Romantic notes: microwaves, NMR and perfumed chemistry

Fragranced notes

Cassis base 345B, undecavertol, 1,3-oxathiane oxane, isospirene... Perfume can be so romantic! But the chemical components underpinning the often-enticing and seductive smells of fragrances are, one might say equal parts art and science. One of the most intriguing elements of several fragrances, including popular perfumes like Le monde est beau by Daniela Andrier and DKNY Be Delicious by Maurice Roucel is the fruity top note - blackcurrant.

Halima Mouhib and Wolfgang Stahl of the Institute of Physical Chemistry, RWTH Aachen University, Aachen, Germany and Monique Luethy, Michelle Buechel and Philip Kraft of Givaudan Schweiz AG, Fragrance Research in Duebendorf, Switzerland, explain how the odour of cassis hinges on stereochemistry. They have now used molecular beam Fourier transform microwave spectroscopy and quantum chemistry to determine the relative stereochemistry and the gas phase structures of cassyrane and its dihydro derivatives. Nuclear Overhauser Effect Spectroscopy NMR corroborated the relative stereochemistry.

Until the discovery of this compound, top blackcurrant notes in perfumery were available by using sulfur-containing compounds. Cassyrane is essentially a seco-etaspirene and lacks the sulfur atoms that can otherwise generate an off-odour. Understanding the structure, the stereochemistry and its effect on odour are crucial to the use of such compounds in perfumery. The volatility and the effect of body heat in generate gas phase structures is also critical. Oddly enough, the most intense cassyrane derivative does not have a fruity odour, the team reports. Instead, the (2S,5R) enantiomer of the parent compound has a mostly agrestic odour, which the team describes as being reminiscent of Provencal herbs. The same compound also has hints of the scent of rosemary and eucalyptus.

Odour patterns

The odour patterns and thresholds correlate with their gas-phase structures as determined by the microwave technique. Given that most odorants are volatile, but non-crystalline. Odour molecules must evaporate to reach the olfactory receptors so their gas-phase structures are of critical importance to fragrance chemists. This makes the technique adopted by Kraft and colleagues an interesting new tool for the fragrance chemist. "The main output [of the research] is, that understanding, that the agrestic, rosemary, eucalyptus side is due to two of the 4 diastereomers only, the odor of cassyrane can now be further improved, and the gas phase structures also allow the new design of cassis (and exotic fuity ingredients in general) from the superposition analysis of these gas phase structures," Kraft told SpectroscopyNOW.

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