Synthesis of oligomethylene-strapped chlorophyll derivatives and optical properties of their stereoisomers in a solution

Methyl pheophorbide-a/a' derivatives covalently linked with oligomethylene chains at the 3-CH2-OCO- and 13(2)-COO- moieties in a molecule were prepared by modifying chlorophyll-a through intramolecular ring-closing metathesis of vinyl groups. At least, a C10-length between the 3(3)- and 13(4)-positions was necessary for the cyclization and connection of a C12-strap was the most suitable to achieve the highest closure yield. The oligomethylene chain in 13(2) R-epimers derived from methyl pheophorbide-a covered the alpha-face of the chlorin pi-plane and the strap in the corresponding 13(2)S-epimers protected the beta-face. Synthetic 13(2)R-epimer with a dodecamethylene chain gave a flat chlorin pi-plane, while the decamethylene chain in the 13(2)R-epimer distorted the pi-system due to its shorter linkage. The distortion by strapping in the 13(2)R-epimer induced a slight blue-shift of Qy peak in dichloromethane. CD spectra of the 13(2)R-epimers were similarly dependent on the chain length, i.e., the distortion of pi-plane. Visible absorption and CD spectra of all the strapped 13(2)S-epimers were almost identical and only slightly different from those of the unstrapped. The strapping in the 13(2)S-epimers shifted the Qy peak bathochromically.