A methodology to determine the enantiomeric excess and the absolute configuration (AC) of natural epoxythymols was developed and tested using five constituents of Ageratina glabrata. The methodology is based on enantiomeric purity determination employing 1,1'-bi-2-naphthol (BINOL) as a chiral solvating agent combined with vibrational circular dichroism (VCD) measurements and calculations. The conformational searching included an extensive Monte Carlo protocol that considered the rotational barriers to cover the whole conformational spaces. (+)-(8S)-10-Benzoyloxy-6-hydroxy-8,9-epoxythymol isobutyrate (1), (+)-(8S)-10-acetoxy-6-methoxy-8,9-epoxythymol isobutyrate (4), and (+)-(8S)-10-benzoyloxy-6-methoxy-8,9-epoxythymol isobutyrate (5) were isolated as enantiomerically pure constituents, while 10-isobutyryloxy-8,9-epoxythymol isobutyrate (2) was obtained as a 75:25 (8S)/(8R) scalemic mixture. In the case of 10-benzoyloxy-8,9-epoxythymol isobutyrate (3), the BINOL methodology revealed a 56:44 scalemic mixture and the VCD measurement was beyond the limit of sensitivity since the enantiomeric excess is only 12%. The racemization process of epoxythymol derivatives was studied using compound 1 and allowed the clarification of some stereochemical aspects of epoxythymol derivatives since their ACs have been scarcely analyzed and a particular behavior in their specific rotations was detected. In more than 30 oxygenated thymol derivatives, including some epoxythymols, the reported specific rotation values fluctuate from -1.6 to +1.4 passing through zero, suggesting the presence of scalemic and close to racemic mixtures, since enantiomerically pure natural constituents showed positive or negative specific rotations greater than 10 units.

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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-al (2) and (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-dien-7-ol (3). Compounds 2 and 3 were analyzed by single-crystal X-ray diffraction, while the structure of 1 was determined by 1D and 2D NMR experiments 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 ε-lactone (4), which was studied by vibrational circular dichroism spectroscopy. Comparison of the experimental VCD spectrum of 4 with the DFT//B3PW91/DGDZVP2 calculated spectrum assigned for the first time the absolute configuration of these seco-oxacassane diterpenes.

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An unprecedented macrocyclic dimeric diterpene containing a C2 symmetry axis was isolated from Acacia schaffneri . This compound, named schaffnerine, was characterized as (5S,7S,8R,9R,10S,17S,5'S,7'S,8'R,9'R,10'S,17'S)-7,8:7,17':16,17:17,7':7',8':16',17'-hexaepoxy-7,8-seco-7',8'-seco-dicassa-13,13'-diene (1) from its spectroscopic data. Comparison of its experimental vibrational circular dichroism spectrum with that calculated using density functional theory, at the B3LYP/DGDZVP level, assigned its preferred conformation and absolute configuration. The latter was confirmed by evaluation of the Flack and Hooft parameters obtained after single-crystal X-ray diffraction analysis.