Identification by GC—MS of cymene isomers and 3,7,7-trimethylcyclohepta-1,3,5-triene in essential oils

Abstract

Retention indices of cymene isomers published in popular GC-MS atlases were found to be erroneous by analyzing synthetic samples. The following retention indices (RI) were found using a nonpolar phase (diphenyl:dimethylpolysiloxane, 5:95) for four essential-oil components with indistinguishable mass spectra: 3,7,3,, m-cymene (RI = 1022), p-cymene (RI = 1024), and o-cymene (RI = 1039). The relative distributions of these compounds were evaluated based on the analysis of about 1000 essential oils. Simple methods were given for preparing standard mixtures of isomeric compounds for identification by GC-MS.Monoterpenes of the p-menthane series are widely distributed in the plant world and are the main components of many essential oils and plant extracts. One of the most common compounds of this type is p-cymene (1), which is the starting material for synthesizing products for various purposes, mainly oxidized derivatives [1], exhibits biological activity [2], is a component of common spices.Identification of 1 in plant extracts is often difficult because the isomeric m-(2) and o-isomers (3) have mass spectra that are practically indistinguishable from that of p-cymene. The situation is further complicated by the fact that GC-MS data in commonly used atlases published in 1989 [3], 1995 [4], and 2001 [5] give erroneous retention indices (RI) and elution orders for o-and p-cymene. As a result, many published scientific studies of essential oil compositions incorrectly identify cymene isomers. An example of this is a technique for precise analysis of lavander oil [6] in which peaks for cymene isomers are incorrectly identified based on the citation of an atlas [4].We synthesized samples of cymene isomers 1-3, confirmed their structures by PMR and 13 C NMR spectra, and determined their GC-MS properties. As it turned out, there is another component, 3,7,7-trimethylcyclohepta-1,3,5-triene (4), with similar properties (see Experimental).

Retention Indices for Frequently Reported Compounds of Plant Essential Oils

“…The most frequently observed isomer is p-cymene. Data presented in Table 1 are largely based on the results of Collin et al, 41 and Romanenko et al 42 Note that the observed elution order is in agreement with the correlation between boiling points and elution order of isomers. It is known that for isomers of the same homologous series the elution order on nonpolar stationary phases corresponds to the order of their boiling points.…”

Rac-2′,3a,6,6,6′,6′-Hexamethyl-3a,3b,6,7-tetra-hydrospiro-[benzo[2,3]cyclopropa[1,2-c]pyrazole-1,1′-cyclo-hepta[2,4]diene]

“…However, upon heating of the reaction mixture at 60 °C the formation of a new species was observed by thin-layer chromatography (TLC) with the reaction reaching completion after 12 h. After quenching the reaction mixture with saturated aqueous ammonium chloride solution and performing an aqueous extraction (DCM/water) the crude product was isolated and purified by silica column chromatography (EtOAc/hexanes 5:95) to yield a colorless solid as the principle product. Analysis of this material by 1 H-NMR revealed that it was not consistent with the reported data for the desired product 2 [6]. Furthermore, HRMS analysis suggested a molecular formula of C20H28N2 implying that a pseudo-dimeric species had alternatively been obtained.…”

Variations in Essential Oils from South Siberian Conifers of the Pinaceae Family: New Data towards Identification and Quality Control

“…Most of essential oils isolated from coniferous twigs by steam distillation do not contain 3,7,7‐trimethylcyclohepta‐1,3,5‐triene which is typical minor component of volatile fractions prepared by distillation of resins [23] . The larch essential oil ( Larix sibirica ) is the exception, and 3,7,7‐trimethylcyclohepta‐1,3,5‐triene was found in all the samples studied ( Table 3).…”

“…Optical activity. Specific rotations ([α] D 23 ) of the essential oil samples vary within different limits (Table 1). For fir (Abies sibirica) and spruce (Picea obovata), the value of the variance is insignificant, As for larch and pine oils, the values of optical rotation vary strongly: from 0.0 to À 34 for Larix sibirica, from À 2.0 to + 17 for Pinus sibirica, and from À 6.7 to + 2.6 for Pinus sylvestris.…”