Anomeric nitro sugars

Anomeric nitro sugars [35] are most efficiently prepared by ozonolysis of the N-glycosylnitrones obtained by reaction of aromatic aldehydes with the corresponding sugar oximes via the tautomeric N-glycosylhydroxylamines [36] 

136 relies on a hydroboration-oxidation sequence [29] based on vinyl stannane 

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Thus, treatment of 11 with paraformaldehyde and TBAF in THF provided the novel branched-chain nitro sugar 54. Reduction of its nitro group to the amino group of 55 followed by deprotection of its anomeric position, resulted in the amine 56, that finally led to compound 57, the first reported branched-chain seven-membered imino sugar, after a synthetic sequence consisting of a reductive amination. 

In order to minimize such possible side-reactions as anomerization of glycosides and acetolysis of acetal structures, alternative techniques of acetylation have been studied. Whereas basic catalysis leads to severe complications in certain nitro sugar derivatives (see Section IV,2a and c pp. 124, 129), boron trifluoride etherate has proved to be an excellent agent for satisfactory acetylation of glycosides, O-benzyl-idenated glycosides, and disaccharides that contain a deoxynitro function.148,149... 

According to the latter approach, reducing sugars 128 were reacted with triethylammonium dimethyl boranophosphate in the presence of bis(2-oxo-3-oxazolidinyl)phosphinic chloride as condensing reagent, 3-nitro- 1,2,4-triazole as nucleophilic catalyst and A,A-diisopropyl-A-ethyl-amine to provide boranophosphate triester derivatives 129 as anomeric... 

The nitro group, as one of the many functional groups with a strong electron-withdrawing effect, plays an extremely important role in the reactivity of the sugar molecule no matter where it is located. The anomeric nitrates introduced by Vasella [53] are particularly useful reactive synthons for introducing additional functionalities via formation of the C-C bond by Henry aldol condensation [54] followed by a radical denitration. 

Aebischer et al. [85] reported in 1983 results of their studies on the anomeric effect of nitro group. They studied the anomeric effect of nitro group in two sugars 1-deoxy-l-nitro-tetra-O-benzyl-D-mannopyranose 106 and 3,4,6-tri-(9-benzyl-l,2-dideoxy-l-nitro-D-arabino-hexopyranose 108 (Fig. 2.35). 

Remarkable selectivity for an attack from the a-side was observed [87a] when the anomeric /7-nitro-benzoate of evernitrose 138 was reacted with the sugar alcohol... 

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