Myrcene synthesis from 0-pinene

Synthesis from fi-Pinene. For a description of this route, see under geraniol. Addition of hydrogen chloride to myrcene (obtained from / -pinene) results in a mixture of geranyl, neryl, and linalyl chlorides. Reaction of this mixture with acetic acid-sodium acetate in the presence of copper(I) chloride gives linalyl acetate in 75-80% yield [37]. Linalool is obtained after saponification. 

The production of myrcene (7) from P-pinene is important commercially for the synthesis of a wide variety of flavor and fragrance materials. Some of those include nerol and geraniol, citroneUol (27) and citral (5). 

Synthesis from (3-Pinene. Pyrolysis of /3-pinene yields myrcene, which is converted into a mixture of predominantly geranyl, neryl, and linalyl chloride by addition of hydrogen chloride in the presence of small amounts of catalyst, e.g., copper(I) chloride and an organic quaternary ammonium salt [29]. After removal of the catalyst, the mixture is reacted with sodium acetate in the presence of a nitrogen base (e.g., triethylamine) and converted to geranyl acetate, neryl acetate, and a small amount of linalyl acetate [30]. 

A synthesis of optically active citronellal uses myrcene (7), which is produced from P-pinene. Reaction of diethyl amine with myrcene gives AyV-diethylgeranyl- and nerylamines. Treatment of the allylic amines with a homogeneous chiral rhodium catalyst causes isomerization and also induces asymmetry to give the chiral enamines, which can be readily hydrolyzed to (H-)-citronellal (151). 

Bark beetles of the genus Ips are pests which attack pine and spruce trees. They use ipsdienols as aggregation pheromones, Ips confusus emitting the (5)-(-l-)-, and Ips paraconfusus the (.K)-(-)-enantiomer The beetles receive the myrcenes (section 2.2) occurring in conifers with their food and metabolize them to ipsdienols some evidence for de-novo synthesis by the bugs is also reported. In order to catch the beetles, pheromone traps are supplied with both enantiomers of ipsdienol which are produced from (-l-)-verbenone, a constituent of the Spanish verbena oil (section 2.4.3). This terpenone, also available by oxidation of a-pinene, is isomerized to the enantiomers of 2(10)-pinen-4-one via three steps (reduction, protonation, oxidation). After separation, both enantiomers are reduced by lithiumaluminumhydride. Pyrolytic cycloreversion of the resulting diastereomeric 2(10)-pinen-4-ols provides the enantiomers of ipsdienol... 

Among the vast number of applications, an important example is the production of the so-called rose alcohols from p-pinene, viz. nerol, geraniol and linalool, key intermediates in the synthesis of many major fragrances. This process involves the thermal isomerization to myrcene, followed by addition of HCI, substitution by acetate and alkaline hydrolysis, as shown in Fig. 2.5 [3]. 

The synthesis starts with myrcene (32), which can be obtained from 3-pinene as described above. Addition of the diethylamide anion gives geranyl diethyl amine (41). Isomerization using ruthenium (S)-BINAP gives the A,A-diethylenamine of enantiomeric excess (ee) of >96% and the catalyst turn over number is 400,000, which makes the process very efficient. The remainder of the synthesis follows that of the standard synthesis of /-menthol from citronellal. The zinc-catalyzed ene reaction converts /-citronellal to /-isopulegol [89-79-2] (172) and this is hydrogenated to /-menthol (42). More detail on this chemistry is given below. 

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