Formation of damascenone derived from glycosidically bound precursors in green tea infusions

Damascenone is well-known for its potent flavour with an extremely low odour threshold. Several glycosidically bound precursors of damascenone have been isolated from several plants, but little is known about their occurrences in green tea infusions. In this work, three major glycosidic precursors of damascenone, 9-O-β-d-glucopyranosyl-megastigma-6,7-dien-3,5,9-triol (1a), 9-O-β-d-glucopyranosyl-3-hydroxy-7,8-didehydro-β-ionol (2a), and 3-O-β-d-glucopyranosyl-3-hydroxy-7,8-didehydro-β-ionol (2b) were isolated and identified in green tea infusions, and the stereochemistries at C-3 and C-9 positions of aglycone parts of the three glycosidic precursors were determined as (3S, 9R)-1a, (3R, 9R)-2a, and (3R, 9R)-2b, respectively. Compounds 1a, 2a, and 2b as well as 3-O-β-d-glucopyranosyl-megastigma-6,7-dien-3,5,9-triol (1b) were hydrolysed to form damascenone in a model system with strong acidic conditions (pH 2.0) and at high temperature (90 °C). In contrast to hydrolysis of 2a and 2b, more damascenone was transformed from 1a and 1b. Furthermore, the β-d-glucosyl moiety at the C-3 position gave a higher dehydration rate from megastigma-6,7-dien-3,5,9-triol to 3-hydroxy-7,8-didehydro-β-ionol than compound 1a carrying the sugar residue at C-9 position. Interestingly, the four glycosidic precursors of damascenone were not hydrolysed to give damascenone under slightly acidic conditions (pH 5.4 and 120 °C for 10 min), but they could be transformed to damascenone in the presence of green tea infusions even under the equal conditions.