Stereospecific numbering system

A stereospecific numbering system (Sn) is used to indicate the location of specific fatty acids in triglyceride molecules such as in 1-stearoyl-2-oleoyl-3-myristoyl-Sn-glycerol the respective fatty acids are indicated in the 1, 2, and 3 positions. This kind of information is very valuable in relating properties of certain fats to compositional data. Table 4.4... 

Fatty acids in oils and fats are found esterified to glycerol. Glycerol (1,2,3-trihy-droxypropane) is a prochiral molecule. It has a plane of symmetry, but if the primary hydroxyls are esterified to different groups, the resulting molecule is chiral and exists as two enantiomers. The stereospecific numbering system is used to... 

It is important to appreciate that, in stereochemical terms, the positions occupied by the acid chains are not identical. Under the stereospecific numbering system the positions are designated sn-1, sn-2 and sn-3, as shown.They are readily distinguished by enzymes and this may lead to preferential reactivity at one or more of the positions. Phosphorylation, for example, always takes place at carbon atom sn-3 rather than at carbon atom sn-1. Although triacylglycerols are predominant, mono- and di-acylglycerols do occur naturally, but in much smaller amounts. 

A large number of compounds containing an extended 3,4-dihydro-/5-carbolinium system (82) have been converted similarly into the corresponding tetrahydro-j8-carboline derivatives (83a and 83b). In many cases the reaction is stereospecific and one or the other of the epimers 83a and 83b has been isolated as the sole or major product. Sodium and ethanoH tin and hydrochloric acid zinc and... 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

Some specific solutes diffuse down electrochemical gradients across membranes more rapidly than might be expected from their size, charge, or partition coefficients. This facilitated diffusion exhibits properties distinct from those of simple diffusion. The rate of facilitated diffusion, a uniport system, can be saturated ie, the number of sites involved in diffusion of the specific solutes appears finite. Many facihtated diffusion systems are stereospecific but, fike simple diffusion, require no metabolic energy. 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators 3 jj... 

Over the last few years the DCA and ACA compounds have been the subject of a number of interesting chemical developments including their use as templates in polymerisation 23,24), and as media for stereospecific chemical reactions 98 100). This work has been carried out on the orthorhombic systems and is therefore outside the scope of this review. 

The most well known and most widely used compounds mentioned in the chapter are the doubly protected monosaccharide derivatives (cf. Schemes 16-18 and 27-29), which serve as building blocks for the regio- and stereospecific synthesis of a host of more complex carbohydrates. A number of newly discovered alkaloids contain fused 5 6 6-triheterocyclic systems these compounds may come from not only traditional (plant) sources, for example, compounds 96, 292-294, 446, and 447, but also insects (compounds 531-533), marine animals of various kinds, for example, sponges (compounds 58 R = H or OH) and ascidians (compound 289), and amphibians (compound 535). Although the majority of these alkaloids are known to be toxic in sufficient quantity, any possible therapeutic applications have apparently not yet been disclosed. 

The addition of singlet oxygen to alkenes also gives dioxetanes. A number of mechanisms have been proposed and the literature abounds with theoretical and experimental results supporting one or more possible intermediates (a) 1,4-diradicals, (b) 1,4-dipolar, (c) perepoxides, or (d) concerted (Scheme 95). Both ab initio and semi-empirical calculations have been done and to date the controversy is still not resolved. These mechanisms have been reviewed extensively (77AHC(21)437, 80JA439, 81MI51500 and references therein) and will not be discussed here, except to point out that any one mechanism does not satisfactorily account for the stereospecificity, solvent effects, isotope effects and trapped intermediates observed. The reaction is undoubtedly substrate-dependent and what holds for one system does not always hold for another. 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

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