Glycopyranosyl radicals

Orbital interactions considered by Koth et al. (1986a) to be responsible for the predilection of glycopyranosyl radicals to adopt conformations with the substituent at C(2) axial or isoclinal... 

When the glycopyranosyl radical adopts a B2 5 conformation then the preference for axial coupling products is readily understood on the basis of well-known steric interactions. Approach of a reagent from the P-face is significantly sterically hindered, particularly by the substituent at C(3), whereas attack from the a-face is less so. If ease of approach to position-2 of the norbornyl system [145] is taken as a model for ease of approach to C(l) of a pyranose ring in the B2 5 conformation, then the favoured approach to the carbohydrate corresponds to the favoured exo-approach to the norbornyl system. If the glycopyranosyl radical adopts the 4H3 conformation, then axial reaction is favoured by the least motion effects set out in Scheme 13. 

Figure 7.6 Rearrangement of 2-acyloxy and related glycopyranosyl radicals (a) tetra-O-acetylglucopyranosyl, (b) tetra-O-acetylmannopyranosyl, (c) a phosphatoxy migration note how the a-face of the sugar is (d) reverse migration where the anomeric centre is stabilised, (e) failure of migration of a 5-thio radical.

C-Glycopyranosides may be obtained from glycopyranosyl halides via intermolecular addition of glycopyranosyl radicals [129]. In a more useful example, the a-aminoacrylate 192 was used as the radical acceptor for preparation of C-glycosyl amino acids 193 and 194 [130] (Scheme 66). In a concise synthesis of showdomycin (197), Barton utilized the trigger reaction of the 7V-hydroxy-2-thiopyridone derivative and the exceptional radicophilicity of tellurides in concocting the conditions for the conversion from the anisyl telluride 195 to the intermediate 196 after oxidative elimination [131] (Scheme 67). In Keck s synthesis of (-t-)-pseudomonic acid C (201), the intermediate 200 was prepared via stereocontrolled intermolecular addition of the radical generated from the iodide 198 to the allylic sulfone 199 [132] (Scheme 68). 

C-Glycosides. Axial C-glycopyranosides arc obtained exclusively by photolysis of a mixture of an ot-D-glycopyranosyl bromide, acrylonitrile, and tributyltin hydride. The diastereoselectivity may involve an anomcric effect or rapid trapping of the initial radical before inversion can occur. 

Radical-initiated reduction of peracetylated glycopyranosyl bromides with tributyltin hydride has provided a route to the corresponding 1,5-anhydro-pentitols and -hexitols, including 2,3,4,6-tetra-0-acetyl-l,5-anhydro-D-iditol. °... 

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