Reactions of Glucosides and Nucleosides

The nucleosidation mechanism of five-membered glycals promoted by /V-iodosuc-cinimide, to give 2 -deoxy-2 -iodo-/i-nuclcosidcs, has been investigated by semiem-pirical methods.11 

In glucosides, hemiacetal hydroxyl activation/substitution can be achieved using a sulfonic anhydride and a nucleophile, plus a base as acid scavenger.12 The reaction is catalysed by dibutyl sulfoxide (Bu2S=0), and shows evidence of sulfur-covalent catalysis. Using benzenesulfonic anhydride [(PhS0)20], it is proposed to involve initial formation of a sulfonium sulfonate (6), the S(IV) centre of which then reacts with 

New DISAL (methyl 3,5-dinitrasa/icylate) glycosyl donors have been prepared and (g) used to carry out /I-selective glycosylations under neutral conditions.13 

Diazeniumdiolate anions as leaving groups at the anomeric position of carbohydrates can act as prodrugs, releasing NO upon hydrolysis by a glycosidase.18 

A number of fundamental studies of the nature of the anomeric effect have been undertaken, probed via kinetics and exo-/endo-regioselectivities. 

The roles of nucleophilic assistance and stereoelectronic control in determining endo-versus exo-cyclic cleavage of pyranoside acetals have been investigated for a series of a- and j8-anomers.15 Exocyclic cleavage of a-anomers, via a cyclic oxocarbenium ion, is predicted by the theory of stereoelectronic control, and was found exclusively for the cases studied. The endocyclic route, with an acyclic ion, is predicted for the /1-structures, and a measurable amount was found in all cases, but its extent was dependent on temperature, solvent, and the nature of the aglycone group. 

The mechanistic role of nucleotides in directing the growth of IR-emitting semiconductor nanocrystals has been investigated for a range of nucleotides, concentrations, stoichiometries, and temperatures.11 

Recent advances in transition-metal-catalysed glycosylations have been reviewed. Plausible transition states for such reactions have been discussed and primary C isotope effects have been determined as a guide to the mechanism of formation of a-manno- and gluco-pyranosides. The influence of protecting groups on the reactivity and selectivity of glycosylation chemistry of 4,6-(9-benzylidene-protected mannopyranosyl donors and related species has been reviewed.  

A commentary on diastereoselectivity in chemical glycosylation reactions has dismissed molecular orbital explanations that invoke stereoelectronic effects analogous to the anomeric effect in kinetically controlled reactions.  

High diastereoselectivity, giving a- and -C-glycosides, respectively, has been reported for reaction of C-nucleophiles with 2-6 -benzyM,6-0-benzylidene-protected 3-deoxy gluco- and manno-pyranoside donors. This does not parallel the preferential formation of -0-glycosides on reaction with alcohols, for which nucleophilic attack by Ojp3 on oxocarbenium ions should be less sterically hindered than for attack by a typical carbon nucleophile.  

4-0-di-t-butylsilylene group induces strict -controlled glycuronylations, without classical neighbouring group participation, by hindering approach of ROH to intermediate oxocarbenium ion.  

A kinetic study of acid hydrolysis of methyl a- and -o-glucopyranosides has revealed direct participation by the counterion (Br or Cl ), which becomes more pronounced as the proportion of 1,4-dioxane is increased.  

Thioglycosides are not subject to acid-catalysed cleavage by glycosyl hydrolases this effect, which allows them to act as inhibitors, is generally ascribed to their lower basicity. However, calculations on conformational changes in the model compounds 

Substituent effects on the endocyclic cleavage of glycosides by trimethylaluminium have been explained in terms of a cyclic C-H O hydrogen-bonded intermediate.  

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The 2,5-anhydro-3,4-diamino-pentose diethyl acetal 72 was synthesized from L-xylose by sequential reaction of epoxide and triflate intermediates with azide ion. After reduction to a 3,4-diamino-2,5-anhydroalditol, oxoruthenium(V) complexes of it and related 2,3-diamino-nucleosides (see Chapter 17) were prepared and evaluated as ribonuclease inhibitors. A number of routes were investigated for the preparation of the 2-amino-3-azido-2,3-dideoxy-D-glucoside 73, a precursor for a mimetic of the cyclopdepsipeptide didemnin B, from 2-acetamido-2-deoxy-D-glucose. Because A -deacetylation was much easier in the presence of a free 3-hydroxy-group, the best route involved preparation of a 2-deoxy-2-trifluoroacetamido-D-allopyranoside, and displacement of a mesylate group from C-3 by azide ion with inversion. ... 

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