Amoenolide A and related acetylated labdane diterpenes from Amphiachyris amoena

Four new labdane diterpenes, amoenolide A [1] and its 19-acetate [2], 6-acetate [3], and 6,19-diacetate [4], were isolated from the above-ground parts of Amphiachyris amoena and their structures established by spectral and chemical methods. High-field 1H- and 13C-nmr assignments were made for each compound using 1D and 2D nmr techniques including 1H single-frequency spin-coupling and nOe difference experiments, one-bond and long-range carbon-hydrogen correlations, and the INADEQUATE carbon-carbon connectivity experiment. The absolute stereochemistry was established by the Horeau partial-resolution chemical method with amoenolide A and racemic 2-phenylbutyric anhydride, and by the cd exciton splitting physical method on the synthetic amoenolide A 2-acetate 6,19-dibenzoate [9].