Synthesis of Small Combinatorial Libraries of Natural Products: Identification and Quantification of New Long-chain 3-Methyl-2-alkanones from the Root Essential Oil of Inula helenium L. (Asteraceae)

ABSTRACT Introduction Recently, a potent anti‐staphylococcal activity of Inula helenium L. (Asteraceae) root essential oil was reported. Also, bioassay guided fractionation of the oil pointed to eudesmane sesquiterpene lactones and a series of unidentified constituents as the main carriers of the observed activity. Objective To identify nine new constituents (long‐chain 3‐methyl‐2‐alkanones) from a fraction of this root essential oil with a low minimum inhibitory concentration value (0.8 µg/mL) by employing a synthetic methodology that leads to the formation of a small combinatorial library of these compounds. Methods The identity of these constituents was inferred from mass spectral fragmentation patterns and GC retention data. A library of 3‐methyl‐2‐alkanones (C11–C19 homologous series) was synthesised in three steps starting from methyl acetoacetate and the corresponding alkyl halides. The synthetic library was also screened for in vitro anti‐microbial activity. Results Gas chromatographic analyses of I. helenium essential oil samples with spiked compounds from the synthesised library corroborated the tentative identifications of the long‐chain 3‐methyl‐2‐alkanones. The availability of these anti‐microbial compounds from this library made it possible to construct GC/FID calibration curves and determine their content in the plant material: 0.08 − 24.2 mg/100 g of dry roots. Conclusion The small combinatorial library approach enabled the first unequivocal identification of long‐chain 3‐methyl‐2‐alkanones as plant secondary metabolites, and, also, allowed determination of not only a single compound and biological properties, but those of a group of structurally related compounds. Copyright © 2013 John Wiley & Sons, Ltd. Synthesis of a small combinatorial library enabled unequivocal and unobstructed identification of nine long‐chain 3‐methyl‐2‐alkanones (C11−C19 homologous series) as new constituents of Inula helenium L. (Asteraceae) root essential oil. This new approach not only gave important and otherwise unavailable spectra data but also permitted quantitation of 3‐methyl‐2‐alkanones in the plant material, as well as, determination of their in vitro antimicrobial activities. These minor I. helenium constituents represent secondary metabolites found for the first time in the Plant Kingdom.