Regioselective Synthesis of 5-Metalated 2-Pyrones by Intramolecular Oxymetalation of Carbonyl-ene-yne Compounds Using Indium Trihalide

Synthesis and Photophysical Properties of β-Alkenyl-Substituted BODIPY Dyes by Indium(III)-Catalyzed Intermolecular Alkyne Hydroarylation

A new atom-economical synthesis of β-alkenyl-substituted BODIPYs via indium(III)-catalyzed intermolecular alkyne hydroarylation with meso-substituted BODIPYs is described. While catalysis with InI3 allows the double β-functionalization of BODIPY, resulting in regioselectively branched β,β'-disubstituted alkenyl BODIPYs, catalytic InCl3 enables the formation of linear β-substituted alkenyl BODIPYs. Subsequent In(III)-catalyzed intermolecular alkyne hydroarylation allows the synthesis of unsymmetrical push-pull BODIPY derivatives. Therefore, indium catalysis offers complementary regioselectivity in good chemical yields and functional group tolerance. The resulting BODIPY dyes displayed bathochromically shifted absorption and emission according to the electron-nature of the substituents in the alkenyl moiety with high molar extinction coefficients (ε up to 88,200 M-1 cm-1) and quantum yields (0.14-0.96).

Iodination-Group-Transfer Reactions to Generate Trisubstituted Iodoalkenes with Regio- and Stereochemical Control

The regio- and stereodefined synthesis of trisubstituted alkenes remains a significant synthetic challenge. Herein, a method is developed for producing regio- and stereodefined trisubstituted iodoalkenes by diverting intermediates from an iodination-electrophilic-cyclization mechanism. Specifically, cyclized sulfonium ion-pair intermediates are diverted to alkenes by ring-opening with nucleophilic iodide. Alternatively, scavenging of the iodide by AgOTf prevents ring-opening, enabling isolation of the sulfonium ion-pair intermediate. Isolation of the ion pair enables access to complementary reactivity, including ring-opening by alternative nucleophiles (i.e., amines), yielding trisubstituted acyclic alkenes and an example acyclic tetrasubstituted alkene. X-ray crystallographic determination of reaction intermediates and products confirms that the initial electrophilic-cyclization step sets the stereo- and regiochemistry of the product. The products serve as synthetic building blocks by readily participating in downstream functionalization reactions, including oxidation, palladium-catalyzed cross-coupling, and nucleophilic displacement.

Efficient Synthesis of 1H-Benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one Derivatives Using Ag2CO3/TFA-Catalyzed 6-endo-dig Cyclization: Reaction Scope and Mechanistic Study

A small library of 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one derivatives was prepared in good to excellent yields, involving a Ag₂CO₃/TFA-catalyzed intramolecular oxacyclization of N-Boc-2-alkynylbenzimidazole substrates. In all experiments, the 6-endo-dig cyclization was exclusively achieved since the possible 5-exo-dig heterocycle was not observed, indicating the high regioselectivity of this process. The scope and limitations of the silver catalyzed 6-endo-dig cyclization of N-Boc-2-alkynylbenzimidazoles as substrates, bearing various substituents, were investigated. While ZnCl₂ has shown limits for alkynes with an aromatic substituent, Ag₂CO₃/TFA demonstrated its effectiveness and compatibility regardless of the nature of the starting alkyne (aliphatic, aromatic or heteroaromatic), providing a practical regioselective access to structurally diverse 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones in good yields. Moreover, the rationalization of oxacyclization selectivity in favor of 6-endo-dig over 5-exo-dig was explained by a complementary computational study.

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Iodo- and Chalcogenoannulation of Morita-Baylis-Hillman Alcohols of Propiolaldehydes: Entry to Functionalized 2-Pyrones

An efficient intramolecular annulation of Morita-Baylis-Hillman (MBH) alcohols of propiolaldehydes is developed in the presence of ICl or PhSeSePh/PhSSPh-CuCl₂. This cyclization offers access to a wide variety of iodinated or chalcogenated 3-(chloromethyl)-2-pyrones in good yields. The chloromethyl group of the obtained 2-pyrones has been easily converted to introduce other handy functionalities, which allowed for further transformations to synthesize diverse 2-pyrone containing molecules.

Indium-Catalyzed C-F Bond Transformation through Oxymetalation/β-Fluorine Elimination to Access Fluorinated Isocoumarins

Fluorinated heterocycles have attracted much attention in the pharmaceutical and agrochemical industries. Many strategies have already been developed to achieve the synthesis of fluorinated heterocycles. Formidable challenges remain, however, in the synthesis of fluorinated isocoumarin derivatives that are among the most alluring structural motifs. Herein, the indium-catalyzed C-F bond transformation of 2-(2,2-difluorovinyl) benzoates is reported, which are readily accessible compounds, to give a diverse array of fluorinated isocoumarins. The present reaction proceeds smoothly using inexpensive reagents: a catalytic amount of indium salt in the presence of zinc salt. A theoretical calculation of potential energy profiles showed that the reaction consists of oxymetalation with the elimination of alkyl halide and the β-fluorine elimination.

A formal [3 + 3] cycloaddition of allenyl imide and activated ketones for the synthesis of tetrasubstituted 2-pyrones

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives have been demonstrated. The allenyl imide was utilized as a C3-synthon, and a ketenyl intermediate was proposed via the process of 1,4-addition of carbon anion to allene followed by elimination of the 2-oxazolidinyl group.

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives were reported.

Innovative Synthetic Procedures for Luminogens Showing Aggregation-Induced Emission

As a consequence of their intrinsic advantageous properties, luminogens that show aggregation-induced emission (AIEgens) have received increasing global interest for a wide range of applications. Whereas general synthetic methods towards AIEgens largely rely on tedious procedures and limited reaction types, various innovative synthetic methods have now emerged as complementary, and even alternative, strategies. In this Review, we systematically highlight advancements made in metal-catalyzed functionalization and metal-free-promoted pathways for the construction of AIEgens over the past five years, and briefly illustrate new perspectives in this area. The development of innovative synthetic procedures will enable the facile synthesis of AIEgens with great structural diversity for multifunctional applications.

Synthetic Access to gem-Difluoropropargyl Vinyl Ethers and Their Application to Propargyl Claisen Rearrangement

With the increasing importance of fluorine to medicinal chemistry and other areas, methods to access various fluorinated compounds are needed. Herein, we report the synthesis of difluoropropargyl vinyl ethers from ketones and aldehydes using difluoropropargyl bromide dicobalt complexes. We applied difluoropropargyl vinyl ethers to the synthesis of difluorodienone or difluoroallene under thermal conditions and trifluoro-pyran under acid-catalyzed conditions.

Triarylborane-Catalyzed Alkenylation Reactions of Aryl Esters with Diazo Compounds

Herein we report a facile, mild reaction protocol to form carbon-carbon bonds in the absence of transition metal catalysts. We demonstrate the metal-free alkenylation reactions of aryl esters with α-diazoesters to give highly functionalized enyne products. Catalytic amounts of tris(pentafluorophenyl)borane (10-20 mol %) are employed to afford the C=C coupled products (31 examples) in good to excellent yields (36-87 %). DFT studies were used to elucidate the mechanism for this alkenylation reaction.

Carbometalation and Heterometalation of Carbon-Carbon Multiple-Bonds Using Group-13 Heavy Metals: Carbogallation, Carboindation, Heterogallation, and Heteroindation

Organogallium and -indium compounds are useful reagents in organic synthesis because of their moderate stability, efficient reactivity and high chemoselectivity. Carbogallation and -indation of a carbon-carbon multiple bond achieves the simultaneous formation of carbon-carbon and carbon-metal bonds. Heterogallation and -indation construct carbon-heteroatom and carbon-metal bonds. Therefore, these reaction systems represent a significant synthetic method for organogalliums and -indiums. Many chemists have attempted to apply various types of unsaturated compounds such as alkynes, alkenes, and allenes to these reaction systems. This minireview provides an overview of carboindation and -gallation as well as heteroindation and -gallation.