Regioselective Synthesis of γ-Lactones by Iron-Catalyzed Radical Annulation of Alkenes with α-Halocarboxylic Acids and Their Derivatives

Configurationally labile α-bromoacid derivatives for asymmetric preparation of heterocycles

α-Bromoacid derivatives are configurationally labile under various conditions, and the dynamic resolution of them has been recognized as an effective strategy in asymmetric synthesis. This article is a concise review of our efforts on the heteroannulation of α-bromoacid derivatives through nucleophilic substitution and subsequent ring formation with diverse carbon, nitrogen, oxygen, and sulfur nucleophiles. Chiral auxiliary (Xc)-bound α-bromoacid derivatives serve as versatile chiral building blocks for the direct incorporation of a two-carbon unit in cyclization reactions. Eight readily available chiral auxiliaries are investigated and identified to be suitable for the dynamic resolution of α-bromoacid derivatives. We have presented selected results about three distinct dynamic resolutions such as dynamic kinetic resolution (DKR), dynamic thermodynamic resolution (DTR) and crystallization induced dynamic resolution (CIDR) that have been successfully employed in the asymmetric synthesis of fourteen different scaffolds of six- or five-membered heterocycles.

Phenothiazine Sulfoxides as Active Photocatalysts for the Synthesis of γ-Lactones

N-substituted phenothiazines are prominent and highly effective organic photoredox catalysts, particularly known for their strong reducing capabilities. Despite their wide utility, the closely related phenothiazine sulfoxides, which easily form upon oxidation, have been largely overlooked and have not been explored in the context of photocatalysis. Herein, we describe the discovery and application of N-phenylphenothiazine sulfoxide as a photocatalyst for the reductive activation of cyclic malonyl peroxides, giving access to complex γ-lactones starting from simple olefins. Detailed mechanistic studies were carried out to better understand the in situ formation of the active catalyst species from a commercial precursor, as well as the catalyst species interconversion and the photocatalytic mechanism for the formation of γ-lactone products. Specifically, we employed a broad range of mechanistic tools, including time-resolved spectroscopy, spectroelectrochemistry, transient UV-vis absorption spectroscopy, cyclic voltammetry, isotopic labeling, radical trapping experiments, NMR spectroscopy, and density functional theory (DFT) calculations. The synthetic utility of this protocol is demonstrated in a substrate scope study, highlighting the facile access to complex spirocyclic γ-lactones, which are widely recognized for their biological importance.

Copper-Catalyzed Dearomative [3 + 2] Annulation of Indoles with 2-Iodoacetic Acid

An efficient copper-catalyzed dearomative [3 + 2] annulation of indoles with 2-iodoacetic acid is developed. By employing Cu(OTf)2/2,2'-bis(2-oxazoline) as the catalyst and LPO as the oxidant, a series of indoline-fused butyrolactones were synthesized in moderate to good yields. The reaction features mild conditions, a broad substrate scope, and readily available starting materials. Furthermore, synthetic transformations of the products were conducted to demonstrate the practical utility of this reaction.

α-Halocarbonyls as a Valuable Functionalized Tertiary Alkyl Source

This review introduces the synthetic organic chemical value of α-bromocarbonyl compounds with tertiary carbons. This α-bromocarbonyl compound with a tertiary carbon has been used primarily only as a radical initiator in atom transfer radical polymerization (ATRP) reactions. However, with the recent development of photo-radical reactions (around 2010), research on the use of α-bromocarbonyl compounds as tertiary alkyl radical precursors became popular (around 2012). As more examples were reported, α-bromocarbonyl compounds were studied not only as radicals but also for their applications in organometallic and ionic reactions. That is, α-bromocarbonyl compounds act as nucleophiles as well as electrophiles. The carbonyl group of α-bromocarbonyl compounds is also attractive because it allows the skeleton to be converted after the reaction, and it is being applied to total synthesis. In our survey until 2022, α-bromocarbonyl compounds can be used to perform a full range of reactions necessary for organic synthesis, including multi-component reactions, cross-coupling, substitution, cyclization, rearrangement, stereospecific reactions, asymmetric reactions. α-Bromocarbonyl compounds have created a new trend in tertiary alkylation, which until then had limited reaction patterns in organic synthesis. This review focuses on how α-bromocarbonyl compounds can be used in synthetic organic chemistry.

Divergent functionalization of alkenes enabled by photoredox activation of CDFA and α-halo carboxylic acids

Herein we present our studies on the solvent-controlled difunctionalization of alkenes utilizing chlorodifluoroacetic acid (CDFA) and α-halo carboxylic acids for the synthesis of γ-lactones, γ-lactams and α,α-difluoroesters. Mechanistic insights revealed that photocatalytic reductive mesolytic cleavage of the C-X bond delivers elusive α-carboxyl alkyl radicals. In the presence of an olefin molecule, this species acts as a unique bifunctional intermediate allowing for stipulated formation of C-O, C-N and C-H bonds on Giese-type adducts via single electron transfer (SET) or hydrogen atom transfer (HAT) events. These protocols exhibit great efficiency across a broad spectrum of readily available α-halo carboxylic acids and are amenable to scalability in both batch and flow. To demonstrate the versatility of this concept, the synthesis of (±)-boivinianin A, its fluorinated analog and eupomatilone-6 natural products was successfully accomplished.

Copper-Catalyzed Radical Enantioselective Carbo-Esterification of Styrenes Enabled by a Perfluoroalkylated-PyBox Ligand

Chiral lactones are found in many natural products. The reaction of simple alkenes with iodoacetic acid is a powerful method to build lactones, but the enantioselective version of this reaction has not been implemented to date. Herein, we report the efficient catalytic radical enantioselective carbo-esterification of styrenes enabled by a newly developed Cu^I -perfluoroalkylated PyBox system. Simple styrenes have been converted to useful chiral lactones, whose synthetic applications are showcased. Mechanistic studies reveal that this reaction is a rare example of an efficient ligand-decelerated system, in which the ligand decelerates the reaction, but the reaction is still efficient with reduced amounts of ligand. This uncommon catalytic system may inspire further consideration of the effect of ligands in asymmetric catalysis.

An access to highly enantioenriched cis-3,5-disubstituted γ-lactones from α-bromoacetate and silyl enol ether

A novel synthetic strategy for highly enantioenriched cis-3,5-disubstituted γ-lactones has been developed by the AgOTf-promoted nucleophilic substitution of α-bromoacetates with silyl enol ethers and subsequent reductive lactonization. The utility of this synthetic method was further demonstrated through the concise stereodivergent synthesis of cis- and trans-2,4-disubstituted tetrahydrofurans.

Iron-catalysed radical cyclization to synthesize germanium-substituted indolo[2,1-a]isoquinolin-6(5H)-ones and indolin-2-ones

A simple and efficient strategy for iron-catalysed cascade radical cyclization was developed, by which an array of germanium-substituted indolo[2,1-a]isoquinolin-6(5H)-ones and indolin-2-ones were obtained in one pot with germanium hydrides as radical precursors. A rapid intramolecular radical trapping mode enabled the selective arylgermylation of alkenes over the prevalent hydrogermylation reaction.

Iron-Catalyzed Radical Annulation of Unsaturated Carboxylic Acids with Disulfides for the Synthesis of γ-Lactones

An efficient aerobic iron-catalyzed annulation of unsaturated carboxylic acids with disulfides has been developed. This procedure proceeds using FeCl₃ as the catalyst and KI as an iodine source under an air atmosphere, which provides practical access to a wide range of substituted γ-lactone derivatives. The disclosed method is quite simple, highly atom-economic, environmentally friendly, and tolerates a broad substrate scope.

Cobalt-catalyzed annulation of styrenes with α-bromoacetic acids

We report a method for addition of α-bromophenylacetic acids to vinyl C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds in styrenes to afford γ-lactones. Reactions employed a simple cobalt catalyst Co(NO₃)₂·6H₂O in the presence of dipivaloylmethane (dpm) ligand. Many functionalities including halogen, ester, and nitro groups were compatible with reaction conditions. If α-bromoesters were used, vinylacetates were the major products.

Cobalt-catalyzed addition of α-bromoacetic acids/acetates to CC bonds in styrenes is reported for the first time. Good tolerance of functional groups was observed.

Iron catalyzed β-C(sp2)–H alkylation of enamides

An attractive and cheap alternative approach was developed for the β-C(sp2)–H (fluoro)alkylation of a range of cyclic and acyclic non-aromatic enamides using either FeCl2 as a catalyst or a stoichiometric amount of nontoxic iron powder.

Synthesis of 2-Isoxazoline N-Oxides by Copper-Mediated Radical Annulation of Alkenes with α-Nitrobenzyl Bromides

Copper-mediated [3 + 2] annulation of alkenes with α-nitrobenzyl bromides has been developed. The reaction is promoted simply by a copper salt to produce the corresponding 2-isoxazoline N-oxides with perfect regioselectivity. The present method can be conducted under mild conditions, affording a diverse array of 2-isoxazoline N-oxides. The obtained products can readily be converted to the related heterocycles such as 2-isoxazoline and isoxazole. A radical-polar crossover pathway initiated by single-electron transfer from nitronate to a copper salt is proposed.

Synthesis of Oxygen-Containing Heterocyclic Compounds by Iron-Catalyzed Alkylative Cyclization of Unsaturated Carboxylic Acids and Alcohols

Iron-catalyzed alkylative cyclization of alkenes bearing oxygen nucleophiles with secondary and tertiary alkyl bromides through carbon-carbon and carbon-oxygen bond formations has been developed. A broad substrate scope is an attractive feature of this synthetic method, providing a variety of potentially bioactive five- and six-membered oxygen-containing heterocycles. The reaction pathway is proposed to involve a radical addition of the in situ-formed alkyl radical to an alkene followed by carbon-oxygen bond-forming intramolecular cyclization.

Radical Strategy for the Transition-Metal-Catalyzed Synthesis of γ-Lactones: A Review

The γ-lactone skeleton is very important component of various natural products, biological molecules, food additives, and perfumes. As a result, much effort has been made towards such compounds. In this review, we summarize recent progress in transition-metal-catalyzed annulation reactions for the formation of γ-lactone derivatives through a radical pathway. Various reagents, such as anhydrides, Togni’s reagent, TMSN3, arenesulfonyl chlorides, arenediazonium salts, dibenzoyl peroxides, O-benzoylhydroxylamine, NFSI, and α-halocarboxylic compounds, used in radical cyclization reactions are described, and the mechanisms of these radical annulation reactions are also discussed.

1 Introduction

2 Annulations of Alkenes with Anhydrides

3 Annulations of Unsaturated Carboxylic Acids with Nucleophiles

4 Annulations of Alkenes with α-Halocarboxylic Compounds

5 Conclusions and Outlook

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

β-C(sp2)–H alkylation of enamides using xanthate chemistry

Access to the γ-amino-β,γ-unsaturated acyl scaffold was established by applying xanthate chemistry to enamides.

trans-Diastereoselective Syntheses of γ-Lactones by Visible Light-Iodine-Mediated Carboesterification of Alkenes

This study aims to develop an intermolecular lactonization reaction of alkenes with carbonyls mediated by visible light and molecular iodine. The one-step reaction involved the carboesterification of alkenes to produce the corresponding lactones in moderate to good yield. It was also revealed that it is possible to control the diastereoselectivity of the reaction by altering the base used and the reaction conditions. When water was added as a solvent, the reaction resulted in the formation of lactones with trans-selectivity. A mechanistic investigation was undertaken and it was found that the reaction requires the generation of an iodine radical from molecular iodine, driven by visible light irradiation, and proceeds via the formation of an iodine radical alkene adduct. The proposed reaction is an example of a rare-metal free intermolecular addition cyclization reaction, which is an environment-friendly chemical process that only uses molecular iodine. In addition, since diastereoselectivity was observed without the use of any specific reagents, the developed methodology is an example of a novel stereoselective transformation using only cost-effective reagents.

Silver-catalyzed oxidative 1,2-alkyletherification of unactivated alkenes with α-bromoalkyl carbonyls: facile access to highly substituted 2,3-dihydrofurans

A silver-catalysed C–Br oxidative functionalization/annulative oxygenation process for producing valuable quaternary-carbon-possessing 2,3-dihydrofuran is presented.