Exploring obscure wine aroma compounds –Thiols and Sesquiterpenes

Abstract

Wine aroma is very important to the enjoyment of wine, and therefore the wine industry. By furthering our understanding of which compounds contribute to wine aroma, and how these compounds are formed in wine, grapes and during fermentation, we can potentially improve wine aroma via viticultural and vinicultural processes. The objective of this research was to further our knowledge of two classes of compounds; thiols and sesquiterpenes. The thiols studied were 3-sulfanylhexan-1- ol (3SH), 3-sulfanylhexylacetate (3SHA), and 4-sulfanylmethyl-pentan-2-one (4SMP) which are all known to be concentration dependent favourable wine aroma compounds. By investigating via which chemical pathways these are formed, the levels of these compounds can be manipulated via changes in viticulture or winemaking processes, resulting in increased quality of wine aroma. Sulfonic acid analogues, putative precursors of these thiols, were prepared with the aim to deepen our understanding of how 3SH, 3SHA and 4SMP are formed in wines. The sulfonic acid group on these compounds renders them highly polar, and as a result, difficult to purify. This problem was overcome, and several sulfonic acids of interest were successfully synthesised. Possible mechanisms of sulfonic acid formation were explored through studies of hex-2-enal with sulfite, and despite several conditions tried, the possible precursor of 3SH was not isolated, likely due to a fast rate of reaction onward to the thermodynamic disulfonic acid product. The second part of the project was the exploration of sesquiterpenes. Though monoterpenes have been extensively studied, the larger sesquiterpenes have not been thoroughly investigated as possible aroma compounds. Recent studies linking a sesquiterpene to wine aroma, as well as increasing amounts of sesquiterpene identifications in wine, makes this class of compounds well worth investigating. Contributions towards the study of this class of compounds were presented by developing methods to introduce deuterium isotopic labels on a number of different sesquiterpenes, followed by development of analytical methods geared toward investigating sesquiterpenes as wine aroma compounds. Due to their low concentrations, highly sensitive and accurate quantitation techniques are needed, therefore a standard isotope dilution assay gas chromatography mass spectrometry/ gas chromatography methodology was developed. Additionally, methods were explored towards the total synthesis of spirolepechinene, a complex spirocyclic sesquiterpene found in grapes. A model spiro compound with the correct relative stereochemistry was successfully synthesised.