Distribution of 8-oxygenated leaf-surface flavones in the genus Ocimum

Nine species of Ocimum (Lamiaceae) were surveyed for leaf-surface flavonoids by means of HPLC with diode array detection and atmospheric pressure chemical ionisation (APCI) mass spectrometry. The analysis revealed the presence of 23 different flavones, most of which were identified by comparing their UV and mass spectra with those of standards. Almost all taxa investigated contained flavones methoxylated in the 6- and 8-positions, such as nevadensin, xanthomicrol and gardenin B. The same taxa also produced flavones methoxylated in the 6-position but hydroxylated in the 8-position, including isothymusin (5,8,4'-trihydroxy-6,7-dimethoxyflavone), pedunculin (5,8-dihydroxy-6,7,4'-trimethoxyflavone) and a new flavone, 5,7,8-trihydroxy-6,4'-dimethoxyflavone, which was given the trivial name pilosin. This compound was isolated from O. americanum var. pilosum and also detected as a minor constituent in O. x citriodorum leaf extracts. Its molecular structure was elucidated by means of NMR spectroscopy. 8-Oxygenated flavones were absent only from O. lamiifolium. APCI mass spectrometry of the flavonoids revealed that the product ions formed by collision induced dissociation of the protonated molecule provided structural information about the substitution pattern of the A-ring. The chemotaxonomic and biogenetic implications of the results are discussed.