Natural Carbohydrate Amino Acids

Sialic acids are important constituents of glycoconjugates, often located peripherally on glycoproteins. This family of natural CAAs consists of N- and O-acyl derivatives of neuraminic acid 2 the main substituents on nitrogen are the iV-acetyl and A-glycolyl groups [14], 

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One of the most unusual carbohydrate - amino acid linkages so far found in Nature is the thio linkage between glucose or galactose and cysteine.35,40 41 This glycopeptide has not yet been isolated or synthesized. Therefore, in order to obtain 13C-n.m.r. chemical-shift data for... 

The physiological relevance together with chnical importance of transamination and deamination is wide-ranging. As an aid to understanding the somewhat complex nature of amino acid metabolism, it can be considered (or imagined) as a metabolic box (represented in Figure 8.13). Some pathways feed oxoacids into the box whereas others remove oxoacids and the ammonia that is released is removed to form urea. The box illustrates the role of transdeamination as central to a considerable amount of the overall metabolism in the liver cell (i.e. protein, carbohydrate and fat metabohsm, see below). 

Nature builds up carbohydrate, amino acids, a-hydroxy acids and other molecules by the use of the aldol reaction. The enzyme aldolases are among the most important biocatalysts for C—C bond formation in nature [3]. 

Substances such as carbohydrates, amino acids, and other small molecules available from natural sources are valuable starting materials for the synthesis of stereochemi-cally defined substances. Suggest a sequence of reactions which could effect the following transformations, taking particular care to ensure that the product would be obtained stereochemically pure. 

Despite its efficiency in numerous cases optical resolution is by no means a trivial operation. In each case the optimum method has to be found by laborious trial and error procedures the optical purity of the material has to be secured and its absolute configuration has to be established before the compound can be used in a synthetic sequence. These drawbacks of optical resolution led chemists to start their syntheses from optically active natural products (the so-called chiral carbon pool ). A variety of suitable ex-chiral-pool compounds including carbohydrates, amino acids, hydroxy acids, and terpenoids are shown. 

Asymmetric C-C bond formation is the most important and most challenging problem in synthetic organic chemistry. In Nature, such reactions are facilitated by lyases, which catalyze the addition of carbonucleophiles to C=0 double bonds in a manner that is classified mechanistically as an aldol addition [1]. Most enzymes that have been investigated lately for synthetic applications include aldolases from carbohydrate, amino acid, or sialic acid metabolism [1, 2]. Because enzymes are active on unprotected substrates under very mild conditions and with high chemo-, regio-, and stereoselectivity, aldolases and related enzymes hold particularly high potential for the synthesis of polyfunctionalized products that are otherwise difficult to prepare and to handle by conventional chemical methods. 

An asymmetric C-C coupling, one of the most important and challenging problems in synthetic organic chemistry, seems to be most appropriate for the creation of a complete set of diastereomers because of the applicability of a convergent, combinatorial strategy [38-40]. In Nature, such reactions are facilitated by lyases which catalyze the (usually reversible) addition of carbo-nucleophiles to C=0 double bonds, in a manner mechanistically most often categorized as aldol and Claisen additions or acyloin reactions [41], The most frequent reaction type is the aldol reaction, and some 30 lyases of the aldol type ( aldolases ) have been identified so far [42], of which the majority are involved in carbohydrate, amino acid, or hydroxy acid metabolism. This review will focus on the current state of development of this type of enzyme and will outline the scope and limitations for their preparative application in asymmetric synthesis. 

IV-alkyl and IV-acyl derivatives has been widely used, the method of choice is that of trimethylsilylation. The procedure is widely applicable to natural products (e.g. carbohydrates, amino acids, steriods, alkaloids, polyhydroxyphenols, fatty acids, etc.), and has found great use in analytical and preparative laboratories which handle such compounds. 

The carbohydrates, amino acids, proteins, and nucleic acids discussed in Chapters 25, 26, and 27 are sometimes called primary natural products because they are found in all types of organisms and are the products of primary metabolism. Secondary natural products are usually produced from primary natural product precursors, such as amino acids or acetate ion, and, in general, are less widespread in occurrence. Today, natural product chemistry usually refers to the structure, reactions, and synthesis of these secondary natural products. 

Primary natural products (Chapter 28) Naturally occurring compounds that are found in all types of organisms and are the products of primary metabolism includes carbohydrates, amino acids, proteins, and nucleic acids. 

Chapters 1 through 21 cover the topics that most instructors will include in their courses, with the possible exception of Chapter 15. The remaining chapters offer a choice for the last part of the course. They include chapters on pericyclic reactions, synthesis, and polymers. The chapters on the more biochemical topics—carbohydrates, amino acids and proteins, nucleotides and nucleic acids, and other natural products— concentrate on the organic chemistry of these important biomolecules. 

Many naturally occurring compounds are chiral and often enantiopure. They may be carbohydrates, amino acids, terpenoids, hydroxy carboxyUc acids, or alkaloids. They can serve as building blocks for the synthesis of chiral compounds. In these compounds nature has already provided the chirahty since natural compounds are... 

The natural standard amino acids are left-handed, whereas carbohydrates and nucleic acids are right-handed. This property has consequences the chiral groups arrange in particular patterns in space relative to each other. Two differently arranged molecules that carry the same substituents are enantiomers. They differ in their ability to rotate plane-polarized light by equal amounts but in opposite directions. A mixture of equal parts of an optically active isomer and its enantiomer is a racemate and does not have a net rotation of plane-polarized light. 

The abundance in nature of amino acids and carbohydrates in optically pure modifications, together with their polyfunctionality, recommends them as candidates for convenient AS units for constructing high-symmetry chiral molecules... 

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