Synthesis of 132,173-cyclopheophorbides and their optical properties

Abstract

A series of 132,173-cyclopheophorbides possessing a variety of 3-substituents were prepared from methyl pyropheophorbide-a (one of the chlorophyll-a derivatives) and its 3-substituted derivatives, via intramolecular mixed Claisen condensation of the 13-COCH2 moiety with the 172-COOCH3 group as the peripheral substitutes of the chlorin chromophore. Similar to the cyclization, methyl pyroprotopheophorbide-a and methyl pyrobacteriopheophorbide-a bearing porphyrin and bacteriochlorin π-skeletons, respectively, were treated with a base to give the corresponding 132,173-cyclopheophorbides-a. All of the 1H NMR spectra of the synthetic cyclopheophorbides showed that they took exclusively enol forms (not diketones) in deuterated chloroform. Their electronic absorption spectra in dichloromethane exhibited characteristically split and expanded Soret bands and red-shifted Qy maxima. The effects of the 3-substitution in cyclopheophorbides on the absorption bands were comparable to those in the starting, non-cyclized pyropheophorbides. Additionally, the 17,18-didehydrogenation of the chlorin to porphyrin π-skeletons sharpened the Soret bands, and the 7,8-dihydrogenation of the chlorin to bacteriochlorin moieties moved the Qy bands to longer wavelengths. No fluorescence emission was observed upon excitation at the Soret maximum in any solutions of the cyclopheophorbides. These results indicated that the exo-cyclization at the 132- and 173-positions would fully suppress their phototoxicities even in an aerated solution at room temperature.