Isoprenoid, acyclic structure

The mobile Jt-electrons of unsaturated systems, responsible for the stabilization of the carbocations, provide equally efficient stabilization for carbanions. Consequently, a retrosynthetic cleavage of a benzylic, allylic, or propargylic C-C bond has additional merits since the resulting fragments can be visualized as either an electrophile or a nucleophile. The dual synthetic value of the allylic moiety has been extensively utilized in the synthesis of a large number of natural acyclic isoprenoids. The structures of many of these compounds look like they were purposely tailored for this type of retrosynthetic analysis. In fact, the 1,5-diene system, usually present in their structure (Scheme 2.19), immediately suggests the cleavage of its central C-3-C-4 bond, which leads to two allylic precursors. 

Since 1971 Maxwell s group at Bristol used these methods to separate and identify the stereochemistry of isoprenoidic acyclic alkanes . Whereas most acyclic isoprenoids in nature appear functionalized (alcohols, acids, alkenes, etc.), they are found in the geoenvironment as fossil alkanes. Hence it is important not only to analyse the structural isomers, but also to determine the stereochemistry. 

The organization of Part Two is according to structural type. The first section, Chapter Seven, is concerned with the synthesis of macrocyclic compounds. Syntheses of a number of heterocyclic target structures appear in Chapter Eight. Sesquiterpenoids and polycyclic higher isoprenoids are dealt with in Chapters Nine and Ten, respectively. The remainder of Part Two describes syntheses of prostanoids (Chapter Eleven) and biologically active acyclic polyenes including leukotrienes and other eicosanoids (Chapter Twelve). 

More than 600 different carotenoids from natural sources have been isolated and characterized. Physical properties and natural functions and actions of carotenoids are determined by their chemical properties, and these properties are defined by their molecular structures. Carotenoids consist of 40 carbon atoms (tetraterpenes) with conjugated double bonds. They consist of eight isoprenoid units j oined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position and the remaining nonterminal methyl groups are in a 1,5-position relationship. They can be acyclic or cyclic (mono- or bi-, alicyclic or aryl). Whereas green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few... 

A unique role is played by chemical communication in the interactions between plants and insects. About half a million insect species feed on plants. The process of reproduction in many plant species is critically dependent upon pollination by insects. It is not surprising, then, to find among the numerous natural products of plants both attractants for useful insects and repellents or even insecticides for plant-eating insects. The remarkable diversity of the these compounds (the list includes acyclic and polycyclic compounds, isoprenoids, aromatic derivatives, heterocyclic compounds, etc.) illustrates the non-selectivity in the structure of the chemical mediators for biological applications. The intimate mechanism of their action is, unfortunately, still insufficiently understood. 

Balaban, A.T., Barasch, M. and Marcus, S. (1980b). Picture Grammars in Chemistry. Generation of Acyclic Isoprenoid Structures. MATCH (Comm.Math.Comp.Chem.), 8,193-213. 

Very recently it was demonstrated that tocopherol moieties in kerogen are likely precursors of prist-l-ene (Figure 7) (30). This idea was supported by the fact that tocopherols are widely distributed in photosynthetic tissues and that they also occur as such in several recent sediments (31). It is tempting to conclude that during "natural pyrolysis" the generated pristene will be transformed to the well known component, pristane, in ancient sediments and oils. This example nicely illustrates that we have to be very careful when we conclude that acyclic isoprenoid hydrocarbons such as pristane originate from the chlorophyll side chain, phytol, based solely on structural similarities. 

This paper describes the structural analysis of naturally occurring ois-1,4 polyisoprenes using 13C NMR spectroscopy. First, the structural characterization of polyprenols, which are linear isoprenoid compounds containing 30 to 100 carbons, was carried out on the basis of information obtained from acyclic terpenes having various ois and trans isoprene units as model compounds. This method was also applied to the structural analysis of polyisoprenes. The elucidation of the structure of the end groups and the arrangement of isoprene units provides information on the mechanism of the biosynthesis of polyprenyl compounds in nature. 

Polyterpenoids.— C N.m.r. has been used to investigate the structure of the C55 polyprenol moraprenol-11 (190) from leaves of Moms alba. The main component has the trans-trans-trans-cis-cis-cis-cis-cis-cis-cis-OH configuration. The monophosphate of this compound was prepared in 34% yield by treatment with o-phenylenephosphorochloridate. Biosynthetic studies have provided evidence for the formation of 2,3-dehydrodolichyl phosphate (191) as an intermediate in dolichyl phosphate (192) biosynthesis. Most of the members of a series of C2S—C40 acyclic hydrocarbons obtained from crude oil have isoprenoid or degraded isoprenoid structures. The mokupalides from a marine sponge have been shown to be hexaprenoids with structures (193)—(195). ... 

The most interesting new polyterpenoids are a series of C40 diols that form part of macrocyclic diglycerol tetraethers which constitute the major membrane lipids of the extremely thermo- and acido-philic bacterium Caldariella. These compounds, (196)—(200), which may be acyclic or may have from one to four isolated cyclopentane rings are formally dimers of C20 isoprenoid structures linked head-to-head rather than the conventional tail-to-tail linkage in, for example, carotenoids.The isoprenoid nature of these compounds has been shown by labelling. 

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

The presence of polar or polarizable groups close to the chiral centre can promote hydrogen bonding or 7i-overlap with the stationary phase and enhance the resolution. This was again shown to be a significant structural feature in the separation of diastereomeric amides of acyclic isoprenoid acids [11]. The introduction of an olefinic group close to the chiral centre increased the separation factors. Similarly, separation factors were decreased... 

Structural analysis of the resinous fraction in crude has indicated the presence of high molecular weight carboxylic acids, esters and porphyrin structures. Cason et al. (12-14) have isolated isoprenoids, cyclic and acyclic carboxylic acids with carbon numbers of to C2q- Seifert and Teeter (15) have discovered 22" 24 carboxylic acids as well as other to petro-... 

The isoprene rule (337) and its evolved version, the biogenetic isoprene rule (336, 337), have proven to be of fundamental consequence in the development of terpene chemistry. According to the biogenetic isoprene rule, which was formally enunciated by Ruzicka in 1953, terpene structures may be rationalized, or preliminary structures deduced, by accepted reaction mechanisms from hypothesized acyclic precursors such as geraniol, farnesol, geranylgeraniol, etc. Biosynthetic investigations (20, 75, 87, 100, 110, 160, 162, 179, 263) over the past three decades have fully confirmed the biogenetic isoprene rule, and have provided detailed information on several fundamental steps in isoprenoid biosynthesis. 

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