V. Site-Selective Reactions

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The regioselectivity discussed thus far has involved reactions in which a compound with a single functional group generates products that include two or more structural isomers. The term regioselectivity also can be used to describe the preference for reaction of a particular atom or group in a molecule that contains at least one other atom or group of the same type. Regioselectivity of this sort is sometimes referred to as site selectivity. An example of a site-selective reaction is shown in Scheme 4, where H‑5 is abstracted even though there are other hydrogen atoms present in the molecule that potentially could have been abstracted.⁶

Although there are a large number of radical reactions in carbohydrate chemistry that involve group and atom replacement, only for hydrogen-atom abstraction is regioselectivity a common consideration. Nearly all carbohydrates have the hydrogen atoms necessary to make site-selective abstraction conceivable, but few carbohydrates have the two or more other groups or atoms required to make selective reaction of one of these groups (or atoms) a possibility. Hydrogen-atom abstraction, therefore, provides the pool from which most site-selective reactions are drawn.