From purpurin-18 to new purpurin imide derivatives with functional groups

Chlorins are among the most interesting photosensitizers for photodynamic therapy because they exhibit a high quantum yield of the singlet oxygen generation and absorbance in the deep red. Indeed the penetration depth of light into skin increases with wavelength from the UV to the near-infrared light range. Even if chlorins can be synthesized, they are present in the nature and the most abundant natural chlorin is the chlorophyll. After acetone extraction of chlorophyll a from spirulina maxima, it is possible to synthesize a derivative, the purpurin 18, which has an absorbance around 700 nm (CHCl[Formula: see text] thanks to an additional anhydride exocyclic ring compared to the chlorin p6, whose maximum absorbance wavelength is at 660 nm (CHCl[Formula: see text]. The disadvantage is that this anhydride exocyclic ring is very reactive and can be easily opened by a nucleophile to lead to a derivative of chlorin p6. To preserve a cycle and thus the absorbance around 700 nm, previous works have shown that it is possible to open purpurin 18 with an amine and to recyclize to lead to purpurin imide derivatives, which have also shown very important phototoxicity. In this work, we have therefore synthesized various derivatives of purpurin imide, bearing different peripherical reactive functional groups such as amino, sulfhydryl, maleimide, azide and alkyne to allow subsequent functionalization and open up the possibilities of using them. The structures of all new derivatives of purpurin imide were characterized by NMR and UV/visible spectroscopy and mass spectrometry.