Antifungal diastereomeric furanones from Mutisia friesiana: structural determination and conformational analysis

Two diastereomeric furanones, (4S,5S)-5-(4′-methyl-3′-pentenyl)-4-hydroxy-5-methyldihydrofuran-2-one 1 and (4S,5R)-5-(4′-methyl-3′-pentenyl)-4-hydroxy-5-methyldihydrofuran-2-one 2 were isolated for the first time from the shrub Mutisia friesiana. The relative stereochemistries of 1 and 2 were ascertained from NOESY NMR data and confirmed by a combination of molecular modeling (molecular mechanics and ab initio molecular orbital calculations) and NMR data. Comparison between experimental and calculated ¹H–¹H vicinal coupling constants revealed that both furanones exist in an equilibrium of two stable conformers of the five-membered ring. Application of Mosher's method suggests that both diastereomeric furanones have the same (S)-configuration at C-(4) and are epimers at C-(5). Furanones 1 and 2 showed antifungal activity against the pathogenic fungus Cladosporium cucumerinum.