New calamenene sesquiterpenes from Tarenna madagascariensis.

Four calamenene sequiterpenes, (+)-(7R,10S)-15-hydroxycalamenene (3), (+)-(7R,10S)-2,15-dihydroxycalamenene (4), (+)-(7R,10S)-2-hydroxy-15-calamenal (5), (+)-(7R,10S)-15-calamenal (6), along with the amorphane sesquiterpene (+)-(1S,6R,7R,10S)-1-hydroxy-3-oxo-amorph-4-ene (16), have been isolated from the Madagascan shrub Tarenna madagascariensis (Rubiaceae) and their structures determined by spectroscopic methods and chemical correlations. Furthermore, five known related sesquiterpenes [(+)-(7R,10S)-2-hydroxycalamenene (1), (+)-(7R,10S)-3-hydroxycalamenene (2), (-)-alpha-cadinol (13), cadinenal (14), 6-epicadinenal (15)], and three known lignans [(-)-hinokinin, (-)-dihydrocubebin, (-)-cubebin] were also isolated from the same plant. This is the first report of compounds 3, 4, 5, 6, and 16 from a natural source.