A Synthetic Strategy toward S-, N-, and O-Heterocyclooctynes Facilitates Bioconjugation Using Multifunctional Handles.

Over the past two decades, the introduction of bioorthogonal reactions has transformed the ways in which chemoselective labeling, isolation, imaging, and drug delivery are carried out in a complex biological milieu. A key feature of a good bioorthogonal probe is the ease with which it can be attached to a target compound through bioconjugation. This paper describes the expansion of the utility of a class of unique S-, N-, and O-containing heterocyclooctynes (SNO-OCTs), which show chemoselective reactivity with type I and type II dipoles and divergent reactivities in response to electronic tuning of the alkyne. Currently, bioconjugation of SNO-OCTs to a desired target is achieved through an inconvenient aryl or amide linker at the sulfamate nitrogen. Herein, a new synthetic approach toward general SNO-OCT scaffolds is demonstrated that enables the installation of functional handles at both propargylic carbons of the heterocycloalkyne. This capability increases the utility of SNO-OCTs as labeling reagents through the design of bifunctional bioorthogonal probes with expanded capabilities. NMR kinetics also revealed up to sixfold improvement in cycloaddition rates of new analogues compared to first-generation SNO-OCTs.

Progress toward the Total Synthesis of Nogalamycin Using a Benzyne Cycloaddition Strategy.

Nogalamycin (NOG) is a member of the anthracycline glycoside natural products; no total syntheses have yet been reported, and there is minimal understanding of how the aglycone substitution pattern and identities of the A- and D-ring sugars impact the anticancer activity and toxicity. This paper reports progress toward a modular approach to NOG that could enable systematic structure-activity relationship studies. Key steps include a regioselective benzyne cycloaddition and reductive ring-opening to assemble a versatile AB core for analogue synthesis.

Solvent Effects on the Chemo- and Site-Selectivity of Transition Metal-Catalyzed Nitrene Transfer Reactions: Alternatives to Chlorinated Solvents.

Transition metal-catalyzed, non-enzymatic nitrene transfer (NT) reactions to selectively transform C-H and C=C bonds to new C-N bonds are a powerful strategy to streamline the preparation of valuable amine building blocks. However, many catalysts for these reactions use environmentally unfriendly solvents that include dichloromethane, chloroform, 1,2-dichloroethane and benzene. We developed a high-throughput experimentation (HTE) protocol for heterogeneous NT reaction mixtures to enable rapid screening of a broad range of solvents for this chemistry. Coupled with the American Chemical Society Pharmaceutical Roundtable (ACSPR) solvent tool, we identified several attractive replacements for chlorinated solvents. Selected catalysts for NT were compared and contrasted using our HTE protocol, including silver supported by N-dentate ligands, dinuclear Rh complexes and Fe/Mn phthalocyanine catalysts.

Iron-Catalyzed Site- and Regioselective 1,2-Azidoamidations of 1,3-Dienes.

The direct 1,2-azidoamidation of unsaturated precursors represents an advantageous approach for the facile synthesis of ß-functionalized azides from readily available starting materials. In this paper, we describe a convenient and mild iron-catalyzed 1,2-azidoamidation of 1,3-dienes that shows excellent functional group compatibility to furnish versatile precursors to 1,2-diamine products with high levels of site, regio-, and stereoselectivity. The reaction is proposed to proceed via a single electron transfer/radical addition/C-N bond formation relay process.

Chemo- and enantioselective intramolecular silver-catalyzed aziridinations of carbamimidates.

Transition metal-catalyzed asymmetric nitrene transfer is a powerful method to generate enantioenriched amines found in natural products and bioactive molecules. A highly chemo- and enantioselective intramolecular silver-catalysed aziridination of 2,2,2-trichloroethoxysulfonyl (Tces)-protected carbamimidates gives [4.1.0]-bicyclic aziridines in good yields and up to 99% ee.

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.

Recent updates and future perspectives in aziridine synthesis and reactivity.

In this review, selected recent advances in the preparation and reactivity of aziridines using modern synthetic approaches are highlighted, while comparing these new strategies with more classical approaches. This critical analysis is designed to help identify current gaps in the field and is showcasing new and exciting opportunities to move the chemistry of aziridines forward in the future.

Divergent C-H Amidations and Imidations by Tuning Electrochemical Reaction Potentials.

Electrochemical C-H functionalizations are attractive transformations, as they are capable of avoiding the use of transition metals, pre-oxidized precursors, or suprastoichiometric amounts of terminal oxidants. Herein an electrochemically tunable method was developed that enabled the divergent formation of cyclic amines or imines by applying different reaction potentials. Detailed cyclic voltammetry analyses, coupled with chronopotentiometry experiments, were carried out to provide insight into the mechanism, while atom economy was assessed through a paired electrolysis. Selective C-H amidations and imidations were achieved to afford five- to seven-membered sulfonamide motifs that could be employed for late-stage modifications.

Re-Evaluation of Product Outcomes in the Rh-Catalyzed Ring Expansion of Aziridines with N-Sulfonyl-1,2,3-Triazoles.

N-heterocycles are prevalent in pharmaceuticals and natural products, but traditional methods often do not introduce significant stereochemical complexity into the ring. We previously reported a Rh-catalyzed ring expansion of aziridines and N-sulfonyl-1,2,3-triazoles to furnish dehydropiperazines with excellent diastereocontrol. However, later studies employing ketone-containing carbene precursors showed that [3,9]-bicyclic aziridine formation competes with production of the desired heterocyclic scaffolds. In light of these surprising results, our initial findings were re-examined both experimentally and computationally to reveal how noncovalent interactions and restricted bond rotation in the aziridinium ylide intermediate promote this unexpected reaction pathway.

Tunable Aziridinium Ylide Reactivity: Non-covalent Interactions Enable Divergent Product Outcomes.

Methods for rapid preparation of densely functionalized and stereochemically complex N-heterocyclic scaffolds are in demand for exploring potential bioactive chemical space. This work describes experimental and computational studies to better understand the features of aziridinium ylides as intermediates for the synthesis of highly substituted dehydromorpholines. The development of this chemistry has enabled the extension of aziridinium ylide chemistry to the concomitant formation of both a C-N and a C-O bond in a manner that preserves the stereochemical information embedded in the substrate. Additionally, we have uncovered several key insights that describe the importance of steric effects, rotational barriers around the C-N bond of the aziridinium ylide, and non-covalent interactions (NCIs) on the ultimate reaction outcome. These critical insights will assist in the further development of this chemistry to generate N-heterocycles that will further expand complex amine chemical space.

Origins of Catalyst-Controlled Selectivity in Ag-Catalyzed Regiodivergent C-H Amination.

Ag-catalyzed nitrene transfer (NT) converts C-H bonds into valuable C-N bonds. These reactions offer a promising strategy for catalyst-controlled regiodivergent functionalization of different types of reactive C-H bonds, as the regioselectivity is tunable by varying the steric and electronic environments around the Ag nitrene, as well as the identity of the nitrene precursors and the tether length. Therefore, a unified understanding of how these individual factors affect the regioselectivity is key to the rational design of highly selective and regiodivergent C-H amination reactions. Herein, we report a computational study of various Ag-catalyzed NT reactions that indicates a concerted H-atom transfer (HAT)/C-N bond formation mechanism. A detailed analysis was carried out on the effects of the C-H bond dissociation enthalpy (BDE), charge transfer, ligand-substrate steric repulsions, and transition state ring strain on the stability of the C-H insertion transition states with different Ag nitrene complexes. The ancillary ligands on the Ag and the nitrene precursor identity both affect transition state geometries to furnish differing sensitivities to the BDE, tether length, and electronic effects of the reactive C-H bonds. Based on our understanding of the dominant factors that control selectivity, we established a rational catalyst and precursor selection approach for regiodivergent amination of diverse C-H bonds. The computationally predicted regiodivergent amination of ß- and γ-C-H bonds of aliphatic alcohol derivatives was validated by experimental studies.

Acylated and alkylated benzo(crown-ethers) form ion-dependent ion channels in biological membranes.

Synthetic ion channels based on benzo(crown-ether) compounds have been previously reported to function as ion-selective channels in planar lipid bilayers, with hydrogen bonding networks implicated in the formation of self-aggregated complexes. Herein, we report the synthesis and characterization of two new families of benzo(crown-ether) compounds, termed monoacylated and monoalkylated benzo(crown-ethers) (MABCE), both of which lack hydrogen bond donors. Depending on the length of alkyl chain substituent and the size of macrocycle, MABCE compounds inhibit bacterial growth and transport ions across biological membranes. Single-channel recordings show that the activity is higher in the presence of K+ as compared with Na+; however, under bionic conditions, open channels do not exhibit any preference between the two ions. These findings reveal that the ionic preference of benzo(crown-ether) compounds is either due to the regulation of assembly of ion-conducting supramolecular complexes or its membrane insertion by cations, as opposed to ion-selective transport through these scaffolds. Furthermore, our data show that the H-bonding network is not needed to form these assemblies in the membrane.

Pd-Catalyzed Heck-Type Reactions of Allenes for Stereoselective Syntheses of Substituted 1,3-Dienes.

A highly stereoselective Pd-catalyzed Heck-type reaction of allenes in which the stereochemistry of both olefins is set simultaneously was developed. The ligand CyJohnPhos was crucial to achieving stereoselectivity, while minimizing isomerization of the starting material through hydropalladation. The stereodetermining factors were proposed to be A1,3 strain between the catalyst and allene substituent, which influences the σ-π-σ equilibrium of the coupled allene intermediate, as well as eclipsing interactions of R groups in the ß-hydride elimination. Good functional group tolerance and stereoselectivities for formation of the Z,E isomer were demonstrated. The methodology was further expanded to include the regioselective formation of 2,4-dienoates and 2,4-dienamides with a variety of substitution patterns, albeit in reduced stereoselectivities favoring the E,E isomer. A plausible mechanism is proposed to account for the observed selectivities and substituent effects.

Recent Developments and Strategies for Mutually Orthogonal Bioorthogonal Reactions.

Over the past decade, several different metal-free bioorthogonal reactions have been developed to enable simultaneous double-click labeling with minimal-to-no competing cross-reactivities; such transformations are termed 'mutually orthogonal'. More recently, several examples of successful triple ligation strategies have also been described. In this minireview, we discuss selected aspects of the development of orthogonal bioorthogonal reactions over the past decade, including general strategies to drive future innovations to achieve simultaneous, mutually orthogonal click reactions in one pot.

Allene Trifunctionalization via Amidyl Radical Cyclization and TEMPO Trapping.

Radical-mediated trifunctionalizations of allenes are virtually unknown, in contrast to well-studied radical difunctionalizations of alkenes and alkynes. In this article, we describe a light-promoted reaction that transforms all three allene carbons to new carbon-heteroatom bonds in one pot with no expensive transition-metal catalyst. Formation of an electron donor-acceptor complex between an electron-deficient aryl and K2CO3, followed by photochemical generation of an amidyl radical and cyclization, yields a vinyl radical that can be trapped by TEMPO to ultimately furnish the product. Insights into the impact of the allene substitution pattern, radical source, and donor are presented, along with studies to unravel the mechanism of this unusual transformation.

Scope and mechanistic investigations of Pd-catalyzed coupling/cyclizations and cycloisomerizations of allenyl malonates.

Pd-catalyzed transformations of allenyl malonates provide convenient access to functionalized carbocycles, but the influence of the ligand, solvent, base, and reaction conditions on the mechanism, regioselectivity, and product outcome of the cyclization are not well-understood. Additionally, from the perspective of synthetic utility, access to either fully substituted or enantioenriched cyclopentane building blocks has not yet been achieved. This work describes how targeted changes to the reaction conditions enable predictable control over the mechanism of Pd-catalyzed allene cross-coupling/cyclization and cycloisomerization, irrespective of the allene substitution pattern. Both enantioenriched cyclopropanes and cyclopentenes can be obtained through axis-to-center chirality transfer from the allene precursor at room temperature, which is not possible using reported Pd-catalyzed methods that result in racemization of the allene. Finally, the ability to divert the reactivity of the allenyl malonate from cross-coupling/cyclization to cycloisomerization by a simple switch of the ligand on Pd from a bidentate phosphine to an electron-poor triphenylphosphite is demonstrated.

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.