Transition-Metal-Free Oxyfluorination of Olefinic Amides for the Synthesis of Fluorinated Heterocycles

Electrochemical oxidative cyclization of N-allylamides for the synthesis of CF3-containing benzoxazines and oxazolines

The introduction of trifluoromethyl (-CF3) groups into compounds is a common synthetic strategy in organic chemistry. Commonly used methods for introducing trifluoromethyl groups are limited by harsh reaction conditions, low regioselectivity, or the need for excess reagents. In this study, a facile electrochemical oxidative and radical cascade cyclization of N-(2-vinylphenyl)amides for the synthesis of CF3-containing benzoxazines and oxazolines was obtained. This sustainable protocol features inexpensive and durable electrodes, a wide range of substrates, diverse functional group compatibility under transition-metal-free, external-oxidant-free, and additive-free conditions, and can be applied in an open environment.

Iridium-Catalyzed Intramolecular Asymmetric Allylation of Vinyl Benzoxazinones for the Synthesis of Chiral 4H-3,1-Benzoxazines via Kinetic Resolution

Chiral benzoxazinones and 4H-3,1-benzoxazines as important motifs are widely found in abundant pharmaceuticals and biological molecules. We herein successfully developed the first kinetic resolution (KR) process of racemic benzoxazinones through Ir-catalyzed asymmetric intramolecular allylation, furnishing a wide range of chiral benzoxazinones and 4H-3,1-benzoxazines with excellent results via outstanding KR performances (with the s factor up to 170). This protocol exhibited broad substrate scope generality and good functional group tolerance, and the chiral 4H-3,1-benzoxazine products could be readily transformed to other useful optically active heterocycles.

Aerial Oxygen-Driven Selenocyclization of O-Vinylanilides Mediated by Coupled Fe3+/Fe2+ and I2/I- Redox Cycles

In the past decade, selenocyclization has been extensively exploited for the preparation of a wide range of selenylated heterocycles with versatile activities. Previously, selenium electrophile-based and FeCl3-promoted methods were employed for the synthesis of selenylated benzoxazines. However, these methods are limited by starting material availability and low atomic economy, respectively. Inspired by the recent catalytic selenocyclization approaches based on distinctive pathways, we rationally constructed an efficient and greener double-redox catalytic system for the access to diverse selenylated benzoxazines. The coupling of I2/I- and Fe3+/Fe2+ catalytic redox cycles enables aerial O2 to act as the driving force to promote the selenocyclization. Control and test redox experiments confirmed the roles of each component in the catalytic system, and a PhSeI-based pathway is proposed for the selenocyclization process.

BF3·OEt2-mediated nucleophilic fluorocyclization of sulfonyl 3-methylene-oxabenzocyclooctan-6-ones. Diastereocontrolled synthesis of benzofused fluorooxabicyclo[4.2.1]nonanes

We describe a facile-operational, high-yield method for the diastereocontrolled preparation of novel sulfonyl benzofused fluorooxabicyclo[4.2.1]nonanes by a straightforward synthetic route, including (i) NaBH₄-mediated reduction of sulfonyl 3-methylene-oxabenzocyclooctan-6-ones and (ii) BF₃·OEt₂-mediated intramolecular nucleophilic fluorocyclization. The plausible mechanism for the preparation is proposed and discussed. This protocol can easily install a fluoro-atom on the bridged head position in a short time and under mild conditions, resulting in one carbon-carbon and one carbon-fluorine bond formation.

Iodine-mediated oxythiolation of o-vinylanilides with disulfides for the synthesis of benzoxazines

An efficient iodine-mediated oxythiolation of o-vinylanilides with disulfides was developed. By virtue of this method, a series of thio-tethered benzoxazine derivatives were synthesized in good to excellent yields. The reaction features high yields, is metal-free, and has a wide substrate scope.

An efficient iodine-mediated oxythiolation of o-vinylanilides with disulfides was developed. The reaction features high yields, is metal-free, and has a wide substrate scope.

Electrochemical oxidative selenocyclization of olefinic amides towards the synthesis of iminoisobenzofurans

We introduced an electrochemical oxidative radical cascade cyclization of olefinic amides and diselenides without a transition-metal catalyst and external oxidant. This selenocyclization reaction provided a facile method to construct C–Se and C–O bonds in one step.

Catalytic enantioselective oxysulfenylation of o-vinylanilides

Tf₂NH-assisted BINAM-derived thiophosphoramide catalysis has been accomplished for the enantioselective oxysulfenylation of o-vinylanilides with N-(aryl/alkylthio)imides. The developed reaction offers access to diverse substituted aryl/alkylthio tethered 3,1-benzoxazines in excellent yields and enantiomeric ratios. Furthermore, synthetic applications of benzoxazines and aryl/alkylthio moieties and a transition state model for the observed enantioselectivity are also discussed.

Catalyst- and Additive-Free Approach to Constructing Benzo-oxazine, Benzo-oxazepine, and Benzo-oxazocine: O Atom Transfer and C═O, C-N, and C-O Bond Formation at Room Temperature

An exclusive synthesis of benzo-oxazine, benzo-oxazepine, and benzo-oxazocine from aryl propanal and 2-(hydroxyamino)phenyl alcohol under metal-free conditions is described. O atom transfer and formation of new C═O, C-N, and C-O bonds occur at room temperature to form six-, seven-, and eight-membered heterocycles under one-pot reaction conditions without using an external oxidant and base. The photophysical properties are studied using ultraviolet-visible absorption and photoluminescence. The mechanistic elucidation is well supported by control experiment and literature precedents.

The sunlight-promoted aerobic selective cyclization of olefinic amides and diselenides

A novel sunlight-promoted approach for the selective synthesis of selenated iminoisobenzofurans or isoindolinones via the aerobic O-cyclization or N-cyclization of olefinic amides with diselenides has been developed.

Anodic Oxidation for the Stereoselective Synthesis of Heterocycles

Stereodefined aliphatic heterocycles are one of the fundamental structural motifs observed in natural products and biologically active compounds. Various strategies for the synthesis of these building blocks based on transition metal catalysis, organocatalysis, and noncatalytic conditions have been developed. Although electrosynthesis has also been utilized for the functionalization of aliphatic heterocycles, stereoselective transformations under electrochemical conditions are still a challenging field in electroorganic chemistry. This Account consists of four main topics related to our recent efforts on the diastereo- and/or enantioselective synthesis of aliphatic heterocycles, especially N-heterocycles, using anodic oxidations as key steps. The first topic is the development of stereoselective synthetic methods for multisubstituted piperidines and pyrrolidines from anodically prepared α-methoxy cyclic amines. Our strategies were based primarily on N-acyliminium ion chemistry, and the key electrochemical transformations were diastereoselective anodic methoxylation, diastereoselective arylation, and anodic deallylative methoxylation. Furthermore, we found a unique property of the N-cyano protecting group that enabled the electrochemical α-methoxylation of α-substituted cyclic amines. The second topic of investigation is memory of chirality in electrochemical decarboxylative methoxylation. We observed that the electrochemical decarboxylative methoxylation of oxazolidine and thiazolidine derivatives with the appropriate N-protecting group occurred in a stereospecific manner even though the reaction proceeded through an sp² planar carbon center. Our findings demonstrated the first example of memory of chirality in N-acyliminium ion chemistry. The third topic is the synthesis of chiral azabicyclo-N-oxyls and their application to chiral organocatalysis in the electrochemical oxidative kinetic resolution of secondary alcohols. The final topic is stereoselective transformations utilizing anodically generated halogen cations. We investigated the oxidative kinetic resolution of amino alcohol derivatives using anodically generated bromo cations. We also developed an intramolecular C-C bond formation of keto amides, a diastereoselective bromoiminolactonization of α-allyl malonamides, and an oxidative ring expansion reaction of allyl alcohols. It is noteworthy that most of the electrochemical reactions were performed in undivided cells under constant-current conditions, which avoided a complicated reaction setup and was beneficial for a large-scale reaction. In addition, we developed some enantioselective electrochemical transformations that are still challenges in electroorganic chemistry. We hope that our research will contribute to the further development of diastereo- and/or enantioselective transformations and the construction of valuable heterocyclic compounds using an electrochemical approach.

Visible-light-induced intramolecular sp3 C-H oxidation of 2-alkyl-substituted benzamides for the synthesis of functionalized iminoisobenzofurans

We have developed a protocol for the synthesis of functionalized iminoisobenzofurans by means of visible-light-induced intramolecular cyclization reactions of 2-alkyl-substituted benzamides. This one step-economical protocol, which involves intramolecular sp3 C-O bond formation, features mild reaction conditions, exclusive chemoselectivity, and high yields.

Silver-Promoted Site-Selective Intramolecular Cyclization of 2-Methylthiobenzamide Through α-C(sp3)-H Functionalization

Silver-mediated intramolecular α-C(sp³)-H bond functionalization of the methylthio group has been established in the presence of Selectfluor as an additive. This novel strategy provides efficient access to various diverse sulfur-based heterocycles with good yields and functional group compatibility. It is noteworthy that the completely novel benzooxathiin-4-imine skeletons were reported for the first time in this study.

Silver or cerium-promoted free radical cascade difunctionalization of o-vinylanilides with sodium aryl- or alkylsulfinates

An efficient cascade oxysulfonylation of o-vinylanilides with sodium aryl- or alkylsulfinates was developed for the synthesis of sulfonated benzoxazines.

Iridium-Catalyzed α-Selective Arylation of Styrenes by Dual C-H Functionalization

An IrI -system modified with a ferrocene derived bisphosphine ligand promotes α-selective arylation of styrenes by dual C-H functionalization. These studies offer a regioisomeric alternative to the Pd-catalyzed Fujiwara-Moritani reaction.

Access to benzo-fused nine-membered heterocyclic alkenes with a trifluoromethyl carbinol moiety via a double decarboxylative formal ring-expansion process under palladium catalysis

Direct access to pharmaceutically attractive benzo-fused nine-membered heterocyclic alkenes 3 with a trifluoromethyl carbinol moiety was achieved via a palladium-catalyzed double-decarboxylative formal ring-expansion process from six-membered trifluoromethyl benzo[d][1,3]oxazinones 1 to nine-membered trifluoromethyl benzo[c][1,5]oxazonines 3 in the presence of vinylethylene carbonates 2. Generation of a Pd-π-allyl zwitterionic intermediate was proposed in the catalytic cycle. The trifluoromethyl group in the benzoxazinanones 1 plays an important role throughout the transformation. Diastereoselective chemical transformations of products 3 were also demonstrated.

Oxidative Difluoromethylation of Tetrahydroisoquinolines Using TMSCF2SPh: Synthesis of Fluorinated Pyrrolo[2,1-a]isoquinolines and Benzo[a]quinolizidines

An efficient C1-difluoromethylation of tetrahydroisoquinolenes was achieved using TMSCF₂SPh as a difluoromethylating agent and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as an oxidant. The process provides an access to a variety of C1-difluoro(phenylsulfanyl)methylated tetrahydroisoquinoline adducts in good yields. These adducts were employed as key precursors for preparing fluorinated pyrrolo[2,1-a]isoquinoline and benzo[a]quinolizidines.

Highly regio- and stereoselective trans-iodofluorination of ynamides enabling the synthesis of (E)-α-fluoro-β-iodoenamides

A highly regio- and stereoselective trans-iodofluorination reaction of ynamides with NIS and Et₃N·3HF has been achieved, affording (E)-α-fluoro-β-iodoenamides in moderate to good yields. The reaction proceeds under mild reaction conditions and exhibits good functional group compatibility.

Copper-catalyzed radical cascade oxyalkylation of olefinic amides with simple alkanes: highly efficient access to benzoxazines

A copper-catalyzed C(sp³)-H bond functionalization of simple alkanes with olefinic amides was developed for the efficient synthesis of important benzoxazine derivatives. It involves new C-C and C-O bond formation in one step via a radical cascade process.

Electronic halocyclization and radical haloazidation of benzene-linked 1,7-dienes for the synthesis of functionalized 3,1-benzoxazines

Electronic halocyclization and radical haloazidation of benzene-linked 1,7-dienes by using TMSN₃ as an azidyl source and NBS as a halogen source has been disclosed in this paper, producing a series of functionalized 3,1-benzoxazines in moderate to good yields.

Fluorination methods in drug discovery

Fluorination reactions of medicinal and biologically-active compounds will be discussed. Late stage fluorination strategies of medicinal targets have recently attracted considerable attention on account of the influence that a fluorine atom can impart to targets of medicinal importance, such as modulation of lipophilicity, electronegativity, basicity and bioavailability, the latter as a consequence of membrane permeability. Therefore, the recourse to late-stage fluorine substitution on compounds with already known and relevant biological activity can provide the pharmaceutical industry with new leads with improved medicinal properties. The fluorination strategies will take into account different fluorinating reagents, either of nucleophilic or electrophilic, and of radical nature. Diverse families of organic compounds such as (hetero)aromatic rings, and aliphatic substrates (sp(3), sp(2), and sp carbon atoms) will be studied in late-stage fluorination reaction strategies.

Synthesis of benzo[1,3]oxazines via copper(i)-catalyzed cascade annulation of nitriles, aldehydes and diaryliodonium salts

A copper(i)-catalyzed one-pot [2 + 2 + 2] cascade annulation reaction of diaryliodoniums, nitriles, and aldehydes has been developed for the efficient synthesis of 2,4-substituted benzoxazine derivatives.

Visible-light-mediated radical oxydifluoromethylation of olefinic amides for the synthesis of CF2H-containing heterocycles

The visible-light-mediated oxydifluoromethylation of olefinic amides with difluoromethyl sulfones has been explored.

Transition-metal free alkylarylation of acrylamides initiated by radical C–C bond cleavage of the tertiary cycloalkanols

An efficient Na₂S₂O₈-promoted radical cyclization reaction of acrylamides with the tertiary cycloalkanols has been developed. This one pot procedure involves a tandem C–C bond cleavage and two C–C bonds formation process.

A Fluorination/Aryl Migration/Cyclization Cascade for the Metal-Free Synthesis of Fluoro-Benzoxazepines

Fluorinated organic molecules are of high interest for many applications across chemical and medical disciplines. Efficient methods for the synthesis of such compounds are thus needed. Within this work, application of the bench-stable cyclic hypervalent iodine(III) fluoro reagent 1 facilitated the development of an efficient, metal-free method for the preparation of the novel class of 4-fluoro-1,3-benzoxazepines starting from readily available styrenes. The efficacy and broad applicability of this concept is demonstrated by the synthesis of 20 structurally diverse congeners in high yields, regio-, and diastereoselectivities. The presented method provides complementary chemoselectivity when compared to the common, commercially available electrophilic fluorination reagents, such as selectfluor. First mechanistic investigations with isotopically labeled substrates reveal a complex reaction mechanism, proceeding via an unusual fluorination/1,2-aryl migration/cyclization cascade.