Jacques Dupuis

Anomeric effect of radicals

Abstract The glucosyl radical 1 is attacked predominately at the axial position. This can be explained by an anomeric effect that stabilizes σ-radical 5.

Synthesis of C-pento, -hexo-, and -heptulo-pyranosyl compounds via radical CC bond-formation reactions

Abstract C -Glycosyl compounds were synthesized by addition of pyranosyl radicals to acrylonitrile, fumarodinitrile, or methacrylonitrile in the presence of tributyltin hydride in 34–75% yields. The pyranosyl radicals were generated by abstraction of a bromine atome or a nitro group by tributyltin radicals. The stereoselectivity of the CC bond to the pyranosyl radicals was, in most cases, remarkably high, but hexopyranosyl and pentopyranosyl radicals showed different selectivity. Whereas the tetra- O -acetylglucosyl radical reacted with acrylonitrile preponderantly to give a C -glycosyl compound having the newly formed CC bond in an axial position, the tri- O -acetylxylosyl radical gave a C -glycosyl compound having an equatorial CC bond. In the tetra- O -acetylmannosyl radical, the axial acetoxy substituent, adjacent to the radical center, led exclusively to a compound having a trans CC bond, whereas the tri- O -acetyllyxosyl radical showed trans - and cis -addition to acrylonitrile in a 7:3 ratio.

The influence of alkoxy groups on the nucleophilicity of alkyl radicals

Abstract One alkoxy substituent increases the nucleophilicity of alkyl radicals in addition reactions at alkenes more than three methyl groups.

Electron spin resonance spectroscopic investigation of carbohydrate radicals. Part 2. Conformation and configuration in pyranos-1-yl radicals

Carbohydrate free radicals wer regiospecifically generated at C-1 of acylated and alkylated pyranosyl derivatives in non-aqueous solution and their conformations were deduced from the hyperfine splittings of their e.s.r. spectra. The preferred conformations of the pyranosyl radicals are discussed in terms of a stabilizing interaction of the singly occupied p-orbital with the σ*-LUMO of the adjacent β-C–OR bond. From the α-13C coupling constant of the tetra-acetylglucosyl radical it is concluded that pyranosyl radicals are of π-type.

Orbital Effects in Carbohydrate Radicals

In cyclic six membered radicals equatorial substituents adjacent to the radical center induce equatorial attack and axial substituents induce axial attack of alkenes if the radical adopt a chair conformation. This is not the case with glucosyl radicals 19 and 24, which change to a boat conformation, so that the s-C-O bond is axial. This conformational change is favored by an interaction between the SOMO of the alkoxyalkyl radical and the LUMO of the adjacent C-0 bond. Equatorial attack at the boat conformation leads to an axial substituted product. This stereoelectronic effect explains also the rel. rates of attack, the easy rearrangement of adjacent acyloxy groups and the rel. rate of chlorine abstraction from glycosyl chlorides. With very flexible systems the conformation of the intermediate, detected by ESR, may change on the way to the transition state.