Minor indoloquinolizidine monoterpene alkaloids from an aqueous extract of the hook-bearing stem of Uncaria rhynchophylla

Six minor undescribed indoloquinolizidine monoterpene alkaloids, uncarrhynchophyllines F - K (1 - 6), were isolated from an aqueous decoction of the hook-bearing stems of Uncaria rhynchophylla (gou-teng). Their structures were determined by spectroscopic data analysis in combination with electron circular dichroism (ECD) and NMR calculations. Compounds 1 - 4 are rare indoloquinolizidine iminium zwitterions, while 5 exhibited protective effects against APAP induced liver cell injury.