Synthesis of allenic ketones and the role of yeast in their conversion to damascenone.

Abstract

The thesis describes the formation of damascenone during fermentation conditions from potential ketone precursors. An introduction to the thesis and aims of the study are described in Chapter 1, the synthesis of the precursors is detailed in Chapter 2, identification of the precursors as natural products and hydrolysis studies are included in Chapter 3, fermentation studies are detailed in Chapters 4 and the damascenone stability studies are in Chapter 5. Chapter 2 describes the synthesis of megastigma-4,6,7-triene-3,9-dione (26) and 3-hydroxymegastigma-4,6,7-trien-9-one (27) from the common starting material, 4-oxoisophorone as well as the synthesis of 9-hydroxymegastigma-4,6,7-trien-3-one (28) from diketone 26. The allene 3-tert-butyldimethylsilyloxy-9-hydroxymegastigma-4,6,7-triene (29) was synthesised first and this was then used to produce ketones 26 and 27. The synthesis of 26 occurred in two steps from the silylated allene diol 29 which involved deprotection, followed by a Dess-Martin oxidation. The synthesis of ketone 27 was achieved by a Dess-Martin oxidation of the silylated allene diol 29 followed by deprotection using TBAF. The ketone 28 was synthesised by a selective reduction of 26. Chapter 3 covers the identification of the ketones 26 and 28 as natural products. It describes the hydrolysis studies performed with ketone 27 in model wine and includes a discussion as to why 27 was not seen in grape juice or honey extracts, in contrast to the analogues 26 and 28. The fermentation study involving the synthesised ketones 26, 27 and 28 is discussed in Chapter 4. The fermentation studies demonstrate that damascenone can be formed by the action of yeast during fermentation, from precursors 26, 27 and 28. The results emphasise that the presence of yeast is essential for the conversion. The concentration of damascenone at the end of fermentation was also shown to be dependent on the yeast strain. Yeast strain AWRI 796 showed to be more efficient in the formation of damascenone compared to yeast strain AWRI 1537. The final section of the thesis (Chapter 5) details the stability of damascenone during fermentation. A substantial loss of damascenone was observed during fermentation and the factors involved were further explored. The possible contributing factors included the loss of damascenone through the action of yeast metabolism, loss from reaction with components in the model or real juice and loss via evaporation of damascenone via the ferment flask during fermentation.