Halogenation substrate-controlled stereoselectivity

In the history of syntheses of halogen-containing molecules, an early approach to effect the stereoselective halogenation is via substrate control. This section will describe various substrate-controlled stereoselective halogenations found in natural product synthesis. 

Enzymatic reactions " and electrolysis have also been applied in the stereoselective halogenation but are limited to specific substrates. Generally, these approaches are based on the use of an electrophilic halogen source or an anionic halide nucleophile. For the approaches that use electrophilic halogen sources, the formation of a halonium intermediate is the key to stereochemical control. A subsequent nucleophilic attack of the halonium intermediate typically yields a product with an anti-relationship (Scheme 42.1). 

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