I. Introduction

Halogen-atom abstraction to give a carbon-centered radical is the first step in many radical re­ac­tions. Among the more common of these are simple reduc­tion, radical addition, and radical cyc­li­zation. A typical example is give in Scheme 1, where chlorine-atom abstraction produces a pyran­os-1-yl radical that then adds to the carbon–carbon double bond in allyltri-n-butyltin.¹ Group mi­gra­tion, ring-opening, elim­in­ation, hydrogen-atom abstrac­tion, and radical combination are less com­mon reac­tions that also often begin with halogen-atom abstraction that forms a carbohydrate radical.

Another way of viewing reac­tions that begin with halogen-atom abstraction is one taken from the perspective of the involvement of such reactions in carbo­hy­drate synthesis. This view describes these reactions in terms of the changes in carbo­hy­drate structure that they make possible; thus, halogen-atom abstraction is involved in synthesizing deoxygenated sugars and nucleo­sides, establishing glycosidic link­ages, extending carbon-atom chains, forming new ring sys­tems, intro­ducing unsat­ur­ation, and modifying pro­tecting group reactivity. This broad range of appli­ca­tions com­bines with generally good product yields to make halogen-atom abstraction the first step in many, useful synthetic reactions.

In addition to the variety of reactions that begin with loss of a halogen atom, there are others that reverse this process; that is, there are reactions that cause a halogen atom to be incor­porated into a carbohydrate. The range of synthetically useful compounds prepared in this way is limited. Radical halogenation is not part of a general synthesis of carbo­hydrate halides, but it some­times provides a route to compounds that are diffi­cult to prepare in other ways. The special­ized, but useful, nature of radical halo­genation is illustrated by the conversion of a 4,6-O-ben­zyl­idene acetal into the corresponding bromodeoxy benzoate (eq 1).²