Synthesis of oligomethylene-strapped chlorophyll derivatives and optical properties of their stereoisomers in a solution
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Methyl pheophorbide-a/a′ derivatives covalently linked with oligomethylene chains at the 3-CH2OCO– and 132-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 33- and 134-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 132 R-epimers derived from methyl pheophorbide-a covered the α-face of the chlorin π-plane and the strap in the corresponding 132 S-epimers protected the β-face. Synthetic 132 R-epimer with a dodecamethylene chain gave a flat chlorin π-plane, while the decamethylene chain in the 132 R-epimer distorted the π-system due to its shorter linkage. The distortion by strapping in the 132 R-epimer induced a slight blue-shift of Qy peak in dichloromethane. CD spectra of the 132 R-epimers were similarly dependent on the chain length, i.e., the distortion of π-plane. Visible absorption and CD spectra of all the strapped 132 S-epimers were almost identical and only slightly different from those of the unstrapped. The strapping in the 132 S-epimers shifted the Qy peak bathochromically.
KeywordsCircular dichroism spectra π-plane distortion Pheophorbide Ring-closing metathesis Site energy Visible absorption spectra
1-Ethyl-3-[3-(N,N-dimethylamino)propyl]carbodiimide (water-soluble carbodiimide)
Flash column chromatography
High performance liquid chromatography
Nuclear magnetic resonance
We thank Dr. Yasuhiko Kureishi for assistance in molecular modeling calculation. This work was partially supported by a Grant-in-Aid for Scientific Research (A) (No. 22245030) (to HT) from the Japan Society for the Promotion of Science (JSPS) and for Young Scientists (B) (No. 21750154) (to SS) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.
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