Synthesis of novel carboranylchlorins with dual application in boron neutron capture therapy (BNCT) and photodynamic therapy (PDT)

Total synthesis of carboranylchlorins 3 and 4, from readily available starting materials, are described and the molecular structures of two key intermediates are presented. Chlorins 3 and 4 show similar spectroscopic behavior but differ considerably in their solubility properties; whereas closo-carboranylchlorin 3 is completely insoluble in water, its nido derivative 4 has good water-solubility. Carboranylchlorin 3 absorbs in the red region of the optical spectrum (at λmax=642 nm) six times more strongly than porphyrin 1, and displays a fluorescence emission band at λmax=651 nm, upon excitation at 642 nm. The water-soluble carboranylchlorin 4 also displays intense absorption and emission bands at λmax=642 and 651 nm, respectively, in ethanol solution. It is concluded that carboranylchlorins 3 and 4 have higher promise for the dual application in PDT and BNCT than do comparable porphyrins.