Application to Carbohydrates

Although the determination of both the configuration and the conformation of carbohydrates involves the same general considerations, the quantitative nature of the latter requires a rather more fundamental approach. Accordingly, the following discussion is divided into two main sections the first refers to the development and applications of nuclear magnetic resonance to stereochemical problems, and deals mainly with substituent resonances the second discusses in some detail the Karplus relationship and the determination of carbohydrate conformations. 

Hudson, Advan. Carbohydrate Chem., 3, 15 (1948), and references therein. 

Comparatively few acetylated furanoses have been investigated. Abraham and CO workers measured the values for the 3-0-acetyl (1) and 3,. ),6-tri-0-acetyl (2) derivatives of 1,2-0-isopropylidene-a-D-glucofur-anose, and their assignments are shown below. 

Similar empirical assignments have been made by Hough and Shute for the acetates of 2,5-anhydro-l-deoxy-l-nitroheptitols (3) and (4), and by Hall and Perry for the acetylated methyl pentofuranosides (5), (6), (7), and (8). These assignments are all empirical and do not appear to be indicative of conformation. 

Although most investigations of substituent resonances have thus far centered on acetoxy groups, other substituents have been investigated. Lemieux and coworkers measured the proton magnetic re.sonance spectra of 2,6-dimethoxycyclohexanol acetates and of cis, trans, cfs-1,2,3-tri-methoxycyclohexane, and found that, although the methyl resonances 

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In line with the policy of Advances to provide periodic coverage of major developments in physical methodology for the study of carbohydrates, A. Dell (London) here surveys the use of fast-atom-bombardment mass spectrometry in application to carbohydrates. This technique has achieved rapid prominence as the soft ionization technique of choice for structural investigation of complex carbohydrate sequences in biological samples. The author s extensive personal involvement in this field makes her chapter a critical, state-of-the-art overview for the specialist, as well as a valuable primer for the reader unfamiliar with this technique. 

Nolls, P., Parella, T. Spin-edited 2D HSQC-TOCSY experiments for the measurement of homonuclear and heteronuclear coupling constants application to carbohydrates and peptides. /. Magn. Reson. 2005, 176, 15-26. 

A third area of development in carbohydrate l.c. analyses is in the combined techniques (see Section IV,3) and other methods that provide qualitative, as well as quantitative, information about sample constituents, such as high-performance liquid affinity chromatography. The use of specific lectin- and monoclonal antibody-based, stationary phases for analytical and preparative applications is now being considered. The basic concepts of these techniques have been reviewed - and their applications to carbohydrates have been discussed. 

Wheat pentosans were dehydrated to furfural with concentrated HC1, and the furfural was dissolved in dibutyl ether in preparation for GC (Folkes, 1980). This method is claimed to be specific, accurate, and precise for pentoses and pentosans (Folkes, 1980), because 5-hydroxymethylfurfural is not produced, as is the case in hexose decomposition. Previously, furfural was distilled and analyzed colorimetrically. Alkalization increases the yield of 1-, 2-, and 3-carbon, vapor-phase compounds at the expense of anhydro sugars and furans (Ponder and Richards, 1993). Hydrolysis-GC is adaptable to the estimation of DP (Morrison, 1975) applicability to carbohydrates is limited to oligomers with DP = 6 (Traitler et al., 1984). 

The location of deuterium in sugars is usually most conveniendy ascertained by physical methods of analysis. The most important of these is nuclear magnetic resonance (n.m.r.) spectroscopy, a general account of whose application to carbohydrates is given in an earlier Volume of this Series84 (see also, This Volume, p. 7). 

Another method of seemingly general applicability to carbohydrates was used by Coat et al. to conjugate uridine to proteins. The isopropyli-dine derivative was allowed to react withp-nitrobenzoyl chloride to yield the 5 ester. Removal of the isopropylidine protecting group and hydrogenation of the nitro group made it possible to link the uridine derivative to the protein by a diazotization reaction. 

There are many other rearrangements utilizing Wittig products as substrates. These, and others, will continue to be addressed in later sections of this chapter because of their diversity and general applicability to carbohydrate chemistry. 

The use of lead tetraacetate for adding acetoxy groups to a double bond, as here used for preparing the pentaacetate (76), has been widely applied in synthetic organic chemistry, but there have been only a few applications to carbohydrates (see Perlin s review in this Series, for other examples). The conversion of (76) to the dicarbonyl derivative (77) (in 65% yield and in the crystalline state) was made by autohydrolysis in water at room temperature for 3 hr. The structure was deduced from elemental analysis, acetyl analysis, and the absence of methoxyl groups, and from its mode of synthesis. The acetyl determination could be made by direct titration with... 

The application of nuclear magnetic resonance to organic compounds is discussed in a number of excellent books " its application to carbohydrates has been reviewed in this Series. ... 

A review on the use of cyclic sulfites and cyclic sulfates as epoxide equivalents contained a number of applications to carbohydrate chemistry. ... 

The future perspectives of this research area depend heavily on the application of CP-FTMW spectrometers, which have changed the scope of rotational spectroscopy in recent years. Chirped-pulse Fourier-transform microwave spectroscopy, combined with laser ablation, opens a new era in the investigation of isolated biomolecules, as can be shown by its application to carbohydrates as discussed in this chapter. The broad frequency and large dynamic range make it possible to extend the range of detectable conformers to less stable forms and to access structural determinations in molecular systems of increasingly larger sizes from measurements of heavy atom ( C, 0) isotopes detected in natural abun-... 

Aerobic incubation of D-glucose with a crude fungal enzyme extract (white rot fungus) resulted in the transient accumulation of D-ara6 io-hexos-2-ulose which was then converted to D-eryt/iro-hexos-2,3-diulose. A single enzyme, pyranose-2-oxidase (EC 1.1.3.10) is believed to be responsible for both oxidation steps. A review on enzymatic oxidoreduction in organic synthesis which includes applications to carbohydrate-related topics is referred to in Section 2 above. 

MM2 has been parameterized for a wide range of molecules, including those exhibiting the anomeric effect. The details of the MM2 and MM3 force fields and their application to carbohydrates have been discussed elsewhere. Of particular relevance to carbohydrates is the ability of these force fields to predict the subtle variations in the Cano C) bond lengths that relate to the anomeric effect. In order to account correctly for the observed relationship between glyco-sidic torsion angle and Cano-O bond lengths, MM2 and MM3 employ a torsion-stretch cross term (equations 21 and 22). 

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