Some methyl 4,6-O-benzylidene monosaccharides can act as strong low molecular weight gelators for various organic solvents. As they are accessible in a variety of homologues, each with a unique molecular architecture, they can be used for systematic studies of gelation phenomena. Structural details of their hydrogen-bond-based fiber network in the gel phase can be resolved by small angle X-ray scattering (SAXS). Analysis of the molecular arrangement in a single crystal can be a valuable tool for the prediction of gelation ability presupposing that the elongated shape of the gel fibers arises from an anisotropic assembly of the gelator molecules into one-dimensional aggregates. It is found that some derivatives act as “supergelators”, which can gelate hydrocarbon solvents with 0.03–0.05 wt %. The recent results emerging from these investigations will be outlined in this article.

Sugar-integrated gelators of organic solvents.

Synthetic applications of the O-H insertion reactions of carbenes and carbenoids derived from diazocarbonyl and related diazo compounds

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor–acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with ...

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The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1–SN2 Interface

Relative stability of alternative chair forms and hydroxymethyl conformations of β-d-glucopyranose

Catalytic glycosylation has been a central reaction in carbohydrate chemistry since its introduction by Fischer 125 years ago, but it is only in the past three to four decades that catalytic methods for synthesizing oligosaccharides have appeared. Despite the development of numerous elegant and ingenious catalytic glycosylation methods, only a few are in general use. This review covers all methods of catalytic glycosylation with the focus on the development and application in oligosaccharide synthesis and provide an overview of the scope and limitations of these. The review also includes relevant mechanistic studies of catalytic glycosylations. The future of catalytic glycosylation chemistry is discussed, including specific, upcoming methods and possible directions for the field of research in general.

Catalytic Glycosylations in Oligosaccharide Synthesis

Glycosylidene Carbenes. Part 14. Glycosidation of partially protected galactopyranose-, glucopyranose-, and mannopyranose-derived vicinal diols

Glycosidation by the diazirine 1, the trichloroacetimidate 4, and the bromide 5 of the altro-diol 2, possessing an intramolecular H-bond (HOC(3) to OC(1)) in solution, but not in the solid state, proceeds with high and complementary regioselectivity. From 2 and 1, one obtains mostly the 1,2-linked disaccharides 10 and 11 (β-D > α-D), together with the 1,3-linked isomers 12 and 13 (α-D > β-D; 1,2-/1,3-linked products ca. 9:1), the demethylated 1,3-linked disaccharides 24–27, the trisaccharides 19–22, the lactone azines 23, and the hydroxyglucal 18, while 2 reacted with 4 or 5 to yield mostly the 1,3-linked disaccharides (1,2-/1,3-linked products ca. 1:9). The disaccharides were additionally characterized as acetates ( 14–17, 28–31). Yields and stereoselectivity depended upon the donor, stoichiometry, solvent, temperature, and concentration. Glycosidation of the 1,3-linked disaccharides with 1 yielded the trisaccharides 19–22. Reaction of the β-D-altro-diol 3 with 1 gave the 1,2- and 1,3-linked disaccharides 32/33 and 34/35 in a 1:1 ratio, characterized as the acetates 36–39, while glycosidation with 5 according to Lemieux proceeded regioselectively (1,2-/1,3-linked products 91:9). The monotosylates 6 and 7 reacted with 1 to yield the anomeric pairs 40/41, and 42/43 of the tosylated disaccharides; the oxiranes 44 and 45 were not observed.

Glycosylidene Carbenes. Part 10. Regioselective Glycosidation of 4,6‐O‐Benzylidene‐D‐altropyranosides

Hydrogen bonding of the triol 4 in chlorinated solvents was studied by IR (CH2Cl2 and CCl4) and 1H-NMR spectroscopy (CDCl3), and the regioselectivity of the glycosidation of the triol 4 by the diazirine 1 is predicted on the basis of two assumptions: preferred protonation of the intermediate glycosylidene carbene by the OH group involved in the weakest intramolecular H-bond, and attack in the π-plane of the thereby generated oxycarbenium cation either by the reoriented oxy anion, or by a properly oriented vicinal OH group. Glycosidation led to the disaccharides 5–10 (Scheme) which were separated and characterized as their acetates 11–16, to the lactone azines 17 and to the 2-(benzyloxy)glucal 18. In agreement with the predictions, glycosidation in non-coordinating solvents gave the 1,2-, 1,3-, and 1,4-linked disaccharides in decreasing relative amounts. Glycosidation in THF proceeded with a lower degree of regioselectivity and led preferentially to the β -D-anomers, except for the minor, 1,4-linked disaccharides, where THF had only a weak influence on stereoselectivity at room temperature and led to a slight increase of the α -D-anomer at −80°.

Glycosylidene Carbenes. Part 16. Glycosidation of methyl 6‐O‐trityl‐α‐D‐altropyranoside

Insertion of the glycosylidene carbenes, derived from the gluco- and the manno-diazirines 1 and 2, into the SnH bond of R3SnH (R = Bu or Ph) leads to the fully substituted stannoglycosides 3–8 (53–77%). The 1,2-cis-configurated products are formed preferentially (α-D/β -D ranges from 2.5:1 to 5.1:1 with 1, and 1:1.3 to 1:4.2 with 2). Relative to CH2Cl2, THF favors formation of the equatorial Sn-glycosides. The stannylated (benzyloxy)glucals 9 and 10 were isolated as side products. The reaction of 1 with (Bu3Sn)2 yielded 9 (17% in CH2Cl2; 36% in CCl4) together with the azines 11 and the benzyloxyglucal 12. NMR Data of the Sn-glycosides 3–8 show evidence for an anomeric effect, 1J(C(1),H) being larger in the axial and 1J(Sn,C(1)) larger in the equatorial anomers.

Glycosylidene Carbenes. Part 18. Insertion of glycosylidene carbenes into the SnH bond of tributyl- and triphenylstannane: A synthesis of stannoglycosides

Eva Bozó

Papers

Glycosylidene Carbenes. Part 14. Glycosidation of partially protected galactopyranose-, glucopyranose-, and mannopyranose-derived vicinal diols

Glycosylidene Carbenes. Part 10. Regioselective Glycosidation of 4,6‐O‐Benzylidene‐D‐altropyranosides

Glycosylidene Carbenes. Part 16. Glycosidation of methyl 6‐O‐trityl‐α‐D‐altropyranoside

Glycosylidene Carbenes. Part 18. Insertion of glycosylidene carbenes into the SnH bond of tributyl- and triphenylstannane: A synthesis of stannoglycosides

Glycosylidene Carbenes. Part 14. Glycosidation of Partially Protected Galactopyranose-, Glucopyranose-, and Mannopyranose-Derived Vicinal Diols.