Synthesis of cytotoxic 2,2-difluoroderivatives of dihydrobetulinic acid and allobetulin and study of their impact on cancer cells - Borkova, L; Jasikova, L; Rehulka, J; Frisonsova, K; Urban, M; Frydrych, I; Popa, I; Hajduch, M; Dickinson, N J; Vlk, M; Dzubak, P; Sarek, J (Eur J Med Chem, 2015)