Revision of the structure of dunnianin and related sesquiterpene lactones from Illicium species

The sesquiterpene lactones dunnianin (1),debenzoyldunnianin (2), 7-deoxy-7-oxodunnianin (3), and pseudoanisatin (4) were isolated from fruits of Illicium floridanum. On the basis of the molecular structure of 4, 1-3 had previously been assigned structures containing 11,14-epsilon-lactone rings (1a-3a). Due to inconsistencies in the H-1-NMR spectra of 1-3, when compared with the spectrum of 4, their structures were reinvestigated by NMR spectroscopic analyses, and the molecular structure of 1 was determined by single-crystal X-ray diffraction. Compounds 1-3 were found to contain an 11,3-delta-lactone ring instead of the previously reported 11,14-epsilon-lactone, which required revision of their structures from 1a-3a to 1b-3b, respectively.