Fluorine-Activated and -Directed Allene Cycloadditions with Nitrile Oxides and Imine Oxides: Synthesis of Ring-Fluorinated Isoxazole Derivatives

Double Selectivity of Cycloaddition Opens a Way to 1,2-Oxazoline-Pyrrole Ensembles via Nitrile Oxide Cycloaddition to N-Allenylpyrrole-2-Carbaldehydes

Cycloaddition of 1,3-dipoles to allenes usually proceeds in a nonselective way. Herein, we report on an efficient metal-free cycloaddition of nitrile oxides to readily available N-allenylpyrrole-2-carbaldehydes with formation of 1,2-oxazoline-pyrrole ensembles. The reaction features a high regioselectivity and occurs only at the β,γ-bond of the allene with a very specific direction of the 1,3-dipole attack. A possible reason for this can be the effect of the formyl group, which remains intact, thus opening up possibilities for further functionalization of the target compounds. The regioselectivity of the cycloaddition has been rationalized by quantum-chemical calculations.

MsOH Promoted Divergent Synthesis of 4-Arylidene Isoxazolidines and Isoxazolines from O-Propargyl Hydroxylamines and Aldehydes

Temperature-dependent, Brønsted-acid-mediated divergent synthesis of 4-arylidene isoxazolidines and isoxazolines from O-propargyl hydroxylamines is developed. A series of control and crossover experiments have been carried out, revealing that the formation of 4-arylidene isoxazoline proceeds through an alkyne-oximium cyclization, ring-opening reaction, and subsequent condensation. Synthetic versatility of the developed methodology was highlighted in the synthesis of 3-aminoacrylaldehyde, β-hydroxy ketone, and isoxazole derivatives.

Gold(I)-Catalyzed [2 + 2] and [3 + 2] Cycloadditions of 1,1-Difluoroallenes with Aldehydes: Switchable Syntheses of Fluorinated Oxetanes and Furans

1,1-Difluoroallenes underwent regioselective [2 + 2] and [3 + 2] cycloadditions with aldehydes using Au(I) catalysts. An AuCl catalyst enabled an α,β-selective [2 + 2] cycloaddition of 1,1-difluoroallenes, yielding (E)-3-alkylidene-2,2-difluorooxetanes. Conversely, an AuCl(IPr)-AgSbF6 catalyst facilitated an α,γ-selective [3 + 2] cycloaddition, followed by dehydrofluorination to produce aromatized 2-fluorofurans. DFT calculations (B3LYP level) suggested that the former reaction proceeded through Au(III)-containing metallacycles, while the latter involved cyclic Au(I) carbenoids.

Gold(I)-catalysed [2+4] cycloaddition of 1,1-difluoroallenes with conjugated enones: synthesis of ring-difluorinated dihydro-2H-pyrans

1,1-Difluoroallenes underwent regioselective [2+4] cycloaddition with α,β-unsaturated ketones (enones) in the presence of an AuCl(IPr)-AgSbF6 catalyst. β-Aurated, charge-localized difluoroallylic carbocations, formed from 1,1-difluoroallenes and the cationic Au catalyst, were stabilised by the two fluorine substituents. This facilitated α-selective nucleophilic attack by the enone oxygen, followed by six-membered ring closure, leading to the formation of ring-difluorinated dihydro-2H-pyrans with (E)-alkylidene groups in high yields.

tert-Butyl Nitrite (TBN)-Enabled Tandem C-H Bond Oxidation/C-N Bond Cleavage/[3+2] Cycloaddition of 1-Nitromethyltetrahydroisoquinoline Derivatives: Construction of Multifunctionalized Isoxazole and Isoxazoline Skeletons

A TBN-enabled tandem C-H bond oxidation/C-N bond cleavage/[3+2] cycloaddition of 1-nitromethyltetrahydroisoquinolines was realized, constructing a series of multifunctionalized isoxazole and isoxazoline skeletons in the presence of alkynes and alkenes, respectively. Various functional groups were smoothly tolerated, and the mechanistic study revealed that the construction of isoxazole and isoxazoline rings is mediated by the in situ generated nitrile oxides.

gem-Difluoroallene (DFA): an emerging versatile synthetic precursor to enable diverse fluorinated molecules

Organofluorine compounds are increasingly found in diverse fields, such as pharmaceuticals, agrochemicals, and materials science. gem-Difluoroallenes, which have gem-difluoro alkenes and allenes-like properties, offer a distinct and adaptable platform for novel synthetic transformations. Their distinctiveness is highlighted by various strategies, where the gem-difluoro group's presence plays a pivotal role in successful reactions. This review article presents a comprehensive overview of the latest progress in utilizing gem-difluoroallenes for selective additions, defluorination, as well as cycloaddition and cyclization reactions. We also discuss the limitations and future directions in this area, inspiring further exploration and innovation.

5-Nitroisoxazoles in SNAr Reactions: A Novel Chemo- and Regioselective Approach to Isoxazole-Based Bivalent Ligands of AMPA Receptors

An efficient regioselective approach to novel functionalized bis(isoxazoles) with a variety of aromatic and aliphatic linkers was elaborated, based on the heterocyclization reaction of electrophilic alkenes under the treatment with tetranitromethane-triethylamine complex affording 3-EWG-5-nitroisoxazoles. The subsequent SNAr reactions of 5-nitroisoxazoles with various O,O-, N,N- and S,S-bis(nucleophiles) provide a wide range of bis(isoxazole) derivatives in good isolated yields. Employing an elaborated method, a series of novel bis(3-EWG-isoxazoles) as the promising allosteric modulators of AMPA receptors were designed and synthesized. The effect of the compounds on the kainate-induced currents was studied in the patch clamp experiments, revealing modulator properties for several of them. The best positive modulator potency was found for dimethyl 5,5'-(ethane-1,2-diylbis(sulfanediyl))bis(isoxazole-3-carboxylate), which potentiated the kainate-induced currents in a wide concentration range (10-12-10-6 M) with maximum potentiation of 77% at 10-10 M. The results were rationalized using molecular docking and molecular dynamics simulations of modulator complexes with the dimeric ligand-binding domain of the GluA2 AMPA receptor. The predicted physicochemical, ADMET, and PAINS properties confirmed that the AMPA receptor modulators based on the bis(isoxazole) scaffold may serve as potential lead compounds for the development of neuroprotective drugs.