New Techniques of Structure Elucidation for Sesquiterpenes
283
O
O
O
O
O
O
O
O
O
O
O
O
H
O
O
O
O
O
O
O
H
O
O
O
HO
O
O
O
O
O
O
O
HO
164
165
166
167
A chemical study of the leaves of Piptocoma rufescens collected in the Dominican
Republic afforded six new germacranolide-type sesquiterpene lactones (168–173),
together with eight known related compounds. The structure elucidation of the new
lactones was determined by extensive NMR measurements, together with the singlecrystal X-ray diffraction analysis of 170 [81]. The absolute configuration of 168 was
established by application of the modified Mosher ester NMR method carried out
in NMR tubes, while those of the whole series of compounds were achieved by a
detailed analysis of their ECD spectra. The negative Cotton effects at 232 and 264 nm
for the π → π* and n → π* transitions of the lactone ring, measured in the ECD
spectrum of 168, was in agreement with the (7R) absolute configuration and with
previous studies of related sesquiterpene lactones, while the negative Cotton effects
at 223 and 272 nm in the ECD spectrum of 170 also indicated an α-configuration for
H-7. All compounds displayed a high cytotoxicity toward the HT29 human colon
cancer cell line and compound 172 exhibited also NF-κB (p65) inhibitory activity
[81].