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 Bu 3 Sn·] or a silicon-centered [usually (Me 3 Si) 3 Si·] 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 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