Myreene

Enone formation-aromatization has been used for the synthesis of 7-hydro-xyalkavinone (716)[456]. The isotlavone 717 was prepared by the elimina-tion[457]. The unsaturated 5-keto allyl esters 718 and 719, obtained in two steps from myreene. were subjected to enone formation. The reaction can be carried out even at room temperature using dinitriles such as adiponitrile (720) or 1,6-dicyanohexane as a solvent and a weak ligand to give the pseudo-ionone isomers 721 and 722 without giving an allylated product(458]. 

Regardless of Llieu apparent structural differences, all terpenoids are related. According to a formalism called the isoprene rule, they can be thought of as arising from head-to-tail joining of 5-carbon isoprene units (2-methyl-1,3-butadiene). Carbon 1 is the head of the isoprene unit, and carbon 4 is the tail. For example, myreene contains two isoprene units joined head to tail, forming an 8-carbon chain with two 1-carbon branches. a-Pinene similarly contains two isoprene units assembled into a more complex cyclic structure, and humulene contains three isoprene units. See if you can identify the isoprene units in a -pinene and humulene. 

While earlier it was generally thought that the acyclic monoterpene alcohols 242-245 are derived from the host tree s oleoresin component, myreene [453], more recent results clearly show that at least in some species they are produced denovo [454-456]. 

An important application of an isomerisation is found in the Takasago process for the commercial production of (-)menthol from myreene. The catalyst used is a rhodium complex of BINAP, an asymmetric ligand based on the atropisomerism of substituted dinaphthyl (Figure 5.5). It was introduced by Noyori [1],... 

Synthesisfrom 7-Hydroxygeranylj-neryl Dialkylamine. The starting material can be obtained by treatment of myreene with a dialkylamine in the presence of an alkali dialkylamide, followed by hydration with sulfuric acid. The 7-hydroxygeranyl/-neryl dialkylamine isomerizes to the corresponding 7-hydro-xyaldehyde enamine in the presence of a palladium(II) phosphine complex as catalyst. Hydrolysis of the enamine gives 7-hydroxydihydrocitronellal [70]. 

Foca CM, Barros HJV, dos Santos EN, Gusevskaya EV, Carles Bayon J (2003) Hydroformylation of myreene metal and ligand effects in the hydroformylation of conjugated dienes. New J Chem 27(3) 533-539... 

Figure 3.2 Monoterpenes (C10H16). (A) Myreene, an acyclic monoterpene. (B) a-Pinene, a dicyclic structure. (C) d-Limonene, a monocyclic structure. Courtesy Spiring Enterprises Ltd.

Myreene is an example of a monoterpene and is found in essential oils of bay, verbena, pine and juniper, and in many others. 

Myreene and ocimene are not arranged in ring structures they are acyclic. However, many monoterpenes link up to form rings or cyclic structures, for example in limonene. 

BOHLMANN, J., STEELE, C. L., CROTEAU, R. Monoterpene synthases from grand fir (Abies grandis) - cDNA isolation, characterization, and functional expression of myreene synthase, (-X4S)-limonene synthase, and (->-<1 S,5S)- pinene synthase, J. Biol. Chem., 1997, 272, 21784-21792. 

The wonderful fragrance of (eaves from the California bay tree is due primarily to myreene, a simple terpene. 

Kig. (1. ). Conversion from myreene into (S)-ipsenol (7 ) and conversion from a-pinene into cis and trans-... 

As expected. Ter values of all compounds were higher than the Ted values. The difference between recognition and detection threshold values was not constant with respect to individual compounds. Large differences were observed with linalool (21-fold), citronellol (22.9-fold), eugenol (35-fold), nerol (49-fold), 1-hexanol (47-fold), nerolidol (21-fold), and (E)-2-hexenol (24-fold). Detection and recognition threshold values were similar with myreene (1.37-fold), (E)-2-hexenal (1.33-fold), a-pinene (1.0-fold), P-pinene (1.77-fold), and 1-octanol (1.64-fold). 

Monoterpenes, on the other hand, possess particularly desirable flavor notes. Three of them, (3-myreene (A.45), limonene (A.46) and p-cymene (A.59) are common to coffee, cocoa and tea, and to numerous plant families (3-myrcene for example constitutes 63% of hop oils. It is worth remembering that they particularly contribute to the flavor of spices. In a study of the effluvia of fresh red coffee berries, Mathieu et al. (1996) identified, in two varieties of robusta, a series of mono- and sesquiterpenes, a blend also present in cold pressed oil from pommelos. In fact, the effluvia of red berries attracts the females of the coffee berry borer, a very important pest. Without giving a list of the hydrocarbons identified in this study, let us note that limonene is the most abundant of them, caryophyllene (4,11,11-trimethyl-8-methylenebicyclo[7.2.0]undec-4-ene), followed by humulene (2,6,6,9-tetramethylundeca-1,4,8-triene) and a-pinene (2,6,6-trimethylbicyclo[3.1.1]hept-2-ene), are rather important in two robusta... 

But it is not this step that makes the synthesis remarkable, but rather Takasago s route focitronel lal. Pinene is another terpene that is produced in only low enantiomeric excess by pine trees (and indeed, which is the major enantiomer depends on whether it is a European or a North Americar pine tree). But in the menthol process none of this matters, and cheap, enantiomerically impure pinene can be used, because the first step is to convert it to an achiral terpene, myreene. Lithium diethylamide adds to this diene to give an allylic amine. 

Oil of Bay. Oil of Myrcia Volatile oil distilled from leaves of Pimento (Myrcia) acris K os tel., Myrtaceae. Constit. 40-55% eugenol myreene, chavico], methyleugenol, methyichavicoi, citral, l-phellandrene total phenols, 50-65% by volume. 

CyclotMlation of myreene. Reaction of myreene (1) with thallium(III) nitrate triJydrate in methanol at -60° followed immediately by neutralization... 

Furanoterpenes. Perillene, (3), one of the simplest furanoterpenes, has been synthesized as shown in scheme (1) from /S-myreene (1) by way of an endoperoxide (2). ... 

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