Stereoselective synthesis and structure elucidation of spiro-ketodisaccharides

Synthesis of D-manno-heptulose via a cascade aldol/hemiketalization reaction

A [4 + 3] synthesis of D-manno-heptulose is described. The cascade aldol/hemiketalization reaction of a C₄ aldehyde with a C₃ ketone provides the differentially protected ketoheptose building block, which can be further reacted to furnish target D-manno-heptulose.

Chemical and enzymatic approaches to carbohydrate-derived spiroketals: di-D-fructose dianhydrides (DFAs)

Di-D-fructose dianhydrides (DFAs) comprise a unique family of stereoisomeric spiro-tricyclic disaccharides formed upon thermal and/or acidic activation of sucrose- and/ or D-fructose-rich materials. The recent discovery of the presence of DFAs in food products and their remarkable nutritional features has attracted considerable interest from the food industry. DFAs behave as low-caloric sweeteners and have proven to exert beneficial prebiotic nutritional functions, favouring the growth of Bifidobacterium spp. In the era of functional foods, investigation of the beneficial properties of DFAs has become an important issue. However, the complexity of the DFA mixtures formed during caramelization or roasting of carbohydrates by traditional procedures (up to 14 diastereomeric spiroketal cores) makes evaluation of their individual properties a difficult challenge. Great effort has gone into the development of efficient procedures to obtain DFAs in pure form at laboratory and industrial scale. This paper is devoted to review the recent advances in the stereoselective synthesis of DFAs by means of chemical and enzymatic approaches, their scope, limitations, and complementarities.

Synthesis of ketopyranosyl glycosides and determination of their anomeric configuration on the basis of the three-bond carbon–proton couplings

Anomeric pairs of ketopyranosyl glycosides with various substituents at C(alpha), C(beta) and C(gamma) were synthesized from the corresponding thioglycosides, and the influence of the C(alpha)-C(beta)-C(gamma)-H(gamma) torsion angle and substituent effects on the three-bond carbon-proton couplings was studied. The cis coupling constants range from 1 to 2Hz. The trans couplings are generally as small as 2.3-2.6Hz; however, for compounds bearing an unsubstituted gamma-carbon, a relatively large trans coupling was measured (4.8Hz). An S-ethyl group at the beta-position increases the cis coupling (up to 3.2Hz) compared to the corresponding O-glycosides.

Inter- and intramolecular glycosylation of exo-glycals promoted by metallic Lewis acids

exo-Glycosyl carbonates were employed for inter- and intramolecular glycosylation reactions. A number of metallic Lewis Acids and solvents were examined to enhance the reactivity. The optimum conditions were found to be the use of AlCl3 in 1-nitropropane. The method was demonstrated to be useful for the intermolecular glycosyl transfer of several nucleophiles, including simple alcohols, sugars, and amino acid derivatives; however, intramolecular glycosylations were not successful.

The o-xylylene protecting group as an element of conformational control of remote stereochemistry in the synthesis of spiroketals

Protection of trans-1,2-diol segments as cyclic o-xylylene ethers strongly favours diequatorial over diaxial dispositions; the possibility of using this grouping for remote control of the stereochemistry in the synthesis of spiroketals is here demonstrated by the stereoselective synthesis of tricyclic spirodisaccharides (di-D-fructose dianhydrides).

Stereoselective glycosylation of exo-glycals accelerated by Ferrier-type rearrangement

[reaction: see text] Owing to the driving force of the Ferrier-type rearrangement, the exo-glycals are highly reactive with various alcohols to afford glycosides and glycoconjugates with exclusive alpha-configuration. The resulting vinyl group in these glycosylation products can be further elaborated for general applications, including the synthesis of spiro derivatives and glycosylation of 2-ketoaldonic acids.