Organic chemistry. Strain-release amination

α-Cyclodextrin Encapsulation of Bicyclo[1.1.1]pentane Derivatives: A Storable Feedstock for Preparation of [1.1.1]Propellane

The bicyclo[1.1.1]pentane (BCP) scaffold is useful in medicinal chemistry, and many protocols are available for synthesizing BCP derivatives from [1.1.1]propellane. Here, we report (1) the α-cyclodextrin (α-CD) encapsulation of BCP derivatives, affording a stable, readily storable material from which BCPs can be easily and quantitatively recovered and (2) new and simple protocols for deiodination reaction of 1,3-diiodo BCP to afford [1.1.1]propellane in protic/aprotic/polar/non-polar solvents. The combination of these methodologies enables simple, on-demand preparation of [1.1.1]propellane in various solvents under mild conditions from α-CD capsules containing 1,3-diiodo BCP.

Generation and trapping of electron-deficient 1,2-cyclohexadienes. Unexpected hetero-Diels-Alder reactivity

Keto-substituted 1,2-cyclohexadienes were generated by base-mediated (KOt-Bu) elimination, and found to dimerize via an unprecedented formal hetero-Diels-Alder process, followed by hydration. These highly reactive cyclic allene intermediates were also trapped in Diels-Alder reactions by furan, 2,5-dimethylfuran, or diphenylisobenzofuran to afford cycloadducts with high regio- and diastereoselectivity, and could also be intercepted in a hetero-Diels-Alder process with enamine dienophiles. Endo/exo stereochemistry was unambiguously determined via X-ray crystallography in the case of nitrile-substituted 1,2-cyclohexadiene. DFT calculations indicate that the novel hetero-Diels-Alder processes observed with these allenes occur via a concerted asynchronous cycloaddition mechanism.

Abiotic scaffolds in medicinal chemistry: not a waste of chemical space

It is well established that medicinal chemists should depart from the flat, sp²-dominated nature of traditional drugs and incorporate complexities of bioactive natural products, such as sp³-richness, 3D topology and chirality. There is a gray area, however, in the relevance of newly developed chemical scaffolds that exhibit these complexities but do not correlate to anything observed in nature. This can leave synthetic methodologists searching for structural similarities between their newly developed products and known natural products in search of justification. This article offers a perspective on how these types of complex 'abiotic' scaffolds can be appreciated purely on the basis of their structural novelty, and identifies the unique advantages arising when a complex chemical entity unrecognized by nature is introduced to biological systems.

Strain-Induced Nucleophilic Ring Opening of Donor-Acceptor Cyclopropenes for Synthesis of Monosubstituted Succinic Acid Derivatives

1,2,3-Trisubstituted donor-acceptor cyclopropenes (DACPs) generated in situ from enoldiazo compounds react with nucleophiles to form α-substituted succinic acid derivatives in high yields. Initial dirhodium(II) carboxylate catalysis rapidly converts enoldiazo-acetates or -acetamides to DACPs that undergo catalyst-free Favorskii ring opening with amines, and also with anilines, alcohols, and thiols, when facilitated by catalytic amounts of 4-dimethylaminopyridine (DMAP). This methodology provides easy access to mixed esters and amides of monosubstituted succinic acids, including derivatives of naturally occurring compounds. It also affords dihydrazide, dihydroxamic acid, and diamide derivatives, as well as α-substituted tetrahydropyridazine-3,6-diones in high yields. Attempts to generate optically enriched DACPs were not successful because their populations having the R and S configurations formed with a chiral dirhodium catalyst are quite similar, and the loss of enantiocontrol likely originates from the DACP ring forming step which is reversible with its intermediate metal carbene.

Synthesis of polysubstituted azetidines via cascade trifluoromethylation/cyclization of N-allyl ynamides

A cascade trifluoromethylation/cyclization of N-allyl sulfonylynamides is developed, providing a direct access to azetidine-fused tricyclic compounds at room temperature.

Electrochemical C-H Amidation of Heteroarenes with N-Alkyl Sulfonamides in Aqueous Medium

The construction of C-N bonds by free radical reactions represents a powerful synthetic approach for direct C-H amidations of arenes or heteroarenes. Developing efficient and more environmentally friendly synthetic methods for C-H amidation reactions remains highly desirable. Herein, metal-free electrochemical oxidative dehydrogenative C-H amidations of heteroarenes with N-alkylsulfonamides have been accomplished. The catalyst- and chemical-oxidant-free C-H amidation features an ample scope and employs electricity as the green and sole oxidant. A variety of heteroarenes, including indoles, pyrroles, benzofuran and benzothiophene, thereby underwent this C(sp² )-H nitrogenation. Cyclic voltammetry studies and control experiments provided evidence for nitrogen-centered radicals being directly generated under metal-free electrocatalysis.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Highly Regioselective Addition of Allylic Zinc Halides and Various Zinc Enolates to [1.1.1]Propellane

We report a range of highly regioselective openings of [1.1.1]propellane with various allylic zinc halides, as well as zinc enolates of ketones, esters and nitriles. The resulting zincated bicyclopentanes (BCPs) were trapped with a range of electrophiles including acyl chlorides, sulfonothioates, hydroxylamino benzoates, tosyl cyanide as well as aryl and allyl halides, generating highly functionalized BCP-derivatives. The unusually high regioselectivity of these reactions has been rationalized using DFT calculations. A bioisostere of the synthetic opioid pethidine was prepared in 95 % yield in one step using this method.

A Single-Step Synthesis of Azetidine-3-amines

The azetidine group is frequently encountered within contemporary medicinal chemistry. However, the introduction of an azetidine can be synthetically challenging. Herein, a straightforward synthesis of azetidine-3-amines, starting from a bench stable, commercial material is presented. The reaction tolerates common functionality and proceeds in moderate-to-high yield with secondary amines, and moderate-to-low yield with primary amines. The methodology compares favorably to alternative procedures and can be utilized in "any-stage" functionalization, including late-stage azetidinylation of approved drugs and other compounds with pharmacological activity.

Bicyclobutane Carboxylic Amide as a Cysteine-Directed Strained Electrophile for Selective Targeting of Proteins

Expanding the repertoire of electrophiles with unique reactivity features would facilitate the development of covalent inhibitors with desirable reactivity profiles. We herein introduce bicyclo[1.1.0]butane (BCB) carboxylic amide as a new class of thiol-reactive electrophiles for selective and irreversible inhibition of targeted proteins. We first streamlined the synthetic routes to generate a variety of BCB amides. The strain-driven nucleophilic addition to BCB amides proceeded chemoselectively with cysteine thiols under neutral aqueous conditions, the rate of which was significantly slower than that of acrylamide. This reactivity profile of BCB amide was successfully exploited to develop covalent ligands targeting Bruton's tyrosine kinase (BTK). By tuning BCB amide reactivity and optimizing its disposition on the ligand, we obtained a selective covalent inhibitor of BTK. The in-gel activity-based protein profiling and mass spectrometry-based chemical proteomics revealed that the selected BCB amide had a higher target selectivity for BTK in human cells than did a Michael acceptor probe. Further chemical proteomic study revealed that BTK probes bearing different classes of electrophiles exhibited distinct off-target profiles. This result suggests that incorporation of BCB amide as a cysteine-directed electrophile could expand the capability to develop covalent inhibitors with the desired proteome reactivity profile.

Re-Examination of Some Carbocations. Structures, Energies, and Charge Distributions

Chemists have had a long-standing interest in reactive intermediates such as carbenes, carbon radicals, carbanions, and carbocations. Carbocations are an interesting part of this group because of their tendency to undergo rearrangement, sometimes forming bridged ions, as well as their ability in many cases of spreading out the positive charge over several atoms. We have re-examined some of these cases using high-level compound procedures, W1BD and G4, as well as by considering the charge distributions making use of the Hirshfeld method that has been shown to uniquely correlate with several types of experimental data.

Difunctionalization of C-C σ-Bonds Enabled by the Reaction of Bicyclo[1.1.0]butyl Boronate Complexes with Electrophiles: Reaction Development, Scope, and Stereochemical Origins

Difunctionalization reactions of C-C σ-bonds have the potential to streamline access to molecules that would otherwise be difficult to prepare. However, the development of such reactions is challenging because C-C σ-bonds are typically unreactive. Exploiting the high ring-strain energy of polycyclic carbocycles is a common strategy to weaken and facilitate the reaction of C-C σ-bonds, but there are limited examples of highly strained C-C σ-bonds being used in difunctionalization reactions. We demonstrate that highly strained bicyclo[1.1.0]butyl boronate complexes (strain energy ca. 65 kcal/mol), which were prepared by reacting boronic esters with bicyclo[1.1.0]butyl lithium, react with electrophiles to achieve the diastereoselective difunctionalization of the strained central C-C σ-bond of the bicyclo[1.1.0]butyl unit. The reaction shows broad substrate scope, with a range of different electrophiles and boronic esters being successfully employed to form a diverse set of 1,1,3-trisubstituted cyclobutanes (>50 examples) with high diastereoselectivity. The high diastereoselectivity observed has been rationalized based on a combination of experimental data and DFT calculations, which suggests that separate concerted and stepwise reaction mechanisms are operating, depending upon the migrating substituent and electrophile used.

Saturated Bioisosteres of ortho-Substituted Benzenes

Saturated bioisosteres of ortho-disubstituted benzenes (bicyclo[2.1.1]hexanes) were synthesized, characterized and validated. These cores were incorporated into the bioactive compounds Valsartan, Boskalid and Fluxapyroxad instead of the benzene ring. The saturated analogues showed a similar level of antifungal activity compared to that of Boskalid and Fluxapyroxad.

Chiral 3-Acylglutaric Acid Derivatives from Strain-Induced Nucleophilic Retro-Claisen Ring-Opening Reactions

A nucleophilic retro-Claisen ring-opening of donor-acceptor cyclobutenes, formed with high stereocontrol by [3 + 1]-cycloaddition of TIPS-protected enoldiazoacetates with α-acyl sulfur ylides, has been developed. Removal of the TIPS group to form the isolable β-keto ester precedes the strain-induced ring-opening. Various amines, alcohols, thiols, and amino acid derivatives are effective nucleophiles, and their products are formed in very high yields via stoichiometric reactions. The chirality of the reactant donor-acceptor cyclobutenes is fully retained in the ring-opening reactions. The 3-acylglutaric acid products are converted to various valuable structures, including amido-diols, γ-aminobutyric acid (GABA) derivatives, and heterocycles.

Copper-Catalyzed Cross-Coupling between Alkyl (Pseudo)halides and Bicyclopentyl Grignard Reagents

The development of a copper-catalyzed cross-coupling between primary and secondary (pseudo)halides and bicyclopentyl Grignard reagents is reported. Highly strained bicyclopentanes can be cross-coupled with a large panel of primary alkyl mesylates and secondary alkyl iodides. The catalytic system is simple and cheap, and the reaction is general and chemoselective.

Iridium-Catalyzed Enantioselective Synthesis of α-Chiral Bicyclo[1.1.1]pentanes by 1,3-Difunctionalization of [1.1.1]Propellane

Bicyclo[1.1.1]pentanes (BCPs) have found application as bioisosteres of aromatic rings in drug development. However, catalytic construction of this motif with adjacent stereocenters with high enantioselectivity from readily available starting materials still constitutes a significant synthetic challenge. Herein we report a direct stereoselective synthesis of α-chiral allylic BCPs by 1,3-difunctionalization of [1.1.1]propellane with Grignard reagents and allyl carbonates using iridium catalysis. This mild protocol proceeds via initial organometallic addition to [1.1.1]propellane followed by asymmetric allylic substitution, providing the products with high enantioselectivities over a broad range of substrates. Further derivatization of the products demonstrates the applicability of this method to the preparation of structurally diverse libraries of chiral BCP derivatives.

Synthesis of azetidines by aza Paternò-Büchi reactions

This review discusses the current scope and limitations of the [2 + 2] photocycloaddition reaction between an imine and an alkene component, the aza Paternò–Büchi reaction, and highlights recent improvements within this area of research.

The [2 + 2] photocycloaddition reaction between an imine and an alkene component, the aza Paternò–Büchi reaction, is one of the most efficient ways to synthesize functionalized azetidines. However, the application of the aza Paternò–Büchi reaction has been met with limited success due to the inherent challenges associated with this approach. This review covers the current scope and limitations of reported examples of aza Paternò–Büchi reactions in organic synthesis. An outlook is provided, which highlights recent improvements and the discovery of new reaction protocols that have overcome some long-standing challenges within this field of research.

Recent advances in the chemistry of bicyclo- and 1-azabicyclo[1.1.0]butanes

Bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes are structurally unique compounds that exhibit diverse chemistry. Bicyclo[1.1.0]butane is a four-membered carbocycle with a bridging C(1)-C(3) bond and 1-azabicyclo[1.1.0]butane is an analog of bicyclo[1.1.0]butane featuring a nitrogen atom at one bridgehead. These structures are highly strained, allowing them to participate in a range of strain-releasing reactions which typically cleave the central, strained bond to deliver cyclobutanes or azetidines. However, despite these molecules being discovered in the 1950s and 1960s, and possessing a myriad of alluring chemical features, the chemistry and applications of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes remain underexplored. In the past 5 years, there has been a resurgent interest in their chemistry driven by the pharmaceutical industry’s increasing desire for new methods to access cyclobutanes and azetidines. This short review intends to provide a timely summary of the most recent developments in the chemistry of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butane to highlight the diverse chemistry they can access, their value as synthetic precursors to cyclobutanes and azetidines, and to identify areas for future research.

Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals

Herein we report the development of a photocatalytic strategy for the divergent preparation of functionalized bicyclo[1.1.1]pentylamines. This approach exploits, for the first time, the ability of nitrogen-radicals to undergo strain-release reaction with [1.1.1]propellane. This reactivity is facilitated by the electrophilic nature of these open-shell intermediates and the presence of strong polar effects in the transition-state for C-N bond formation/ring-opening. With the aid of a simple reductive quenching photoredox cycle, we have successfully harnessed this novel radical strain-release amination as part of a multicomponent cascade compatible with several external trapping agents. Overall, this radical strategy enables the rapid construction of novel amino-functionalized building blocks with potential application in medicinal chemistry programs as p-substituted aniline bioisosteres.

Rationalizing the diverse reactivity of [1.1.1]propellane through σ-π-delocalization

[1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this molecule places an inter-bridgehead C-C bond along its central axis; 'strain relief'-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ-π-delocalization of electron density in [1.1.1]propellane. Using ab initio and DFT calculations, we show that its reactions with anions and radicals are facilitated by increased delocalization of electron density over the propellane cage during addition, while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize experimental observations of propellane reactivity, opening up opportunities for the exploration of new chemistry of [1.1.1]propellane and related strained systems that are useful building blocks in organic synthesis.

Sodium Bicyclo[1.1.1]pentanesulfinate: A Bench-Stable Precursor for Bicyclo[1.1.1]pentylsulfones and Bicyclo- [1.1.1]pentanesulfonamides

Herein, we present the synthesis of the bench-stable sodium bicyclo[1.1.1]pentanesulfinate (BCP-SO2 Na) and its application in the synthesis of bicyclo[1.1.1]pentyl (BCP) sulfones and sulfonamides. The salt can be obtained in a four-step procedure from commercially available precursors in multigram scale without the need for column chromatography or crystallization. Sulfinates are known to be useful precursors in radical and nucleophilic reactions and are widely used in medicinal chemistry. This building block enables access to BCP sulfones and sulfonamides avoiding the volatile [1.1.1]propellane which is favorable for the extension of SAR studies. Further, BCP-SO2 Na enables the synthesis of products that were not available with previous methods. A chlorination of BCP-SO2 Na and subsequent reaction with a Grignard reagent provides a new route to BCP sulfoxides. Several products were analyzed by single-crystal X-ray diffraction.

Water-Soluble Non-Classical Benzene Mimetics

A new generation of saturated benzene mimetics, 2-oxabicyclo[2.1.1]hexanes, was developed. These compounds were designed as analogues of bicyclo[1.1.1]pentane with an improved water solubility. Crystallographic analysis of 2-oxabicyclo[2.1.1]hexanes revealed that they occupy a novel chemical space, but, at the same time, resemble the motif of meta-disubstituted benzenes.

Polarity-Reversal Strategy for the Functionalization of Electrophilic Strained Molecules via Light-Driven Cobalt Catalysis

Strain-release-driven methodology is a powerful tool for accessing structural motifs, highly desirable by the pharmaceutical industry. The reactivity of spring-loaded cyclic reagents is dominated by transformations relying on their inherent electrophilic reactivity. Herein, we present a polarity-reversal strategy based on light-driven cobalt catalysis, which enables the generation of nucleophilic radicals through strain release. The applicability of this methodology is demonstrated by the design of two distinct types of reactions: Giese-type addition and Co/Ni-catalyzed cross-coupling. Moreover, a series of electrochemical, spectroscopic, and kinetic experiments as well as X-ray structural analysis of the intermediate alkylcobalt(III) complex give deeper insight into the mechanism of the reaction.

Structurally Diverse Acyl Bicyclobutanes: Valuable Strained Electrophiles

Bicyclo[1.1.0]butanes (BCBs) are highly strained carbocycles that have emerged as versatile synthetic tools, particularly for the construction of functionalized small molecules. This work reports two efficient pathways for the rapid preparation of over 20 structurally diverse BCB ketones, encompassing simple alkyl and aryl derivatives, as well as unprecedented amino acid, dipeptide, bioisostere, and bifunctional linchpin reagents currently inaccessible using literature methods. Analogues are readily forged in two steps and in high yields from simple carboxylic acids or through unsymmetrical ketone synthesis beginning with a convenient carbonyl dication equivalent. The utility of this novel toolbox of strained electrophiles for the selective modification of proteinogenic nucleophiles is highlighted.

Copper-mediated synthesis of drug-like bicyclopentanes

Multicomponent reactions (MCRs) have become a mainstay in both academic and industrial synthetic organic chemistry due to their step- and atom-economy advantages over traditional synthetic sequences¹. Recently, bicyclo[1.1.1]pentane (BCP) motifs have come to the fore as valuable pharmaceutical bioisosteres of benzene rings, and, in particular, 1,3-disubstituted BCP moieties have become widely adopted in medicinal chemistry as para-phenyl ring replacements². Often these structures are generated from [1.1.1]propellane via opening of the internal C─C bond, either through the addition of radicals or metal-based nucleophiles ^3-13. The resulting propellane-addition adducts are subsequently transformed to the requisite polysubstituted BCP compounds via a range of synthetic sequences that traditionally involve multiple chemical steps. While this approach has been effective to date, it is clear that a multicomponent reaction that enables single-step access to complex and diverse polysubstituted BCP products would be synthetically advantageous over the current stepwise approaches. Herein we report a one-step three-component radical coupling of [1.1.1]propellane to afford diverse functionalized bicycles using various radical precursors and heteroatom nucleophiles via a metallaphotoredox catalysis protocol. The reaction operates on short time scales (five minutes to one hour) across multiple (>10) nucleophile classes and can accommodate a diverse array of radical precursors, including those which generate alkyl, α-acyl, trifluoromethyl, and sulfonyl radicals. This method has been used to rapidly prepare BCP analogues of known pharmaceuticals, one of which has substantially different pharmacokinetic properties to those of its commercial progenitor.

Readily accessible sp3-rich cyclic hydrazine frameworks exploiting nitrogen fluxionality

Increased molecular complexity correlates with improved chances of success in the drug development process. Here, a strategy for the creation of sp3-rich, non-planar heterocyclic scaffolds suitable for drug discovery is described that obviates the need to generate multiple stereogenic centers with independent control. Asymmetric transfer hydrogenation using a tethered Ru-catalyst is used to efficiently produce a range of enantiopure cyclic hydrazine building blocks (up to 99% ee). Iterative C-N functionalization at the two nitrogen atoms of these compounds produces novel hydrazine and hydrazide based chemical libraries. Wide chemical diversification is possible through variation in the hydrazine structure, use of different functionalization chemistries and coupling partners, and controlled engagement of each nitrogen of the hydrazine in turn. Principal Moment of Inertia (PMI) analysis of this small hydrazine library reveals excellent shape diversity and three-dimensionality. NMR and crystallographic studies confirm these frameworks prefer to orient their substituents in three-dimensional space under the control of a single stereogenic center through exploitation of the fluxional behavior of the two nitrogen atoms.

Strain release – an old tool for new transformations

This Feature Article provides an overview of research advances in the chemistry of spring-loaded molecules, focusing mainly on strain-release transformations.

The renaissance of strained 1-azabicyclo[1.1.0]butanes as useful reagents for the synthesis of functionalized azetidines

Since their discovery in the late 1960s, 1-azabicyclo[1.1.0]butanes have demonstrated to be interesting precursors of azetidines, because of the peculiar reactivity of the C3–N bond that allows double functionalization in the 1,3 positions.