Telluroglycosides as source of glycosyl radicals

Telluroglycosides generate glycosyl radicals by homolitic C-Te bond cleavage promoted by photolysis with a UV lamp, or by thermolysis at 140°C in the dark.  

A rapid equilibration of the j3-isomer into the a-isomer takes place by a unimolecular radical mechanism, giving a 83 17 a, -mixture. The same mixture is formed by irradiation of the pure a-isomer. 

These results reveal that the a-isomer is thermodynamically more stable than the )3-iso-mer. The radicals so generated are trapped with TEMPO and diaryl diselenides. The addition of alkynes gives rise to vinylic tellurides. 

In the last case, the a-isomers are the main products and the reactivity is practically insensitive to the nature of the substituents of the alkynes. 

Reaction of telluroglycosides with alkynes (typical procedure) A mixture of the teUuro-glycoside (R=Ac) and phenylacetylene (2.55 g, 25.0 nunol) in a sealed Pyrex tube was irradiated with a UV lamp (Rayonet RMR-600 equipped with RMR-3500 A lamp (4.5 W X 8)) at 120°C for 20 h. Purification of the crude mixture by silica gel chromatography afforded the addition product (R=Ph) in 93% yield (3.02 g, 4.65 mmol) as a 47 29 13 11 mixture of the a-E, a-Z, jS-E and j8-Z isomers. The reaction in the dark at 120°C also gave the adduct in good yield. By the addition of 10 mol% of AIBN, the reaction proceeded under milder conditions (80°C). 

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