Tropane and related alkaloid skeletons via a radical [3+3]-annulation process.

Tropanes and related bicyclic alkaloids are highly attractive compounds possessing a broad biological activity. Here we report a mild and simple protocol for the synthesis of N-arylated 8-azabicyclo[3.2.1]octane and 9-azabicyclo[3.3.1]nonane derivatives. It provides these valuable bicyclic alkaloid skeletons in good yields and high levels of diastereoselectivity from simple and readily available starting materials using visible-light photoredox catalysis. These bicyclic aniline derivatives are hardly accessible via the classical Robinson tropane synthesis and represent a particularly attractive scaffold for medicinal chemistry. This unprecedented annulation process takes advantage of the unique reactivity of ethyl 2-(acetoxymethyl)acrylate as a 1,3-bis radical acceptor and of cyclic N,N-dialkylanilines as radical 1,3-bis radical donors. The success of this process relies on efficient electron transfer processes and highly selective deprotonation of aminium radical cations leading to the key α-amino radical intermediates

Intermolecular Radical C–H Bond Activation: A Powerful Tool for Late Stage Functionalization

The synthesis of complex molecules via radical reactions involving carbon–carbon and carbon–heteroatom bonds has become a very successful approach. Radical chemistry has long been dominated by the use of tin-based reagents. Those strongly contributed to the development of the field, allowing one to achieve spectacular transformations, most of which being difficult or impossible to achieve under ionic conditions, and giving access to invaluable kinetics data that paved the way for the development of improved protocols and the design of new synthetic strategies. However, tin reagents and tin byproducts are often toxic and they proved to make purification steps sometimes tedious. In this context, tin-free methods have progressively gained in interest. This short review aims at providing the reader with alternative methods employing C–H bonds in place of the classical alkyl halides to generate, via an intermolecular hydrogen atom transfer (HAT), the radical species. Examples of carbon–carbon and carbon–heteroatom bond formation using this type of C–H bond activation approach will be provided, from early reports to the more recent developments

Preparation of 5-Membered Rings via 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 Conclusio

Easy Access to γ-Iodo-gem-Diborylated-Cyclopentanes and Bicyclic Cyclopropanes

A formal atom transfer radical [3+2] annulation (ATRAn) reaction between different homoallyl radical precursors and 1,1-diborylethene was developed. It provides a rapid access to polysubstituted cyclopentanes containing a gem-diboronic ester moiety. The synthetic utility of theses uniquely functionalized 5-membered rings is highlighted by their easy conversion to attractive borylated building blocks such as 1-borylated bicyclo[3.1.0]hexanes. The ATRAn reaction was extended to homopropagylic radicals giving access to unique allylic gem-diboronic esters that could be used in allylboration of aldehydes

Thiophenol-Mediated 1,5-Hydrogen Transfer for the Preparation of Pyrrolizidines, Indolizidines, and Related Compounds

1523-706

Radical Cyclization of Haloacetals: The Ueno-Stork Reaction

International audienceThe formation of the CC bond using radical cyclization of haloacetals (Ueno-Stork reaction) has proven to be an extremely efficient method to access g-lactones and related compounds. This reaction is also highly attractive for the regio-and stereoselective introduction of side chains to cyclic and acyclic allylic alcohols. It has been used as a key step in many natural product syntheses and has proven to be particularly efficient for the stereoselective generation of quaternary carbon centers. This review focuses on the different methods available to carry out this radical cyclization, as well as on the stereochemical aspect of the reaction and its applications in total synthesis

Thiols, thioethers, and related compounds as sources of C-centred radicals

Due to their stability, availability and reactivity, sulfides are particularly attractive sources of carbon-centered radicals. However, their reactivity in homolytic substitution processes is strongly reduced when compared with the corresponding selenides or halides. Despite this, sulfur-containing compounds can be engineered so that they become effective agents in radical chain reactions. A detailed description of the reactivity of organo-sulfur compounds is reported here with the aim of providing clear guidance on the scope and limitation of their use as radical precursors in chain reactions