Modular functionalization of allenes to aminated stereotriads

The Development of a Sulfamate-Tethered Aza-Michael Cyclization Allows for the Preparation of (-)-Negamycin tert-Butyl Ester

We present the first examples of intramolecular aza-Michael cyclizations of sulfamates and sulfamides onto pendant α,β-unsaturated esters, thioesters, amides, and nitriles. Stirring the substrate with catalytic quantities of the appropriate base delivers the product in good yield and excellent diastereoselectivity. The reactions are operationally simple, can be performed open to air, and are tolerant of a variety of important functional groups. We highlight the utility of this technology by using it in the preparation of a (-)-negamycin derivative.

Progress toward the Total Synthesis of Jogyamycin Using a Tandem Ichikawa/Winstein Rearrangement

Jogyamycin is a densely functionalized aminocyclopentitol that displays potent antiprotozoal activity. Herein, we report a route toward this natural product that utilizes an unprecedented transformation involving a tandem Ichikawa-Winstein rearrangement to install the C-1/C-2 diamine core. Attempts to further functionalize the C-3/C-4 alkene en route to jogyamycin are also discussed.

Ring Opening of Aziridines by Pendant Sulfamates Allows for Regioselective and Stereospecific Preparation of Vicinal Diamines

The ring opening of aziridines by pendant sulfamates is a viable strategy for the rapid preparation of vicinal diamines. Our reaction is compatible with both disubstituted cis- and trans-aziridines; unsubstituted, N-alkyl, and N-aryl sulfamates engage effectively. In all cases examined, the cyclization reaction is perfectly regioselective and stereospecific. Once activated, the product oxathiazinane heterocycles can be ring opened with a diverse range of nucleophiles.

Lewis Acid-Mediated Electrophilic Thiolative Difunctionalization of Enimides: Rapid Access to β-Amino Sulfides

An efficient and practical route for the synthesis of β-amino sulfides by Lewis acid-mediated electrophilic thiolative difunctionalization of enimides is disclosed. A series of free phenols, electron-rich arene, alcohol, azide, and hydride, are successfully incorporated into the substrates in high regio- and stereoselectivities under mild conditions. The obtained products possess multiple functional groups and can be easily transformed to other valuable molecules.

Modular and practical diamination of allenes

Vicinal diamines are privileged scaffolds in medicine, agrochemicals, catalysis, and other fields. While significant advancements have been made in diamination of olefins, diamination of allenes is only sporadically explored. Furthermore, direct incorporation of acyclic and cyclic alkyl amines onto unsaturated π systems is highly desirable and important, but problematic for many previously reported amination reactions including the diamination of olefins. Herein, we report a modular and practical diamination of allenes, which offers efficient syntheses of β,γ-diamino carboxylates and sulfones. This reaction features broad substrate scope, excellent functional group tolerability, and scalability. Experimental and computational studies support an ionic reaction pathway initiated with a nucleophilic addition of the in situ formed iodoamine to the electron deficient allene substrate. An iodoamine activation mode via a halogen bond with a chloride ion was revealed to substantially increase the nucleophilicity of the iodoamine and lower the activation energy barrier for the nucleophilic addition step.

Rh(I)-Catalyzed Coupling of Azides with Boronic Acids Under Neutral Conditions

Because of the importance of polyfunctional amines, C-N bond formation is important in synthetic organic chemistry. Here we present a neutral amination reaction using azides as the nitrogen source and arylboronic acids with a rhodium(I) catalyst to afford alkyl-aryl and aryl-aryl secondary amines. Natural products and pharmaceutical derivatives were applied, and gram-scale reactions were performed, which demonstrated the utility. Mechanistic experiments and DFT calculations suggested that the reaction involves a metal-nitrene intermediate.

In Situ Generation of N-Triflylimino-λ3-iodanes: Application to Imidation of Phosphines and Catalytic α-Amidation of 1,3-Dicarbonyl Compounds

We describe the imidation of phosphines and α-amidation of 1,3-dicarbonyl compounds using N-triflylimino-λ³-iodane, which is generated in situ from iodosylarene and triflylamide without any other additives. Furthermore, the imino-λ³-iodane catalytically generated from an iodoarene precatalyst with oxone and triflylamide promotes α-amidation of 1,3-dicarbonyl compounds, representing the first method catalyzed by imino-λ³-iodane.

Combined radical and ionic approach for the enantioselective synthesis of β-functionalized amines from alcohols

Chiral amines are among the most important organic compounds and have widespread applications. Enantioselective construction of chiral amines is a major aim in organic synthesis. Among synthetic methods, direct functionalization of omnipresent C-H bonds with common organic nitrogen compounds represents one of the most attractive strategies. However, C-H amination strategies are largely limited to constructing a specific type of N-heterocycles or amine derivatives. To maximize the synthetic potential of asymmetric C-H amination, we report here an approach that unites the complementary reactivities of radical and ionic chemistry for streamlined synthesis of functionalized chiral amines. This synthesis merges the development of an enantioselective radical process for 1,5-C(sp ³)-H amination of alkoxysulfonyl azides via Co(II)-based metalloradical catalysis with an enantiospecific ionic process for ring-opening of the resulting five-membered chiral sulfamidates by nucleophiles. Given that alkoxysulfonyl azides are derived from the corresponding alcohols, this approach offers a powerful synthetic tool for enantioselective β-C-H amination of common alcohols while converting the hydroxy group to other functionalities through formal nucleophilic substitution.

Strategies for the Syntheses of Pactamycin and Jogyamycin

Pactamycin and jogyamycin are aminocyclopentitol natural products, where each core carbon bears a stereodefined alcohol or amine moiety. Their structural complexity, coupled with the diversity of functional groups coexisting in a condensed space, make them fascinating synthetic targets in their own right. Pactamycin and its derivatives bind to the 30S ribosomal subunit and display activity against parasites responsible for drug-resistant malaria and African sleeping sickness; however, efforts to develop their therapeutic potential have been hampered by their cellular toxicity. Interestingly, bioengineered analogues display differences in selectivity and toxicity towards mammalian cells, spurring efforts to develop flexible strategies to thoroughly probe structure-activity relationships (SAR), particularly in analogues lacking the C7 hydroxyl group of pactamycin. This review compares and contrasts approaches towards pactamycin and jogyamycin, including two successful total syntheses of the former. The implications of each route for preparing analogues to inform SAR and lead to compounds with increased selectivity for binding malarial over human ribosomes are briefly discussed.

Beyond osmium: progress in 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes

Olefin 1,2-difunctionalization has emerged as a popular strategy within modern synthetic chemistry for the synthesis of vicinal amino alcohols and derivatives. The advantage of this approach is the single-step simplicity for rapid diversification, feedstock nature of the olefin starting materials, and the possible modularity of the components. Although there is a vast number of possible iterations of 1,2-olefin difunctionalization, 1,2-amino oxygenation is of particular interest due to the prevalence of both oxygen and nitrogen within pharmaceuticals, natural products, agrochemicals, and synthetic ligands. The Sharpless amino hydroxylation provided seminal results in this field and displayed the value in achieving methods of this nature. However, a vast number of new and novel methods have emerged in recent decades. This review provides a comprehensive review of modern advances in accomplishing 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes that move beyond osmium to a range of other transition metals and more modern strategies such as electrochemical, photochemical, and biochemical reactivity.

Synthesis and applications of methyleneaziridines

Methyleneaziridines (MAs) are a special subset of aziridines featuring an exocyclic C–C double bond on the three-membered ring. They have found great potential in organic synthesis. In this review, the structural characterization of MAs, synthetic methods, chemical transformations and mechanisms, especially the advances achieved over the past decade are comprehensively summarized.

This review surveys the structural characterization, synthetic methods, chemical transformations and mechanisms of methyleneaziridines, especially the advances achieved over the past decade.

Aziridinium Ylides: Underutilized Intermediates for Complex Amine Synthesis

Harnessing the chemistry of onium ylide intermediates generated from transition metal catalysis is a powerful strategy to convert simple precursors into complex scaffolds. While the chemistry of onium ylides has been studied for over three decades, transformations of aziridinium ylides have just recently emerged as a versatile way to exploit the strain of these reactive intermediates to furnish densely functionalized N-heterocycles in a highly stereocontrolled manner. Herein, we provide a short overview of the key concepts and recent developments in this area, with a focus on how mechanistic studies to delineate the factors controlling the reactivity of aziridinium ylides can stimulate fruitful future investigations.

Intermolecular [3+3] ring expansion of aziridines to dehydropiperi-dines through the intermediacy of aziridinium ylides

The importance of N-heterocycles in drugs has stimulated diverse methods for their efficient syntheses. Methods that introduce significant stereochemical complexity are attractive for identifying new bioactive amine chemical space. Here, we report a [3 + 3] ring expansion of bicyclic aziridines and rhodium-bound vinyl carbenes to form complex dehydropiperidines in a highly stereocontrolled rearrangement. Mechanistic studies and DFT computations indicate that the reaction proceeds through formation of a vinyl aziridinium ylide; this reactive intermediate undergoes a pseudo-[1,4]-sigmatropic rearrangement to directly furnish heterocyclic products with net retention at the new C-C bond. In combination with asymmetric silver-catalyzed aziridination, enantioenriched scaffolds with up to three contiguous stereocenters are rapidly delivered. The mild reaction conditions, functional group tolerance, and high stereospecificity of this method are well-suited for appending piperidine motifs to natural product and complex molecules. Ultimately, our work establishes the value of underutilized aziridinium ylides as key intermediates for converting small, strained rings to larger N-heterocycles.

Stereocontrolled Synthesis of the Aminocyclopentitol Core of Jogyamycin via an Ichikawa Rearrangement Reaction

Jogyamycin is a member of the aminocyclopentitol class of natural products that exhibits significant antiprotozoal activities against diseases that include African sleeping sickness and malaria. Herein, we report a route to the core of this natural product via an underutilized Ichikawa rearrangement as a key step. This route efficiently forms the cyclopentane ring from simple and easily accessible starting materials and rapidly installs the C1/C4/C5 polar functional groups. In addition, this strategy shows excellent potential for the preparation of analogues of jogyamycin to study how structural changes impact the selectivity in binding to the ribosome.

Oxidative allene amination for the synthesis of nitrogen-containing heterocycles

The prevalence of stereochemically complex amines in natural products, pharmaceuticals and other bioactive compounds, coupled with the challenges inherent in their preparation, has inspired our work to develop new and versatile methodologies for the synthesis of amine-containing stereotriads ('triads'). The key step is a highly chemo-, regio-, and stereoselective transition-metal catalyzed nitrene transfer reaction that transforms one of the cumulated double bonds of an allene precursor into a bicyclic methyleneaziridine intermediate. This account summarizes our strategies to rapidly elaborate such intermediates into stereochemically rich, densely functionalized amine triads, nitrogen heterocycles, aminated carbocycles and other useful synthetic building blocks.

Ring Expansion of Bicyclic Methyleneaziridines via Concerted, Near-Barrierless [2,3]-Stevens Rearrangements of Aziridinium Ylides

The synthesis of densely functionalized azetidinesin a highly stereocontrolled manner is challenging, but interest in the bioactivities of these small heterocycles has stimulated methods for their preparation. We recently reported a one-carbon ring expansion of bicyclic methylene aziridines under dirhodium catalysis capable of delivering enantioenriched azetidines. This work explores this ring expansion using computational and experimental studies. DFT computations indicate that the reaction proceeds through formation of an aziridinium ylide, which is precisely poised for concerted, asynchronous ring-opening/closing to deliver the azetidines in a [2,3]-Stevens-type rearrangement. The concerted nature of this rearrangement is responsible for the stereospecificity of the reaction, where axial chirality from the initial allene substrate is transferred to the azetidine product with complete fidelity. The computed mechanistic pathway highlights the key roles of the olefin and the rigid structure of the methylene aziridine in differentiating our observed ring expansion from competing cheletropic elimination pathways noted with ylides derived from typical aziridines.

Reaction of Ynamides with Iminoiodinane-Derived Nitrenes: Formation of Oxazolones and Polyfunctionalized Oxazolidinones

This article describes the reaction of ynamides with metallanitrenes generated in the presence of an iodine(III) oxidant. N-(Boc)-Ynamides are converted to oxazolones via a cyclization reaction. The reaction is mediated by a catalytic dirhodium-bound nitrene species that first behaves as a Lewis acid. The oxazolones can be converted in a one pot manner to functionalized oxazolidinones following a regio- and stereoselective oxyamination reaction with the same nitrene reagent generated in stoichiometric amounts.

A Stereoselective [3+1] Ring Expansion for the Synthesis of Highly Substituted Methylene Azetidines

The reaction of rhodium-bound carbenes with strained bicyclic methylene aziridines results in a formal [3+1] ring expansion to yield highly substituted methylene azetidines with excellent regio- and stereoselectivity. The reaction appears to proceed through an ylide-type mechanism, where the unique strain and structure of the methylene aziridine promotes a ring-opening/ring-closing cascade that efficiently transfers chirality from substrate to product. The resultant products can be elaborated into new azetidine scaffolds containing vicinal tertiary-quaternary and even quaternary-quaternary stereocenters.

Fluorinated Amine Stereotriads via Allene Amination

The incorporation of fluorine into organic scaffolds often improves the bioactivity of pharmaceutically relevant compounds. C-F/C-N/C-O stereotriad motifs are prevalent in antivirals, neuraminidase inhibitors, and modulators of androgen receptors, but are challenging to install. An oxidative allene amination strategy using Selectfluor rapidly delivers triply functionalized triads of the form C-F/C-N/C-O, exhibiting good scope and diastereoselectivity for all syn products. The resulting stereotriads are readily transformed into fluorinated pyrrolidines and protected α-, β-, and γ-amino acids.

Rh2(II)-Catalyzed Ring Expansion of Cyclobutanol-Substituted Aryl Azides To Access Medium-Sized N-Heterocycles

A new reactivity pattern of Rh₂(II)-N-arylnitrenes was discovered that facilitates the synthesis of medium-sized N-heterocycles from ortho-cyclobutanol-substituted aryl azides. The key ring-expansion step of the catalytic cycle is both chemoselective and stereospecific. Our mechanistic experiments implicate the formation of a rhodium N-arylnitrene catalytic intermediate and reveal that sp³ C-H bond amination of this electrophilic species is competitive with the ring-expansion process.

Square sugars: challenges and synthetic strategies

The synthesis of square sugars requires innovative strategies based on efficient stereoselective methodologies, from organocatalysis to metal carbene insertion.

Stereocontrolled Syntheses of Seven-Membered Carbocycles by Tandem Allene Aziridination/[4+3] Reaction

A tandem allene aziridination/[4+3]/reduction sequence converts simple homoallenic sulfamates into densely functionalized aminated cycloheptenes, where the relative stereochemistry at five contiguous asymmetric centers can be controlled through the choice of the solvent and the reductant. The products resulting from this chemistry can be readily transformed into complex molecular scaffolds which contain up to seven contiguous stereocenters.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Diastereoselective Synthesis of the Aminocyclitol Core of Jogyamycin via an Allene Aziridination Strategy

Oxidative allene amination provides rapid access to densely functionalized amine-containing stereotriads through highly reactive bicyclic methyleneaziridine intermediates. This strategy has been demonstrated as a viable approach for the construction of the densely functionalized aminocyclitol core of jogyamycin, a natural product with potent antiprotozoal activity. Importantly, the flexibility of oxidative allene amination will enable the syntheses of modified aminocyclitol analogues of the jogyamycin core.

Oxidative allene amination for the synthesis of azetidin-3-ones

Regioselectivity in the aziridination of silyl-substituted homoallenic sulfamates is readily diverted to the distal double bond of the allene to yield endocyclic bicyclic methyleneaziridines with excellent stereocontrol. Subsequent reaction with electrophilic oxygen sources initiates facile rearrangement to densely functionalized, fused azetidin-3-ones in excellent d.r., effectively transferring the axial chirality of the allene to central chirality in the products. The steric nature of the silyl group dictates which of the two rings of the fused azetidin-3-one will undergo further functionalization, providing an additional element of diversity for the preparation of enantioenriched azetidine scaffolds with potential biological activity.

Spontaneous resolution upon crystallization of allenyl-bis-phosphine oxides

The first example of ‘spontaneous resolution by crystallization’ in allene chemistry, by means of crystal structures and solid state CD spectra of the R and S enantiomers, is presented.