"Off-Label Use" of the Siderophore Enterobactin Enables Targeted Imaging of Cancer with Radioactive Ti(IV)

The development of inert, biocompatible chelation methods is required to harness the emerging positron emitting radionuclide 45Ti for radiopharmaceutical applications. Herein, we evaluate the Ti(IV)-coordination chemistry of four catechol-based, hexacoordinate chelators using synthetic, structural, computational, and radiochemical approaches. The siderophore enterobactin (Ent) and its synthetic mimic TREN-CAM readily form mononuclear Ti(IV) species in aqueous solution at neutral pH. Radiolabeling studies reveal that Ent and TREN-CAM form mononuclear complexes with the short-lived, positron-emitting radionuclide 45Ti(IV), and do not transchelate to plasma proteins in vitro and exhibit rapid renal clearance in naïve mice. These features guide efforts to target the 45Ti isotope to prostate cancer tissue through the design, synthesis, and evaluation of Ent-DUPA, a small molecule conjugate composed of a prostate specific membrane antigen (PSMA) targeting peptide and a monofunctionalized Ent scaffold. The [45Ti][Ti(Ent-DUPA)]2- complex forms readily at room temperature. In a tumor xenograft model in mice, selective tumor tissue accumulation (8±5 %, n=5), and low off-target uptake in other organs is observed. Overall, this work demonstrates targeted imaging with 45Ti(IV), provides a foundation for advancing the application of 45Ti in nuclear medicine, and reveals that Ent can be repurposed as a 45Ti-complexing cargo for targeted nuclear imaging applications.

Crystal structure and characterization of a new one-dimensional copper(II) coordination polymer containing a 4-amino-benzoic acid ligand

A CuII coordination polymer, catena-poly[[[aqua-copper(II)]-bis-(μ-4-amino-benz-o-ato)-κ2 N:O;κ2 O:N] monohydrate], {[Cu(pABA)2(H2O)]·H2O}n (pABA = p-amino-benzoate, C7H4NO2 -), was synthesized and characterized. It exhibits a one-dimensional chain structure extended into a three-dimensional supra-molecular assembly through hydrogen bonds and π-π inter-actions. While the twinned crystal shows a metrically ortho-rhom-bic lattice and an apparent space group Pbcm, the true symmetry is monoclinic (space group P2/c), with disordered Cu atoms and mixed roles of water mol-ecules (aqua ligand/crystallization water). The luminescence spectrum of the complex shows an emission at 345 nm, cf. 349 nm for pABAH.

Divergent Bimetallic Mechanisms in Copper(II)-Mediated C-C, N-N, and O-O Oxidative Coupling Reactions

Copper-catalyzed aerobic oxidative coupling of diaryl imines provides a route for conversion of ammonia to hydrazine. The present study uses experimental and density functional theory computational methods to investigate the mechanism of N-N bond formation, and the data support a mechanism involving bimolecular coupling of Cu-coordinated iminyl radicals. Computational analysis is extended to CuII-mediated C-C, N-N, and O-O coupling reactions involved in the formation of cyanogen (NC-CN) from HCN, 1,3-butadiyne from ethyne (i.e., Glaser coupling), hydrazine from ammonia, and hydrogen peroxide from water. The results reveal two different mechanistic pathways. Heteroatom ligands with an uncoordinated lone pair (iminyl, NH2, OH) undergo charge transfer to CuII, generating ligand-centered radicals that undergo facile bimolecular radical-radical coupling. Ligands lacking a lone pair (CN and CCH) form bridged binuclear diamond-core structures that undergo C-C coupling. This mechanistic bifurcation is rationalized by analysis of spin densities in key intermediates and transition states, as well as multiconfigurational calculations. Radical-radical coupling is especially favorable for N-N coupling owing to energetically favorable charge transfer in the intermediate and thermodynamically favorable product formation.

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 β-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.

Tuning Structure and Excitonic Properties of 2D Ruddlesden-Popper Germanium, Tin, and Lead Iodide Perovskites via Interplay between Cations

The compositional tunability of 2D metal halide perovskites enables exploration of diverse semiconducting materials with different structural features. However, rationally tuning the 2D perovskite structures to target physical properties for specific applications remains challenging, especially for lead-free perovskites. Here, we study the effect of the interplay of the B-site (Ge, Sn, and Pb), A-site (cesium, methylammonium, and formamidinium), and spacer cations on the structure and optical properties of a new series of 2D Ruddlesden-Popper perovskites using the previously unreported spacer cation 4-bromo-2-fluorobenzylammonium (4Br2FBZ). We report eight new crystal structures and study the consequence of varying the B-site (Pb, Sn, Ge) and dimension (n = 1, 2, vs 3D). Dimension strongly influences local distortion and structural symmetry, and the increased octahedral tilting and lone pair effects in Ge perovskites lead to a polar n = 2 perovskite that exhibits second harmonic generation, (4Br2FBZ)2(Cs)Ge2I7. In contrast, the analogous Sn and Pb perovskites remain centrosymmetric, but the B-site metal influences the photoluminescence properties. The Pb perovskites exhibit broad, defect-mediated emission at low temperature, whereas the Sn perovskites show purely excitonic emission over the entire temperature range, but the carrier recombination dynamics depend on dimensionality and dark excitonic states. Wholistic understanding of these differences that arise based on cations and dimensionality can guide the rational materials design of 2D perovskites for targeting physical properties for optoelectronic applications based on the interplay of cations and the connectivity of the inorganic framework.

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.

De Novo Approaches to the Solid-Phase Separation of Titanium(IV) and Scandium(III): Translating Speciation Data to Selective On-Bead Chelation toward Applications in Nuclear Medicine

The solution chemistry of the hydrolytic, early-transition-metal ions Ti4+ and Sc3+ represents a coordination chemistry challenge with important real-world implications, specifically in the context of 44Ti/44Sc and 45Ti/NatSc radiochemical separations. Unclear speciation of the solid and solution phases and tertiary mixtures of mineral acid, organic chelators, and solid supports are common confounds, necessitating tedious screening of multiple variables. Herein we describe how thermodynamic speciation data in solution informs the design of new solid-phase chelation approaches enabling separations of Ti4+ and Sc3+. The ligands catechol (benzene-1,2-diol) and deferiprone [3-hydroxy-1,2-dimethyl-4(1H)-pyridone] bind Ti4+ at significantly more acidic conditions (2-4 pH units) than Sc3+. Four chelating resins were synthesized using either catechol or deferiprone with two different solid supports. Of these, deferiprone appended to carboxylic acid polymer-functionalized silica (CA-Def) resin exhibited excellent binding affinity for Ti4+ across a wide range of HCl concentrations (1.0-0.001 M), whereas Sc3+ was only retained in dilute acidic conditions (0.01-0.001 M HCl). CA-Def resin produced separation factors of >100 (Ti/Sc) in 0.1-0.4 M HCl, and the corresponding Kd values (>1000) show strong retention of Ti4+. A model 44Ti/44Sc generator was produced, showing 65 ± 3% yield of 44Sc in 200 μL of 0.2 M HCl with a significant 44Ti breakthrough of 0.1%, precluding use in its current form. Attempts, however, removed natSc in loading fractions and a dilute (0.4 M HCl) wash and recovered 80% of the loaded 45Ti activity in 400 μL of 6 M HCl. The previously validated 45Ti chelator TREN-CAM was used for comparative proof-of-concept reactions with the CA-Def eluent (in HCl) and literature-reported hydroxamate-based resin eluents (in citric acid). CA-Def shows improved radiolabeling efficiency with an apparent molar activity (AMA) of 0.177 mCi nmol-1, exceeding the established methods (0.026 mCi nmol-1) and improving the separation and recovery of 45Ti for positron emission tomography imaging applications.

Highly Enantioselective 6π Photoelectrocyclizations Engineered by Hydrogen Bonding

Photochemical electrocyclization reactions are valued for both their ability to produce structurally complex molecules and their central role in elucidating fundamental mechanistic principles of photochemistry. We present herein a highly enantioselective 6π photoelectrocyclization catalyzed by a chiral Ir(III) photosensitizer. This transformation was successfully realized by engineering a strong hydrogen-bonding interaction between a pyrazole moiety on the catalyst and a basic imidazolyl ketone on the substrate. To shed light on the origin of stereoinduction, we conducted a comprehensive investigation combining experimental and computational mechanistic studies. Results from density functional theory calculations underscore the crucial role played by the prochirality and the torquoselectivity in the electrocyclization process as well as the steric demand in the subsequent [1,4]-H shift step. Our findings not only offer valuable guidance for developing chiral photocatalysts but also serve as a significant reference for achieving high levels of enantioselectivity in the 6π photoelectrocyclization reaction.

Exploring a β-Amino Acid with a Seven-Membered Ring Constraint as a Foldamer Building Block for Nontraditional Helices

We explored trans- and cis-2-aminocycloheptanecarboxylic acid (ACHpC) as potential building blocks for helical foldamers. trans-ACHpC does not show sufficient folding propensity in unnatural peptides. cis-ACHpC promotes nontraditional helices of two unnatural peptide backbones: the 11/9-helix for 1:1 α/β-peptides and the 12/10-helix for β-peptides with interconvertible handedness. The two opposite-handed 12/10-helices rapidly interconvert in solution by pseudorotation of the two twist chair forms of the cycloheptane moiety in each cis-ACHpC residue.

Diastereoselective Synthesis of Cyclopropanes from Carbon Pronucleophiles and Alkenes

Cyclopropanes are desirable structural motifs with valuable applications in drug discovery and beyond. Established alkene cyclopropanation methods give rise to cyclopropanes with a limited array of substituents, are difficult to scale, or both. Herein, we disclose a new cyclopropane synthesis through the formal coupling of abundant carbon pronucleophiles and unactivated alkenes. This strategy exploits dicationic adducts derived from electrolysis of thianthrene in the presence of alkene substrates. We find that these dielectrophiles undergo cyclopropanation with methylene pronucleophiles via alkenyl thianthrenium intermediates. This protocol is scalable, proceeds with high diastereoselectivity, and tolerates diverse functional groups on both the alkene and pronucleophile coupling partners. To validate the utility of this new procedure, we prepared an array of substituted analogs of an established cyclopropane that is en route to multiple pharmaceuticals.

Benzylic C-H Esterification with Limiting C-H Substrate Enabled by Photochemical Redox Buffering of the Cu Catalyst

Copper-catalyzed radical-relay reactions provide a versatile strategy for selective C-H functionalization; however, reactions with peroxide-based oxidants often require excess C-H substrate. Here, we report a photochemical strategy to overcome this limitation by using a Cu/2,2'-biquinoline catalyst that supports benzylic C-H esterification with limiting C-H substrate. Mechanistic studies indicate that blue-light irradiation promotes carboxylate-to-copper charge transfer, reducing resting-state CuII to CuI, which activates the peroxide to generate an alkoxyl radical hydrogen-atom-transfer species. This "photochemical redox buffering" introduces a unique strategy to sustain the activity of Cu catalysts in radical-relay reactions.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp²)-C(sp³) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp²)-C(sp³) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO_(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.

Regiospecific Alkene Aminofunctionalization via an Electrogenerated Dielectrophile

Modular strategies to rapidly increase molecular complexity have proven immensely synthetically valuable. In principle, transformation of an alkene into a dielectrophile presents an opportunity to deliver two unique nucleophiles across an alkene. Unfortunately, the selectivity profiles of known dielectrophiles have largely precluded this deceptively simple synthetic approach. Herein, we demonstrate that dicationic adducts generated through electrolysis of alkenes and thianthrene possess a unique selectivity profile relative to more conventional dielectrophiles. Specifically, these species undergo a single and perfectly regioselective substitution reaction with phthalimide salts. This observation unlocks an appealing new platform for aminofunctionalization reactions. As an illustrative example, we implement this new reactivity paradigm to address a longstanding synthetic challenge: alkene diamination with two distinct nitrogen nucleophiles. Studies into the mechanism of this process reveal a key alkenyl thianthrenium salt intermediate that controls the exquisite regioselectivity of the process and highlight the importance of proton sources in controlling the reactivity of alkenyl sulfonium salt electrophiles.

Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

Compared with their 3D counterparts, 2D hybrid organic-inorganic halide perovskites (HOIPs) exhibit enhanced chemical stabilities and superior optoelectronic properties, which can be further tuned by the application of external pressure. Here, we report the first high-pressure study on CMA₂PbI₄ (CMA = cylcohexanemethylammonium), a 2D HOIP with a soft organic spacer cation containing a flexible cyclohexyl ring, using UV-visible absorption, photoluminescence (PL) and vibrational spectroscopy, and synchrotron X-ray microdiffraction, all aided with density functional theory (DFT) calculations. Substantial anisotropic compression behavior is observed, as characterized by unprecedented negative linear compressibility along the b axis. Moreover, the pressure dependence of optoelectronic properties is found to be in strong contrast with those of 2D HOIPs with rigid spacer cations. DFT calculations help to understand the compression mechanisms that lead to pressure-induced bandgap narrowing. These findings highlight the important role of soft spacer cations in the pressure-tuned optoelectronic properties and provide guidance to the design of new 2D HOIPs.

Metabolomics and Genomics Enable the Discovery of a New Class of Nonribosomal Peptidic Metallophores from a Marine Micromonospora

Although microbial genomes harbor an abundance of biosynthetic gene clusters, there remain substantial technological gaps that impair the direct correlation of newly discovered gene clusters and their corresponding secondary metabolite products. As an example of one approach designed to minimize or bridge such gaps, we employed hierarchical clustering analysis and principal component analysis (hcapca, whose sole input is MS data) to prioritize 109 marine Micromonospora strains and ultimately identify novel strain WMMB482 as a candidate for in-depth "metabologenomics" analysis following its prioritization. Highlighting the power of current MS-based technologies, not only did hcapca enable the discovery of one new, nonribosomal peptide bearing an incredible diversity of unique functional groups, but metabolomics for WMMB482 unveiled 16 additional congeners via the application of Global Natural Product Social molecular networking (GNPS), herein named ecteinamines A-Q (1-17). The ecteinamines possess an unprecedented skeleton housing a host of uncommon functionalities including a menaquinone pathway-derived 2-naphthoate moiety, 4-methyloxazoline, the first example of a naturally occurring Ψ[CH2NH] "reduced amide", a methylsulfinyl moiety, and a d-cysteinyl residue that appears to derive from a unique noncanonical epimerase domain. Extensive in silico analysis of the ecteinamine (ect) biosynthetic gene cluster and stable isotope-feeding experiments helped illuminate the novel enzymology driving ecteinamine assembly as well the role of cluster collaborations or "duets" in producing such structurally complex agents. Finally, ecteinamines were found to bind nickel, cobalt, zinc, and copper, suggesting a possible biological role as broad-spectrum metallophores.

Control of Redox‐Active Ester Reactivity Enables a General Cross‐Electrophile Approach to Access Arylated Strained Rings**

Strained rings are increasingly important for the design of pharmaceutical candidates, but cross‐coupling of strained rings remains challenging. An attractive, but underdeveloped, approach to diverse functionalized carbocyclic and heterocyclic frameworks containing all‐carbon quaternary centers is the coupling of abundant strained‐ring carboxylic acids with abundant aryl halides. Herein we disclose the development of a nickel‐catalyzed cross‐electrophile approach that couples a variety of strained ring N‐hydroxyphthalimide (NHP) esters, derived from the carboxylic acid in one step, with various aryl and heteroaryl halides under reductive conditions. The chemistry is enabled by the discovery of methods to control NHP ester reactivity, by tuning the solvent or using modified NHP esters, and the discovery that ^t‐BuBpyCam^CN, an L2X ligand, avoids problematic side reactions. This method can be run in flow and in 96‐well plates.

Cooperative Stereoinduction in Asymmetric Photocatalysis

Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

Hybrid Synthetic and Computational Study of an Optimized, Solvent-Free Approach to Curcuminoids

A green and optimized protocol has been developed for the preparation of symmetric 1,7-bis(aryl)-1,6-heptadiene-3,5-diones and asymmetric 2-aryl-6-arylidenecyclohexanones with modified substrate scope and good functional group tolerance. Syntheses proceed smoothly under solvent-free conditions, providing moderate to excellent product yields with a minimal workup procedure. Control experiments, spectroscopic, and computational studies support a mechanism involving the boron-assisted in situ generation of imine intermediates. Crystal structures of three curcuminoids and isolated mechanistic intermediates are reported. The data provide insight for the further development of solvent-free protocols toward diverse curcumin derivatives in the fields of pharmaceutical and synthetic chemistries.

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.

Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

Control over the stereochemistry of excited-state photoreactions remains a significant challenge in organic synthesis. Recently, it has become recognized that the photophysical properties of simple organic substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple organic substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asymmetric photoreactions. Herein, we show that chiral Brønsted acids can also catalyze asymmetric excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddition catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature a trans-cis stereochemistry that is complementary to other enantioselective catalytic [2+2] photocycloadditions reported to date.

Lewis acids have recently been shown to enable stereocontrol in photochemical cycloadditions, a difficult task due to the reactivity of excited-state compounds. Here the authors show that chiral Brønsted acids are competent chromophore activators in [2+2] cycloadditions, forming diastereomers disfavored in similar Lewis acid catalyzed photochemical reactions.

Diastereoselective, Multicomponent Synthesis of Pyrrolopyrazinoquinazolinones via a Tandem Quinazolinone Rearrangement/Intramolecular Ring Closure of Tautomeric (Z)-Benzamidines

An expedient route to enantiopure, diastereomeric pyrrolopyrazinoquinazolinones was developed following the discovery of a domino quinazolinone rearrangement-intramolecular cyclization of N-H benzamidines. A Ugi-Mumm-Staudinger sequence employing an optically pure proline derivative gave quinazolinones that, upon N-Boc deprotection, rearranged to tautomeric Z-benzamidines. Subsequent spontaneous cyclization afforded 15 diastereomeric pyrazinoquinazolinone pairs in up to 83% overall yield and 89:11 d.r which were separated easily via routine chromatographic purification-the only one required in the entire process.

Helical Structures of Nylon-Like Oligomers Consisting of 1,2-Diamine and 1,2-Dicarboxylic Acid Building Blocks Containing a Five-Membered Ring Constraint

A series of nylon-like oligomers was synthesized, which consisted of alternating cyclic 1,2-diamine and 1,2-dicarboxylic acid building blocks with a five-membered ring constraint. The nylon 2 4 oligomers are symmetric and display helical structures similar to the β-peptide 12-helix with intramolecular 12-membered ring hydrogen bonds. The cyclopentane moiety allows each building block to promote 12-helical folding. In addition, a tartaric acid derivative with the acetonide moiety increases the solubility of oligomers in common organic solvents and promotes helical folding.

Ring opening polymerization of d,l-lactide and ε-caprolactone catalysed by (pyrazol-1-yl)copper(ii) carboxylate complexes

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L) reacts with [Cu(OAc)₂] and C₆H₅COOH, 4-OH-C₆H₄COOH, 2-Cl-C₆H₄COOH and (3,5-NO₂)₂-C₆H₃COOH to afford the copper complexes [Cu₂(C₆H₅COO)₄(L)₂] (1), [Cu₂(4-OH-C₆H₄COO)₄(L)₂] (2), [Cu₂(2-Cl-C₆H₄COO)₄(L)₂]_n (3) and [Cu{(3,5-NO₂)₂-C₆H₃COO}₂L]_n (4) which are characterised by IR, mass spectrometry, elemental analyses, and X-ray crystallography. The structural data revealed two geometries that are adopted by the complexes: (i) paddle wheel in 1, 2·7H₂O, 3 and (ii) regular chains in 3 and 4. Magnetic studies show strong antiferromagnetic couplings in the paddle wheel complexes and a weak antiferromagnetic coupling in the monometallic chain one. Catalysis studies performed with these complexes (1–4) showed that they initiate ring opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions and d,l-lactide in toluene at elevated temperatures. Polycaprolactone (PCL) and poly(d,l-lactide) (PLA) obtained from the polymerization reactions are of low molecular weights (858 for PCL and 602 Da for PLA for initiator 1) and polydispersity indices (typically 2.16 for PCL and 1.64 for PLA with 1 as the initiator). End group analysis of the polymers, determined by MALDI-ToF MS, indicates that the polymers have benzoate, hydroxyl, methoxy and cyclic end groups.

We report the synthesis, structure and complete characterization of four new pyrazolyl carboxylate-based copper(ii) complexes that catalyze the ring opening polymerization of ε-caprolactone under solvent-free conditions and of d,l-lactide in toluene.

A marine microbiome antifungal targets urgent-threat drug-resistant fungi

New antifungal drugs are urgently needed to address the emergence and transcontinental spread of fungal infectious diseases, such as pandrug-resistant Candida auris. Leveraging the microbiomes of marine animals and cutting-edge metabolomics and genomic tools, we identified encouraging lead antifungal molecules with in vivo efficacy. The most promising lead, turbinmicin, displays potent in vitro and mouse-model efficacy toward multiple-drug-resistant fungal pathogens, exhibits a wide safety index, and functions through a fungal-specific mode of action, targeting Sec14 of the vesicular trafficking pathway. The efficacy, safety, and mode of action distinct from other antifungal drugs make turbinmicin a highly promising antifungal drug lead to help address devastating global fungal pathogens such as C. auris.

Disentangling Second Harmonic Generation from Multiphoton Photoluminescence in Halide Perovskites using Multidimensional Harmonic Generation

Layered two-dimensional Ruddlesden-Popper (RP) halide perovskites are an intriguing class of semiconductors being explored for their linear and nonlinear optical and ferroelectric properties. Second harmonic generation (SHG) is commonly used to screen for noncentrosymmetric and ferroelectric materials. However, SHG measurements of perovskites can be obscured by their intense multiphoton photoluminescence (mPL). Here, we apply multidimensional harmonic generation as a method to eliminate the complications from mPL. By scanning and correlating both excitation and emission frequencies, we unambiguously assess whether a material supports SHG by examining if an emission feature scales as twice the excitation frequency. Measurements of a series of n = 2, 3 RP perovskites reveal that, contrary to previous belief, n-butylammonium (BA) RP perovskites are not SHG-active and thus centrosymmetric, but RP perovskites with phenylethylammonium (PEA) and 2-thiophenemethylammonium (TPMA) spacer cations display SHG. This work establishes multidimensional harmonic generation as a definitive method to measure SHG in halide perovskites.

Structural diversity in copper(I) iodide complexes with 6-thioxopiperidin-2-one, piperidine-2,6-di-thione and isoindoline-1,3-di-thione ligands

Copper(I) iodide complexes are well known for displaying a diverse array of structural features even when only small changes in ligand design are made. This structural diversity is well displayed by five copper(I) iodide compounds reported here with closely related piperidine-2,6-di-thione (SNS), isoindoline-1,3-di-thione (SNS6), and 6-thioxopiperidin-2-one (SNO) ligands: di-μ-iodido-bis-[(aceto-nitrile-κN)(6-sulfanylidenepiperidin-2-one-κS)copper(I)], [Cu2I2(CH3CN)2(C5H7NOS)2] (I), bis-(aceto-nitrile-κN)tetra-μ3-iodido-bis-(6-sulfanylidenepiperidin-2-one-κS)-tetra-hedro-tetra-copper(I), [Cu4I4(CH3CN)4(C5H7NOS)4] (II), catena-poly[[(μ-6-sulfanylidenepiperidin-2-one-κ2 O:S)copper(I)]-μ3-iodido], [CuI(C5H7NOS)] n (III), poly[[(piperidine-2,6-di-thione-κS)copper(I)]-μ3-iodido], [CuI(C5H7NS2)] n (IV), and poly[[(μ-isoindoline-1,3-di-thione-κ2 S:S)copper(I)]-μ3-iodido], [CuI(C8H5NS2)] n (V). Compounds I and II crystallize as discrete dimeric and tetra-meric complexes, whereas III, IV, and V crystallize as polymeric two-dimensional sheets. To the best of our knowledge, compound III is the first instance of an extended hexa-gonal [Cu3I3] structure that is not supported by bridging ligands. Structures I, II, and IV display weak to moderately strong Cu⋯Cu cuprophilic inter-actions [Cu⋯Cu inter-nuclear distances range between 2.5803 (10) and 2.8485 (14) Å]. All structures except III display weak hydrogen-bonding inter-actions between the N-H of the ligand and the μ2 and μ3-I- atoms. Structure III contains classical N-H⋯O inter-actions between the SNO ligands that connect the mol-ecules in a three-dimensional framework. Complex V features π-π stacking inter-actions between the aryl rings of the SNS6 ligands within the same polymeric sheet. In structure IV, there were three partially occupied solvent mol-ecules of di-chloro-methane and one partially occupied mol-ecule of aceto-nitrile present in the asymmetric unit. The SQUEEZE routine [Spek (2015 ▸). Acta Cryst. C71, 9-18] was used to correct the diffraction data for diffuse scattering effects and to identify the solvent mol-ecules. The given chemical formula and other crystal data do not take into account the solvent mol-ecules.

Silver-Catalyzed Enantioselective Propargylic C-H Bond Amination through Rational Ligand Design

Asymmetric C-H amination via nitrene transfer is a powerful tool to prepare enantioenriched amine precursors from abundant C-H bonds. Herein, we report a regio- and enantioselective synthesis of γ-alkynyl γ-aminoalcohols via a silver-catalyzed propargylic C-H amination. The protocol was enabled by a new bis(oxazoline) (BOX) ligand designed via a rapid structure-activity relationship (SAR) analysis. The method utilizes accessible carbamate esters bearing γ-propargylic C-H bonds and furnishes versatile products in good yields and excellent enantioselectivity (90-99% ee). The putative Ag-nitrene is proposed to undergo enantiodetermining hydrogen-atom transfer (HAT) during the C-H amination event. Density functional theory calculations shed insight into the origin of enantioselectivity in the HAT step.

Site-Selective Copper-Catalyzed Azidation of Benzylic C-H Bonds

Site selectivity represents a key challenge for non-directed C-H functionalization, even when the C-H bond is intrinsically reactive. Here, we report a copper-catalyzed method for benzylic C-H azidation of diverse molecules. Experimental and density functional theory studies suggest the benzyl radical reacts with a CuII-azide species via a radical-polar crossover pathway. Comparison of this method with other C-H azidation methods highlights its unique site selectivity, and conversions of the benzyl azide products into amine, triazole, tetrazole, and pyrrole functional groups highlight the broad utility of this method for target molecule synthesis and medicinal chemistry.

Catalytic Behavior of Mono-N-Protected Amino-Acid Ligands in Ligand-Accelerated C-H Activation by Palladium(II)

Mono-N-protected amino acids (MPAAs) are increasingly common ligands in Pd-catalyzed C-H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C-H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand-accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand-to-metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C-H activation and catalytic C-H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50-100-fold increase in rate when only 0.05 equivalents of MPAA are present relative to PdII . These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to PdII enables a single MPAA to support C-H activation at multiple PdII centers.

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.

Divergent 2-Chloroquinazolin-4(3H)-one Rearrangement: Twisted-Cyclic Guanidine Formation or Ring-Fused N-Acylguanidines via a Domino Process

A highly efficient 2-chloroquinazolin-4(3H)-one rearrangement was developed that predictably generates either twisted-cyclic or ring-fused guanidines in a single operation, depending on the presence of a primary versus secondary amine in the accompanying diamine reagent. Exclusive formation of twisted-cyclic guanidines results from pairing 2-chloroquinazolinones with secondary diamines. Use of primary amine-containing diamines permits a domino quinazolinone rearrangement/intramolecular cyclization, gated through (E)-twisted-cyclic guanidines, to afford ring-fused N-acylguanidines. This scalable, structurally tolerant transformation generated 55 guanidines and delivered twisted-cyclic guanidines with robust plasma stability and an abbreviated total synthesis of an antitumor ring-fused guanidine (4 steps, 55 % yield).

MS-Derived Isotopic Fine Structure Reveals Forazoline A as a Thioketone-Containing Marine-Derived Natural Product

Forazoline A is a structurally complex PKS-NRPS hybrid produced by marine-derived Actinomadura sp. During the course of studies highlighting the application of IFS analysis as a powerful tool for natural products analysis, we were alerted to an earlier misinterpretation with respect to forazoline A structure elucidation. In particular, IFS reveals that forazoline A contains a thioketone moiety rarely seen in secondary metabolites and, thus, constitutes an even more intriguing structure than originally thought.

Efficient electrochemical synthesis of robust, densely functionalized water soluble quinones

Conjugate addition of thiols to benzoquinones has been coupled to in situ electrochemical oxidation of the resulting hydroquinone to enable full substitution of quinone C-H bonds. The sulfonated thioether-substituted quinones exhibit high solublity and stability in aqueous solution and have redox potentials ranging from 440-750 mV vs. SHE. The electrosynthetic protocol is effective on >100 g scale.

Site-Selective and Stereoselective O-Alkylation of Glycosides by Rh(II)-Catalyzed Carbenoid Insertion

Carbohydrates are synthetically challenging molecules with vital biological roles in all living systems. Selective synthesis and functionalization of carbohydrates provide tremendous opportunities to improve our understanding on the biological functions of this fundamentally important class of molecules. However, selective functionalization of seemingly identical hydroxyl groups in carbohydrates remains a long-standing challenge in chemical synthesis. We herein describe a practical and predictable method for the site-selective and stereoselective alkylation of carbohydrate hydroxyl groups via Rh(II)-catalyzed insertion of metal carbenoid intermediates. This represents one of the mildest alkylation methods for the systematic modification of carbohydrates. Density functional theory (DFT) calculations suggest that the site selectivity is determined in the Rh(II)-carbenoid insertion step, which prefers insertion into hydroxyl groups with an adjacent axial substituent. The subsequent intramolecular enolate protonation determines the unexpected high stereoselectivity. The most prevalent trans-1,2-diols in various pyranoses can be systematically and predictably differentiated based on the model derived from DFT calculations. We also demonstrated that the selective O-alkylation method could significantly improve the efficiency and stereoselectivity of glycosylation reactions. The alkyl groups introduced to carbohydrates by OH insertion reaction can serve as functional groups, protecting groups, and directing groups.

Telescoped synthesis of C3-functionalized (E)-arylamidines using Ugi-Mumm and regiospecific quinazolinone rearrangements

An efficient four-step, six-transformation protocol was developed to afford bioactive N-alkyl- or N-arylamide (E)-arylamidines featuring strategic amidine C3 modifications which were inaccessible or low yielding by previous methods. This synthetic approach, exemplified with 24 amidines and requiring only a single purification, highlights a multicomponent Ugi-Mumm rearrangement to afford highly diversified quinazolinones which undergo regiospecific rearrangement to afford new amidines. The method extensively broadens the structural scope of this new class of trisubstituted amidines and demonstrates the tolerance of regional C3 amidine steric bulk, visualized with X-ray crystallographic analysis.

Incorporating Large A Cations into Lead Iodide Perovskite Cages: Relaxed Goldschmidt Tolerance Factor and Impact on Exciton-Phonon Interaction

The stability and formation of a perovskite structure is dictated by the Goldschmidt tolerance factor as a general geometric guideline. The tolerance factor has limited the choice of cations (A) in 3D lead iodide perovskites (APbI₃), an intriguing class of semiconductors for high-performance photovoltaics and optoelectronics. Here, we show the tolerance factor requirement is relaxed in 2D Ruddlesden-Popper (RP) perovskites, enabling the incorporation of a variety of larger cations beyond the methylammonium (MA), formamidinium, and cesium ions in the lead iodide perovskite cages for the first time. This is unequivocally confirmed with the single-crystal X-ray structure of newly synthesized guanidinium (GA)-based (n-C₆H₁₃NH₃)₂(GA)Pb₂I₇, which exhibits significantly enlarged and distorted perovskite cage containing sterically constrained GA cation. Structural comparison with (n-C₆H₁₃NH₃)₂(MA)Pb₂I₇ reveals that the structural stabilization originates from the mitigation of strain accumulation and self-adjustable strain-balancing in 2D RP structures. Furthermore, spectroscopic studies show a large A cation significantly influences carrier dynamics and exciton-phonon interactions through modulating the inorganic sublattice. These results enrich the diverse families of perovskite materials, provide new insights into the mechanistic role of A-site cations on their physical properties, and have implications to solar device studies using engineered perovskite thin films incorporating such large organic cations.

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.

Crystal structures of a manganese(I) and a rhenium(I) complex of a bi-pyridine ligand with a non-coordinating benzoic acid moiety

The structures of two facially coordinated Group VII metal complexes are reported, namely: fac-bromido-[2-(2,2'-bipyridin-6-yl)benzoic acid-κ2N,N']tricarbonyl-manganese(I) tetra-hydro-furan monosolvate, [MnBr(C17H12N2O2)(CO)3]·C4H8O, I, and fac-[2-(2,2'-bipyridin-6-yl)benzoic acid-κ2N,N']tricarbonyl-chlorido-rhenium(I) tetra-hydro-furan monosolvate, [ReCl(C17H12N2O2)(CO)3]·C4H8O, II. In both complexes, the metal ion is coordinated by three carbonyl ligands, a halide ion, and a 2-(2,2'-bipyridin-6-yl)benzoic acid ligand, in a distorted octa-hedral geometry. In manganese complex I, the tetra-hydro-furan (THF) solvent mol-ecule could not be refined due to disorder. The benzoic acid fragment is also disordered over two positions, such that the carb-oxy-lic acid group is either positioned near to the bromide ligand or to the axial carbonyl ligand. In the crystal of I, the complex mol-ecules are linked by a pair of C-H⋯Br hydrogen bonds, forming inversion dimers that stack up the a-axis direction. In the rhenium complex II, there is hydrogen bonding between the benzoic acid moiety and a disordered co-crystallized THF mol-ecule. In the crystal, the mol-ecules are linked by C-H⋯Cl hydrogen bonds, forming layers parallel to (100) separated by layers of THF solvent mol-ecules.