Betulinic Acid: Isolation from Triphyophyllum peltatum and Ancistrocladus heyneanus, Antimalarial Activity, and Crystal Structure of the Benzyl Ester* - Bringmann, Gerhard; Saeb, Wael; Assi, Laurent Aké; François, Guido; Sankara Narayanan, A S; Peters, Karl; Peters, Eva-Maria (Planta Medica, 1997)