(+)/(−)-Phaeocaulin A-D, four pairs of new enantiomeric germacrane-type sesquiterpenes from Curcuma phaeocaulis as natural nitric oxide inhibitors

Germacrane-type sesquiterpenes, with a flexible 10-membered ring unit as the structural and conformational features, play a central role in the biosynthesis and synthesis of other sesquiterpenes. In this report, two pairs of new sesquiterpene alkaloids, (+)/(-)-phaeocaulin A [(+)-1/(-)-1] and B [(+)-2/(-)-2], and two pairs of new sesquiterpenes, (+)/(-)-phaeocaulin C [(+)-3/(-)-3] and D [(+)-4/(-)-4], along with one related known analog (5), were isolated from the rhizomes of Curcuma phaeocaulis. The absolute configurations of (+)-1/(-)-1, (+)-2/(-)-2, (+)-3/(-)-3 and (+)-4/(-)-4 were unambiguously determined by analysis of single-crystal X-ray diffractions and quantum chemical electronic circular dichroism (ECD) method. It is noteworthy that (+)/(-)-phaeocaulin A [(+)-1/(-)-1] and B [(+)-2/(-)-2] are two pairs of rare N-containing germacrane-type sesquiterpenes. A possible biogenetic pathway for 1-5 was postulated. All of the isolated compounds were tested for their inhibitory activity against LPS-induced nitric oxide production in RAW 264.7 macrophages.