Hemiterpene

Synonyms BRN 0969158 CCRIS 6253 EINECS 201-143-3 Hemiterpene Isopentadiene Iso-prene p-Methylbivinyl 2-Methyldivinyl 2-Methylbutadiene NSC 9237 UN 1218. 

Hemellitol, see 1,2,3-Trimethylbenzene Hemimellitene, see 1,2,3-Trimethylbenzene Hemimellitol, see 1,2,3-Trimethylbenzene Hemiterpene, see 2-Methyl-l,3-butadiene Hemostyp, see 1-Butanol HEOD, see Dieldrin HEPT, see Tetraethyl pyrophosphate Heptachlorane, see Heptachlor... 

Hu JY, Qiao W, Takaishi Y, Duan HQ (2006) A New Hemiterpene Derivative from Prinsepia utilis. Chin Chem Lett 17 198... 

The terpenoids form a large and structurally diverse family of natural products derived from C5 isoprene units (Figure 5.1) joined in a head-to-tail fashion. Typical structures contain carbon skeletons represented by (Cs) , and are classified as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C2o), sesterterpenes (C25), triterpenes (C30) and tetraterpenes (C40) (Figure 5.2). Higher polymers are encountered in materials such as rubber. Isoprene itself (Figure 5.1) had been characterized as a decomposition product from various natural cyclic hydrocarbons, and was suggested as the fundamental building block for these compounds, also referred to as isoprenoids . Isoprene is produced naturally but is not involved in the formation of... 

IPP and DMAPP are reactive hemiterpene intermediates in the pathways leading to more complex terpenoid structures. They are also used as alkylating agents in the formation of meroterpenoids as indicated above, but examples of these structures are discussed under the section appropriate to the major substructure, e.g. alkaloids, shiki-mate, acetate. Relatively few true hemiterpenes are produced in nature, with isoprene, a volatile compound which is released by many species of plants, especially trees, being the notable example. Isoprene is formed by loss of a proton from the allylic cation (Figure 5.7). 

Terpenes are built out of Cs-isoprene units. There is a variety of terpenes ranging from Cs-terpenes which are called hemiterpenes to tetraterpenes with C40 atoms. There are also terpenes with more Cs-units, such as natural rubber, which is a polyterpene. Terpenes are also classified by the number of carbon cycles in the molecule, e.g., monoterpenes can have up to two cyclic systems in one molecule. Examples of some monoterpenes are shown in Scheme 2 myrcene is acyclic,... 

IP-PP and DMA-PP can yield volatile C3 hemiterpenes. At the other extreme, extensive polymerization of the C3-pyrophosphates (with release of pyrophosphate, PP ) yields the formation of the plant latex polymers such as eis-polyisoprenes (rubber) and trans-polyisoprenes (gutta-percha). In between these extremes, a variety of monoterpenes, sesquiterpenes, triterpenes and C3() carotenes derive from these C3-pyrophosphate precursors. 

Not all terpene synthases catalyse complex reactions. Isoprene synthase converts DMAPP to the hemiterpene (G5), isoprene (Fig. 5.1), a comparatively simple process involving the ionization of the diphosphate group, followed by double-bond migration and proton elimination (Silver and Fall, 1991). Present in chloroplasts in both stromal and thylakoid-bound forms, isoprene synthase is a homodimer that differs from other terpene synthases in many properties, such as subunit architecture, optimum pH and kinetic parameters... 

Most terpenoids are derived from mevalonic acid (MVA) through the universal precursor isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP). Thus, the vast majority of terpenoids contain the basic structural residue 2-methylbutane, often less precisely referred to as isoprene units. These C5 hemiterpene units combine with... 

A small number of hemiterpenes are used in perfumery, the most important of which are prenyl acetate and benzoate. Thioesters, such as those shown in Scheme 4.11, have extremely intense green odours reminiscent of galbanum, in which they occur naturally. The esters are usually prepared from prenyl chloride and the thioesters from the corresponding thiol and acid chloride. 

Silyl enol ethers also react well with allylic halides, or with the hemiterpene fragment prenyl acetate24 79 or with secondary25 80 and tertiary26 81 benzylic halides (Zn salts are used as catalysts in these examples). In each case, regioselectivity has been demonstrated with the same two isomers 22 and 25. 

The restricted space available for these Reports unfortunately does not allow the discussion of a number of topics of interest to monoterpenoid chemists useful reviews of such topics include monoterpenoid alkaloids/ carbazole alkaloids/ isoprenoids and alkaloids of tobacco/" naturally occurring plant coumarins/ the biosynthesis of aromatic hemiterpenes/ and recent developments in the field of naturally occurring aroma components/ The poor quality of the Chemical Abstracts makes it difficult to assess the significance of a number of reviews of potential industrial interest/ A volume in the Methodicum Chimicum series includes a very brief discussion of some monoterpenoids/"... 

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