Liquid-liquid extraction of cinchona alkaloids by using some metal complexes of zinc(II), cobalt(II) and copper(II) with (+)- and (-)-usnic acids was studied. The synergic effect of the chelates was studied by adduct formation with neutral Lewis bases such as tri-n-octylphosphine oxide and optically active isomers of cinchona alkaloids. It was possible to differentiate between the adduct formation of quinine and that of quinidine, and also of cinchonine and of cinchonidine, in the synergic extraction of zinc(II) and cobalt(II) with (+)- and (-)-usnic acids. © 1991.
Tsurubou, S., Sakai, T., Kihara, S., & Matsui, M. (1991). Liquid-liquid extraction of cinchona alkaloids by using some metal complexes of optically pure usnic acids. Analytica Chimica Acta, 248(2), 501–506. https://doi.org/10.1016/S0003-2670(00)84667-X