Synthetic Carbohydrate Amino Acids

Other approaches have been to attach acid residues as substituents. Examples are a CAA 10, derived from activated chitin in four steps (26%) [28], or the demethyl 

The coupling of a protected serine aldehyde, in which the amino acid functionalities are preformed, to a substituted furan was the key step of the preparation of 

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Thus, the tissue cholesterol can be synthetized from any materials whose break-down leads to acetyl-CoA. These include carbohydrates, amino acids, fatty acids, and glycerol. 

Easily available advanced synthons, such as the carbohydrates, amino acids, hydroxyacids, and terpenoids, make the synthetic task easier than the complexity metrics of the target suggests this is especially true for the glycosides, if the carbohydrate portion can be introduced intactly. It must also be borne in mind that the S metric is counted in a linearly additive hion, neglecting interactions between the functional groups (Whitlock 1998) such interactions are not treated adequately by any method so far proposed to calculate the molecular complexity. Moreover, no attention was paid here to the graphic analysis of the synthesis plan based on the molecular complexity of the intermediates these aspects have recently been reviewed (Bertz 1993 Whitlock 1998 Chanon 1998). 

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. 

The wide variety of known oxidases reflects the complexities of the different substrate classes such as carbohydrates, amino acids, lipids, amines, metabolites, alcohols, acids, and other chiral building blocks. An overview of synthetic applications of oxidases is given in Figure 20.3. 

Biopolymers are the naturally occurring macromolecular materials that are the components of all living systems. There are three principal categories of biopolymers, each of which is the topic of a separate article in the Eniyclopedia proteins (qv) nucleic acids (qv) and polysaccharides (see Carbohydrates Microbial polysaccharides). Biopolymers are formed through condensation of monomeric units ie, the corresponding monomers are amino acids (qv), nucleotides, and monosaccharides, for proteins, nucleic acids, and polysaccharides, respectively. The term biopolymers is also used to describe synthetic polymers prepared from the same or similar monomer units as are the natural molecules. 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

At that time, as now, the enantiomers of many chiral amines were obtained as natural products or by synthesis from naturally occurring amines, a-amino acids and alkaloids, while others were only prepared by introduction of an amino group by appropriate reactions into substances from the chiral pool carbohydrates, hydroxy acids, terpenes and alkaloids. In this connection, a recent review10 outlines the preparation of chiral aziridines from enantiomerically pure starting materials from natural or synthetic sources and the use of these aziridines in stereoselective transformations. Another report11 gives the use of the enantiomers of the a-amino acid esters for the asymmetric synthesis of nitrogen heterocyclic compounds. 

Cortisol, the most important g/ucocorticoid, is synthesized by the adrenal cortex, it is involved in regulating protein and carbohydrate metabolism by promoting protein degradation and the conversion of amino acids into glucose. As a result, the blood glucose level rises (see p. 152). Synthetic glucocorticoids (e.g., dexamethasone) are used in drugs due to their anti-inflammatory and immunosuppressant effects. 

As chiral cyanohydrins are important synthetic intermediates for the preparation of chiral amino acids, carbohydrates, and so on, this process would be a useful asymmetric one—carbon homologation procedure widely employable for organic synthesis. 

The liver is involved in a variety of both synthetic and catabolic functions, including metabolism of amino acids, lipids, carbohydrates, protein synthesis and detoxification [ 1 ]. These metabolic functions are performed mainly by hepatocytes, although the liver is made of three major cell types (hepatocytes, biliary epithelial cells and Kupffer cells). Exerting many different metabolic functions, the liver contains several different and specific enzymes, leakage of which into the bloodstream occurs in hepatic diseases. 

However, now it is well established that the appropriate a-keto acids giving rise to a particular higher alcohol arise mostly from carbohydrate sources through the synthetic pathways by which yeast synthesizes its amino acid requirements. 

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