Deoxy-nitrosugars. 6th communication. Stereoelectronic control in the reductive denitration of tertiary nitro ethers. A synthesis of ?C-glycosides?

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

Probing the influence of protecting groups on the anomeric equilibrium in sialic acid glycosides with the persistent radical effect

A method for the investigation of the influence of protecting groups on the anomeric equilibrium in the sialic acid glycosides has been developed on the basis of the equilibration of O-sialyl hydroxylamines by reversible homolytic scission of the glycosidic bond following the dictates of the Fischer-Ingold persistent radical effect. It is found that a trans-fused 4O,5N-oxazolidinone group stabilizes the equatorial glycoside, i.e., reduces the anomeric effect, when compared to the 4O,5N-diacetyl protected systems. This effect is discussed in terms of the powerful electron-withdrawing nature of the oxazolidinone system, which in turn is a function of its strong dipole moment in the mean plane of the pyranose ring system. The new equilibration method displays a small solvent effect and is most pronounced in less polar media consistent with the anomeric effect in general. The unusual (for anomeric radicals) poor kinetic selectivity of anomeric sialyl radicals is discussed in terms of the planar π-type structure of these radicals and of competing 1,3-diaxial interactions in the diastereomeric transition states for trapping on the α- and β-faces of the radical.

Dissecting the influence of oxazolidinones and cyclic carbonates in sialic acid chemistry

At a moment's notice: Thermal equilibration of 1 and mass spectral analysis of sialyl phosphates suggest that the 4O,5N-oxazolidinone and the 4,5-O-carbonate systems influence the anomeric effect and the mechanisms of sialidation by virtue of their dipole moment in the mean plane of the pyranose ring. The electron-withdrawing effect destabilizes 2 and promotes associative glycosylation mechanisms. TEMPO = 2,2,6,6-tetramethylpiperidine N-oxide.

Fe(III)/NaBH4-mediated free radical hydrofluorination of unactivated alkenes

A powerful Fe(III)/NaBH(4)-mediated free radical hydrofluorination of unactivated alkenes is disclosed using Selectfluor reagent as a source of fluorine and resulting in exclusive Markovnikov addition. In contrast to the traditional and unmanageable free radical hydrofluorination of alkenes, the Fe(III)/NaBH(4)-mediated reaction is conducted under exceptionally mild reaction conditions (0 °C, 5 min, CH(3)CN/H(2)O). The reaction can be conducted open to the air and with water as a cosolvent and demonstrates an outstanding substrate scope and functional group tolerance.

Iron(III)/NaBH4-mediated additions to unactivated alkenes: synthesis of novel 20'-vinblastine analogues

An Fe(III)/NaBH(4)-mediated reaction for the functionalization of unactivated alkenes is described defining the alkene substrate scope, establishing the exclusive Markovnikov addition, exploring a range of free radical traps, examining the Fe(III) salt and initiating hydride source, introducing H(2)O-cosolvent mixtures, and exploring catalytic variants. Its use led to the preparation of a novel, potent, and previously inaccessible C20'-vinblastine analogue.

Methodology development and physical organic chemistry: a powerful combination for the advancement of glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.

C-Glycoside Clustering on Calix[4]arene, Adamantane, and Benzene Scaffolds through 1,2,3-Triazole Linkers

A route has been paved toward the preparation of triazole glycocluster libraries via the copper(I)-catalyzed modern version of the classical Huisgen 1,3-dipolar cycloaddition of azides to alkynes. Up to four 1,4-disubstituted 1,2,3-triazole rings bearing carbon-linked glycosyl fragments were constructed on various scaffolds via multiple cycloadditions of suitably polyfunctionalized calix[4]arene, adamantane, and benzene derivatives with ethynyl and azidomethyl C-glycosides. Each cycloaddition occurred with high regioselectivity to give exclusively the 1,4-disubstituted triazole ring in very high yield up to an average value of 98%. The high degree of efficiency of this approach and its wide scope constitute a simple and practical means for the attachment of various sugar units to polyfunctionalized substrates.

New Constituents from the Heartwood of Picea morrisonicola HAYATA

Three new constituents, including a p-menthane type monoterpene, trans-p-menthane-7,8,9-triol (1), an aromatic, 2,6-dihydroxy-3,4-dimethylbenzoic acid methyl ester (2), and a lignan, (+)-morrisonicolanin (3), were isolated from the low polar layer of heartwood extracts of Picea morrisonicola, and their structures were elucidated on the basis of spectroscopic analysis. Of these compounds identified, 1 was obtained as its diacetylated derivative 1a, and 2 was the chemical entity first isolated from a natural source.

Transformation of D-glucose into 1D-3-deoxy-3-hydroxymethyl-myo-inositol by stereocontrolled intramolecular Henry reaction

[reaction: see text] An intramolecular Henry cyclization provides a promising new strategy for the transformation of nitroheptofuranoses into deoxyhydroxymethylinositols. This method has allowed a stereospecific transformation of d-glucose into 1D-3-deoxy-3-hydroxymethyl-myo-inositol.

meso-Tetrahydropyranylperoxides: molecular structures in solution, in the crystal, and by DFT calculations and their isomerization to the racemate

The crystalline peroxide 3a is the main product (out of 10 theoretically possible) from the aerial peroxidation of all-cis-2,4,6-trimethyltetrahydropyran (2a). It has a similar structure both in solution and in the crystal as shown by nuclear Overhauser effects and X-ray analysis, respectively. Theoretical calculations at a density functional theory level (B3LYP/6-31G) provide insight into the stabilities of the different stereoisomers of this peroxide, accounting for the facile, acid-catalyzed isomerization from the meso form to the racemate. Peroxide 3b, which is the 2-tert-butyl analogue of 3a, out of 22 theoretically possible isomers, crystallizes in a similar meso form. As a result of crystal packing effects and the intrinsically (axial) chiral peroxy "chromophore" that deviates slightly from the antiperiplanar conformation, both enantiomorphic forms of 3b are encountered in the lattice.

The total synthesis of C-glycosides with completely resolved seven-carbon backbone polyol stereochemistry: stereochemical correlations and access to L-configured and other rare carbohydrates

The de novo synthesis of a full set of hydroxymethyl C-glycosides from only two precursors is described. The seven-carbon target molecules contain five stereocentres and bridge the stereochemical gap between natural D-configured and non-natural L-configured series of hexoses. Key steps include hydroxylation, differential protection, stereoselective reduction and desymmetrization of 8-oxabicyclo[3.2.1]oct-6-enes. C-Terminus differentiation and C-terminus excision of the seven-carbon polyol backbone lead to hexoses, including those of the L-series. A stereochemical and genetic classification of C-glycosides is presented.