Monosaccharides structurally modified

Monosaccharides with modified structures are often biologically important. Examples include sugar phosphates, deoxy sugars, amino sugars, and ascorbic acid (vitamin C). 

In many cases the monosaccharides found in these complex structures are present as one of their chemical derivatives, which may be an oxidation or reduction product, a phosphate or sulphate ester or an amino derivative, etc. However, these modified forms of monosaccharides may themselves have important biochemical roles and are not always found incorporated in polysaccharides. 

Preparation of modified, bacterial polysaccharides having monosaccharide analogs inserted into the polymeric chain is of interest for study of the structure-properties relationship in these biopolymers. Incorporation of chemically prepared, modified, biosynthetic precursors of the polymers in enzymic reactions seems a promising approach for achieving this aim. Such an approach, which may be termed chemical-enzymic synthesis, has now been studied by our group,439-441 using O-specific polysaccharides (10-12) of Salmonella serogroups B and E as an example. 

The IUPAC-IUBMB short form symbolism18 2-Carb-38.5 is used here and elsewhere in this chapter for designation of oligosaccharide structures. Parent monosaccharide residues are the d enantiomers except for IdoA, which is the l enantiomer. Uronic acid residues modified by 44-unsaturation are prefixed by A4tS, and the associated anomeric symbol relates to the configuration at the 5-position of that residue before 4,5-elimination. The abbreviation HexA is used to denote either d-G1cA or L-IdoA. 

One of the primary dividends resulting from this disconnection was its exposure of a possible relationship between the target and a monosaccharide precursor. After systematic analysis, Pearson saw that the stereochemistry of 5 correlated perfectly with lactone 7. Furthermore, lactone 7 had a structure appropriate for easy synthetic elaboration into 6 since it was functionally differentiated at C(l) and C(4). It therefore appeared fully capable of being selectively modified in the manner required to obtain 5. 

The configurations of monosaccharides are described by several types of formulas. As an example, the following formulas are shown for a-D-glucopyranose structural, Haworth, modified Fischer, and Fischer. In Fischer formulas the -H or -OH groups are above or below the plane formed by the hemiacetal bonds and the carbon chain (-H bonds are shown shorter). 

As the accompanying structure shows, the carbohydrate portion of the peptidoglycan is a polymer of alternating units of two modified glucose molecules called N-acetylglucosamine and N-acetylmuramk acid. These two unusual monosaccharides... 

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