ionone

Ionones
IUPAC name	α: (3E)-4-(2,6,6-Trimethylcyclohex-2-en-1-yl)but-3-en-2-one β: (3E)-4-(2,6,6-Trimethylcyclohex-1-en-1-yl)but-3-en-2-one γ: (3E)-4-(2,2-Dimethyl-6-methylenecyclohexyl)but-3-en-2-one
Other names	Cyclocitrylideneacetone, irisone, jonon
Properties
Molecular formula	C13H20O
Molar mass	192.30 g/mol
Density	α: 0.933 g/cm3 β: 0.945 g/cm3
Melting point	β: -49 °C
Boiling point	β: 126-128 °C at 12 mmHg
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

The ionones are a series of closely related chemical substances that are part of a group of compounds known as rose ketones, which also includes damascones and damascenones. Ionones are aroma compounds found in a variety of essential oils, including rose oil. beta-Ionone is a significant contributor to the aroma of roses, despite its relatively low concentration, and is an important fragrance chemical used in perfumery.^[1] The ionones are derived from the degradation of carotenoids.

The carotenes alpha-carotene, beta-carotene, gamma-carotene, and the xanthophyll beta-cryptoxanthin, all contain beta-ionone, and thus have vitamin A activity because they can be converted by plant-eating animals to retinol and retinal. Carotenoids that do not contain the beta-ionone moiety cannot be converted to retinol, and thus have no vitamin A activity.

Organic synthesis

Ionone can be synthesised from citral and acetone with calcium oxide as a basic heterogeneous catalysis and serves as an example of an aldol condensation followed by a rearrangement reaction.^[2][3]

The nucleophilic addition of the carbanion 3 of acetone 1 to the carbonylgroup on citral 4 is base catalysed. The aldol condensation product 5 eliminates water through the enolate ion 6 to the pseudoionone 7.

The reaction proceeds by acid catalysis where the double bond in 7 opens to form the carbocation 8. A rearrangement reaction of the carbocation follows with ring closure to 9. Finally a hydrogen atom can be abstracted from 9 by an acceptor molecule (Y) to form either 10 (extended conjugated system) or 11.

References