## 3 Coumarins in the Context of Phenolic Composition  
The name \"coumarin\" derives from the French term \"coumarou\" for the tonka bean, which are the seeds of _Dipteryx odorata_ (Aubl.) Forsyth f. (or _Coumarouna odorata_ Aubl. ), in which coumarin was first isolated by [PERSON] [85,86]. This compound has a sweet odour that is easily identified as the aroma of freshly known hay; as a result, it has been used in perfumes since the late 19 th century [87,88]. Coumarins are important secondary metabolites that can be found abundantly in plants (detailed in Section 4). Numerous studies have been conducted on the isolation, structural characterization, synthesis, and biological activity of an extensive variety of natural coumarins, derived from trees and plants, including some used in the production of beverages [89, 90, 91, 92, 93], as well as coumarins obtained by chemical synthesis [94]. Regarding their chemical structure, natural coumarins are unsaturated lactones that belong to the C6C3 chemical class, that is, a class of benzopyranes (1,2-benzopyranes or 2H-1-benzopyran-2-ones, consisting of a benzene ring fused to an \(\alpha\)-pyrone ring), which can be thought of structurally as _ortho_-hydroxy-cinnamic acid derivatives (Figure 8). The majority of natural coumarins, including umbelliferone, scopoletin, and aesculetin, among others, include an oxygenated substituent at position 7 [95]. This substituent can exist either in its free form, as seen in hydroxylated umbelliferone, or in combination with other compounds (methyl,  
Figure 7: Scheme of grape marc spirit production process. ethyl, saccharides, etc.) in different derivatives. These compounds exhibit a conjugated system that is rich in electrons, resulting in favourable charge-transport features [91,96]. Several methodologies for classifying coumarins can be found in the literature. Among them, the most comprehensive, which includes coumarins with the greatest complexity and substituted coumarins, often found in combination with other heterocycles [88], categorises these compounds into the following groups: simple coumarins, funcoumarins, dihydrofuranocoumarins, pyranocoumarins (both linear and angular), phenylcumarins, bicoumarins, and coumarin glycosides, as depicted in Figure 9[97]. Simple hydroxycumarins, funcoumarins and isofuracoumarins, pyranocoumarins, bicoumarins, and dihydroisocoumarins are the most important compounds of this chemical family isolated from plants. In plants, coumarins often exist in the free form, due to their polar nature, and a significant number of these compounds have the ability to undergo sublimation. Additionally, they can also be present in the form of glycosides, including psoralen-correlated compounds. These entities exhibit a distinct blue fluorescence, owing to their absorption of UV light, and they possess a high degree of photosensitivity, making them susceptible to modification by natural light. These properties are exploited for separation and analysis purposes, as well as in unconventional therapeutic approaches such as photochemotherapy and the chemical sensor industry (detailed in Sections 5 and 7). Simple coumarins are synthesised biogenetically by the transformation of shikimic acid into cinnamic acid. The underlying primary enzymatic transformation is the C-2  
Figure 8: Chemical structure of coumarin and the IUPAC numbering system for its framework. Figure 9: Classification and generic structures of the main categories of plant-derived coumarins (some examples are given in green). hydroxylation, which results in the cleavage (\(\beta\)-oxidation) of the side chain, or alternatively, chain isomerization followed by lactonization, leading to the formation of umbelliferone (Figure 10). Shikimate dehydrogenase (AroE) promotes the transformation of shikimate and NADP+ into 3-dehydroshikimate, NADPH, and H+. The shikimate pathway is a metabolic system consisting of seven steps and it is involved in the synthesis of phenylalanine, tyrosine, and tryptophan.