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9: Chemoselectivity

Chemoselectivity is a term that describes the ability of a reagent or inter­mediate to react with one group or atom in a mole­cule in preference to another group or atom present in the same molecule. Since most carbohydrate radicals trace their begin­nings to reactions involving either a tin-centered [usually Bu3Sn·] or a silicon-centered [usually (Me3Si)3Si·] radical, the chemo­selec­tiv­ity in reactions of these radicals plays a central role in carbo­hydrate radical formation. An example of a chemoselective reaction is found in Scheme 1, where the tri-n-butyltin radical ab­stracts the O-thiocarbonyl group from the thioglycoside 1 while the potentially reactive ethylthio group remains in place.1

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Chemoselective reaction also can occur when a carbohydrate radical reacts with another mole­cule present in the reaction mixture. Such a process is shown in Scheme 2, where the pyran­os-1-yl radical 2 adds to the C–C double bond in 3 rather that reacting with the chlorine atom or phenylseleno group also present in this molecule (3).2

s2.png

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