Nootkatone Valencene

The allylic oxidation of the sesquiterpenoid (+)-valencene has been performed using f-BuOOH as the oxidant and BiCl3 as catalyst. Nootkatone was the major product, isolated in 35% yield by flash chromatography (ethyl acetate - light petroleum, boiling point 40-60 °C) (Scheme 17) [87, 88]. 

Nootkatone can be prepared by oxidation of valencene, a sesquiterpene hydrocarbon isolated from orange oils. 

Nootkatone is an important constituent from grapefruit flavour. It is synthesised by oxidation of valencene, which is obtained and isolated from orange peel oil where it occurs at a maximum level of 0.4% [9]. 

For the oxidation of valencene to nootkatone, strong oxidation agents like oxygen in the presence of metal salts, peroxides or chromate compounds are required. There are also several patents on the bioconversion of valencene to nootkatone which results in natural nootkatone (Scheme 13.9) [10,11]. 

Franssen et al. [24] pointed out an alternative method of production of nootkatone from valencene catalysed by (-i-)-germacrene A hydroxylase, an enzyme of the cytochrome P450 monooxygenase type that was isolated from chicory roots. In general, this enzyme appeared to accept a broad range of sesquiterpenes and hydroxylates exclusively at the side-chain s isopropenyl group. Valencene is an exception it was not hydroxylated at the side chain, but -nootkatol was formed in the first step (Scheme 22.5) it is not yet clear if the second step is enzyme-catalysed. 

Scheme 22.5 Production of nootkatone from valencene catalysed by (+)-germacrene A hydroxylase [81]...

Scheme 23.12 Biotransformation of (+)-valencene to (+)-nootkatone via a-nootkatol and/or fi-nootkatol...

Recently, an industrial process development for nootkatone production from valencene by microbial transformation (bacteria, fungi) was mentioned [199, 200]. Although no details were given, the author claimed the development of an in situ product-removal technique by which an extremely selective recovery of nootkatone from the reaction mixture and the excess precursor during the proceeding production was achieved and which was said to be essential for an economically viable bioprocess. 

The same rational P450cam mutants which have already been described for limonene and pinene oxyfunctionalisations were also successfully applied to valencene. In whole-cell biotransformations -nootkatol and nootkatone formed as main products with up to 25% overall yield, corresponding to activities of up to 9.9 nmol (nmol P450) min [201]. Higher activities (up to 43 min ) but lower selectivities than those with P450cam were obtained with mutants derived from Bacillus megaterium P450 BM3. 

Valencene (5), a sesquiterpene hydrocarbon isolated from orange oils is used as starting material for the synthesis of nootkatone (6), which is used for flavouring beverages [26] and which is a much sought-after aromatic substance [131]. 

Two bacterial strains, one from soil and the other from infected local beer, which utilised calarene as the sole source of carbon and energy have been isolated by enrichment culture techniques [149]. Both these bacteria were adapted to grow on valencene as the sole carbon source. Fermentations of valencene (5) by these bacteria of the genus Enterobacter in a mineral salts medium yielded several neutral metabolic products dihydro alpha-agarofuran (200) (7.5%), nootkatone (6) (12%), another ketone (201) (18%) and a-cyperone (202) (8%), Fig. (40). 

Very recently, the chemoenzymatic preparation of nootkatone from valencene was described [150]. Nootkatone was prepared from valencene by copper(I) iodide catalysed oxidation with tert-butyl hydroperoxide and hydroxylated at C-9 by Mucor plumbeus and Cephalosporium aphidicola. 

The most important (and also the most expensive) grapefruit aroma compound is the bicyclic terpene nootkatone. It is manufactured by oxidation of valencene, which is extracted from Valencia oranges. Figure 1.23 shows two routes for this oxidation, a stoichiometric reaction using chromium trioxide, and a catalytic alternative using sodium hypochlorite (bleach) in the presence of 1 mol% osmium tetraoxide catalyst. 

Figure 1.23 Alternative synthesis routes from valencene to nootkatone.

For the synthesis of flavour-active compounds numerous methodologies have been developed [17], In many cases natural products served as starting materials such as eugenol from clove oil for the synthesis of vanillin. In case of complex stereochemistry natural materials are still welcome for the synthesis of valuable flavour compounds such as nootkatone, which is obtained by oxidation from valencene. 

Valencene, another olefinic sesquiterpene, has been studied in the same context using microorganisms isolated from soil1 741. It was observed that these biotransformations led in reasonable yields to a mixture of three main metabolites, including an epoxide and nootkatone, an interesting flavoring compound. 

Highly Ef cient Production of Nootkatone (2) from Valencene (1).908... 

Chlorella sp. Valencene (1) 2a-Nootkatol (3) 2P-Nootkatol (4) Nootkatone (2) Conversion Ratio (%)... 

In the time course of the biotransformation of 1 by C. pyrenoidosa, the yield of nootkatone (2) and nootkatol (4) without 2a-hydroxy valencene (3) increased with the decrease in that of 1, and sub sequently, the yield of 2 increased with decrease in that of 3. In the metabolic pathway of valencene... 

The expensive grapefruit aromatic nootkatone (2) used by cosmetic and ber manufacturers was obtained in high yield by biotransformation of (+)-valencene (1), which can be cheaply obtained from Valencia orange, by Chlorella species, fungi such as Mucor species, B. dothidea, and B. theobromae. This is a very inexpensive and clean oxidation reaction, which does not use any heavy metals, and thus, this method is expected to nd applications in the industrial production of nootkatone. 

Aspergillus wentii and Epicoccum purpurascens converted valencene (1) to 11,12-epoxide (14a) and the same diol (6, 7) (Takahashi and Miyazawa, 2005) as well as nootkatone (2) and 2a hydroxyvalencene (3) (Takahashi and Miyazawa, 2006). 

The metabolites of valencene, nootkatone (2), (3), and (14a), indicated grapefruit with sour and citrus with bitter odor, respectively. Nootkatone 11,12-epoxide (14) showed no volatile fragrant properties. 

FIGURE 20.7 Biotransforaiation of valencene (1) and nootkatone (2) by Aspergillus wentii, Epicoccum purpurascens, and Chaetomium globosum. 

Although Mucor species could give nootkatone (21) from valencene (1), this fungus biotrans formed the same substrate (25) to the same alcohol (30, 13.2%) obtained from the same starting compound (25) in A. sojae, a new epoxide (34, 5.1%), and a diol (35, 9.9%). 

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