Applications of oxaziridines in organic synthesis

A novel method of alkoxyamine homolysis activation via photochemical rearrangement

We proposed the nitrone-oxaziridine rearrangement as a novel method for photochemical activation for the homolysis of alkoxyamine in nitroxide-mediated polymerization. The photoisomerization of the aldo-/ketonitrone-group into the oxaziridine one in 2,5-dihydroimidazole 3-oxide-based alkoxyamines was studied; the products of photolysis have been identified, and quantum yields were measured. Conversion of the nitrone group into the oxaziridine one was found to decrease the activation energy of alkoxyamine homolysis by ca. 10 kJ mol-1.

Diastereoselective Hydroxylation of N-tert-Butanesulfinyl Imines with 2-(Phenylsulfonyl)-3-phenyloxaziridine (Davis Oxaziridine)

The diastereoselective α-hydroxylation of N-tert-butanesulfinyl metallodienenamine and metalloenamines with Davis oxaziridine affords α-hydroxy N-sulfinyl imines with 50-88% yield and up to 98:2 diastereomeric ratio. Dramatic changes in diastereoselectivity and stereoselectivity were observed by choice of metal bases. The mechanistic understanding for the switch in diastereoselectivity was assisted by DFT computational modeling, which suggests the facial approach is governed by aza-enolate geometry. A one-pot protocol for the asymmetric synthesis of 1,2-amino alcohols is described.

Enhancing the Extent of Enolization for α-C-H Bonds of Aliphatic Carboxylic Acid Equivalents via Ion Pair Catalysis: Application toward α-Chalcogenation

In general, the α-functionalization of carboxylic acid derivatives requires either a transition metal catalyst or a stoichiometric activating agent/strong base/external additive. A transition metal free α-chalcogenation of aliphatic carboxylic acid equivalents is reported herein via ion pair formation using K₃PO₄ as a catalyst. Mild conditions, broad scope, scalability of the process, attaining bioactive glucokinase activators, and some synthetic intermediates establish merits of the strategy.

Biomimetic Total Synthesis of Enterocin

The first chemical total synthesis of the highly oxygenated polyketide enterocin has been accomplished. The key step of the synthesis was a late‐stage biomimetic reaction cascade involving two intramolecular aldol reactions in which each step proceeded in 52 % yield (averaged) and which established four of the seven stereogenic centers. The pivotal precursor for the cascade reaction was assembled from three readily available building blocks. A chiral dithioacetal with two stereogenic centers originating from L‐arabinose represented the core fragment to both ends of which the other building blocks were attached by aldol reactions. The remaining stereogenic center was installed by Davis oxygenation immediately prior to the key step.

Stereoselective Synthesis of the I-L Fragment of the Pacific Ciguatoxins

The I-L ring system found in all the Pacific ciguatoxins has been prepared from a tricyclic precursor in a highly stereoselective manner. Subtle differences in the reactivity of the enones present in the seven- and eight-membered rings of the tricyclic ether starting material have been exploited to allow selective protection of the enone in the eight-membered ring. Subsequent distereoselective allylation of the seven-membered ring has been accomplished by a palladium-mediated Tsuji-Trost reaction. The K-ring methyl and hydroxyl groups have been installed in a highly stereoselective manner by sequential conjugate reduction and enolate oxidation reactions. Ring L has been constructed by a use of a novel relay ring-closing metathesis reaction to complete the tetracyclic framework, which possesses the functionality necessary for elaboration of rings I and L and the introduction of ring M.

Gas Phase Identification of the Elusive N-Hydroxyoxaziridine (c-H2CON(OH)): A Chiral Molecule

The hitherto elusive N-hydroxyoxaziridine molecule (c-H₂CON(OH)), a chiral, high energy isomer of nitromethane (CH₃NO₂) and one of the simplest representatives of an oxaziridine, is detected in the gas phase. Electronic structure calculations propose an impending synthesis eventually via addition of carbene (CH₂) to the nitrogen-oxygen double bond of nitrous acid (HONO). The oxaziridine ring demonstrates an unusual kinetic stability toward ring opening compared to the isoeletronic cyclopropane (C₃H₆) counterpart. This system defines a fundamental benchmark to explore the formation and stability of racemic derivatives of strained oxaziridines (c-H₂CONH) and changes our perception how we think about fundamental decomposition and isomerization mechanisms in (model compounds of) energetic materials.

Gas phase identification of the elusive oxaziridine (cyclo-H2CONH) – an optically active molecule

First detection of the chiral oxaziridine (cyclo-H₂CONH) molecule.

Expanding the Toolbox of Chemoselective Modifications of Protein-Like Polymers at Methionine Residues

Selective modifications at methionyl residues in proteins have attracted particular attention in recent years. Previously described methods to chemoselectively modify the methionine side chain in elastin-like polypeptides (ELPs) involved nucleophilic addition using alkyl halides or epoxides yielding a sulfonium group with a positive charge strongly affecting ELPs' physicochemical properties, in particular their thermal responsiveness. We herein explored the recently reported ReACT method (Redox-Activated Chemical Tagging) based on the use of oxaziridine derivatives, yielding an uncharged sulfimide as an alternative route for chemoselective modifications of methionine-containing ELPs in aqueous medium. The different synthetic strategies are herein compared in order to provide a furnished toolbox for further biorthogonal postmodifications of any protein polymers.

Unified Approach to the Chemoselective α-Functionalization of Amides with Heteroatom Nucleophiles

Functionalization at the α-position of carbonyl compounds has classically relied on enolate chemistry. As a result, the generation of a new C–X bond, where X is more electronegative than carbon requires an oxidation event. Herein we show that, by rendering the α-position of amides electrophilic through a mild and chemoselective umpolung transformation, a broad range of widely available oxygen, nitrogen, sulfur, and halogen nucleophiles can be used to generate α-functionalized amides. More than 60 examples are presented to establish the generality of this process, and calculations of the mechanistic aspects underline a fragmentation pathway that accounts for the broadness of this methodology.

Intermolecular alkene difunctionalizations for the synthesis of saturated heterocycles

Saturated heterocycles are important structural motifs in a range of pharmaceuticals and agrochemicals. As a result of their importance, syntheses of these molecules have been extensively investigated. Despite the progress in this area, the most adopted strategies are still often characterized with inefficiency or relying on functionalizations with specialized precursors and pre-existing cores. This review highlights a dynamic synthetic strategy for the direct synthesis of saturated heterocycles from intermolecular alkene difunctionalizations. These coupling processes are highly modular, and therefore, offer perhaps the most convenient means to prepare diverse heterocyclic structures in compound libraries for bioactivity evoluations.

Oxygen transfer in electrophilic epoxidation probed by 17O NMR: differentiating between oxidants and role of spectator metal oxo

Peroxide compounds are used both in laboratory and industrial processes for the electrophilic epoxidation of olefins. Using NMR-spectroscopy, we investigate why certain peroxides engage in this type of reaction while others require activation by metal catalysts, e.g. methyltrioxorhenium (MTO). More precisely, an analysis of ¹⁷O NMR chemical shift and quadrupolar coupling parameters provides insights into the relative energy of specific frontier molecular orbitals relevant for reactivity. For organic peroxides or H₂O₂ a large deshielding is indicative of an energetically high-lying lone-pair on oxygen in combination with a low-lying σ*(O-O) orbital. This feature is particularly pronounced in species that engage in electrophilic epoxidation, such as peracids or dimethyldioxirane (DMDO), and much less pronounced in unreactive peroxides such as H₂O₂ and ROOH, which can however be activated by transition-metal catalysts. In fact, for the proposed active peroxo species in MTO-catalyzed electrophilic epoxidation with H₂O₂ an analysis of the ¹⁷O NMR chemical shift highlights specific π- and δ-type orbital interactions between the so-called metal spectator oxo and the peroxo moieties that raise the energy of the high-lying lone-pair on oxygen, thus increasing the reactivity of the peroxo species.

Oxidation of 5-methylaminomethyl uridine (mnm⁵U) by Oxone Leads to Aldonitrone Derivatives

Oxidative RNA damage is linked to cell dysfunction and diseases. The present work focuses on the in vitro oxidation of 5-methylaminomethyl uridine (mnm⁵U), which belongs to the numerous post-transcriptional modifications that are found in tRNA. The reaction of oxone with mnm⁵U in water at pH 7.5 leads to two aldonitrone derivatives. They form by two oxidation steps and one dehydration step. Therefore, the potential oxidation products of mnm⁵U in vivo may not be only aldonitrones, but also hydroxylamine and imine derivatives (which may be chemically more reactive). Irradiation of aldonitrone leads to unstable oxaziridine derivatives that are susceptible to isomerization to amide or to hydrolysis to aldehyde derivative.

N-Sulfonyl α-imino ester-derived chiral oxaziridines: catalytic asymmetric synthesis and application as a modular chiral organic oxidant

A novel class of chiral N-sulfonyl oxaziridines is catalytically prepared and introduced for use as highly reactive and enantiospecific oxidants.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Hypolipidemic effect of dihydroisoquinoline oxaziridine in high-fat diet-fed rats

Obesity is a serious health problem that increases the risk of many complications, including diabetes and cardiovascular disease. This study aims to evaluate, for the first time, the effects of oxaziridine 3 on lipoprotein lipase activity in the serum of rats fed with a high-fat diet (HFD) on body weight, lipid profile and liver-kidney functions. The administration of oxaziridine 3 to HFD-rats lowered body weight and inhibited the lipase activity of obese rats leading to notable decrease of T-Ch, TGs and LDL-Ch levels accompanied with an increase in HDL-Ch concentration in serum. Moreover, the findings of this study revealed that oxaziridine 3 helped to protect liver tissue from the appearance of fatty cysts. Additionally, oxaziridine 3 administration to HFD-rats induces antioxidant activity proven by the increase of superoxide dismutase (SOD) and catalase (CAT) activities and the decrease in Thiobarbituric acid reactive substances (TBARS) levels. It also induces the protection of liver-kidney functions confirmed by a decrease in the levels of toxicity parameters in blood.

Enantioselective organocatalytic oxidation of ketimine

with promoted the enantioselective organocatalytic oxidation of to afford oxaziridines bearing a tetrasubstituted carbon stereogenic center in high yields with up to 87% ee.

A simple and efficient method for the preparation of 5-hydroxy-3-acyltetramic acids

Oxidation of the bisenolates of 3-acyltetramic acid to the corresponding 5-hydroxylated compounds using molecular oxygen is reported. The deprotection of the resulting compounds was also achieved.

Synthesis and Evaluation of Antimicrobial Activity of Nitrones from Derivatives of Aryl-Substituted Dihydroisoquinoline

This paper describes the synthesis of a series of dihydroisoquinoline nitrones by isomerization of the corresponding oxaziridines. Nitrones4a–cwere obtained in excellent yields and high purity by a simple and effective method from the isomerization of oxaziridines. The synthesized compounds were also evaluated for their antibacterial activity against Gram-positive and Gram-negative bacteria and fungus.