Natural products isoprene units

Numerous natural products contain units derived from a terpene precursor. Built up of five carbon isoprene ... 

The structural feature that distinguishes terpenes from other natural products is the iso prene unit The carbon skeleton of myrcene (exclusive of its double bonds) corresponds to the head to tail union of two isoprene units... 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Natural products containing the isoprene unit can be prepared in this way. Methyl famesoates have been synthesized from geranyl halides and 1-carbomethoxyallylnickel halides (example 4, Table III). 

The compound dimethylallyl diphosphate provides an excellent example of a natural product with a diphosphate leaving group that can be displaced in a nucleophilic substitution reaction. Suitable nucleophiles are hydroxyl groups, e.g. a phenol, though frequently an electron-rich nucleophilic carbon is employed. Dimethylallyl diphosphate is a precursor of many natural products that contain in their structures branched-chain C5 subunits termed isoprene units. 

Many of the natural-product molecules synthesized by plants are formed by the joining together of isoprene monomers via an addition polymerization. A good example is the nutrient beta-carotene, which consists of eight isoprene units. Find and circle these units within the structure shown here. 

Camphor is a 10-carbon odoriferous natural product made from the joining of two isoprene units plus the addition of a ketone. Shown here is its chemical structure. Find and circle the two isoprene units. 

A word of warning is also necessary. Some natural products have been produced by processes in which a fundamental rearrangement of the carbon skeleton has occurred. This is especially common with structures derived from isoprene units, and it obviously disguises some of the original building blocks from immediate recognition. The same is true if one or more carbon atoms are removed by oxidation reactions. 

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... 

Terpenes are natural products usually obtained from the essential oils of plants. They contain multiples of five-carbon atoms (5, 10, 15, and so on). Each five-carbon arrangement is called an isoprene unit, a four-carbon chain with a one-carbon branch at C-2. Terpenes are frequently used in fragrances and perfumes. Steroids are lipids that contain a unique four-ring structure and are biosynthetically related to terpenes. Important examples of steroids include cholesterol, the bile acids, and the sex hormones. 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

Many natural products are derived from terpenes, even though they do not have carbon skeletons composed exclusively of C5 isoprene units. These terpene-like compounds are called terpenoids. They may have been altered through rearrangements, loss of carbon atoms, or introduction of additional carbon atoms. Cholesterol is an example of a terpenoid that has lost some of the isoprenoid carbon atoms. 

Structure of Natural Rubber Like many other plant products, natural rubber is a terpene composed of isoprene units (Section 25-8). If we imagine lining up many molecules of isoprene in the. v-cis conformation, and moving pairs of electrons as shown in the following figure, we would produce a structure similar to natural rubber. This polymer results from 1,4-addition to each isoprene molecule, with all the double bonds in the cis configuration. Another name for natural rubber is cis-1,4-polyisoprene. 

The prevalence of 2,2-dimethylchromene derivatives (arising biosynthetically via addition of an isoprene unit to phenols) in a variety of natural products led to extensive examination of the asymmetric epoxidation of numerous such substrates, the yields and enantioselectivies of the thus-obtained fused oxiranes typically being good to excellent <1991JA7063, 1991TL5055, 1993SL261, 1995TL5457>. 

A fascinating area of research linking organic chemistry and biology is the study of the biogenesis of natural products the detailed sequence of reactions by which a compound is formed in living systems, plant or animal. All the isoprene units in nature, it appears, originate from the same compound, isopentenyl pyrophosphate. 

The diverse, widespread and exceedingly numerous class of natural products that are derived from a common biosynthetic pathway based on mevalonate as parent, are synonymously named terpenoids, terpenes or isoprenoids. Essentially, they are derived from the basic 5-carbon isoprene unit, biosynthetically as isopentenyl pyrophosphate, which is itself derived from aeetate via mevalonic acid. They may be classified into diverse groups according to the number of isoprene units, e.g.. 

Terpenoids form a large and the most structurally diverse family of natural products derived from repetitive fusion of branched C5 isoprene units. The name terpenoid, or terpene, derives from the fact that the first members of the class were isolated from turpentine ( terpentin in German). They play numerous essential roles in both plant and animal life. Those from plant origin are essential for several plant processes, including respiration, photosynthesis, growth,... 

The isoprene unit is the most important building block for lipids, steroids, terpenoids, and a wide variety of natural products. The only chemical reaction of commercial importance (other than polymerization) is its conversion to terpenes. Isoprene is used in the manufacture of synthetic natural rubber, butyl rubber, and as a copolymer in the production of synthetic elastomers. 

Terpenoids are widespread natural products that are formed from C5 isoprene units leading to their characteristic branched chain structure. Terpenoids are divided into families on the basis of the number of isoprene units from which they are formed. Thus there are monoterpenoids (Cio), sesquiterpenoids (C15) diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30) and carotenoids (C40). The isoprene units are normally linked together in a head-to-tail manner. However, the C30 triterpenoids and C40 carotenoids are formed by the dimerization of two Ci5 and C20 units, respectively. Hence, in these cases the central isoprene units are linked in a head-to-head manner. The presence of tertiary centres in the isoprenoid backbone of the terpenoids facilitates skeletal rearrangements in the biosynthesis of these natural products. As a consequence, on first inspection some structures appear not to obey the isoprene rule. 

Scheme 3.8 shows how the isoprene units and the original backbone can be traced out in a number of terpenes that are important in perfumery. Sometimes skeletal rearrangements occur which make this process more difficult and fragmentation or degradation reactions can reduce the number of carbon atoms so that the empirical formula does not contain a simple multiple of five carbons. Nonetheless, the natural product chemist quickly recognizes the characteristic terpene framework of the structure. 

Many natural products contain an isolated isoprene unit derived from mevalonic acid hence, these compounds may be considered as hemiterpenoid derivatives. In most cases the immediate precursor is probably dimethyl allyl pyrophosphate... 

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