Isolation and characterization of Yunnaneic acids A-D, four novel caffeic acid metabolites from Salvia yunnanensis

In the course of chemical studies on medicinal plants having uremic toxin-decreasing effects, the novel caffeic acid metabolites yunnaneic acids A-D were isolated from the roots of Salvia yunnanensis. Yunnaneic acids C and D were found to be caffeic acid trimers having a bicyclo[2.2.2]octene skeleton, which is biogenetically formed by a Diels-Alder-type addition between rosmarinic acid and caffeic acid. Yunnaneic acid A has a dimeric structure composed of yunnaneic acids C and D, which are linked by formation of a spiroacetal ring. Yunnaneic acid B consists of two molecules of yunnaneic acid C. The structures of the yunnaneic acids were elucidated on the basis of spectroscopic evidence, and their absolute configurations were established by application of the circular dichroic exciton chirality method.