Synthesis of a chlorophyll-a derivative fused with an additional exo-five-membered ring and its optical properties

A C20-free chlorophyll-a derivative with an additional exo-five-membered ring was successfully prepared using an ethylene linkage at the C3- and C5-positions. A bromination at the C20-position was requisite for the cyclization of a 1-hydroxyethyl or vinyl group at the C3-position of methyl bacteriopheophorbide-d or methyl pyropheophorbide-a, respectively. By comparing optical properties of the cyclized product with those of its 3-ethyl uncyclized analog in a diluted dichloromethane solution, it was shown that the cyclization shifted the Q_x and B_x absorption maxima to longer wavelengths and reduced the Stokes shift.