Site-Selective Pyridyl Alkyl Ketone Synthesis from N-Alkenoxypyridiniums through Boekelheide-Type Rearrangements

Chemo- and Regioselective Alkylation of Pyridine N-Oxides with Titanacyclopropanes

While titanacyclopropanes are used to react mainly with ester, amide, and cyano to undergo cyclopropanation, herein they react preferentially with pyridine N-oxide to accomplish C2-H alkylation beyond these functionalities with double regioselectivity. After being pyridylated at the less hindered C-Ti bond, the remaining C-Ti bond of titanacyclopropanes can be further functionalized by various electrophiles, allowing facile introduction of complex alkyls onto the C2 of pyridines. Its synthetic potential has been demonstrated by late-stage diversification of drugs.

Mechanistic duality of indolyl 1,3-heteroatom transposition

A novel mechanistic duality has been revealed from the indolyl 1,3-heteroatom transposition (IHT) of N-hydroxyindole derivatives. A series of in-depth mechanistic investigations suggests that two separate mechanisms are operating simultaneously. Moreover, the relative contribution of each mechanistic pathway, the energy barrier for each pathway, and the identity of the primary pathway were shown to be the functions of the electronic properties of the substrate system. Based on the mechanistic understanding obtained, a mechanism-driven strategy for the general and efficient introduction of a heteroatom at the 3-position of indole has been developed. The reaction developed exhibits a broad substrate scope to provide the products in various forms of the functionalised indole. Moreover, the method is applicable to the introduction of both oxygen- and nitrogen-based functional groups.

An umpolung strategy for chemically selective intermolecular cross-enolate-type coupling of N-alkenoxypyridinium salts with aldehydes

An efficient method for the synthesis of 1,4-ketoaldehydes via the cross-coupling of N-alkenoxyheteroarenium salts and primary aldehydes is developed. This method provides a broad substrate scope and excellent functional group compatibility. The utility of this method is demonstrated via the diverse transformations of heterocyclic compounds and cycloheptanone, as well as the late-stage functionalization of biorelevant molecules.

Site-Selective Pyridylic C-H Functionalization by Photocatalytic Radical Cascades

An efficient pyridylic C(sp³ )-H functionalization has been developed through photocatalytic radical-mediated fluoroalkylation or cascade reactions. This method is enabled by the reversible formation of alkylidene dihydropyridine intermediates via the facile enolate formation of C4-alkyl N-amidopyridinium salts in the absence of an external base, thereby establishing the conditions necessary for subsequent intermolecular radical trapping. Rapid structural diversification of the pyridylic site can be achieved through photocatalytic multicomponent cascade reactions involving alkene trifluoromethylation, SO₂ -reincorporation, and sulfonyl radical addition. This operationally simple method features a broad substrate scope and high chemoselectivity and offers a unique approach for the rational modification of the heterobenzylic C-H bonds of pyridines and quinolines with uniform site-selective control. Furthermore, experimental and theoretical studies were performed to elucidate the reaction mechanism.

A photoinduced transient activating strategy for late-stage chemoselective C(sp3)–H trifluoromethylation of azines

The direct functionalization of C(sp³)–H bonds is an ultimately ideal synthetic strategy with high atom economy and step efficiency. However, the direct trifluoromethylation of electron-deficient heteroaryl adjacent C(sp³)–H bonds remains a formidable challenge. We have described a transient activating strategy involving a Tf-shift process and π–π stacking interaction for catalyst-free direct benzylic C(sp³)–H trifluoromethylation of azines, such as pyridine, pyrimidine, quinoline, dihydropyridinone, tetrahydroisoquinoline and tetrahydroquinazoline, with an air-stable crystalline imidazolium sulfonate reagent IMDN-Tf. This bench-stable cationic reagent offers a scalable and practical protocol for the late-stage modification of drug molecules with high site selectivity, which avoids the prefunctionalization and the use of stoichiometric metals and strong oxidants. Furthermore, comprehensive mechanistic studies revealed the determining effect of π–π stacking for the activation of azinylic C(sp³)–H bonds.

Late-stage C(sp³)–H functionalization of unactivated azines: the traceless Tf switching process offers ample opportunities for site-selective derivatization of heteroaryls, allowing for the rapid increase of molecular complexity.

Recent Progress in Enolonium Chemistry under Metal-Free Conditions

Umpolung approach through inversion of the polarity of conventional enolates, has opened up an unprecedented opportunity in the cross-coupling via alkylation. The enolonium equivalents can be accessed either by hypervalent iodine reagents, activation/oxidation of amides, or the oxidation of alkynes. Under umpolung conditions, highly basic conditions required for classical enolate chemistry can be avoided, and they can couple with unmodified nucleophiles such as heteroatom donors and electron-rich arenes.

Rapid and Effective Reaction of 2-Methylpyridin-N-oxides with Triphosgene via a [3,3]-Sigmatropic Rearrangement: Mechanism and Applications

A facile and effective synthesis of 2-chloromethylpyridines was developed by a one-pot reaction of 2-alkylpyridin-N-oxides and triphosgene at room temperature. As starting materials, N-oxides of 2-alkylpyridine derivatives, including 2-alkylpyridines, 2-methyl quinolines, and phenanthroline, can react rapidly with triphosgene in the presence of triethylamine, affording 2-chloromethylpyridines in good to excellent yields (52-95%). Using the 2-methylquinoline substrate for the mechanistic study, it has been well demonstrated that the chlorination reaction undergoes a [3,3]-sigmatropic rearrangement, which can be observed as a reversible process by monitoring the intermediates. Moreover, the chlorination reaction can be used to construct a rapid and sensitive fluorescent probe for the detection of phosgene.