Determination of the absolute configuration of the fragments composing the phytotoxin phomalide

The absolute configurations of the residues constituting phomalide (1) were determined on the products resulting from acidic hydrolysis. The configurations of the amino acids Val and Leu were determined as (S) and (R), respectively, by chiral GC analysis (Chirasil-Val capillary column), employing N-trifluoroacetyl (TFA) amino acid methyl ester derivatives. The configurations of the hydroxy acids O-Phe and O-Leu were determined as (S) by H-1 NMR of the Mosher's esters of the isopropyl ester derivatives. The method described allowed for the unambiguous assignment of the absolute configuration of the alpha-amino and alpha-hydroxy acid residues composing phomalide (1).