Nine New Sesquiterpenes from Dendrobium nobile

Nine new sesquiterpenes, i.e. , dendronobilins A–I ( 1 – 9 ), with copacamphane‐type ( 1 ), picrotoxane‐type ( 2 – 6 ), muurolene‐type ( 7 ), alloaromadendrane‐type ( 8 ), and cyclocopacamphane‐type ( 9 ) skeletons, were isolated from the 60% EtOH extract of the stems of Dendrobium nobile. Their structures were established as (1 R , 2 R,4 S ,5 S ,6 S , 8 S,9 R )‐2,8‐dihydroxycopacamphan‐15‐one ( 1 ), (2 β ,3 β ,4 β ,5 β )‐2,4,11‐trihydroxypicrotoxano‐3(15)‐lactone ( 2 ), (2 β ,3 β ,5 β ,9 α ,11 β )‐2,11‐epoxy‐9,11,13‐trihydroxypicrotoxano‐3(15)‐lactone ( 3 ), (2 β ,3 β ,5 β ,12 R *)‐2,11,13‐trihydroxypicrotoxano‐3(15)‐lactone ( 4 ), (2 β ,3 β ,5 β ,12 S *)‐2,11,13‐trihydroxypicrotoxano‐3(15)‐lactone ( 5 ), (2 β ,3 β ,5 β ,9 α )‐9,10‐cyclo‐2,11,13‐trihydroxypicrotoxano‐3(15)‐lactone ( 6 ), (9 β ,10 α )‐muurol‐4‐ene‐9,10,11‐triol ( 7 ), (10 α )‐alloaromadendrane‐10,12,14‐triol ( 8 ), and (5 β )‐cyclocopacamphane‐5,12,15‐triol ( 9 ) on the basis of spectroscopic analysis. The absolute configuration of compound 1 was tentatively assigned as (1 R ,2 R ,4 S ,5 S ,6 S ,8 S ,9 R ) according to its CD spectrum and the octant rule. Compounds 1 and 4 – 9 were inactive in our preliminary in vitro immunomodulatory bioassay.