Rose ketones

The rose ketones are a group of structurally very closely related fragrance chemicals which are truly key aromachemicals in the successful recreation of rose scents.

rose-ketones-group
The different interpretations of the structure of rose ketones. The commercially available and IFRA regulated compounds can be extended with some chemically related but less common damascone and damascenone isomers, and with the chemically different but olfactory related Ionones.

Depending on the interpretation, the group may contain the isomers of the damascones, the isodamascones, the damascenones and/or the ionones. The commercially available, olfactory interesting, “true” rose ketones are IFRA regulated and consist of:

  • α-Damascone
  • β-Damascone
  • γ-Damascone
  • δ-Damascone
  • Iso-Damascone
  • β-Damascenone

During the course of the development of these rose ketones, a few very close analogues were evaulated for fragrance use, but not further pursued. ε-Damascone displays the typical damascone character, but is much weaker than the other isomers. The same can be said for ε,α-Damascenone; while  ε,β-Damascenone was found to convey a more fatty, minty, leathery wood impression.

Due to their somewhat similar character and structure, Ionones are often mentioned in one breath with the rose ketones, but strictly speaking they are not part of this class of aromachemicals. As such, they will not be covered here.


Discovery: two wrongs make a right.

The discovery of the rose ketones dates back to the early 1960’s. Following the discovery of Rose oxide, a new project was launched at Firmenich, joint with Professor Ruzicka and Dr. Kovats (ETH Zürich), to discover the remaining key aromachemicals characterizing the smell of Bulgarian rose oil. At that time, reconstructions of rose oil were still miles apart from the true natural material, despite the fact that 80 aromachemicals had been identified already. During this project Bulgarian rose oil was fractionated into 200 samples, of which one had a honey-like note thought to be the missing link. From this sample, 0.0008 grams of an unknown chemical was isolated, which was in fact not responsible for the honey-like note, but which conveyed a powerful rose odor. At that time, the analytical equipment was not powerful enough to perform an exact structure determination on such small amounts of products, and as such, an incorrect proposal (1) was made. This proposed structure in its turn, was beyond the capabilities of the synthetic chemists working on the project. As such, they proposed to make a close analogue (2), which was easier to make in the lab due to its similarities with the known ionones. During the synthetic process of this analogue, intermediate 4 already had a powerful “rose” odor, and what was highly surprising: once the analogue 2 was made, it turned out to be exactly the unknown chemical originally isolated by Dr. Kovats from the rose oil! A series of misinterpretations had led to the successful discovery of Damascenone (“Damasc-” from Rosa Damascena; “-enone” from the chemical functionality found in the structure). Certainly the researchers who have worked on this project will disagree, but it seems as though Damascenone actually wanted to be discovered.

Discovery synthesis of rose ketones

Given it’s intense rose odor, the synthetic intermediate 4 was similarly named Damascone, and together with damascenone introduced in the perfumers’ pallets. Both aromachemicals were quickly recognized as being indespensible for the creation of rose scents and perfumers were uni-soundly enthusiastic about their discovery. Having established damascenone and β-damascone, Firmenich rapidly synthesized a series of isomers for olfactive evaluation, and the whole class has now become known as the “Rose Ketones”.


Perfume use

The introduction of the rose ketones in the fragrance industry has been extremely successful. E.g. Diors “Poison” (1985) is based heavily on the use of damascenone and the damascones. Within Firmenich, the consumption of damascenone rose from a few grams in 1971, to over 100 kg in 1979. “Rosessence 17.600” was the first commercial base which contained a rose ketone (damascenone). It found immediate success because it delivered synthetic Bulgarian rose oil of an unprecedented quality, compared to anything that had been blended before. This success continued and nowadays, it would be a struggle to find any rose scented material which does not contain a rose ketone.

Taming the immense power of the rose ketones has been a slow process and it took years before the perfumers at Firmenich managed to understand and appreciate all the facets of the rose ketones. The different rose ketones now allow to highlight many different aspects of a rose scent, but are also used to deepen “culinary” fragrance compositions based around honey, chocolate, or even olive or tomato. They are said to add “radiance, bloom and naturalness”. A more specific discussion for each analogue can be found through these links:

  • α-Damascone
  • β-Damascone
  • γ-Damascone
  • δ-Damascone
  • Iso-Damascone
  • β-Damascenone
  • ε,α-Damascenone
  • ε,β-Damascenone

The rose ketones are subject to some restrictions on their use in perfumery . Per category, the combined content of all IFRA regulated rose ketones should not exceed:

  • Category 1:   0.003 %
  • Category 2:   0.004 %
  • Category 3:   0.02 %
  • Category 4:   0.02 %
  • Category 5:   0.02 %
  • Category 6:   0.07 %
  • Category 7:   0.008 %
  • Category 8:   0.002 %
  • Category 9:   0.002 %
  • Category 10: 0.002 %

(data March 17th 2017)


Natural sources

rose ketone biosynthesis
The biological origin of rose ketones.

Rose ketones are quite common in nature andwhen disregarding roses, they were found to contribute to the scent of apricots, beer, blackberries, grapes and wine, kiwi, mango, papaya, passion fruit, rum, raspberries, tobacco, tomato, tea, and many more. This is actually not very surprising, because the biological origin of the rose ketones is the oxidation of carotenoids. These plant pigments are omnipresent, and they are the reason why carrots, autumn leaves and (yes!) oranges are orange.


Synthetic sources

Over the course of their development, many different synthetic pathways have been used to produce the rose ketones. Each of them were plagued with their own difficulties and impurities, and due to the high olfactory impact of these aromachemicals, a new synthetic pathway or even the differences in between batches meant that  the older creations containing the ketone  had to be olfactively reworked. Initially, a separate synthesis for each rose ketone was developed, but in the mid 1980’s, the process was optimized to use one common intermediate for each analogue.

Rose ketone synthesis
The unified synthesis of rose ketones using (α- and β-) methyl cyclogeranate 8 as a common intermediate.

Starting from methyl heptenone 5, methyl cyclogeranate 8 can be easily obtained by Horner-Wittig reaction and a subsequent acid catalyzed cyclization. This cyclization step delivers a mixture of α- and β-methyl cyclogeranate. The α-methyl cyclogeranate is  used as the raw material for the production of α-damascone and damascenone; and the β-isomer is used for the production of β-damascone and γ-damascone.


Alternatives

The rose ketones may be used alongside the ionones, romascone, givescone and rose oxide to assemble typical rose scents. The (2R,4S)-enantiomer of Clarycet by IFF is an analogue of rose oxide which is olfactory closer to the rose ketones.


Properties of rose ketones

See compound specific pages.


Further reading:

The history and chemistry of the rose ketones was extensively discussed by Alvin Williams, Firmenich in Perfumer and Flavorist.

 

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