Structure–Odor Relationship Study on Geraniol, Nerol, and Their Synthesized Oxygenated Derivatives

Abstract

Despite being isomers having the same citrus-like, floral odor, geraniol, 1, and nerol, 3, show different odor thresholds. To date, no systematic studies are at hand elucidating the structural features required for their specific odor properties. Therefore, starting from these two basic structures and their corresponding esters, namely, geranyl acetate, 2, and neryl acetate, 4, a total of 12 oxygenated compounds were synthesized and characterized regarding retention indices (RI), mass spectrometric (MS), and nuclear magnetic resonance (NMR) data. All compounds were individually tested for their odor qualities and odor thresholds in air (OT). Geraniol, the Z-isomer, with an OT of 14 ng/L, was found to be more potent than its E-isomer, nerol, which has an OT of 60 ng/L. However, 8-oxoneryl acetate was the most potent derivative within this study, exhibiting an OT of 8.8 ng/L, whereas 8-oxonerol was the least potent with an OT of 493 ng/L. Interestingly, the 8-oxo derivatives smell musty and fatty, whereas the 8-hydroxy derivatives show odor impressions similar to those of 1 and 3. 8-Carboxygeraniol was found to be odorless, whereas its E-isomer, 8-carboxynerol, showed fatty, waxy, and greasy impressions. Overall, we observed that oxygenation on C-8 affects mainly the odor quality, whereas the E/ Z position of the functional group on C-1 affects the odor potency.

Influence of the chemical structure on the odor characters of β-citronellol and its oxygenated derivatives

“…This is in line with our previous observation that this type of oxidation of the parent acetates does not decrease odor potency, but that the parent molecules sometimes exert a more pleasant smell, (Elsharif & Buettner, 2016;Elsharif et al, 2015). From the above results, several insights into structure-odor relationships can be deduced for the investigated b-citronellol and citronellyl acetate derivatives.…”

“…In view of this it is interesting to note that 4 was perceived as musty with a citrusy background by two panelists, so that one might speculate that the musty impression was interpreted as a meaty tone by this single panelist. Moreover, it is interesting to note that in our previous study on geraniol and its oxygenated derivatives (Elsharif & Buettner, 2016) comparable observations were made for the 8-oxo-derivatives, where some panelists reported the odor as being musty with a citrus-like nuance.…”

“…The assessors were trained for at least half a year in recognizing orthonasally about 140 selected known odorants at different concentrations according to their odor qualities and in naming these according to an in-house developed flavor language. The detailed procedure is described in our previous publication (Elsharif & Buettner, 2016).…”

“…Apart from that, our comprehensive revision of reports of such substances on their occurrence in nature, and of other potential effects of these derivatives in plant and animal kingdom, revealed that such derivatives may be underrated, and that some of these compounds might still await their future discovery. For that reason, we previously established a comprehensive database on oxygenated derivatives of linalool, geraniol, nerol and their acetates (Elsharif, Banerjee, & Buettner, 2015;Elsharif & Buettner, 2016). The aim of the present study was to further expand this database, now focusing on derivatives of citronellol and its acetate.…”

“…To close this gap, the aim of our study was to systematically evaluate the odor qualities and OTs of these substances as we previously did in a comparative approach for the related derivatives of linalool (Elsharif et al, 2015) and geraniol (Elsharif & Buettner, 2016). To achieve this aim we synthesized, starting from 1 and 2, derivatives with oxygenated functional groups at C-8, and determined their respective odor qualities and OTs in air.…”