Geranium (Pelargonium sp.) Essential Oil from Reunion. Minor Compounds: Acids, Phenols, Pyridines
Vernin, Gaston A
One Kg of essential oil was kindly supplied by the CAHEB (Agricultural Cooperative of Essential oils Bourbon).
The chemical composition of Geranium oils have been widely studied. These works have been summarized by Lawrence (1) and in several theses (2-5). The first important work on the acidic fraction of geranium Bourbon was made by Ter Heide et al. (6) who identified 29 acids as free or methylated forms. Besides the usual alkanoic acids (C1 to C10), they reported the presence of some rnonoterpenic (neric, geranic, citronellic) and aromatic (benzoic, salicyclic and phenylacetic) acids. In addition to the acids, some pyridines have been identified in natural products (7-13). Our previous works on geranium oil (14,15) led us to undertake a further study on the minor compounds (acids, phenols and pyridines) of a geranium Bourbon oil.
The basic fraction of the geranium oil was obtained according to the method of Toyoda et al. (16). After three extractions with a 10% solution of hydrogen chloride, acidic extracts were carefully washed (two times) with diethyl ether. After neutralization with a 20% diluted solution of sodium hydroxide, basic compounds were extracted with ether. The ethereal solution was dried over anhydrous sodium sulfate and then the solvent was evaporated under vacuum using a Rotavapor (yield: 10 ppm).
The acidic/phenolic fraction was separated according to the above reverse extraction procedure and methylated according to the method of Shaw and Kunerth (17). The two residues obtained were analyzed by GC on an OV-I nonpolar column (yield
GC/MS analysis was performed on a DELSI 700 gas Chromatograph combined with a Ribermag RlO-IO quadrupole mass spectrometer connected to a computerized system and the EPA/NIH data bank. The WCOT DB-I capillary column (50 m × 0.32 nun, film thickness 0.25 µm) was programmed from 70°-160°C at a rate of 2°C/min and then from 160°-300°C at 8°C/min. Carrier gas was helium with a flow rate of 1 mL/min and an inlet pressure of 0.8 bar. The identification of components was based on a comparison of recorded mass spectra and the calculated retention indices from scans (each 200 units)(18) either with those reported in the literature (19,20) or with our SPECMA 2000 data bank (21).
The identified aliphatic and terpenic methyl esters; phenol methyl ethers are listed in Table I (yield: 0.1%). New or tentatively identified compounds are indicated by an asterisk. Besides the well known linear aliphatic methyl esters, we also found the corresponding unsaturated and branched methyl derivatives, respectively (22). Among the terpenic acids: geranic, isogeranic, citronellic, chrysanthemic, neric and some unidentified acids were also found as methyl esters. In the weak phenolic fraction, only the presence of phenol, guaiacol, methyl benzoate, methyl salicylate, 4-methyl anisole, 2- methoxy acetophenone was detected. In the basic fraction obtained in very low amount (10 ppm) two pyridines were newly identifiedin geranium oils. They were 2-isopropyl-4-methylpyridine and (Z)-S-(l-butenyl) pyridine, accounting for 0.6% andO.8% of the mixture, respectively. Their mass spectra are identical to those previously reported by Tsuneya et al. (23) in spearmint oils. The last pyridine was also reported by Maurer and Hauser (12) in jonquil absolute. VVe also found the 2-methyl3-amino-l-pentene, some unidentified amino compounds, and several terpene alcohols, oxides and acetates. Owing to their veiy low threshold values, no doubt that the basic compounds should play an important role on the olfactive properties of geranium Bourbon oil.
The citronellyl diethylamine previously reported (24) in this oil was not found. Dimethyl-sulfoxide and -sulfone have also been identified. They arise from oxidation of dimethyl sulfide present in the geranium oil.
The authors are indebted to C. Chariot for recording GC/MS analysis.
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