Oxidation of N,N-Disubstituted Hydroxylamines to Nitrones with Hypervalent Iodine Reagents

Palladium-Catalyzed Synthesis of Nitrones Via Redox Cross-Coupling of Nitro Compounds and Alcohols

A novel methodology for the synthesis of nitrones via palladium-catalyzed redox cross-coupling of nitro compounds and alcohols is established. The protocol is a mild, convenient, ligand-free, and scalable synthesis method that can be compatible with various nitro compounds and alcohols. Nitrone is a significant multifunctional platform synthon which can be synthesized directly and efficiently via this tactic from commercially available and cheap raw materials.

Synthesis of Hypervalent Iodine Diazo Compounds and Their Application in Organic Synthesis

Diazomethyl-substituted iodine(III) compounds with electron-withdrawing groups (EWG) connected to diazo methyl center were a type of donor-acceptor diazo compounds with potential reaction abilities similar to ordinary diazo compounds. Although several diazomethyl-substituted iodine(III) compounds were synthesized and used in the nucleophilic substitution reactions as early as 1994, the synthesis and application of new iodine(III) diazo compounds have only been reported to a certain extent in recent years. In the presence of rhodium catalyst, photocatalyst, or nucleophiles, diazomethyl-substituted iodine(III) compounds can be converted into rhodium-carbenes, diazomethyl radicals, ester radicals or nucleophilic intermediates, which can be used as key intermediates for the formation of chemical bonds. The aim of this review is to give an overview of diazomethyl-substituted iodine(III) compounds in organic synthesis.

Synthesis of (±)-Setigerumine I: Biosynthetic Origins of the Elusive Racemic Papaveraceae Isoxazolidine Alkaloids*

The biosynthetic origins of the structurally related racemic isoxazolidine Papaveraceae alkaloids Setigerumine I, Dactylicapnosinine and Dactylicapnosine have remained elusive since their original isolation over two decades ago. Herein we report the first biosynthetic hypothesis for their formation and, inspired by it, the first synthesis of (±)-Setigerumine I with accompanying computational rationale. Based on the results, these isoxazolidine alkaloids arise from racemizing oxidative rearrangements of prominent isoquinoline alkaloids Noscapine and Hydrastine. The key steps featured in this synthesis are a room temperature Cope elimination and a domino oxidation/inverse-electron demand 1,3-dipolar cycloaddition of an axially chiral, yet configurationally unstable, intermediate. The work opens this previously inaccessible family of natural products for biological studies.

Metal-Free Solvent Promoted Oxidation of Benzylic Secondary Amines to Nitrones with H2O2

An environmentally benign protocol for the generation of nitrones from benzylic secondary amines via catalyst-free oxidation of secondary amines using H2O2 in MeOH or CH3CN is described. This methodology provides a selective access to a variety of C-aryl nitrones in yields of 60 to 93%. Several studies have been performed to shed light on the reaction mechanism and the role of the solvent.

Piperidine Azasugars Bearing Lipophilic Chains: Stereoselective Synthesis and Biological Activity as Inhibitors of Glucocerebrosidase (GCase)

We report a straightforward synthetic strategy for the preparation of trihydroxypiperidine azasugars decorated with lipophilic chains at both the nitrogen and the adjacent carbon as potential inhibitors of the lysosomal enzyme glucocerebrosidase (GCase), which is involved in Gaucher disease. The procedure relies on the preparation of C-erythrosyl N-alkylated nitrones 10 through reaction of aldehyde 8 and primary amines 13 followed by oxidation of the imines formed in situ with the methyltrioxorhenium catalyst and urea hydrogen peroxide. The addition of octylMgBr to nitrone 10e provided access to both epimeric hydroxylamines 21 and 22 with opposite configuration at the newly created stereocenter in a stereodivergent and completely stereoselective way, depending on the absence or presence of BF₃·Et₂O. Final reductive amination and acetonide deprotection provided compounds 14 and 15 from low-cost d-mannose in remarkable 43 and 32% overall yields, respectively, over eight steps. The C-2 R-configured bis-alkylated trihydroxypiperidine 15 was the best ligand for GCase (IC₅₀ = 15 μM), in agreement with MD simulations that allowed us to identify the chair conformation corresponding to the best binding affinity.

Visible-light-promoted nitrone synthesis from nitrosoarenes under catalyst- and additive-free conditions

A green and sustainable nitrone formation reaction via visible-light-promoted reaction of aryl diazoacetates with nitrosoarenes is described. This protocol exhibits good functional group tolerance and broad substrate scope for both aryl diazoacetates with nitrosoarenes. Comparing the reported methods for the synthesis of nitrones from nitrosoarenes, the reaction described herein occurs under sole visible-light irradiation without the need of any catalysts and additives.

Single-Step Approach toward Nitrones via Pyridinium Ylides: The DMAP-Catalyzed Reaction of Benzyl Halides with Nitrosoarenes

A single-step approach is reported for the preparation of nitrones from benzyl halides and nitrosoarenes via pyridinium ylides, utilizing 4-dimethylaminopyridine (DMAP) catalyst and mild reaction conditions (Li₂CO₃, dimethylacetamide, and room temperature). The reaction provides both keto- and aldonitrones in high yields with a wide scope for benzyl halides and nitrosoarenes. In the same reaction system, 2-methyl-2-nitrosopropane, which does not have an aryl group, also affords the corresponding N-tert-butyl nitrones from primary benzyl bromides that have an electron-withdrawing group. As an application of the reaction, methyl 2-bromo-2-phenylacetate was used to prepare the corresponding isoxazolidine by a sequential one-pot synthesis.

The Development of Reaction Cascades to Synthesize Dimeric Coccinellid Alkaloids

Over the course of the past decade, our group has been intensely interested in achieving the laboratory synthesis of varied members of the coccinellid alkaloid family of natural products. These compounds, produced by varied species of ladybugs throughout the world as defensive agents, include several polycyclic members that can formally be considered as either monomeric or dimeric with architectures that contain between 3 and 7 ring systems along with an array of stereocenters. As a result of their fascinating structures, many groups have achieved syntheses of varied monomeric members using a variety of synthetic strategies and tactics. However, no efforts to synthesize any of the dimeric structures had been reported at the time we began our studies, and only a modest amount of study had been performed as relates to their biosynthesis, with little knowledge of how the larger structures might actually arise in Nature. In this Account, we provide an overview of our general synthetic considerations to achieve a global synthesis of the collection, efforts that have led to date to the formal and total synthesis of 12 different members, 4 at the dimer level. Critical was (1) the identification of a key, common intermediate to enable access to a large number of monomeric substructures in short order, (2) careful thinking as to how the larger structures might arise biosynthetically to fuel building block design, and (3) the development of several reaction cascades that rapidly assembled the majority of their molecular complexity in single-pot operations. Key discoveries in the program include the finding that when efforts to achieve intermolecular dimerizations fail with advanced intermediates, attempts to couple more functionalized fragments earlier and then fold them into the desired structure can be an effective strategy. We also highlight suggestive evidence that a non-natural isomer we originally prepared from one of those cascades may, in fact, be a natural product. And, in particular, we will focus on how two key cascades were developed, as a result of synthetic challenges at varied points in our explorations, which proved capable of forging multiple bonds, rings, and stereocenters in the target structures. One of these includes a designed event that combined 9 different chemical reactions in a single pot and may prove useful for the synthesis of other targets.

Intramolecular [3 + 2] nitrone cycloaddition reaction: highly regio and diastereoselective synthesis of bicyclo[3.2.1]octane scaffolds

A regio and diastereoselective strategy has been developed for the synthesis of complex bicyclo[3.2.1]octane scaffolds from the reaction of readily accessible vinylogous carbonates with N-substituted hydroxylamine hydrochlorides via intramolecular 1,3-dipolar nitrone cycloaddition reaction. Wide varieties of bicyclic isoxazolidines were synthesized in high yields under catalyst free conditions through a highly efficient and operationally simple protocol.

A Concise, Enantiospecific Total Synthesis of Chilocorine C Fueled by a Reductive Cyclization/Mannich Reaction Cascade

Among defensive alkaloids isolated from ladybugs, the heterodimeric member chilocorine C possesses an alluring monomeric unit that combines quinolizidine and indolizidine substructures. Indeed, the overall stereochemical disposition of its ring fusions is distinct from those of related natural products. Herein we show that a carefully orchestrated sequence with several chemoselective transformations, including a designed cascade that accomplishes nine distinct chemical reactions in one-pot, can smoothly forge that domain and ultimately enable a 15-step, 11-pot enantiospecific synthesis of the natural product. Mechanistic studies, density functional theory calculations, and the delineation of several other unsuccessful approaches highlight its unique elements.

Glucocerebrosidase (GCase) activity modulation by 2-alkyl trihydroxypiperidines: Inhibition and pharmacological chaperoning

The enzyme glucocerebrosidase (GCase) has become an important therapeutic target due to its involvement in pathological disorders consequent to enzyme deficiency, such as the lysosomal storage Gaucher disease (GD) and the neurological Parkinson disease (PD). Pharmacological chaperones (PCs) are small compounds able to stabilize enzymes when used at sub-inhibitory concentrations, thus rescuing enzyme activity. We report the stereodivergent synthesis of trihydroxypiperidines alkylated at C-2 with both configurations, by means of the stereoselective addition of Grignard reagents to a carbohydrate-derived nitrone in the presence or absence of Lewis acids. All the target compounds behave as good GCase inhibitors, with IC₅₀ in the micromolar range. Moreover, compound 11a behaves as a PC in fibroblasts derived from Gaucher patients bearing the N370/RecNcil mutation and the homozygous L444P mutation, rescuing the activity of the deficient enzyme by up to 1.9- and 1.8-fold, respectively. Rescues of 1.2-1.4-fold were also observed in wild-type fibroblasts, which is important for targeting sporadic forms of PD.

Synthesis and Transformations of Nitrones for Organic Synthesis

Nitrones are important compounds and are highly useful in many aspects. The first part describes the methods for synthesis of nitrones, which are useful and environmentally friendly. Catalytic oxidations, condensations, and other useful reactions are described. The nitrones thus obtained are key intermediates for the synthesis of biologically important nitrogen compounds. The second part describes the fundamental transformations of nitrones, which will provide the strategies and means for the construction of nitrogen compounds. The reactions with nucleophiles or radicals, C-H functionalization, and various addition reactions are described. The last reactions are particularly important for highly selective carbon-carbon bond formations. 1,3-Dipolar cycloaddition reactions are excluded because the size of the review is limited and excellent reviews have been published in Chemical Reviews.

Regiospecific formation of sugar-derived ketonitrone towards unconventional C-branched pyrrolizidines and indolizidines

The synthesis of unprecedented branched pyrrolizidines and indolizidines was accomplished via nitrone chemistry.

Mannich-type Reactions of Cyclic Nitrones: Effective Methods for the Enantioselective Synthesis of Piperidine-containing Alkaloids

Even though there are dozens of biologically active 2-substituted and 2,6-disubstituted piperidines, only a limited number of approaches exist for their synthesis. Herein is described two Mannich-type additions to nitrones, one using β-ketoacids under catalyst-free conditions and another using methyl ketones in the presence of chiral thioureas, which can generate a broad array of such 2-substituted materials, as well as other ring variants, in the form of β-N-hydroxy-aminoketones. Both processes have broad scope, with the latter providing products with high enantioselectivity (up to 98 %). The combination of these methods, along with other critical steps, has enabled 8-step total syntheses of the 2,6-disubstituted piperidine alkaloids (-)-lobeline and (-)-sedinone.

1-Oxo-2,2,6,6-tetramethylpiperidinium bromide converts α-H N,N-dialkylhydroxylamines to nitrones via a two-electron oxidation mechanism

Herein we provide experimental proof that 1-oxo-2,2,6,6-tetramethylpiperidinium bromide converts α-H N,N-dialkylhydroxylamines to nitrones via a two-electron oxidation mechanism. The reactions reported are rapid, proceed under mild conditions, and afford nitrones in excellent yields.

Construction of α-methoxyimidoyl ketonitrones via phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes

A dimethyl phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes was developed. Nitrosoarenes were successfully used as electrophiles to trap aza-enolate intermediates that were generated from nucleophilic addition of deprotonated phosphite to α-keto N-tert-butanesulfinyl imidates and following phospha-Brook rearrangement, allowing efficient construction of ketonitrones with excellent (Z)-geometries.

Triphenylbismuth carbonate-mediated oxidation of hydroxylamines to nitrones and in situ 1,3-dipolar cycloaddition

Triphenylbismuth carbonate was shown to act as a mild, selective and efficient reagent for the oxidation of hydroxylamines.