Rationalizing the formation of damascenone: synthesis and hydrolysis of damascenone precursors and their analogues, in both aglycone and glycoconjugate forms

Storage of megastigma-4,6,7-trien-3,9-diol (5), and megastigma-3,4-dien-7-yn-9-ol (6) in aqueous ethanol solution at pH 3.0 and 3.2 gave exclusively damascenone (1) and damascenone adducts at room temperature. The diol (5) had half-lives for the conversion of 32 and 48 h at pH 3.0 and pH 3.2, respectively. The acetylenic alcohol (6) had half-lives of 40 and 65 h at the same pH levels. In order to study the reactivity of the C-9 hydroxyl function in 5 and in the previously investigated allenic triol 2, two model compounds, megastigma-4,6,7-trien-9-ol (7) and megastigma-6,7-dien-9-ol (8) were synthesized. No 1,3-transposition of oxygen to form analogues of damascenone was observed when 7 and 8 were subjected to mild acidic conditions. Such transposition takes place only with highly conjugated acetylenic precursors such as 6 or tertiary allenic alcohols such as 2. The placement of glucose at C-3 of 5 and at C-9 of 6 gave the glycosides 9 and 10, respectively. The effect of such glucoconjugation was to increase the observed half-lives by a factor of only 1.6-1.7 for the allenic glucoside 9, and by 2.1-2.2 for the acetylenic glucoside 10. These studies indicate that the effect of glycosylation on damascenone formation is probably not important on the time scale of wine making and maturation.