Stereocontrol in a combined allylic azide rearrangement and intramolecular Schmidt reaction

CuI-Amidobis(phosphine)-Catalyzed Direct Amidation of Unactivated Alkanes via C(sp3)-H Activation

Herein, we describe an acid-base-free, sustainable, and efficient method for direct amidation of unactivated alkanes and toluene derivatives, using the dimeric CuI complex [CuI{o-Ph2PC6H4CONC6H4PPh2-o}2] (here onward referred to as [PNP-Cu]2). Using this method, C(sp3)-N bond formation was achieved through the activation of very challenging C(sp3)-H bonds in cycloalkanes, alkenes, allyl groups, and benzyl groups, with tolerance toward ketonic groups, heterocycles, and halide functionalities. One of the precatalysts, (PNHP-Cu-Npht) was isolated and structurally characterized. Isomerization in allyl-functionalized alkanes and selective benzylic alkylation in ketones were observed. This is a novel method for C(sp3)-N bond formation via direct N-alkylation of phthalimide, sulfonamide, benzamide, and phosphamidate.

Stereocontrol in an intermolecular Schmidt reaction of equilibrating hydroxyalkyl allylic azides

A selective intermolecular Schmidt reaction of equilibrating hydroxyalkyl allylic azides is reported to afford N-hydroxyalk-1-en-3-yl lactams in modest to high yields. For prochiral and chiral ketones, modest to high 1,5-diastereoselectivity was achieved, and the mechanistic analysis is supported by DFT calculation.

Asymmetric total synthesis strategies of halichlorine and pinnaic acid

Halichlorine and pinnaic acid are structurally related natural alkaloids isolated from different marine organisms. These two marine alkaloids bearing a 6-azaspiro[4.5]decane skeleton demonstrate a wide range of biological effects. It is this kind of unique structure and potentially valuable biological activity that have prompted strong synthetic interest, making it a research focus in recent years. Since the first total synthesis of halichlorine and pinnaic acid completed by Danishefsky's group, many groups have reported their outstanding synthesis methods especially the asymmetric synthesis strategies. This review summarizes the asymmetric synthesis strategies of halichlorine and pinnaic acid using a 6-azaspiro[4.5]decane skeleton as the key intermediate, which can provide some guidance for related work.

Synthesis of a New α-Azidomethyl Styrene from Safrole via a Dearomative Rearrangement

There is a growing interest in developing more efficient synthetic alternatives for the synthesis of nitrogen-containing allylic compounds. This article presents a straightforward two-step protocol to produce 5-(3-azidoprop-1-en-2-yl)benzo[d][1,3]dioxole 4 from the natural product safrole. The method yielded the expected α-azidomethyl styrene 4, in good yield, via a dearomative rearrangement.

Substituent effects on intramolecular Schmidt reactions: a theoretical study on the formation of bridged lactams

The competitive formation of isomeric bridged lactams via acid-catalyzed intramolecular Schmidt reactions from 3-azidoethylcyclopentanones is explored using density functional theory (DFT) calculations, primarily performed at the M06-2X/6-311++G(d,p) level of theory. The results indicate that specific substituents installed at α-carbons can efficiently control the regioselectivity of the reaction by lone pair-cation interactions or steric hindrance reversing the main product preference, whereas cation-π interactions are not so effective.

Visible light-initiated radical 1,3-difunctionalization of β,γ-unsaturated ketones

Radical-mediated 1,2-difunctionalization of olefins is a well-established synthetic technique widely used in the rapid construction of structurally diverse molecular entities. However, radical-mediated 1,3-difunctionalization reactions are rare, and the substrates are generally limited to strained skeletons. Here, we report a practical approach for 1,3-difunctionalization of available β,γ-unsaturated ketones via a radical cascade process including visible light-irradiated radical addition, thermodynamic stability-driven 1,2-carbonyl migration from unactivated all-carbon quaternary center, and terminal C-radical varied transformations. Various highly functionalized alkyl skeletons with different valuable functional groups at positions 1 and 3 and the carbonyl group at position 2 have been synthesized through a radical chain pathway or Cu-catalyzed Ritter-type reaction. Moreover, this protocol provides a real case of diversity-oriented radical rearrangement for drug discovery. We identified a previously unknown chemotype of dual inhibitors for hypoxia-inducible factor (HIF) and WNT signaling pathways from products. These small-molecule inhibitors could suppress HIF and WNT signaling-dependent HCT116 cell growth in 2D and 3D culture systems.

Construction of azaspirocyclic skeletons mediated by the carbonyl of the Weinreb amide: formal total synthesis of (±)-cephalotaxine

A facile Stevens rearrangement of the Weinreb amide and the subsequent key steps mediated by the carbonyl of the Weinreb amide led to the construction of azaspirocyclic skeletons of some typical alkaloids. And the formal total synthesis of (±)-cephalotaxine was completed via a shorter synthetic route by this efficient method. Further studies on the development and asymmetric synthesis application of this strategy are underway.

Selective hydroboration of equilibrating allylic azides

The iridium(I)-catalyzed hydroboration of equilibrating allylic azides is reported to provide only the anti-Markovnikov product of alk-1-ene isomers in good yields and with good functional group tolerance.

Stereoselective and Stereospecific Triflate-Mediated Intramolecular Schmidt Reaction: Ready Access to Alkaloid Skeletons*

The stereoselectivity and stereospecificity of the triflate-mediated intramolecular Schmidt reaction of substituted 3-(1-azidocyclohexyl)propanol derivatives leading to octahydro-1H-pyrrolo[1,2-a]azepine, the structural skeleton of several important families of alkaloids such as the Stemona alkaloids, has been examined. The reaction involves an initial intramolecular S_N 2 reaction between the azide moiety and the triflate affording an intermediate spirocyclic aminodiazonoium salt that undergoes the expected 1,2-shift/N₂ -elimination followed by hydride-mediated iminium salt reduction. Remarkably, chiral alcohols are converted to the azabicyclic derivative with no or limited racemization. The initial asymmetric alcohol center controls the diastereoselectivity of the whole process, leading to the formation of one out of the four possible diastereoisomers of disubstituted octahydro-1H-pyrrolo[1,2-a]azepine. The origin of the stereoselectivity is rationalized based on theoretical calculations. The concise synthesis of (-)-(cis)-3-propylindolizidine and (-)-(cis)-3-butyllehmizidine, two alkaloids found in the venom of workers of the ant Myrmicaria melanogaster, is reported.

Highly selective hydrosilylation of equilibrating allylic azides

The Pt-catalyzed hydrosilylation of equilibrating allylic azides is reported. The reaction provides only one out of four possible hydrosilylation products in good yields and with very high chemoselectivity (alk-1-ene vs. alk-2-ene), regioselectivity (linear vs. branched), and excellent functional group tolerance.

Stereocontrolled Microwave-Assisted Domino [3,3]-Sigmatropic Reactions: A Winstein-Overman Rearrangement for the Formation of Differentiated Contiguous C-N Bonds

A domino [3,3]-sigmatropic rearrangement sequence employing a sequential reversible allylic azide rearrangement followed by an irreversible Overman reaction provides a new route to the formation of two contiguous C-N bonds. The reaction occurs in a stereocontrolled fashion in two steps from readily available alkenyl epoxides via initial azide anion ring opening of the epoxides.

Intramolecular Csp3-H/C-C bond amination of alkyl azides for the selective synthesis of cyclic imines and tertiary amines

The intramolecular Csp³–H and/or C–C bond amination is very important in modern organic synthesis due to its efficiency in the construction of diversified N-heterocycles. Herein, we report a novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines through selective Csp³–H and/or C–C bond cleavage. Two C–N single bonds or a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N double bond are efficiently constructed in these transformations. The carbocation mechanism differs from the reported metal nitrene intermediates and therefore enables metal-free and new transformation.

A novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines has been developed. The aliphatic C–H or C–C bond was selectively cleaved with the efficient formation of two C–N single bonds or a CN double bond.

Insight into the factors controlling the equilibrium of allylic azides

Several allylic azides with different double bond substitutions were studied to understand the factors, governing their equilibrium using density functional theory along with the quantum theory of atoms in molecules, non-covalent interactions and natural bond orbital approaches. The results showed that the hydroxyl group or heteroatoms in allylic azides interact with the molecule through an electrostatic weak interaction in each pair of regioisomers. The equilibrium shifts of substituted allylic azides, compared to non-substituted allylic azides, were not attributed to the presence of specific interactions, such as hydrogen bonds. The observed equilibrium shifts stemmed mainly from the strengthening and weakening of negative hyperconjugative interactions, which were affected by the weak interaction involving the proximal substituent in each regioisomer. A good linear correlation was obtained between the hyperconjugative energies of πC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C→σ*Z_b interactions and the calculated percentages of the secondary azide and tertiary azide in the equilibrium mixture. Also, the effect of the aromatic ring substituent was analysed using such approaches. This study not only provides insights into the factors controlling the stabilities of the substituted allylic azides, but also settles the basis to predict the regioisomer predominance in the equilibrium mixture.

The factors controlling the allyl azides equilibrium has been studied by different theoretical approaches setting the basis to predict the regioisomers predominance in the equilibrium mixture.

Iron(ii)-chloride-catalyzed regioselective azidation of allenamides with TMSN3

The first report of iron catalyzed azidation of allenamides via radical process to provide an efficient route for accessing allyl azides.

On the Winstein rearrangement: equilibrium and mechanism

Allylic azides are underutilized in organic synthesis when compared to other organic azides or other allylic functionality. This is likely because allylic azides rearrange at room temperature, resulting in a potentially complex mixture of azides. This rearrangement has been termed the Winstein rearrangement. Understanding the mechanism and basic principles governing the allylic azide equilibrium may aid in developing applications for these molecules based on either alkene or azide functionalization. Presented herein is a compilation of the key observations regarding the nature of the allylic azide rearrangement. Mechanistic considerations are explicitly addressed with key examples from the literature.

Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Organic azides are useful synthetic intermediates, which demonstrate broad reactivity. Unlike most organic azides, allylic azides can spontaneously rearrange to form a mixture of isomers. This rearrangement has been named the Winstein rearrangement. Using allylic azides can result in low yields and azide racemization in some synthetic contexts due to the Winstein rearrangement. Effort has been made to understand the mechanism of the Winstein rearrangement and to take advantage of this process. Several guiding principles can be used to identify which azides will produce a mixture of isomers and which will resist rearrangement. Selective reaction conditions can be used to differentiate the azide isomers in a dynamic manner. This review covers all aspects of allylic azides including their synthesis, their reactivity, the mechanism of the Winstein rearrangement, and reactions that can selectively elaborate an azide isomer. This review covers the literature from Winstein's initial report to early 2019.

Automatic mapping of atoms across both simple and complex chemical reactions

Mapping atoms across chemical reactions is important for substructure searches, automatic extraction of reaction rules, identification of metabolic pathways, and more. Unfortunately, the existing mapping algorithms can deal adequately only with relatively simple reactions but not those in which expert chemists would benefit from computer's help. Here we report how a combination of algorithmics and expert chemical knowledge significantly improves the performance of atom mapping, allowing the machine to deal with even the most mechanistically complex chemical and biochemical transformations. The key feature of our approach is the use of few but judiciously chosen reaction templates that are used to generate plausible "intermediate" atom assignments which then guide a graph-theoretical algorithm towards the chemically correct isomorphic mappings. The algorithm performs significantly better than the available state-of-the-art reaction mappers, suggesting its uses in database curation, mechanism assignments, and - above all - machine extraction of reaction rules underlying modern synthesis-planning programs.

Enantioselective Copper Catalyzed Alkyne-Azide Cycloaddition by Dynamic Kinetic Resolution

The copper(I) catalyzed alkyne-azide cycloaddition (CuAAC), a click reaction, is one of the most powerful catalytic reactions developed during the last two decades. Conducting CuAAC enantioselectively would add a third dimension to this reaction and would enable the direct synthesis of α-chiral triazoles. Doing so is demanding because the two precursors have linear geometries, and the triazole product is a flat heterocycle. Designing a chiral catalyst is further complicated by the complex mechanism of CuAAC. We report an enantioselective CuAAC (E-CuAAC), enabled by dynamic kinetic resolution (DKR). The E-CuAAC is high yielding and affords up to 99:1 er. The E-CuAAC can directly generate α-chiral triazoles in a complex molecular environment.

Practical regio- and stereoselective azidation and amination of terminal alkenes

There is significant interest in developing more rapid and efficient production of nitrogen-containing allylic compounds, as widely used in various syntheses. This work reports a variety of allylic azides and allylic amines synthesized by an efficient, new one-pot protocol that employs readily available terminal alkenes as starting materials. This method is highly regio- and stereoselective, affording the linear (E)-isomer, under metal-free conditions. This process tolerates several functional groups including halogen-containing molecules; it is general for azides and amine nucleophiles; and, adducts were obtained in good yields.

Catalytic Racemization of Activated Organic Azides

The first detailed description of the catalytic racemization of activated benzylic and allylic azides under mild conditions is reported. A kinetic analysis of the observed racemization indicates a first-order dependence on azide, a first-order dependence on catalyst, and that the rate of racemization correlates to σ⁺. A variety of azides with varying substitution patterns undergo facile racemization, and catalyst selection can tune this process.

"It's a (Kinetic) Trap!" - Selectively Differentiating Allylic Azide Isomers

Allylic azides are known to undergo the Winstein rearrangement and are often isolated as an equilibrating mixture of isomers. While this process has been known for almost 60 years, very few synthetic applications of this process have been reported. The absence of methods exploiting these intermediates likely stems from a paucity of approaches for gaining the required selectivity to differentiate the isomers. Our lab has made some progress in leveraging this unusual reaction into practical synthetic methodology. Presented herein is a summary of our lab's recent accomplishments in selectively trapping allylic azides.

Evidence for a Sigmatropic and an Ionic Pathway in the Winstein Rearrangement

The spontaneous rearrangement of allylic azides is thought to be a sigmatropic reaction. Presented herein is a detailed investigation into the rearrangement of several allylic azides. A combination of experiments including equilibrium studies, kinetic analysis, density functional theory calculations, and selective ¹⁵N-isotopic labeling are included. We conclude that the Winstein rearrangement occurs by the assumed sigmatropic pathway under most conditions. However, racemization was observed for some cyclic allylic azides. A kinetic analysis of this process is provided, which supports a previously undescribed ionic pathway.

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization

Highly functionalized aminocyclopentadienes can be formed through the rearrangement of anions generated from readily prepared 6-silyl-1,2-dihydropyridines by desilylation with fluoride. The scope and generality of the reaction was defined, and diverse transformations were performed on the highly functionalized products. A mechanism and driving force for the rearrangement were identified from experiments and DFT calculations.

Base-Controlled Regiodivergent Azidation of Trifluoromethyl Alkenyl Triflates: Transition-Metal-Free Access to CF3-Containing Allyl Azides and Alkenyl Azides

A base-controlled regiodivergent azidation of trifluoromethyl alkenyl triflates providing either (E)-3-azido-1-aryl-4,4,4-trifluorobut-1-ene (CF₃-containing allyl azides) or (Z)-1-azido-1-aryl-4,4,4-trifluorobut-1-ene (CF₃-containing alkenyl azides) is described. Catalyzed by Et₃N, the azidation of trifluoromethyl alkenyl triflates with TMSN₃ gave CF₃-containing allyl azides. On the other hand, using stoichiometric DBU, the regioisomeric azidation products, CF₃-containing alkenyl azides, were obtained in good yield. A further transformation for CF₃-containing amines, triazoles, and azirines highlights the practical applicability of this transition-metal-free protocol.

Stereoselective Dynamic Cyclization of Allylic Azides: Synthesis of Tetralins, Chromanes, and Tetrahydroquinolines

This report describes the stereoselective synthesis of 3-azido-tetralins, -chromanes, and -tetrahydroquinolines via a tandem allylic azide rearrangement/Friedel-Crafts alkylation. Exposure of allylic azides with a pendant trichloroacetimidate to catalytic quantities of AgSbF6 proved optimal for this transformation. This cascade successfully differentiates the equilibrating azide isomers, providing products in excellent yield and selectivity (>25 examples, up to 94% yield and >25:1 dr). In many cases, the reactive isomer is only a trace fraction of the equilibrium mixture, keenly illustrating the dynamic nature of these systems. We demonstrate the utility of this process via a synthesis of hasubanan.

Catalytic enantioselective synthesis of α-chiral azides

The catalytic asymmetric synthesis of α-chiral azides is of current interest and three synthetic strategies have been developed. This review summarizes the recent progress in this research area, discusses the advantages and limitations of each strategy, and outlines synthetic opportunities for future research.

Dynamic Kinetic Resolution of Allylic Azides via Asymmetric Dihydroxylation

The catalytic enantioselective preparation of densely functionalized amines is a fundamental synthetic challenge. To address this challenge, we report for the first time that the Winstein rearrangement can be enlisted as the racemization pathway in a dynamic kinetic resolution of allylic azides. Alkene functionalization by Sharpless dihydroxylation affords tertiary azides in excellent enantioselectivity (up to 99:1 er). This approach establishes the chirality of the tertiary azide, obviates the need to directly forge either a congested C-N or C-C bond at the new nitrogenous stereocenter, and establishes additional functionality. Several examples demonstrate further elaboration of this functionality.

Allylic Azide Rearrangement in Tandem with Huisgen Cycloaddition for Stereoselective Annulation: Synthesis of C-Glycosyl Iminosugars

Allylic azide rearrangement is used in tandem with intramolecular azide-alkene cycloaddition to give a triazoline that when subsequently decomposed in the presence of a nucleophile gives piperidines. The tandem reaction gives two stereocenters that are generated with high control. The formation of the piperidines required the presence of innate conformational constraint. The applicability of the annulation reaction is demonstrated by the synthesis of iminosugars. A proposal is included to account for the observed stereoselectivity, which is influenced by the precursor structure.

Practical Electrochemical Anodic Oxidation of Polycyclic Lactams for Late Stage Functionalization

Electrochemistry provides a powerful tool for the late-stage functionalization of complex lactams. A two-stage protocol for converting lactams, many of which can be prepared through the intramolecular Schmidt reaction of keto azides, is presented. In the first step, anodic oxidation in MeOH using a repurposed power source provides a convenient route to lactams bearing a methoxy group adjacent to nitrogen. Treatment of these intermediates with a Lewis acid in dichloromethane permits the regeneration of a reactive acyliminium ion that is then reacted with a range of nucleophilic species.

Synthesis of novel N-cyclopentenyl-lactams using the Aubé reaction

A novel and convenient method utilizing the Aubé reaction to access a new class of compounds that are similar to carbocyclic nucleosides is reported. The azido alcohol derived from Vince lactam undergoes the Aubé reaction with various cyclic ketones to give cyclopentenyl-substituted lactams. Upon dihydroxylation, this affords the N-cyclopentenyl-lactam compounds in racemic form. Given the numerous uses of nucleosides and related compounds, we were interested in the synthesis of carbocylic nucleoside mimics. The attempts and results are described herein.

A concomitant allylic azide rearrangement/intramolecular azide-alkyne cycloaddition sequence

An intramolecular Huisgen cycloaddition of an interconverting set of isomeric allylic azides with alkynes affords substituted triazoles in high yield. The stereoisomeric vinyl-substituted triazoloxazines formed depend on the rate of cycloaddition of the different allylic azide precursors when the reaction is carried out under thermal conditions. In contrast, dimerized macrocyclic products were obtained when the reaction was done using copper(I)-catalyzed conditions, demonstrating the ability to control the reaction products through changing conditions.