A UV-guided fractionation of the AcOEt extract of the fermentation broth of Guignardia sp., an endophytic fungus from the leaves of the tropical tree Spondias mombin, resulted in the identification of the new metabolite (À)-(2S,5Z)-2-(1-methylethyl)-4-oxo-5-(phenylmethylene)-1,3-dioxolane-2-carboxylic acid (1), isolated as NH 4 salt 1a. The metabolite 1 was designated (À)-(S)-guignardic acid. This first member of a new class of natural compounds contains a dioxolanone moiety formed by fusion of 2-oxo-3-phenylpropanoic acid (phenylpyruvic acid) and 3-methyl-2-oxobutanoic acid (dimethylpyruvic acid), products of the oxidative deamination of phenylalanine and valine, respectively. The structure of 1a was deduced from spectral data (UV, IR, MS, 1 H-and 13 C-NMR) and confirmed by asymmetric synthesis.
Results and Discussion. ± (À)-(S)-Guignardic acid (1), isolated as its NH 4 salt 1a, was obtained as amorphous solid and identified as (À)-(2S,5Z)-2-(1-methylethyl)-4oxo-5-(phenylmethylidene)-1,3-dioxolane-2-carboxylic acid, as deduced from UV, IR, MS, 1 H-, 13 C-NMR spectral data.
We failed to obtain useful data of 1 by the usual positive mode in EI, CI, or ESI mass spectrometric methods, but the ESI-MS data of ammonium guignardate 1a in the negative mode furnished a [M À H] À peak at