Sugars configuration

The compound is named as a disubstituted deoxy sugar. Configuration is determined by regarding the substituent having priority according to the Sequence Rule ([13], Section E), as equivalent to OH. Any potential ambiguity should be dealt with by the alternative use of the R,S system to specify the modified stereocentre. 

Stereoelectronic effects can be invoked for the radical reaction at anomeric centre of carbohydrates. The high stereoselective preparation of a-substituted C-glycosyl phosphonates in a a p ratio of 98 2 was achieved by reductive addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in parentheses) depend on the sugar configuration D-galacto (80%), D-manno (47 %), D-gluco (30 %) and L-fuco (62 %). 

Pedersen, A.T. et al., Anomeric sugar configuration of anthocyanin 0-pyranosides determined from heteronuclear one-bond coupling constants, Phytochem. Anal, 6, 313, 1995. 

Glycosyl fluorides are excellent precursors for glycosylations. Lewis acids promote the defluorination leaving a stabilized carbenium species which is able to react with nucleophiles, e.g. N-nucleophiles. The configuration at the newly formed C-N bond is not only influenced by the sugar configuration but also by the utilized Lewis acid. This means that this is not a clear SNl-type reaction SN2-type pathways also have to be considered.15 18... 

The sugar configuration about the T, 2, 3, and 4 positions can be changed by synthesis. A variety of pyrimidine nucleoside cyclic phosphates have been made. Ukita et al. (425) prepared -D-lyxo-uridine 2 3 -cyclic phosphate (Fig. 21a). The configuration about the 2 and 3 positions is inverted and the two OH groups are now cis to the base rather than trans as in the D-ribose series. No hydrolysis at all of this compound was observed in the presence of RNase. However, both the cyclic phosphate and the free 2 (3 )-nucleotides inhibit the enzyme. The... 

The effect of variation of sugar configuration on the rate of hydrolysis of a number of methyl glycosides has been studied by several groups of work-ers.7n.7i The relative reactivities collected by Shafizadeh and Thompson are shown in Table XII. These results have been discussed by Foster and Overend and by Edward, on the basis of mechanism a of Fig. 4. 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

Org. Chem. 39, 3847. Carbon-13 Nuclear Magnetic Resonance Spectra of Branched-Chain Sugars, Configurational Assignment of the Branching Carbon Atom of Methyl Branched-Chain Sugars. 

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