Preparation of Five-Membered Rings via the Translocation-Cyclization of Vinyl Radicals

Vinyl radicals have a propensity to rearrange via intramolccular 1,5-hydrogen transfer to the more-stable alkyl radicals, which themselves possess a suitable structure to undergo rapid 5-exo-trig cyclizations. This tandem radical translocation-cyclization process represents a useful tool for the preparation of a wide range of five-membered rings. Moreover, this reaction is highly selective and, due to its radical nature, tolerates a variety of functional groups. During the last decade, tin-free procedures have emerged, rendering this process very attractive for the synthesis of natural products and their analogues. Selected examples of this strategy illustrating the scope of the method are described. 1 Introduction 2 Vinyl Radicals from Vinyl Halides 3 Vinyl Radicals Generated by Addition to Alkynes and Allenes 3.1 Addition of Carbon-Centered Radicals 3.1.1 Intermolecular Addition of Carbon-Centered Radicals 3.1.2 Intramolecular Addition of Carbon-Centered Radicals 3.2 Addition of Tin-Centered Radicals 3.3 Addition of Oxygen-Centered Radicals 3.4 Addition of Nitrogen-Centered Radicals 3.5 Addition of Sulfur-Centered Radicals 3.6 Addition of Phosphorus-Centered Radicals 4 Translocation-Cyclization Processes Involving a Stereoselective Hydrogen Atom Transfer 5 Conclusion