Carbohydrates, accessibility

Of the many physical and chemical procedures thus far applied to enhance lignocellulose reactivity (1), fine grinding appears to offer the most direct response to both lignin and crystallinity. When the grinding is done in a vibratory ball mill, particle size can be reduced to micron dimensions, with attendant expansion of external surface area, and crystallinity can be essentially eliminated (2). This combination of events markedly influences carbohydrate accessibility and, hence, the degree of response of a milled lignocellulosic material to chemical, enzymatic, and microbiological attack. 

Our reviewer felt the molecule builder was easy to use. It is set up for organic molecules. Specialized building modes are available for peptides, nucleotides, and carbohydrates. It is also possible to impose constraints on the molecular geometry. Functions are accessed via a separate window with buttons labeled with abbreviated names. This layout is convenient to use, but not completely self-explanatory. The program is capable of good-quality rendering. At the time of this book s publication, a new three-dimensional graphic user interface called Maestro was under development. 

The most stable protected alcohol derivatives are the methyl ethers. These are often employed in carbohydrate chemistry and can be made with dimethyl sulfate in the presence of aqueous sodium or barium hydroxides in DMF or DMSO. Simple ethers may be cleaved by treatment with BCI3 or BBr, but generally methyl ethers are too stable to be used for routine protection of alcohols. They are more useful as volatile derivatives in gas-chromatographic and mass-spectrometric analyses. So the most labile (trimethylsilyl ether) and the most stable (methyl ether) alcohol derivatives are useful in analysis, but in synthesis they can be used only in exceptional cases. In synthesis, easily accessible intermediates of medium stability are most helpful. 

At low relative humidities, adsorption is due to interaction of water with accessible hydroxyl groups. These are present on the lignin and on the carbohydrates ia the noncrystalline or poorly crystalline regions. The high differential heat of adsorption by dry wood, - 1.09 kJ/g (469 Btu/lb) water. 

The formal lUPAC rules for a and 3 notation in carbohydrates are more detailed and less easily understood than most purposes require. These rules can be accessed at http //www.chem.qmw.ac. uk/iupac/2carb/06n07.html. 

Since the early 1970s a panel convened by the International Union of Pure and Applied Chemistry and the International Union of Biochemistry and Molecular Biology has been working to formulate recommendations for carbohydrate nomenclature that meet developing needs of research and electronic data handling, while retaining links to the established literature base on carbohydrates. The realization of these endeavors is presented here in the final document Nomenclature of Carbohydrates, which provides a definitive reference for current researchers, both in the text version and in the version accessible on the World Wide Web (http //www.chem.qmw.ac.uk/iupac/2carb/), where amendments and revisions are maintained. 

Figure 10.21 Aldolase-catalyzed asymmetric synthesis of uncommon L-configured sugars (a), and selected examples of carbohydrate-related product structures that are accessible by enzymatic aldolization (b).

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

A useful and simple method for the one-pot preparation of highly functionalized, enanhomerically pure cyclopentanes from readily accessible carbohydrate precursors has been designed by Chiara and coworkers [73]. The procedure depends on a samarium(II) iodide-promoted reductive dealkoxyhalogenahon of 6-desoxy-6-iodo-hexopyranosides such as 7-160 to produce a 6,e-unsaturated aldehyde which, after reductive cyclization, is trapped by an added electrophile to furnish the final product. In the presence of acetic anhydride, the four products 7-161 to 7-164 were obtained from 7-160. 

Click chemistry has been particularly active in various fields this year. For example, ample applications of click chemistry have been seen in carbohydrate chemistry. Various /weiido-oligosacchardies and amino acid glycoconjugates were synthesized via an intermolecular 1,3-dipolar cycloaddition reaction using easily accessible carbohydrate and amino acid derived azides and alkynes as building blocks <06JOC364>. The iterative copper(I)-catalyzed... 

Oxidation is a widely used procedure in carbohydrate chemistry, mainly to access sugars that contain a carbonyl function to serve as valuable intermediates for a variety of derivatizations. Many procedures have been developed, employing either chemical or biochemical methodologies.14 148 While most of these methodologies rely on homogeneous catalysis, the use of heterogeneous catalysts has proved to be a feasible alternative.123c However, the utilization of catalysts based on silicon porous materials for the oxidation of carbohydrates is still a field to be further explored. 

Synthetic aspects for access to monovalent fullerene-carbohydrate hybrids were highlighted, but only a few biological applications were mentioned. In contrast, multivalent presentation of saccharides by multiple anchorages to the same structure, or their presentation as antennary glycodendrons, has generated promising results. 

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