Note pubs.acs.org/jnp
Fragrant Sesquiterpene Ketones as Trace Constituents in Frankincense Volatile Oil of Boswellia sacra Johannes Niebler,† Katharina Zhuravlova,† Mirjana Minceva,‡ and Andrea Buettner*,† †
Department of Chemistry and Pharmacy, Food Chemistry, Emil Fischer Center, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Schuhstraße 19, 91052 Erlangen, Germany ‡ Biothermodynamics, TUM School of Life Sciences Weihenstephan, Technische Universität München, Gregor-Mendel Straße 4, 85354 Freising, Germany S Supporting Information *
ABSTRACT: In a previous study, two highly potent yet unidentified odorants were detected that were present at trace levels in the volatile fraction of Boswellia sacra gum resin. These two compounds were isolated semipreparatively from the volatile oil by a sensory-guided fractionation process involving microscale bulb-to-bulb distillation, countercurrent chromatography, and preparative gas chromatography. In this manner, the two oxygenated sesquiterpenes could be identified as rotundone (1) and mustakone (2). Compound 2 is described for the first time as a potent odorant with a very low odor threshold.
M
embers of the plant family Burseraceae are well-known for their production of a variety of fragrant resins, inclusive of frankincense from Boswellia and myrrh from Commiphora. The genus Boswellia comprises 19 different species,1 among which Boswellia sacra Flueck. is regarded as a source of high-quality frankincense. Its volatile oil is obtained commonly by hydrodistillation and used in perfumery, aromatherapy, and fragrance applications. Recently, we have investigated odor-active constituents from the gum resin of Boswellia sacra.2 During this study, the chemical structures relating to two odor-active zones in the chromatogram, which exhibited very high flavor dilution factors in combination with an extremely low mass spectrometric signal, could not be resolved successfully. A structural hypothesis based on a preliminary mass spectrum suggested that these compounds are oxygenated sesquiterpenoids. Although B. sacra frankincense is known to contain a large variety of sesquiterpenes,3,4 this was the first evidence that oxygenated sesquiterpenes might be important contributors to the smell of frankincense. In the present study, semipreparative and analytical methods were used to enrich and isolate compounds 1 and 2 from the sesquiterpene fraction. Microscale bulb-to-bulb (also known as Kugelrohr) distillation is a convenient tool for the fractionation of minor amounts of volatiles and semivolatiles. Thus, in the first step, B. sacra volatile oil was subjected to microscale bulb-to-bulb distillation and fractionated by rising boiling point ranges. In the temperature range 125−170 °C at a vacuum of 200 ng/L air) or at least highly insensitive to 2. Second, the remaining eight panelists appeared to belong to two different subgroups. One group (five panelists) exhibited very low odor thresholds (
