Anomeric proton stereochemistry

The stereochemistry of 3-C-nitro glycals has been studied in some detail [210]. For example, when nitroanhydroglucitol 106 was subjected to reaction with triethylamine, it gave the elimination product 107, which was then rearranged to an equilibrated mixture of glycals 108 and 109 (O Scheme 36) [211,212]. Some researchers have tried to explain this equilibrium shift by arguing that the quasi-equatorial anomeric proton is made more acidic by the stereoelectronic effect [213]. 

In the case of 3-amino-l,2,6-thiadiazine 1,1-dioxide 131, glycosylation was achieved using a mixture of potassium nonaflate, hexamethyldisilylazide, and trimethylsilyl chloride to obtain analogs 132 and 133 (Equation 13) <1995H(41)87>. Assessment of the relative stereochemistry was based on the H NMR shifts and multiplicity of the anomeric protons that appear as a doublet in the /ra r-isomer about 0.3 ppm downfield from the doublet of doublets for the rw-isomer. 

Whereas the anomeric protons of 2 -deoxypurine nucleosides 4 have long been known to appear as triplets in H-n.m.r. spectra recorded in polar solvents, it has only now been found that they resonate as doublets of doublets in non-polar solvents, due to intramolecular H-bonding between 0-5 and N-3. Careful attention to solvent polarity in interpreting such splitting patterns has therefore been recommended appearance of H-1 as doublets of doublets has previously been associated with a-stereochemistry. ... 

Under strongly acidic conditions, glycals add alcohols. The acid of choice is triphenylphosphine hydrobromide. The stereochemistry at Cl appears to be dictated by the position of anomeric equilibrium, with axial products predominating, except in the case of tribenzylallal, where they would be disfavoured by 1,3-diaxial interactions. Running the reaction in deuterated solvents gave a mixture of axial and equatorial deuterium at C2, with between 5 1 and 2 -l predominance of equatorial protonation, except for dibenzyl-3-deoxyglucal. ... 

In this regard, the authors probed different substituents and stereochemistries, mainly in position 5, 6, and 9 of the pyranose core of 242-(Z), to explore the role of the stereoelectronic interactions [102-105], conformational restrictions [106-108], and formation of intramolecular hydrogen bonds in the stereocontrol of this reaction. For the anomeric effect, see [109]. For the anomeric effect of protons, see [110], and for the formation of intramolecular hydrogen bonds [111] in the stereocontrol of this reaction. 

Franck [12] has shown that the major axial stereochemistry at the anomeric center is not due to a trans addition. No strong bridging of the proton can be really expected and the oxycarbenium ion 11 should be the reactive intermediate (Scheme 2). The use of deuterated alcohols ROD with catalytic amounts of Ph3P HBr showed that deuteron delivery occurs largely from below the plane of the double bond. The predominant axial stereochemistry at C-1 and C-1 of the glycoside products 12 and 15 probably arises from the kinetic anomeric effect. 

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