Strip of hazelnuts

FILBERTONE

(the smell of hazelnuts)

Strip of hazelnuts

Simon Cotton
University of Birmingham

Strip of hazelnuts

Also available: JMol, and VRML versions.

Strip of hazelnuts

hazelnuts

3D structure of filbertone

Hazel treeThe smell of filberts?

Yes, though in many places they are known as hazelnuts (Corylus maxima and Corylus avellana), the fruit of the hazeltree (photo, right). And the molecule was only identified in hazelnuts in 1989. Hazelnuts are used as a flavouring in cakes, famously Viennese hazelnut torte and Kiev cake; confectionary (pralines) and chocolates; liqueurs; and other food products (like Nutella).

Hazelnut pralines A slice of kiev cake Nutella

Hazelnut prlaines

Kiev cake

Nutella

Filbertone is shown as enantiomers, do they both occur in nature?

In nature, there is a slight excess of the (S)-isomer, the amount varies depending upon the source of the filberts. The enantiomeric excess in the extracted filbertone is greater under mild extraction conditions.

E-R-filbertone ES filbertone

(-)-(E,R)-filbertone

(+)-(E,S)-filbertone

Can you tell the isomers apart?

They have been separated by chromatography using a chiral column. The two mirror-image isomers (enantiomers) have significantly different smells. Both smell of "hazelnuts", but the (+)-(E,S)-isomer has a more fatty smell than the (-)-(E,R)-isomer, which has notes of butter and chocolate, as well as having a stronger impact, with a 10 times greater odour threshold.

How do you make it?

Synthesis of filbertone

In the original laboratory synthesis, (+)-(E,5S)-filbertone was prepared starting from (S)-(-)-2-methylbutan-1-ol. First it was oxidised to the corresponding aldehyde, which underwent nucleophilic attack at the carbonyl carbon by propynyl lithium, forming a mixture of the isomers (4R,5S)-5-methylhept-2-yn-4-ol and (4S,5S)-5-methylhept-2-yn-4-ol. Reduction with LiAlH4 proceeded stereospecifically at the triple bond to give (E,4R,5S)-5-methylhept-2-en-4-ol and (E,4S,5S)-5-methylhept-2-en-4-ol, which on oxidation with MnO2 formed just (E,5S)-5-methylhept-2-en-4-one.

original synthesis

A second route uses oxidation in the first step, with pyridinium chlorochromate oxidant, but introduces the additional chain in a different way, by a coupling with allyl bromide. The alcohol product was oxidised using PCC again to afford (5S)-5-methylhept-1-en-4-one, which was isomerised on treatment with p-toluenesulfonic acid.

In a modification of the original route, NaOCl was used as the stoichiometric oxidant in the first step, with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) catalyst. Yields were improved by better work-up procedures.

Is filbertone the only molecule responsible for the smell of hazelnuts?

Not at all. A recent paper on Italian hazelnuts (2010) identified 37 odour-active compounds in the raw nuts. Important ones include three substituted pyrazines, especially 2-methoxy-3-isopropylpyrazine and 2-methoxy-3,5-dimethylpyrazine, as well as 5-methyl-4-heptanone and ethyl 2-methylbutanoate, in addition to 5-methyl-(E)-2-hepten-4-one.

Other molecules that contribute to hazelnut smell
Raw hazelnut odourants

Rather more (46) aroma-active compounds were detected in roasted nuts, with some additional new molecules present, notably unsaturated aldehydes (Z)-2-octenal , (Z)-2-nonenal, (Z)-2-decenal and (E,E)-2,4-decadienal, 2-thienylthiol, 2-acetyl-1-pyrroline and 2-propionyl-1-pyrroline.

Roasted hazelnuts

Roast hazelnut odourants
Roast hazelnut odourants

And what's this about using Hazelnut oil to adulterate olive oil?

It's true. Olive oil is a cornerstone of the Mediterranean diet. It's more expensive than hazelnut oil, so unscrupulous people adulterate olive oil with cheaper hazelnut oil, as small amounts can be hard to detect, though GC-MS has been successfully used.

The Abbey at Tournus
The Abbey at Tournus

And why are they called filberts?

They are linked with St. Philibert, the 7th-century monk who founded several monasteries in France, and whose feast day of August 20th coincides with the nutting season. He was buried at Noirmoutier, on the western coast of France, but the 9th-century Viking attacks caused the monks to move. They eventually settled at Tournus in the Rhone valley, where the great abbey church of St. Philibert still holds his relics.

The reliquary holding the relics of St Philibert
The casket (or reliquary) holding the relics of St Philibert.

Strip of hazelnuts

Bibliography

General

Syntheses

Hazelnut odorants

The enantiomers and their separation

Roasted hazelnuts

Hazelnut oil in adulterated olive oils

Tournus and St Philbert

Strip of hazelnuts

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