Absolute configuration of (13R)- and (13S)-labdane diterpenes coexisting in Ageratina jocotepecana.

Chemical investigation of the hexanes extracts of Ageratina jocotepecana afforded (-)-(5S,9S,10S,13S)-labd-7-en-15-oic acid (1), methyl (-)-(5S,9S,10S,13S)-labd-7-en-15-oate (2), (+)-(5S,8R,9R,10S,13R)-8-hydroxylabdan-15-oic acid (3), and (-)-(5S,9S,10S,13Z)-labda-7,13-dien-15-oic acid (5). The coexistence of (13R)- and (13S)-labdanes in this member of the Asteraceae family was demonstrated by vibration circular dichroism measurements of ester 2 and methyl (+)-(5S,8R,9R,10S,13R)-8-hydroxylabdan-15-oate (4) in comparison to the DFT B3LYP/DGDZVP-calculated spectra. In addition, transformation of 1 and 3 with HClO4 in MeOH yielded epimeric methyl (+)-(5S,10S,13S)-labd-8-en-15-oate (6) and methyl (+)-(5S,10S,13R)-labd-8-en-15-oate (7), respectively, confirming the presence of C-13 epimers in this plant. Diterpene 1 showed remarkable antibacterial activity against Bacillus subtilis (MIC 0.15 mg/mL) and Staphylococcus aureus (MIC 0.78 mg/mL), while diterpene 3 exhibited moderate activities against the same organisms.