Spirovetivane- and Eudesmane-Type Sesquiterpenoids Isolated from the Culture Media of Two Cyanobacterial Strains

As part of a study examining polar
metabolites produced by cyanobacterial
strains, we examined media extracts of a Calothrix sp. (strain R-3-1) and a Scytonema sp. (strain
U-3-3). The cell mass of each was separated from the media, and HP20
resin was added for adsorption of secreted metabolites, a relatively
unexplored area of cyanobacterial chemistry. HPLC-UV-LCMS-guided isolation
led to the discovery of seven sesquiterpenoid compounds with five
new, one known, and one previously isolated as the methyl ester. Through
a complement of 1D and 2D NMR spectroscopic techniques, the planar
structures and relative configurations of the seven compounds were
elucidated. Spironostoic acid (1), 11,12-didehydrospironostoic
acid (2), and 12-hydroxy-2-oxo-11-epi-hinesol (4) are spirovetivane-type compounds from R-3-1,
while stigolone (5), 11R,12-dihydroxystigolone
(6), and 11S,12-dihydroxystigolone (7) are three eudesmane-type compounds from U-3-3. Circular
dichroism was utilized to decipher the absolute configurations of
new compounds 1, 2, 4, 5, 6, and 7. Due to the structural
variety observed among the spirovetivane- and eudesmane-type compounds
in the literature and often a lack of clarity in how determinations
were made, computational spectra and model compounds were used to
support the interpretation of ECD and NMR spectra. A straightforward
process to determine the configuration of these systems is presented.