Absolute Configuration of 7,8-seco-7,8-Oxacassane Diterpenoids from Acacia schaffneri

Chemical investigations of Acacia schaffneri led to the isolation of the new diterpenoid (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-diene-7,17-diol (1), together with the known (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-dien-7-ol-17-ol (2) and (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-dien-7-ol (3). Compounds 2and 3 were analyzed by single-crystal X-ray diffraction, while the structure of 1 was determined by 1D and 2D NMRexperiments and by chemical correlation with 2. Oxidation of 3 afforded conformationally restricted (5S,8R,9R,10S)-(-)-8-hydroxy-7,8-seco-cassa-13,15-dien-7-oic acid epsilon-lactone (4), which was studied by vibrational circulardichroism spectroscopy. Comparison of the experimental VCD spectrum of 4 with the DFT//B3PW91/DGDZVP2calculated spectrum assigned for the first time the absolute configuration of these seco-oxacassane diterpenes.