New Flavanol Dimers from the Bark of Celtis tetrandra and Their TRAIL Resistance-Overcoming Activity

In the courses of our search for bioactive natural products on TRAIL resistance-overcoming activity, the active EtOAc extract from the bark of Celtis tetrandra was chemically investigated. The fractionation of the extract resulted in the isolation of flavanols; (+)-afzelechin (1), (-)- epiafzelechin (2), (-)-catechin (3), and two new flavanol dimers; (-)- epiafzelechin-(4α→8)-(-)-catechin (4) and (-)- epiafzelechin-(4α→8)-(-)- epicatechin (5). All isolated compounds 1-5 were tested for their TRAIL resistance-overcoming activity against AGS cells at the concentrations from 5–20 μM. It is likely that the flavanol dimers 4 and 5 exhibited weak activity in the range of the concentration of 10–20 μM as compared with the standard.