(R)-Limonene is a monoterpene found in citrus fruits. It smells like oranges and is a commonly used flavor and fragrance compound. Other aroma compounds, such as menthol or carvone, can be synthesized from limonene via natural oxidation reactions. Using synthetic chemistry could allow the formation of other, new aroma compounds from limonene, which is a readily available starting material. Electrochemical oxidation, in particular, could be a sustainable way to achieve these reactions.

Holger Zorn, University of Gießen and Fraunhofer Institute for Molecular Biology and Applied Ecology, Gießen, Germany, and colleagues have electrochemically oxidized limonene in the presence of ethanol to create new aroma compounds, which were then isolated, structurally characterized, and tested for their odor impressions. The team used a solution of (R)-limonene and methyl-tri-n-butylammonium methylsulfate in ethanol, which was electrolyzed at 25 °C using graphite anodes and steel cathodes.

The products were separated using column chromatography and preparative high-performance liquid chromatography (HPLC). They were analyzed using gas chromatography–mass spectrometry (GC–MS) and NMR spectroscopy. The team found that the product mixture has a fruity, herbal, citrus, and resinous odor. They identified 20 different oxidation products in the mixture. According to the researchers, eleven of those compounds had not been described in the literature before. Several products showed pleasant aroma properties. They would, of course, need safety evaluations before their use. This electrochemical approach could be a useful method for the sustainable production of aroma compounds.

• Generation of Flavor-Active Compounds by Electrochemical Oxidation of (R)-Limonene,
  Florian Birk, Heike Hausmann, Marco A. Fraatz, Axel Kirste, Nicola C. Aust, Ralf Pelzer, Holger Zorn,
  J. Agric. Food Chem. 2022.
  https://doi.org/10.1021/acs.jafc.2c01301