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Several types of Quantum Dots (QDs) (CdS, CdSe and InP, as well as core-shell QDs such as type I InP-ZnS, quasi type-II CdSe-CdS and inverse type-I CdS-CdSe) were considered for generating α-aminoalkyl free radicals. The feasibility of the oxidation of the N-aryl amines and the generation of the desired radical was evidenced experimentally by quenching of the photoluminescence of the QDs and by testing a vinylation reaction using an alkenylsulfone radical trap. The QDs were tested in a radical [3+3]-annulation reaction giving access to tropane skeletons and that requires the completion of two consecutive catalytic cycles. Several QDs such as CdS core, CdSe core and inverted type I CdS-CdSe core-shell proved to be efficient photocatalysts for this reaction. Interestingly, the addition of a second shorter chain ligand to the QDs appeared to be essential to complete the second catalytic cycle and to obtain the desired bicyclic tropane derivatives. Finally, the scope of the [3+3]-annulation reaction was explored for the best performing QDs and isolated yields that compares well with classical iridium photocatalysis were obtained.

The monoalkylation of N-methoxypyridinium salts with alkyl radicals generated from alkenes (via hydroboration with catecholborane), alkyl iodides (via iodine atom transfer) and xanthates is reported. The reaction proceeds under neutral conditions since no acid is needed to activate the heterocycle and no external oxidant is required. A rate constant for the addition of a primary radical to N-methoxylepidinium >107 M−1 s−1 was experimentally determined. This rate constant is more than one order of magnitude larger than the one measured for the addition of primary alkyl radicals to protonated lepidine demonstrating the remarkable reactivity of methoxypyridinium salts towards radicals. The reaction has been used for the preparation of unique pyridinylated terpenoids and was extended to a three-component carbopyridinylation of electron-rich alkenes including enol esters, enol ethers and enamides., N-Methoxypyridinium salts are exceptionally reactive radical traps that can be used in efficient radical chain reactions with organoboranes.

Due to the wide spectrum of biological activities of steroids, the detection and quantification of steroidal residues in various biological materials are crucial for drug development, doping prevention, and environmental protection. In addition, the analytical technique of stable isotopic dilution (SID) by Liquid Chromatography-Mass Spectrometry (LC-MS) requires 13C-labeled steroids as standards to provide accurate and reproducible steroid quantification. In this context, the synthesis of 13C-labeled steroids is reviewed. The approaches based on partial synthesis starting from commercially available steroids have been, by far, the most commonly employed strategy.1 Introduction2 Hemisynthesis of 13C3-Labeled Steroids via Partial Degradation of the A Ring2.1 Degradation of the A Ring to an Enol Lactone2.1.1 Introduction of 13C Atom(s) via Claisen Condensation: Turner’s Strategy2.1.2 Introduction of 13C Atom(s) with 13C-Labeled 5-(Diethylphosphono)pentan-2-one Ethylene Ketal Based on the Fujimoto–Belleau Reaction2.2 Degradation of the A Ring and Introduction of 13C Atom(s) with [13C3]-1-(Triphenylphosphoranylidene)propan-2-one3 Construction of the A Ring from an α,β-Unsaturated Ketone with 13C-Labeled 1-Iodo-3,3-(ethylenedioxy)butane as Electrophile4 Construction of the A and B rings: Stork’s Strategy5 Hemisynthesis with Introduction of 13C Atom(s) in the C17 Side Chain6 Conclusion

In this contribution, we describe two simple and efficient routes for the preparation of keto-aldehyde 1, a key intermediate for the synthesis of

We developed a new approach to γ-lactols and methylene-γ-lactols based upon the radical cyclization of aluminium acetals obtained by reduction of α-bromoesters with DIBAL-H. The cyclic aluminium acetals resulting from the cyclization process could engage in situ in further functionalization, as illustrated by the Oppenauer-type oxidation to give the corresponding lactones and γ-butenolides. The preparation of butenolides using this strategy compared favourably with the direct, tin-mediated cyclization of α-bromoesters, for which side reactions such as epimerization via [1,5]-HAT processes have been observed.

The rapid and regioselective synthesis of a series of 2,6-disubstituted dihydropyranic building-blocks bearing an oxygenated side chain is described. The corresponding 4-iodo tetrahydropyran precursors, easily prepared by Prins cyclization, underwent regioselective elimination in the presence of an In(OAc)(3)/LiI system to provide the title compounds. The one-pot Prins cyclization-elimination process was also studied and could be achieved with the TMSBr/Lil/In(OAc)(3) system. (C) 2015 Elsevier Ltd. All rights reserved.

An overview of the synthetic methods allowing the preparation of heterocyclic structures based upon the use of heteroatom-centred radicals (R–X•) is presented here. The discussion will be limited to specific examples for which the heteroatom (X = SiR2, GeR2, SnR2, PR, NR, O, S, SO2, Se) is included in the heterocyclic structure itself during the cyclisation process leading to its formation. These include the cyclisation of heteroatom-centred radicals onto unsaturated partners such as alkenes, alkynes, arenes, ketones or nitriles and the formation of the heterocycle via a sequence involving remote functionalisation reactions leading to halogenated intermediates, followed by ionic cyclisation (e.g. Hofmann–Loffler–Freytag reaction), as well as cascade reactions terminating with an intramolecular homolytic substitution (SHi) at the heteroatom in the final step.

International audience; In this contribution, we describe two simple and efficient routes for the preparation of keto-aldehyde 1, a key intermediate for the synthesis of (13)C3-androstanes and (13)C3-estranes. In the first route, the targeted aldehyde 1 was obtained in 40% overall yield from 1,4-androstadien-3,17-dione (3 mmol scale) via a two-step sequence involving a one-pot, abnormal ozonolysis/sulfur oxidation/retro-Michael/ozonolysis process. Alternatively, a second route from 4-androsten-3,17-dione, using a six-step sequence, was optimized to produce 40 mmol batches of the key intermediate 1 in 42% overall yield. At the final stage, the A-ring was reconstructed through a Wittig reaction with the 1-triphenylphosphoranylidene-(13)C3-2-propanone 2, followed by an intramolecular condensation assisted by thioacetic acid via a Michael addition/retro-Michael reaction sequence to provide 2,3,4-(13)C3-1,4-androstadien-3,17-dione.

A new synthetic approach to thiolactones that employs an efficient acyl thiol–ene (ATE) or acyl thiol–yne (ATY) cyclization to convert unsaturated thiocarboxylic acid derivatives into thiolactones under very mild conditions is described. The high overall yields, fast kinetics, high diastereoselectivity, excellent regiocontrol, and broad substrate scope of these reaction processes render this a very useful approach for diversity-oriented synthesis and drug discovery efforts. A detailed computational rationale is provided for the observed regiocontrol.

Aluminum acetals are easily obtained from esters and lactones by reduction with aluminum hydrides. These thermally unstable tetrahedral intermediates have found applications in organic synthesis, with various methodologies taking advantage both of the stabilities of these aluminum species at low temperatures and of their tendencies to undergo rearrangement into the corresponding aldehydes. Efficient one-pot transformations can be achieved from aluminum acetals under ionic and radical conditions. A comprehensive overview of the reactivity of these species, including the most recent advances in this field, is presented in this microreview.

An efficient and simple radical chain reaction to convert terminal alkynes into arenesulfonylmethylcyclopentanes is described. The reaction involves a radical addition-translocation-cyclization process and necessitates solely the use of readily available arenesulfonyl chlorides in tetrahydrofuran. Interestingly, this radical-mediated C-H activation process took place with a high level of retention of configuration when an enantiomerically pure starting material was used.

A thiophenol-mediated method for the conversion of propargylamines to pyrrolidines under acidic conditions is described. This cascade reaction involves addition of a thiyl radical to the terminal alkyne followed by a 1,5-hydrogen transfer (radical translocation) and a rapid cyclization affording the pyrrolidine ring. Our studies reveal that complete protonation of the tertiary amine with 10 equivalents of trifluoroacetic acid avoids undesired hydrogen atom abstractions by the thiyl radicals.

The rapid and regioselective synthesis of a series of 2,6-disubstituted dihydropyranic building-blocks bearing an oxygenated side chain is described. The corresponding 4-iodo tetrahydropyran precursors, easily prepared by Prins cyclization, underwent regioselective elimination in the presence of an In(OAc)3/LiI system to provide the title compounds. The one-pot Prins cyclization-elimination process was also studied and could be achieved with the TMSBr/LiI/In(OAc)3 system.

Cognac in the jar! An efficient one-pot sequence for the preparation of γ-lactones is described. Following reduction of α-bromo ester precursors with DIBAL-H and radical cyclization of the resulting O-aluminum acetals, a preparative in-situ Oppenauer-type oxidation of the cyclic O-aluminum acetal using simple aldehydes or ketones gives access to γ-lactones in high yields.

A practical method for the formation of thiophosphonates bearing functionalized monocyclic, fused bicyclic and spirocyclic residues is presented. The procedure requires the easily available terminal alkynes as starting materials as well as commercially and readily available reagents such as diethyl thiophosphite. The experimental procedure consists of a one-pot process without any slow addition of one of the reagents.

An efficient preparation of gamma-lactols and methylene-gamma-lactols is described. Highly acid-sensitive lactols are prepared in a concise manner by using a radical cyclization of aluminum acetals. The precursors for the radical reactions are readily prepared from allyl or propargyl alcohols and alpha-bromo acids. Functionalization of the resulting gamma-lactols and methylene-gamma-lactols can be achieved following isolation, leading to synthetically useful building blocks, such as 1,4-diols, 1,4-dienes, gamma-lactones, and polysubstituted tetrahydrofurans.

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

The effects of memory of chirality (MoC) in reactions involving monoradical species are reviewed here. Reactions involving a nonracemic chiral starting material bearing a single stereogenic element such as a chiral center or chiral axis directly involved in the new bond formation are discussed. These reactions lead to a nonracemic product via an intermediate susceptible to rapid racemization. Memory of chirality has been observed in cyclic radicals, aryl, ester/amide substituted acyclic radicals, and benzylic radicals at temperatures up to 130 °C.

An original and efficient stereocontrolled synthesis of ribonucleosidic homo- and heterodimers has been achieved from inexpensive d-xylose. This successful strategy involved the sequential introduction of nucleobases, using two stereocontrolled N-glycosidation reactions, from a common two-furanoside amide-linked scaffold offering the possibility of obtaining any given base sequence. The pertinence of this approach is illustrated through the preparation of the homodimers UU-34 and TT-35 in 18 steps with an excellent overall yield of more than 10% from d-xylose, while the heterodimer route led to UT-39 in 19 steps with around 10% overall yield.

A four-step one-pot procedure involving generation of α-bromo aluminum acetals, their subsequent radical cyclization followed by oxidation of the resulting cyclic aluminum acetals and base promoted double bond migration is developed to convert α-bromoesters derived from propargylic alcohols to butenolides.

International audience; Polysubstituted butenolides were obtained in good to high yields from α-bromoesters derived from propargyl alcohols by a one-pot reaction involving the radical cyclization of α-bromo aluminium acetals, followed by the oxidation of the resulting cyclic aluminium acetals in an Oppenauer-type process and migration of the exocyclic CC bond into the α,β-position. Comparison with the direct cyclization of α-bromoesters at high temperature and under high dilution conditions is described. Deuterium-labelling experiments allowed us to uncover “invisible” 1,5-hydrogen atom transfers (1,5-HATs) that occur during these cyclization processes, together with the consequences of the latter in the epimerization of stereogenic centres. Compared to the classical approach, the cyclization of aluminium acetals proved to be highly chemoselective and its efficiency was illustrated by the short total syntheses of optically enriched γ-butenolides isolated from Plagiomnium undulatum and from Kyrtuhrix maculans.

A thiophenol-mediated method for cyclization of alkynes is described. The reaction cascade involves the intermolecular addition of a phenylthiyl radical to a terminal triple bond generating an alkenyl radical, followed by a 1,5-hydrogen atom transfer and a 5-exo-trig radical cyclization. This very efficient tin-free procedure allows one to prepare highly functionalized cyclopentane derivatives as well as fused bicyclic and spirocyclic compounds from easily available precursors. During this cyclization process, a phenylthio moiety is incorporated into the final cyclized products. This functionalization is particularly attractive for further transformation of the products.

International audience; The 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.

Aluminum acetals are easily obtained from esters and lactones by reduction with aluminum hydrides. These thermally unstable tetrahedral intermediates have found applications in organic synthesis, with various methodologies taking advantage both of the stabilities of these aluminum species at low temperatures and of their tendencies to undergo rearrangement into the corresponding aldehydes. Efficient one-pot transformations can be achieved from aluminum acetals under ionic and radical conditions. A comprehensive overview of the reactivity of these species, including the most recent advances in this field, is presented in this microreview.

The review covers the uses of thiyl radicals in organic synthesis and provides mechanistic information to understand these processes and to design new synthetic applications. It describes the main elementary reactions involving thiyl radicals and discusses the factors directing the rates and selectivity of these processes. The reaction of sulfur-containing molecules with another radical is a preeminent source of thiyl radicals. Thiols can rapidly transfer a hydrogen atom to most types of radical X having a corresponding higher X-H BDE. All the commonly used initiators such as azo-compounds or peroxides that generate alkyl or alkoxyl radicals are efficient to initiate thiol-mediated radical transformations.

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.

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.

In continuation of recently published studies on the cyclization of in situ generated α-bromo aluminum acetals, an efficient protocol for preparation of γ-lactones is presented.

Under Lewis acid activation, the new α-hydroxy-spiro epoxide scaffold 1a underwent an original tandem Payne/Meinwald rearrangement affording the cyclopentyl hydroxymethylketone 6 in a stereospecific manner, while a Meinwald-type epoxide rearrangement occurred when the derived α-trimethylsilyloxy-spiro epoxide 2a was treated with MABR, yielding stereoselectively the cyclohexane carbaldehyde 9.

A practical method for the formation of thiophosphonates bearing functionalized monocyclic, fused bicyclic and spirocyclic residues is presented. The procedure requires the easily available terminal alkynes as starting materials as well as commercially and readily available reagents such as diethyl thiophosphite. The experimental procedure consists of a one-pot process without any slow addition of one of the reagents.

A new access to γ-lactols and 4-methylene-γ-lactols, based on a one-pot sequence, involves the radical cyclization of thermally labile α-bromo aluminum acetals.

The efficient preparation of 1-azabicyclic alkanes is described. Highly functionalized skeletons are prepared in a concise manner using a radical tin-free 1,5-hydrogen transfer−cyclization process. The precursors for the radical reactions are readily assembled either from pyrrolidine/piperidine/hexahydro-1H-azepine or via condensation of a properly designed N-alkylimine with an allenylzinc species.

The stereochemical outcome of intramolecular radical mediated hydrogen transfer (= radical translocation) is discussed. Low to excellent levels of stereocontrol are observed making such processes attractive for applications in target-oriented synthesis.

The efficient preparation of 1-azabicyclic alkanes is described. Highly functionalized skeletons are prepared in a concise manner using a radical tin-free 1,5-hydrogen transfer−cyclization process. The precursors for the radical reactions are readily assembled either from pyrrolidine/piperidine/hexahydro-1H-azepine or via condensation of a properly designed N-alkylimine with an allenylzinc species.

[reaction: see text] A highly stereoselective synthesis of (-)-erythrodiene starting from 4-isopropylcyclohexanone is described. The key reactions are an asymmetric methoxycarbonylation of the starting ketone and a highly diastereoselective radical cascade involving addition of a phenylthiyl radical to a terminal alkyne followed by a 1,5-hydrogen transfer and a 5-exo-cyclization.

The formation of the C-C bond using radical cyclization of haloacetals (Ueno-Stork reaction) has proven to be an extremely efficient method to access γ-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.

[reaction: see text] An efficient procedure for running a cascade reaction involving 1,5-abstraction of a hydrogen atom followed by a radical cyclization is reported. Alkenyl radicals are generated from easily available terminal alkynes and thiophenol. This procedure eliminates the need of using the toxic tributyltin hydride and gives a greater amount of radical translocation products.