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

Abstract

Vinyl radicals have a propensity to rearrange via intramolecular1,5-hydrogen transfer to the more-stable alkyl radicals, which themselvespossess a suitable structure to undergo rapid 5- EXO-trigcyclizations. This tandem radical translocation-cyclizationprocess represents a useful tool for the preparation of a wide rangeof five-membered rings. Moreover, this reaction is highly selectiveand, due to its radical nature, tolerates a variety of functional groups.During the last decade, tin-free procedures have emerged, renderingthis process very attractive for the synthesis of natural productsand their analogues. Selected examples of this strategy illustratingthe scope of the method are described. 1 Introduction 2 Vinyl Radicals from Vinyl Halides 3 Vinyl Radicals Generated by Addition to Alkynes and [nl]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 Stereo-selectiveHydrogen Atom Transfer 5 Conclusion