Tellurium-mediated addition of carbohydrates to olefins

Carbohydrate anisyl tellurides are easily prepared by treatment of the corresponding mesylates or tosylates with the anisyl teUurolate anion. By irradiation of these tellurocarbohy-drates in the presence of M-acetoxythiopyridone and the electrophilic olefin, the tandem addnct is formed. The oxidative elimination of the thiopyridine moiety leads to the trans-olefms.  

Radical addition (general procedure)d To the tellurocarbohydrate (1 mmol) and the appropriate olefin (5 mmol) in dry CH2CI2 (4 mL) under argon at 5°C is added acetoxy-2-thiopy-ridone (0.5 mmol). Photolysis with a 150 W tungsten lamp for 10 min is followed by further addition of the reagent (0.25 mmol). This is repeated until all the carbohydrate disappears (TLC -1.5 mmol of the reagent is usually required). 

The synthesis of the antibiotic showdomycin, starting from a ribose derivative, is illus-trative.  

The intramolecular version of the described radical reaction provides an easy approach to the synthesis of six-membered carbocycles.  

In accordance with the following scheme the hydroxyaldehyde A was submitted to a Wittig olefmation followed by the mesylation of the hydroxyl group to give the a,)3-unsat-urated compound B which was in turn converted into the telluride C by treatment with the appropriate aryltellurolate. Irradiation of C in the presence of iV-acetoxy-2-thiopyridone gave rise to the cyclic compound D. 

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