Terpenes, acyclic

Hdschle B, V Gnau, D Jendrossek (2005) Methylcrotonyl-CoA and geranyl-CoA carboxylases are involved in leucine/isovalerate utilization (Liu) and acyclic terpene utilization (Atu), and are encoded by liuB/liuD and atuC/atuF, in Pseudomonas aeruginosa. Microbiology (UK) 151 3649-3656. 

In order to obtain lactones from natural alkenols, we investigated the cyclocarbonylation of monoterpenic alcohols. The catalytic precursor is [PdCl2L2] in the presence of a slight excess of tin chloride and phosphine ligands. Dihydromyrcenol, a representative acyclic terpene containing a termi-... 

In females of the genus Agriotes, several esters of acyclic terpenes have been identified as pheromone components. Typical examples are geranyl 3-methyl-butyrate 95, the first pheromone identified from an Agriotis species [193] or ( , )-farnesyl butyrate 96, which together with geranyl butyrate is the major component of the sex pheromone of A. brevis [194]. In A. lineatus, the activity... 

The acyclic terpenes are discussed separately in Section 2.2. Some of the cycloaliphatic fragrance and flavor materials are structurally related to the cyclic terpenes and are, therefore, discussed in Section 2.4 after the cyclic terpenes. 

Acyclic terpene (Cjo) and sesquiterpene (C15) hydrocarbons find little use in flavor and fragrance compositions. They are relatively unstable and some have a slightly aggressive odor due to their highly unsaturated structure. 

Acyclic terpene and sesquiterpene alcohols occur in many essential oils. These alcohols were formerly isolated from oils in which they are major components. 

Among the acyclic terpene aldehydes, citral and citronellal hold key positions as fragrance and flavor chemicals, as well as starting materials for the synthesis of other terpenoids. Hydroxydihydrocitronellal is one of the most important fragrance materials. Derivatives of these aldehydes, particularly the lower acetals, are also used as fragrance materials. Acyclic sesquiterpene aldehydes are not very important as such, but they contribute to the characteristic fragrance and aroma of essential oils, for example, in the case of a- and /3-sinensal in sweet orange oil. 

Unlike the terpene alcohols, aldehydes, and esters, acyclic terpene ketones are not particularly important as fragrance or flavor substances thus, they are not discussed here in detail. 

Although a small amount of acyclic terpene acids such as geranic acid and citronel-lic acid occurs in many essential oils, often as esters, they are rarely used in perfume and flavor compositions. Methyl geranate is an intermediate in a-damascone synthesis and is sometimes needed in the reconstitution of essential oils. 

However, the lower fatty acid esters (particularly the acetates) of the acyclic terpene alcohols geraniol, linalool, and citronellol are extremely important both as fragrance and as flavor substances. The acetates occur in many essential oils, sometimes in rather high amounts. Formates, propionates, and butyrates occur less frequently. As a result of the development of large-scale production processes for terpenes, the esters of acyclic terpene alcohols are nearly always made synthetically. All acyclic terpene esters that are used as fragrance and flavor materials can be prepared by direct esterification of the appropriate alcohols. However, special precautions are required for the esterification of linalool. 

In aroma compositions, fatty acid esters of the acyclic terpene alcohols are used for obtaining citrus notes and for rounding off other flavor types. 

The most important and most frequently used acyclic terpene esters are described below. 

Pinene is similar to a-pinene in its reactions. Pyrolytic cleavage to myrcene, the starting material for acyclic terpenes, is used on an industrial scale. Addition of formaldehyde results in the formation of nopol nopyl acetate is used as a fragrance material. /3-Pinene is produced in large quantities by distillation of turpentine oils. It is used as a fragrance material in household perfumery. However, most /3-pinene is used in the production of myrcene. 

Apart from the terminal oxidation and cu-hydroxylation, hydroxylations in the chain of acyclic terpenes were also reported [112]. In a limited screen some microorganisms were found capable of introducing a hydroxy group in 8- or 9-position of the substrate -nerolidol. 

Exercise 30-1 a. Write out all of the possible carbon skeletons for acyclic terpene and sesquiterpene hydrocarbons that follow the isoprene rule. Do not consider doublebond position isomers. 

The chemical interrelationships among these seemingly widely different types of compounds justify the recognized segregation of the terpenes from other fields of organic chemistry. The structural significance of the isoprene residue ( C-C-C) to the different types of terpene carbon skeletons has already been indicated. The simplest type of terpene is an open-chain olefinic structure formed by the union of two isoprene units. Such compounds are called the acyclic terpenes. From two units of isoprene or from the simple acyclic structures, under proper conditions, simple... 

Name the acyclic terpene hydrocarbons in a manner similar to other unsaturated aliphatic hydrocarbons (I UP AC rules) when pure compounds are involved. 

Chart 3. Names and Formulas of Acyclic Terpene Hydrocarbons... 

The common terpene names for acyclic terpene hydrocarbons are not sufficiently precise to indicate their structures or relationships among derivatives. It is recommended therefore that use of these names be discouraged, although the names allo-ocimene and myrcene will still be convenient for limited usage for commercial products. 

The monocyclic terpene hydrocarbons, many of which are formed readily from the acyclic terpenes by ring closure or from the bicyclic terpenes by ring fission, contain a six-carbon ring and may all be considered as derivatives of either cyclohexane or benzene. They may also be classified, on the basis of common larger fundamental structures, into two distinct types of substituted six-carbon ring structures ... 

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