Absolute configuration of podophyllotoxin related lignans from Bursera fagaroides using vibrational circulardichroism

The ethanol extract from the dried exudate of Bursera fagaroides (Burseraceae) showed significant cytotoxicactivity in the HT-29 (human colon adenocarcinoma) test system. The extract provided four podophyllotoxin relatedlignans, identified as (7'R,8R,8'R)-(-)-deoxypodophyllotoxin (3), (7'R,8R,8'R)-(-)-morelensin (4), (8R,8'R)-(-)-yatein (5),and (8R,8'R)-(-)-5'-desmethoxyyatein (6), whose spectroscopic and chiroptical properties were compared with those of(7R,7'R,8R,8'R)-(-)-podophyllotoxin (1) and its acetyl derivative (2). Their absolute configurations were assigned bycomparison of the vibrational circular dichroism spectra of I and 3 with those obtained by density functional theorycalculations.