III. Tellurides

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The organotellurium compounds that are used as radical precursors in carbohydrate chemistry usually are synthesized by a nucleophilic displacement reaction such as that pictured in eq 14.⁷⁹

II4(14).png

The majority of compounds prepared in this way are anomeric tellurides. Furanosyl tellurides are relatively unstable and tend to decompose within a few days,^80–82 but although their pyranosyl counterparts can exist unchanged in the solid state for months,⁷⁹ heating or exposing pyranosyl tellurides to UV light causes epimerization (eq 15).⁸³

II4(15).png

Two procedures, both of which involve photolysis, cause radical reaction of carbohydrate tellurides. The first of these is photochemical homolysis of a carbon–tellurium bond, a reaction that generates the more stable of the two possible, carbon-centered radicals (Scheme 11).

An example of reaction brought about in this way is found eq 16.⁸³ The second procedure for radical formation from a carbohydrate telluride calls for photochemical decomposition of N‑acetoxy-2-thiopyridone to produce a methyl radical that then reacts with the telluride (Scheme 12).

II4(16).png

Equation 17 describes a cyclization reaction initiated in this way.⁸⁴ Reactions of carbohydrate radicals formed from tellurides include cyclization,^84,85 addition,^86–89 reduction,⁸³ and group migration.⁸³ It is reasonable to assume that reaction of a carbohydrate telluride with a methyl radical involves, as molecular orbital calculations indicate, formation of an intermediate with a hypervalent tellurium atom (Scheme 13).⁹⁰

II4(17).png