Stereospecific 1,2-ethyl(phenyl)thio group migration in sugars: stereocontrolled synthesis of precursors to α- and β-2-deoxyglycosides

Scope and limitations of 2-deoxy- and 2,6-dideoxyglycosyl bromides as donors for the synthesis of β-2-deoxy- and β-2,6-dideoxyglycosides

It is shown that 2-deoxy- and 2,6-dideoxyglycosyl bromides can be prepared in high yield (72-94%) and engaged in glycosylation reactions with β:α selectivities ≥6:1. Yields of product are 44-90%. Fully armed 2-deoxyglycoside donors are viable, while 2,6-dideoxyglycosides require one electron-withdrawing substituent for high efficiency and β-selectivity. Equatorial C-3 ester protecting groups decrease β-selectivity, and donors bearing an axial C-3 substituent are not suitable. The method is compatible with azide-containing donors and acid-sensitive functional groups.

2-nitroglycals as powerful glycosyl donors: application in the synthesis of biologically important molecules

[Reaction: see text]. The biological significance of oligosaccharides and glycoconjugates is profound and wide-ranging. For example, the mucins have attracted attention because of their role in fundamental cellular processes such as fertilization, parasitic infection, inflammation, immune defense, cell growth, and cell-cell adhesion. Increased expression of mucins is implicated in malignant transformation of cells. Antifreeze glycoproteins also are of interest because they are important for the survival of many marine teleost fishes that live in polar and subpolar waters. The synthesis of glycoconjugates requires methods for glycoside bond formation, the most difficult aspect of which is the assembly of monosaccharide building blocks. This Account discusses a valuable addition to the repertoire of methods for glycoconjugate synthesis: an approach that involves 2-nitroglycal concatenation. For a long time, methods for glycosylation via glycosyl donor generation required either an anomeric oxygen exchange reaction or anomeric oxygen retention. In the case of an anomeric oxygen exchange reaction, activation of the glycosyl donors demands a promoter in at least equimolar amounts. However, anomeric oxygen retention, such as base-catalyzed formation of O-glycosyl trichloroacetimidates, can be activated by catalytic amounts of acid or Lewis acid. Alternatively, glycals, which are readily available from sugars, can be an attractive starting material for glycoside bond formation. Their nucleophilic character at C-2 permits reactions with oxygen, nitrogen, and sulfur electrophiles that under high substrate stereocontrol generally lead to three-membered rings; ring opening under acid catalysis furnishes the corresponding glycosides, whichdepending on the electrophile Xare also employed for 2-deoxyglycoside synthesis. Glycals also can be transformed into derivatives that have at C-2 an electron-withdrawing group and are amenable to Michael-type addition. A good example are 2-nitroglycals. In this case, glycoside bond formation is achieved under base catalysis and leads to 2-deoxy-2-nitroglycosides. These intermediates are readily converted into 2-amino-2-deoxyglycosides, which are constituents of almost all glycoconjugates. This 2-nitroglycal concatenation has been extensively investigated with 2-nitrogalactal derivatives. When alcohols are used as nucleophiles and strong bases used as catalysts, the result is primarily or exclusively the alpha-galacto-configured adducts. Some studies show that weaker bases may lead to preferential formation of the beta-galacto-configured products instead. The reaction was very successfully extended to other nucleophiles and also to other 2-nitroglycals that undergo base-catalyzed stereoselective Michael-type additions. Thus, 2-nitroglycals are versatile synthons in glycoconjugate and natural-products synthesis, and it is foreseeable that many more applications will be based on these readily available and highly functionalized skeletons.

Stereoselective synthesis of 2-S-ethyl(phenyl)-2-thio-beta-glucopyranosides via 1,2-migration and concurrent glycosidation of ethyl(phenyl) 2,3-orthoester-1-thio-alpha-mannopyranosides

1,2-Migration and concurrent glycosidation of ethyl(phenyl) 2,3-orthoester-1-thio-alpha-D- and L-mannopyranosides under the action of TMSOTf readily afforded the corresponding 2-S-ethyl(phenyl)-2-thio-beta-glucopyranosides, ready precursors to 2-deoxy-arabino-hexopyranosides (2-deoxy-beta-glucopyranosides).

Stereoselective synthesis of 2-S-phenyl-2-deoxy-beta-glycosides using phenyl 2,3-O-thionocarbonyl-1-thioglycoside donors via 1,2-Migration and concurrent glycosidation

[figure: see text] 1,2-Migration and concurrent glycosidation of phenyl 2,3-O-thionocarbonyl-1-thlo-alpha-L-rhamnopyranosides under the action of methyl trifluoromethanesulfonate (MeOTf) afforded in high yields the 3-O-(methylthio)carbonyl-2-S-phenyl-2,6-dideoxy-beta-L-glucopyranosides, ready precursors to the corresponding 2-deoxy-beta-glycosides.

Synthesis of deoxyfluoro sugars from carbohydrate precursors

Results obtained over the past decade concerning the introduction of the fluorine atom into carbohydrate molecules, either by nucleophilic substitution or electrophilic addition reactions, are summarised. The first section mainly deals with the triflate/fluoride tandem sequence and the DAST-reaction. In the discussion, emphasis is given to the dependency of the reaction course on the stereochemical and protecting group features. Possible reaction pathways are direct substitution (with inversion or retention of configuration), rearrangement (combined with substitution and inversion of configuration at both of the centres involved) and elimination. Based on the assumption of cyclic transition states or transient intermediates (formed through participation of neighbouring groups), far-reaching mechanistic generalisations were made. On this basis, isolated examples from the literature, which are not in accordance with these generalisations, are specifically brought to attention. Results from the recently introduced reaction of safe and easy to handle N-F fluorinating agents with glycals are also reported. This approach allows the simple and stereoselective access to a series of 2-deoxy-2-fluoro aldopyranoses, as well as the synthesis of various C-1-substituted derivatives by an easy one-pot reaction. However, the same method applied to furanoid glycals is rather poor with respect to stereoselectivity. Finally, considerations on the importance of fluorine-specific reactions of the S(N)-type in related fields of organic synthesis are made.