Radical pyranosyl

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

These reactions are believed to involve pyranosyl radical intermediates 146 and 147128 (Fig. 7). 

Alternatively, pyranosyl radicals can be generated through the reduction of 3,4,6-tri-O-benzyl glucal epoxide with Cp2TiCl2 and manganese metal.136 With the conformationally restricted 1-phenylseleno-D-xylose derivatives 151 and 152 (4Ci conformation) their reaction with Bu3SnCH2 CH=CH2 in the presence of AIBN (Scheme 51) affords the corresponding a-C-pyranosyl derivatives (153) preferentially.137... 

According to ESR measurements, the intermediate glycosyl radical exists in the boat-like conformation [20], which was previously observed in some photoproducts of spirocyclization (Sect 2.2). This preferred conformation of the pyranosyl radical results from the stabilizing interaction of the single occupied p orbital with the a -Lumo of the adjacent p-C-OR bond (Scheme 6). 

It is clear that attempts to rationalise the axial stereochemistry of coupling reactions of pyranosyl radicals in terms of configurationally stable a-radicals are incorrect. The preferences for formation of axial C-glycosides by attack of radicals on olefins (Adlington et al., 1983 Albrecht and ScheflFold, 1985 ... 

Glycosyl radicals produced by reaction of various alkylated and acylated glycosyl derivatives (1-Br, -SePh, or -C(O)Bu ) with tln(II) derivatives have been examined by e.s.r. Preferred conformations were assigned and rationalized. Pyranosyl radicals were... 

The photochemical reaction of 2,3,4,6-tetra-0-acetyl-/3-D-gluco-pyranosyl sulfone (57) in benzene under nitrogen has been carefully studied, and a number of products identified116 (see Scheme 22). A mechanism that involves a photochemically initiated series of free-radical processes has been proposed that is consistent not only with product formation but also with the extent of incorporation of deuterium found in the various products following irradiation of 57 in benzene-d6. The mechanism shown in Scheme 22 is compatible with proposals offered to explain sulfone photochemistry in noncarbohy-... 

B. Giese, J. Dupuis, M. Leising, M. Nix, and H. J. Lindner, Synthesis of C-pento, -hexo, anc -heptulo-pyranosyl compounds via radical C-C bond-formation reaction, Carbohydr. Res 171 379 (1987). 

Another well-adapted method for removing the oxygen functionality at Cl uses a radical reduction of a protected pyranosyl halide (O Scheme 28) [211]. Formally, this is not a deoxygenation since the hydroxy group at Cl has already been replaced by a halide. However, gly-cosyl halides are easily available from aldoses by a one-pot procedure [212] and combined with the radical reduction this two-step route gives easy access to a number of 1,5-anhydroalditols. 1,4-Anhydroalditols, on the other hand, are more difficult to obtain by this method since the corresponding furanosyl halides require more steps for their preparation. The rad-... 

It should be noted that, under radical-generating conditions, a number of vicinal 1,2-substituents in cyclic sugar derivatives will tend to undergo elimination. This tendency has been demonstrated in a series of pyranosyl 1-phenylselenide-2-azides, which form corresponding glycals in yields close to quantitative upon treatment with tributyltin hydride [84]. 

Danishefsky formulated a strategy to staurosporine [41], in which epoxidation of glycal (-)-98 with maleimide 97 resulted in one of the indole N-glycosidic linkages to form 99. Treatment of olefin 99 using Barton deoxygenation, iodine and f-BuOK followed by radical dehalogenation provided the pyranosylated indolocarbazole 101 with 64% yield. Deprotection and methylation followed as shown in Scheme 14 (i.e., 101—102), after which reduction of imide 102 led to a 1 1 mixture of 1 and la. 

Olefins andAlkynes. The increased scope of the silicon tether in radical chemistry, which is still the most important reactivity allowed by the (bromomethyl)dimethylsilane moiety, has been reviewed. Some authors have again illustrated the synthetic interest of Nishiyama-Stork radical cyclizations of allylic (bromomethyl)dimethylsilyl ethers. Jenkins achieved stereoselective cyclizations on a fused cyclopentanol, while Herdewijn prepared pyranosyl nucleosides. Starting from /-substituted... 

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