NMR study of fumarprotocetraric acid, a complex lichen depsidone derivative from Cladonia furcata†

A lichen depsidone derivative, fumarprotocetraric acid, was obtained as the major constituent of Cladonia furcata (Huds.) Schrad. collected in Iceland. In the 1H NMR spectrum (in DMSO-d6) of this compound, the trans double bond proton signals of the fumaryl unit appeared as overlapping singlets. To confirm the presence of a trans double bond in the molecule, fumarprotocetraric acid was treated with triethylamine. The 1H NMR spectrum of the resultant triethylamine salt displayed a pair of clear doublets with coupling constants of 15.7 Hz, which are characteristic of a trans double bond. Unambiguous 1H and 13C NMR spectral data for both fumarprotocetraric acid and its triethylamine salt were assigned for the first time based on the interpretation of their 2D NMR (HMQC, HMBC and NOESY) spectra. Copyright © 2003 John Wiley & Sons, Ltd.