Absolute configuration of a diterpene lactone from Parinari capensis

Phytochemical investigation of Parinari capensis (Chrysobalanaceae) led to the isolation of three diterpene lactones 1-3. The structures of these compounds have been established by NMR spectroscopy and X-ray crystal-structure analysis. Addition of bromine onto the exocyclic methylidene group of 1 yielded the brominated derivative 1-Br. X-Ray analysis of 1-Br established unambiguously the absolute configuration, and 1 was then identified as the (4R,9R)-10-hydroxy-13-methoxy-9-methyl-15-oxo-20-norkaur-16-en-18-oic acid gamma-lactone. Diterpenes 2 and 3 were identified as the C(13)-demethoxylated and the C(13)-demethylated derivatives of diterpene 1, respectively. Antifungal activity against Cladosporium cucumerinum was determined for 1-3.