Damascenone
Damascenones are a series of closely related chemical compounds that are components of a variety of essential oils. The damascenones belong to a family of chemicals known as rose ketones, which also includes damascones and ionones. beta-Damascenone is a major contributor to the aroma of roses, despite its very low concentration, and is an important fragrance chemical used in perfumery.
The damascenones are derived from the degradation of carotenoids.
β-damascenone is well-known for contributing to the strong aroma of red wines and has been found to enhance the floral, fruity notes of wine and contribute to its characteristic aroma.
In 2008, -β-damascenone was identified as a primary odorant in Kentucky bourbon.
Biosynthesis
The biosynthesis for β-damascenone begins when farnesyl pyrophosphate and isopentenyl pyrophosphate react to produce geranylgeranyl pyrophosphate. The enzyme phytoene synthase condenses two GGPP molecules together to produce phytoene, removing diphosphate with a proton shift.Phytoene then undergoes a series of dehydrogenation reactions. phytoene desaturase first desaturates phytoene to produce phytofluene, then ζ-carotene. Other enzymes have been found to catalyze this reaction including CrtI and CrtP.
Then ζ-Carotene desaturase catalyzes further desaturation to produce neurosporene followed by lycopene. Other enzymes that are able to catalyze this reaction include CtrI and CrtQ. The desaturation concludes when lycopene β-cyclase catalyzes lycopene cyclization, first producing γ-carotene, then β-carotene:
The cyclization mechanism is as follows:
Next, β-carotene reacts with and the enzyme β-carotene ring hydroxylase, producing zeaxanthin.
Zeaxanthin then reacts with, NADPH, a reduced ferredoxin cluster, and the enzyme zeaxanthin epoxidase to produce antheraxanthin which reacts in a similar fashion to produce violaxanthin. Violaxanthin then reacts with the enzyme neoxanthin synthase to form neoxanthin, the main β-damascenone precursor:
In order to generate β-damascenone from neoxanthin there are a few more modifications needed. First, neoxanthin undergoes an oxidative cleavage to create the grasshopper ketone. The grasshopper ketone then undergoes a reduction to generate the allenic triol. At this stage, there are two main pathways the allenic triol can take to produce the final product. The allenic triol can undergo a dehydration reaction to generate either the acetylenic diol or the allenic diol. Finally, one last dehydration reaction of either the acetylenic diol or the allenic diol produces the final product β-damascenone:
The proposed mechanism for the conversion of the allenic triol to the acetylenic diol is:
The proposed mechanism for the conversion of the acetylenic diol to the final product is:
This mechanism is known as a Meyer-Schuster rearrangement.