Carbon-Nitrogen Multiple Bond Radical Acceptors

Carbon-Nitrogen Multiple Bond Radical Acceptors... 

Radical cyclizations to carbon-nitrogen multiple bonds resemble additions to carbon-carbon multiple bonds in that they usually give products of irreversible exo cyclization. To date, the most useful acceptors have been oximes189 and nitriles,190 and one example of each type of cyclization is given in Scheme 45.191 Nitriles are useful because the intermediate imines are readily hydrolyzed by mild acid to ketones. Although this route to ketones is shorter than the two-step sequence of alkyne cyclization/ozo-nolysis, nitriles are slightly poorer acceptors than terminal alkynes, and much poorer acceptors than activated alkynes. Thus, when slow cyclizations are involved, the two-step protocol is preferable. 

Carbon-nitrogen multiple bonds serve as intramolecular radical acceptors for oxacycle synthesis. Many oxacyclic natural products are synthesized via carbocycle-forming radical reactions of oxacyclic substrates. Oxygen-centered radicals play an important role in oxacycle synthesis, which frequently involves initial hydrogen abstraction and subsequent displacement reactions. Many different kinds of inter-molecular radical reactions are also of considerable practical value in the synthesis of a plethora of oxacyclic natural products. 

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