Xanthones are secondary metabolites derivating from the 9H-xanthen-9-one scaffold
produced by a wide range of plant, bacterial and fungal species. In lichens, they
occur mostly as members of two classes: lichexanthones and norlichexanthones, differing
by the methylation of positions 3 and 5 for lichexanthones. Chlorination can occur
on positions 2, 4, 5 and 7, resulting in a total of sixteen different norlichexanthones
for only five different exact masses. This high number of isomers makes them tedious
to identify. High resolution tandem mass spectrometry is of no help without HLPC separation
combined to standard compounds availability, and even NMR on pure compounds could
be tricky, due to their high proton- deficiency.
However, they are of chemotaxonomic interest for lichenologists to identify crustaceous
lichens with very similar morphology, such as some species of Lecanora.
We therefore constituted a library of the sixteen norlichexanthones, which were obtained
by a combined strategy of synthesis and isolation. An HPLC/MS method was set up to
separate them efficiently by their m/z and retention times ([Fig. 1a]). In parallel, we acquired NMR data to perform the dereplication of these compounds
in lichen extracts, based on the HSQC correlations observed for the non-chlorinated
positions ([Fig. 1b]).
Fig. 1 Norlichexanthones in lichens: two complementary dereplication strategies.
While LC/MS is sensitive and easy to interpret, NMR is robust and highly reproducible.
A combination of both methods provides a good overview of the xanthone content in
lichens.
