Synthesis of purpurin-18 imide derivatives from chlorophyll-a and -b by modifications and functionalizations along their peripheries

The methyl pheophorbide-a and methyl pheophorbide-b were used as starting materials and converted to purpurin-18 ester by ring-opening and rearrangement reaction in their exocyclic ring. N-Substituted purpurin-18 imides were obtained from purpurin-18 ester through amidation reaction of six-membered cyclic anhydride. Further chemical modifications along their peripheries were carried out by a variety of common reactions, including electrophilic substitution, Wittig reaction, allomerization and Vilsmeier reaction, to afford the title compounds with long-wavelength absorption. The structures of all new chlorins were characterized by elemental analysis, IR, UV–vis and ¹H NMR spectra.