Guaianolides and unusual 3-oxa-guaianolides from Artemisia macrocephala

Eight undescribed guaianolides (macrocephalolides A-H) and two known analogues (parishin C, artabsinolide E) were isolated from the whole plant of Artemisia macrocephala growing in Xinjiang, China. Their structures were determined on the basis of extensive spectroscopic analysis, with absolute configurations established by comparison of experimental and calculated ECD data, as well as confirmation of single-crystal X-ray diffraction crystallography. Macrocephalolides A-B featured an unusual type of 3-oxa-guaianolide with a cyclopentenone moiety. Macrocephalolides C-E possessed a dihydro-2H-pyran acetal segment, representing an unprecedented 2, 3-secoguaianolide skeleton with 6/7/5 tricyclic ring system in natural sesquiterpenes. The X-ray crystal structures of parishin C and artabsinolide E were reported for the first time.