[4+1] cyclization of benzohydrazide and ClCF2COONa towards 1,3,4-oxadiazoles and 1,3,4-oxadiazoles-d5

Difluorocarbene Enables Access to 2,2-Difluorohydrobenzofurans and 2-Fluorobenzofurans from ortho-Vinylphenols

2-Fluorobenzofurans are the backbone structures of many drug molecules and have many potential therapeutic bioactivities. Despite the potential applications in medicinal chemistry, practical and efficient synthetic methods for the construction of 2-fluorobenzofuran are very limited. Herein, we report an efficient and general method for the construction of 2-fluorobenzofurans. Contrary to the previous functionalizations of the existing backbone of benzofuran, our strategy directly constructs benzofuran scaffolds alongside the incorporation of fluorine atom on C2 position in a formal [4 + 1] cyclization from readily accessible ortho-vinylphenols and difluorocarbene. In our strategy, ClCF2H decomposes into difluorocarbene in the presence of base, which is further captured by the oxygen anion from the hydroxy group in ortho-hydroxychalcones; subsequent intramolecular Michael addition to the α, β-unsaturated system leads to 2,2-difluorohydrobenzofurans, and further fluorine elimination renders 2-fluorobenzofurans by forming one C-O bond and one C-C double bond. Of note, various complex 2,2-difluorohydrobenzofurans and 2-fluorobenzofurans could be readily accessed through our protocol via the late-stage elaborations.

Palladium-Catalyzed Iodine Assisted Carbonylation of Indoles with ClCF2CO2Na and Alcohols

A palladium-catalyzed iodine-assisted carbonylation reaction of indoles with readily available ClCF2CO2Na and alcohols has been developed. This protocol provides a practical and efficient approach to highly regioselective indole-3-carboxylates via a preiodination strategy of indoles. Different from classic carbonylation using toxic and difficult-to-handle carbon monoxide, this operationally simple and scalable reaction employed difluorocarbene as the carbonyl surrogate.

Anti-inflammatory and Antimicrobial Potential of 1, 3, 4-oxadiazoles and its Derivatives: A Review

1, 3, 4-oxadiazole and its derivatives have significant anti-inflammatory and antimicrobial property. Their precise mechanism of action is not known but it is postulated that they act by inhibiting the biosynthesis of certain prostaglandins. 1, 3, 4-oxadiazoles are a class of heterocyclic compounds with wide variety of biological and pharmacological activities. They have been reported to possess analgesic, antimicrobial, antipyretic and anti-inflammatory properties. These compounds are also active against a number of other inflammatory conditions such as arthritis, gout etc. A wide variety of these compounds have been synthesized and some of them are under clinical trials. In this review article, anti-inflammatory and antimicrobial activity of the 1, 3, 4- oxadiazole shall be discussed.

Rhodium-Catalyzed Intramolecular Cyclization to Synthesize 2-Aminobenzofurans via Carbene Metathesis Reactions

Herein, we report a new method of synthesizing of 2-aminobenzofuran 3-enes via the formal C-S insertion reaction of alkyne-tethered diazo compounds. Metal carbene, as a kind of active synthetic intermediate, plays a very important role in organic synthesis. Through the carbene/alkyne metathesis strategy, a new donor carbene is produced in situ as a key intermediate, which has different reactions from the donor receptor carbene.

Unconventional Transformations of Difluorocarbene with Amines and Ethers

ConspectusFluorine-containing compounds are extensively involved in various fields originating from intriguing and unique characteristics of fluorine atom; notably, in pharmaceuticals, the involvement of a fluorine atom or a fluorine-containing group is a chief technique for improving the pesticide effect and developing new drugs. Difluorocarbene, one of the most important and powerful fluorine-containing reagents, is widely employed and studied in many areas mainly to assemble gem-difluoromethyl molecules, including but not limited to the abundant reactions between difluorocarbene with nucleophilic substrates, Wittig reaction with ketones or aldehydes, cascade reaction with both a nucleophile and an electrophile, or [2+1] cycloaddition with alkenes or alkynes. However, its unconventional and intriguing protocols beyond as a difluoromethyl synthon have rarely been studied, and thus, it is highly desired given its abundance, inexpensiveness and peculiar properties. In this Account, we mainly discuss our discovery with unconventional transformations of difluorocarbene, instead of as a sole difluoromethyl source (different from other dihalocarbene), actually can serve as an electron acceptor to activate C-X bonds (X = N and O) and thus promote a myriad of fascinating transformations for the assembly of versatile valuable products with various aza-compounds (primary/secondary/tertiary amines as well as NH₃ and NaNH₂ and so on) and aliphatic ethers in the absence of transition metals and expensive ligands. Inspired by the electron-deficient characteristics of difluorocarbene, we first found that the isocyanides could be readily formed in situ when the unoccupied orbital of difluorocarbene meets the lone-pair of primary amines; in basic condition, a cascade defluorination and cyclizations could afford plethora of valuable N-containing heterocycles. Meanwhile, we disclosed that cyano anion could be accessible in situ as well when difluorocarbene and NaNH₂ or NH₃ were mixed up in suitable basic conditions, and thus a series of aryl nitrile compounds were obtained in the presence of Pd catalysis and ArI. Interestingly, when difluorocarbene encountered secondary amines, formamides were rendered under mild reactions. Of note, concomitant functionalizations of C and N moieties via cleavage of the unstrained C(sp³)-N bond in the absence of metal and oxidant are sparce, which indeed significantly add versatility and diversity to products. Gratifyingly, by uitilizing difluorocarbene and cyclic tertiary amines, we achieved difluorocarbene-mediated deconstructive functionalizations for the first time, showing successive C(sp³)-N bond scission of amines and simultaneous functionalization of C and N atoms which would be introduced into the products in the absence of transition metals and oxidants. This method provides a brand-new while very universal synthetic pathway to selectively cleave inert unactivated Csp³-N bonds, in which halodifluoromethyl reagents act as both C1 synthon and halo (Cl, Br, I) sources. Fascinatingly, nitrogen ylides are generated in situ from difluorocarbene and tertiary amines, and an intriguing and universal approach for deaminative arylation or alkenylation of tertiary amines was disclosed for the first time in appropriate basic conditions, which represents an intriguing reaction mode to lead to a formal transition-metal free Suzuki cross coupling. Besides, we also disclosed that difluorocarbene could proceed novel atom recombination to render meaningful 2-fluoroindoles or 3-(2,2-difluoroethyl)-2-fluoroindoles from ortho-vinylanilines, 3-fluorined oxindoles from 2-aminoarylketones, in which difluorocarbene acts as a C1 synthon and F1 source simultaneously. Last but not the least, we recently found that the lone-pair-electron of oxygen could trap difluorocarbene as well to form oxonium ylide, which eventually leads to C-O bond cleavage with the formation of difluoromethyl ethers.

Difluorocarbene-Enabled Synthesis of 3-Alkenyl-2-oxindoles from ortho-Aminophenylacetylenes

Herein, we report a transition-metal-free [4 + 1] cyclization pathway from difluorocarbene and ortho-amino aryl alkynone, rendering an effective and universal strategy for the construction of 3-alkenyl-2-oxindoles. Our strategy starts from cheap and accessible ortho-amino aryl alkynone instead of the direct indole skeleton; moreover, in situ generated difluorocarbene from commercially available halogenated difluoroalkylative reagents enables the cleavage of a C-N bond and formation of new C-N bonds and C-C bonds.

Cu-Catalyzed Three-Component Cascade Synthesis of 1,3-Benzothiazines from ortho-Aminohydrazones and Bromodifluoroacetamides

An efficient and convenient synthesis of benzo[d][1,3]thiazine has been developed by employing a copper-catalyzed transformation of readily available ketone-derived hydrazones with elemental sulfur and bromodifluoroalkylative reagents. The strategy involves an S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides, which acts as a C1 synthon at the 2-position of benzo[d][1,3]thiazine. A mechanism proceeding through a Cu-carbene intermediate is proposed for the C-S bond formation.

Palladium-Catalyzed Regioselective [5 + 1] Annulation of Vinyl Aziridines/Epoxides with ClCF2COONa

Palladium-catalyzed regioselective [5 + 1] annulation reactions of vinyl aziridines/epoxides with ClCF₂COONa have been developed. Significantly, vinyl aziridines/epoxides act as heteroatom-containing five-atom synthons, and commercially available and cheap ClCF₂COONa acts as the source of carbonyl serving as a difluorocarbene precursor. This protocol provides an efficient and practical method for the synthesis of δ-lactams and δ-lactones in good yields.

Multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds: fluorine as a detachable chemical handle

We highlighted the recent advances in the field of multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds.

Formal [4+1] cyclization of (thio/imido)hydrazides and ethyl 3,3,3-trifluoropropanoate: unified synthesis of 1,3,4-oxadiazoles, 1,3,4-thiadiazoles and 1,2,4-triazoles

We have developed a unified approach to 1,3,4-oxadiazoles, 1,3,4-thiadiazoles and 1,2,4-triazoles with ethyl acetate decoration by treating hydrazides, thiohydrazides and imidohydrazide with ethyl 3,3,3-trifluoropropanoate in the presence of a suitable base.

Difluorocarbene-enabled access to 1,3-oxazin-6-ones from enamides

1,3-Oxazin-6-ones as important structural scaffolds widely exist in many bioactive or therapeutic agents.

Double Capture of Difluorocarbene by 2-Aminostyrenes Enables the Construction of 3-(2,2-Difluoroethyl)-2-fluoroindoles

We report herein an efficient strategy to construct 3-(2,2-difluoroethyl)-2-fluoroindoles from activated o-aminostyrenes with ethyl bromodi-fluoroacetate as a difluorocarbene source. Through double capture of a difluorocarbene, two different types of fluorine motifs are incorporated into the products with simultaneous construction of one C-N and two C-C bonds, without the need for transition metals. This reaction features high efficiency and excellent functional group compatibility and has great potential in the late-stage modifications of pharmaceutical molecules and natural products.