Comprehensive analysis of natural products of Galium odoratum (Rubiaceae), focusing on the formation of coumarin during the drying process

Changes in the contents of coumarin and its precursors were monitored during the post-harvest drying process of Galium odoratum (L.) Scop. (Rubiaceae) to get scientific support for the traditional preparation of "Maibowle" (an alcoholic beverage made of white wine, sparkling wine, and G. odoratum shoots and sometimes additional ingredients) in Germany, which involves pre-drying of the freshly harvested herb for two to 24 h. The contents of coumarin, cis-melilotoside, and trans-melilotoside in G. odoratum were determined over a period of 72 h after harvesting using quantitative UHPLC-DAD analysis. The drying process was finished after 48 h, and afterwards no further weight loss of the plant material was detectable. During this 48-h-period, trans-melilotoside seemed to isomerize gradually to cis-melilotoside. For the first time, we demonstrated that coumarin decreases during drying due to sublimation, which supports the use of fresh or minimally air-dried material in traditional recipes with specific drying times. Moreover, we demonstrated for the first time that reduction in coumarin during prolonged periods of drying is explained by sublimation of coumarin. Flavonoid glycosides, quercetin 3-O-[α-l-rhamnosyl-(1 →6)-β-d-glucosyl]-7-O-β-D-glucoside 12 and kaempferol 3-O-[α-l-rhamnosyl-(1→6)-β-d-glucosyl]-7-O-β-D-glucoside 13 as well as coumarin precursors cis-melilotoside 9 and trans-melilotoside 10 were for the first time isolated from G. odoratum. Moreover, monotropein 1, geniposidic acid 2, scandoside 3, asperulosidic acid 4, deacetylasperuloside 5, asperuloside 6, 3-O-caffeoylquinic acid (neochlorogenic acid) 7, and 5-O-caffeoylquinic acid (chlorogenic acid) 8, which were already known from this source species, were isolated from a methanolic extract of G. odoratum.