Design, synthesis and photophysical properties of bodipy-labeled lupane triterpenoids

The BODIPY family fluorophores have been covalently linked to numerous classes of biomolecules, including proteins, carbohydrates, fatty acids, and steroids. Still, only two research papers on the synthesis and fluorescent biological analysis of BODIPY-labeling of triterpenoid compounds have been reported so far. In these works, fluorescent pentacyclic triterpene conjugates have been prepared by covalent binding to the known BODIPY-FL fluorophore through the 3-OH and 17-COOH functional groups. Here we aimed to work out a new approach for the synthesis of BODIPY-triterpenoid acids conjugates avoiding covalent binding of the triterpene core to the BODIPY-platform at the C-3 and C-17 positions. Novel BODIPY-lupane triterpenoid conjugates bearing a fluorescent marker at the C-2 position of ring A of the triterpene core were obtained via the Sonogashira reaction as a key step. The starting compounds in the cross-coupling reaction were C-2 propynyl derivatives of betulinic or betulonic acids and fluorescent dyes with an iodo-group at C-2 or meso position of BODIPY-platform. The elaborated coupling procedure might have applicability in the synthesis of fluorescent-labeled triterpenoid conjugates suitable for biological assays