyl pyropheophorbide a, we propose a reliable process for the transformation of this chlorophyll derivative. Our methodology represents a simple and efficient way for the preparation of novel, differently functionalized long-wavelength absorbing chlorins, those can be applied for photodynamic therapy, solar cells, and other relevant purposes.