Advances in the chemical synthesis of C-pyranosides/furanosides are summarized, covering the literature from 2000 to 2016. The majority of the methods take advantage of the construction of the glycosidic C—C bond. These C-glycosylation methods are categorized herein in terms of the glycosyl donor precursors, which are commonly used in O-glycoside synthesis and are easily accessible to nonspecialists. They include glycosyl halides, glycals, sugar acetates, sugar lactols, sugar lactones, 1,2-anhydro sugars, thioglycosides/sulfoxides/sulfones, selenoglycosides/telluroglycosides, methyl glycosides, and glycosyl imidates/phosphates. Mechanistically, C-glycosylation reactions can involve glycosyl electrophilic/cationic species, anionic species, radical species, or transition-metal complexes, which are discussed as subcategories under each type of sugar precursor. Moreover, intramolecular rearrangements, such as the Claisen rearrangement, Ramberg–Backlund rearrangement, and 1,2-Wittig rearrangement, which usuall...

Recent Advances in the Chemical Synthesis of C-Glycosides

: Catching Photons: Visible light, an amine reductant, and a Ru(bpy)32+ photocatalyst can be used to mediate the addition of glycosyl halides into alkenes to synthesize important C-glycosides. This method highlights the growing potential of photocatalysis to effectively drive useful and difficult chemical transformations.

Intermolecular Addition of Glycosyl Halides to Alkenes Mediated by Visible Light

In less than the last two decades a fundamentally new role of carbohydrates in living organisms1 has emerged: conjugates of oligosaccharides with proteins or lipids proved to be the major information carriers between cells and their surroundings.2-4 The need for these specific substances in quantities larger than isolable from biological sources in order to thoroughly understand their biological functions has brought about the renaissance of synthetic carbohydrate chemistry.5-8 A quest for compounds which can mimic the above natural substances by way of similar structure and/or biological action (the so-called mimetics) has also arisen and stimulated elaboration

Carbanionic reactivity of the anomeric center in carbohydrates.

The first total synthesis of a major component of the microbial biosurfactant sophorolipid has been achieved This approach to the 26-membered macrolide 1 containing a Z-configured alkene group in its lipidic tether spanning the sophorose backbone is based on a ring-closing metathesis reaction of diyne 21 catalyzed by Mo[N(t-Bu)(Ar)]3 (5; Ar = 3,5-dimethylphenyl) activated in situ by CH2Cl2, followed by Lindlar reduction of the resulting cycloalkyne 22 The two β-glycosidic linkages of compound 21 were installed by means of the glucal epoxide method and a modified Koenigs−Knorr reaction promoted by AgOTf/lutidine, respectively

Ring-closing alkyne metathesis. Application to the total synthesis of sophorolipid lactone.

The fundamental nature of Ti(III) complexes generated in tetrahydrofuran by reduction of Cp(2)TiCl(2) has been clarified by means of cyclic voltammetry and kinetic measurements. While the electrochemical reduction of Cp(2)TiCl(2) leads to the formation of Cp(2)TiCl(2)(-), the use of metals such as Zn, Al, or Mn as reductants affords Cp(2)TiCl and (Cp(2)TiCl)(2) in a mixture having a dimerization equilibrium constant of 3 x 10(3) M(-)(1), independent of the metal used. Thus, we find it unlikely that the trinuclear complexes or ionic clusters known from the solid phase should be present in solution as previously suggested. The standard potentials determined for the redox couples Cp(2)TiCl(2)/Cp(2)TiCl(2)(-), (Cp(2)TiCl)(2)(+)/(Cp(2)TiCl)(2), Cp(2)TiCl(+)/Cp(2)TiCl, and Cp(2)Ti(2+)/Cp(2)Ti(+) increase in the order listed. However, the reactivity of the different Ti(III) complexes is assessed as (Cp(2)TiCl)(2) greater, similar Cp(2)TiCl approximately Cp(2)Ti(+) >> Cp(2)TiCl(2)(-) in their reactions with benzyl chloride and benzaldehyde. None of the reactions proceed by an outer-sphere electron transfer pathway, and clearly the inner-sphere character is much higher in the case of Cp(2)Ti(+) than for (Cp(2)TiCl)(2), Cp(2)TiCl, and in particular Cp(2)TiCl(2)(-). As to the electron acceptor, the inner-sphere character increases, going from benzyl chloride to benzaldehyde, and it is suggested that the chlorine atom in benzyl chloride and the oxygen atom in benzaldehyde may function as bridges between the reactants in the transition state.

http://www.juelenemaerke.dk/rasmus/Download-Filer/A7.pdf

Revelation of the nature of the reducing species in titanocene halide-promoted reductions.

Titanium(III) Reagents in Carbohydrate Chemistry: Glycal and C-Glycoside Synthesis

Glycosyl chlorides and bromides can be rapidly converted to glycals in high yield by reaction with (Cp2Ti[III]Cl)2. This reagent tolerates a wide range of common carbohydrate protecting groups, including silyl ethers, acetals, and esters; the methodology provides a general route for the preparation of glycals substituted with both acid- and base-labile functionality. A reaction mechanism is proposed that is based on heteroatom abstraction to give an intermediate glycosyl radical. This radical reacts with a second equivalent of Ti(III) to yield a glycosyltitanium(IV) species. β-Heteroatom elimination from the glycosyltitanium(IV) complex gives the glycal.

Roxanne P. Spencer

Papers

Titanium(III) Reagents in Carbohydrate Chemistry: Glycal and C-Glycoside Synthesis

Rapid preparation of variously protected glycals using titanium(iii)

Titanium(III)-Promoted Stereoselective Synthesis of Simple C-Glycosides.

Variously substituted glycals are readily prepared from glycosyl bromides using (Cp2TiCl)2

Preparation of Aldehydo Sugars and Sugar Acids via Ozonolysis of Sugar Hydrazones.