Scalable synthesis and coupling of quaternary α-arylated amino acids: α-aryl substituents are tolerated in α-helical peptides

Quaternary amino acids are important tools for the modification and stabilisation of peptide secondary structures. Here we describe a practical and scalable synthesis applicable to quaternary alpha-arylated amino acids (Q4As), and the development of solid-phase synthesis conditions for their incorporation into peptides. Monomeric and dimeric α-helical peptides are synthesised with varying degrees of Q4A substitution and their structures examined using biophysical methods. Both enantiomers of the Q4As are tolerated in folded monomeric and oligomeric α-helical peptides, with the (R)-enantiomer slightly more so than the (S).

Both R and S enantiomers of Fmoc-protected amino acids bearing α-aryl substituents may be made on gram scale. Solid-phase synthesis leads to helical peptides unperturbed by the presence of these additional α-aryl groups.     

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

Described here is the de novo design and synthesis of a series of 6H-dipyrido[1,2-e:2′,1′-i]purin-6-ones (DPs) as a new class of visible-light photoredox catalysts (PCs). The synthesized DP1–5 showed their λ_Abs(max) values in 433–477 nm, excited state redox potentials in 1.15–0.69 eV and −1.41 to −1.77 eV (vs. SCE), respectively. As a representative, DP4 enables the productive guanylation of various amines, including 1°, 2°, and 3°-alkyl primary amines, secondary amines, aryl and heteroaryl amines, amino-nitrile, amino acids and peptides as well as propynylamines and α-amino esters giving diversities in biologically important guanidines and cyclic guanidines. The photocatalytic efficacy of DP4 in the guanylation overmatched commonly used Ir and Ru polypyridyl complexes, and some organic PCs. Other salient merits of this method include broad substrate scope and functional group tolerance, gram-scale synthesis, and versatile late-stage derivatizations that led to a derivative 81 exhibiting 60-fold better anticancer activity against Ramos cells with the IC₅₀ of 0.086 μM than that of clinical drug ibrutinib (5.1 μM).

A novel visible-light photocatalyst was designed and its photocatalytic efficacy in the guanylation of amines overmatched common metal-core and organic photocatalysts.     

Catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate via kinetic resolution or enantioconvergent reaction pathways

We report herein catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate. The Brønsted acid-catalyzed kinetic resolution–allylboration reaction sequence of the racemic reagent gave (Z)-δ-hydroxymethyl-anti-homoallylic alcohols with high Z-selectivities and enantioselectivities upon oxidative workup. In parallel, enantioconvergent pathways were utilized to synthesize chiral nonracemic 1,5-diols and α,β-unsaturated aldehydes with excellent optical purity.

We report herein catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate.     

Stereodivergent entry to β-branched β-trifluoromethyl α-amino acid derivatives by sequential catalytic asymmetric reactions

Currently, conventional reductive catalytic methodologies do not guarantee general access to enantioenriched β-branched β-trifluoromethyl α-amino acid derivatives. Herein, a one-pot approach to these important α-amino acids, grounded on the reduction – ring opening of Erlenmeyer–Plöchl azlactones, is presented. The configurations of the two chirality centers of the products are established during each of the two catalytic steps, enabling a stereodivergent process.

A one-pot approach to β-branched β-trifluoromethyl α-amino acids, grounded on the reduction – ring opening of Erlenmeyer–Plöchl azlactones, and complementary to conventional catalytic asymmetric hydrogenation, is presented.     

An expeditious route to sterically encumbered nonproteinogenic α-amino acid precursors using allylboronic acids

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yields and diastereoselectivities. Gram-scale synthesis, broad tolerance of functional groups, excellent stereodivergence, post-synthetic modifications, and easy removal of the chiral auxiliary are some of the key highlights. The protocol is applicable to various amino acids and short peptides, resulting in the incorporation of these precursors at the N-terminal position.

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yield and diastereoselectivities.     

Ligand-controlled regioselective and chemodivergent defluorinative functionalization of gem-difluorocyclopropanes with simple ketones

Modulating the reaction selectivity is highly attractive and pivotal to the rational design of synthetic regimes. The defluorinative functionalization of gem-difluorocyclopropanes constitutes a promising route to construct β-vinyl fluorine scaffolds, whereas chemo- and regioselective access to α-substitution patterns remains a formidable challenge. Presented herein is a robust Pd/NHC ligand synergistic strategy that could enable the C–F bond functionalization with exclusive α-regioselectivity with simple ketones. The key design adopted enolates as π-conjugated ambident nucleophiles that undergo inner-sphere 3,3′-reductive elimination warranted by the sterically hindered-yet-flexible Pd-PEPPSI complex. The excellent branched mono-defluorinative alkylation was achieved with a sterically highly demanding IHept ligand, while subtly less bulky SIPr acted as a bifunctional ligand that not only facilitated α-selective C(sp³)–F cleavage, but also rendered the newly-formed C(sp²)–F bond as the linchpin for subsequent C–O bond formation. These examples represented an unprecedented ligand-controlled regioselective and chemodivergent approach to various mono-fluorinated terminal alkenes and/or furans from the same readily available starting materials.

A robust Pd/NHC ligand synergistic strategy that enables the exquisite regioselective and chemodivergent C–F bond functionalization of gem-difluorocyclopropanes with simple ketones, is reported.     

Modulation of amyloid-β aggregation by metal complexes with a dual binding mode and their delivery across the blood–brain barrier using focused ultrasound

One of the key hallmarks of Alzheimer''s disease is the aggregation of the amyloid-β peptide to form fibrils. Consequently, there has been great interest in studying molecules that can disrupt amyloid-β aggregation. While a handful of molecules have been shown to inhibit amyloid-β aggregation in vitro, there remains a lack of in vivo data reported due to their inability to cross the blood–brain barrier. Here, we investigate a series of new metal complexes for their ability to inhibit amyloid-β aggregation in vitro. We demonstrate that octahedral cobalt complexes with polyaromatic ligands have high inhibitory activity thanks to their dual binding mode involving π–π stacking and metal coordination to amyloid-β (confirmed via a range of spectroscopic and biophysical techniques). In addition to their high activity, these complexes are not cytotoxic to human neuroblastoma cells. Finally, we report for the first time that these metal complexes can be safely delivered across the blood–brain barrier to specific locations in the brains of mice using focused ultrasound.

We report a series of non-toxic cobalt(iii) complexes which inhibit Aβ peptide aggregation in vitro; these complexes can be safely delivered across the blood–brain barrier in mice using focused ultrasound.     

De novo generation of a bright blue fluorophore from 2-oxoglutarate in biological samples

We discovered the generation of a new bright blue fluorophore from a particular type of amine and 2-oxoglutarate (2-OG) under mild conditions without any chemical additives. Two β-aminoethylamine molecules and three 2-OG molecules form an unprecedented 2-pyridone structure with a fused γ-lactam ring (DTPP) via complex reactions including double decarboxylation and quintuple dehydration. The DTPP fluorophore shows a high quantum yield (80%) and photostability. The great potential of the present DTPP generation in the quantitative analysis of 2-OG in biosamples is demonstrated.

DTPP, a bright fluorophore with 2-pyridone and γ-lactam, is generated in a de novo manner from non-fluorogenic 2-oxoglutarate and specific amines. The DTPP-based fluorometry is applied to the analysis of 2-oxoglutarate in a biological sample.     

Cooperativity as quantification and optimization paradigm for nuclear receptor modulators

Nuclear Receptors (NRs) are highly relevant drug targets, for which small molecule modulation goes beyond a simple ligand/receptor interaction. NR–ligands modulate Protein–Protein Interactions (PPIs) with coregulator proteins. Here we bring forward a cooperativity mechanism for small molecule modulation of NR PPIs, using the Peroxisome Proliferator Activated Receptor γ (PPARγ), which describes NR–ligands as allosteric molecular glues. The cooperativity framework uses a thermodynamic model based on three-body binding events, to dissect and quantify reciprocal effects of NR–coregulator binding (K^I_D) and NR–ligand binding (K^II_D), jointly recapitulated in the cooperativity factor (α) for each specific ternary ligand·NR·coregulator complex formation. These fundamental thermodynamic parameters allow for a conceptually new way of thinking about structure–activity-relationships for NR–ligands and can steer NR modulator discovery and optimization via a completely novel approach.

A cooperativity framework describes the formation of nuclear receptor ternary complexes and deconvolutes ligand and cofactor binding into intrinsic affinities and a cooperativity factor, providing a conceptually new understanding of NR modulation.     

Intermolecular CDC amination of remote and proximal unactivated Csp3–H bonds through intrinsic substrate reactivity – expanding towards a traceless directing group

An intermolecular radical based distal selectivity in appended alkyl chains has been developed. The selectivity is maximum when the distal carbon is γ to the appended group and decreases by moving from γ → δ → ε positions. In –COO– linked alkyl chains, the same distal γ-selectivity is observed irrespective of its origin, either from the alkyl carboxy acid or alkyl alcohol. The appended groups include esters, N–H protected amines, phthaloyl, sulfone, sulfinimide, nitrile, phosphite, phosphate and borate esters. In borate esters, boron serves as a traceless directing group, which is hitherto unprecedented for any remote C_sp³–H functionalization. The selectivity order follows the trend: 3° benzylic > 2° benzylic > 3° tertiary > α to keto > distal methylene (γ > δ > ε). Computations predicted the radical stability (thermodynamic factors) and the kinetic barriers as the factors responsible for such trends. Remarkably, this strategy eludes any designer catalysts, and the selectivity is due to the intrinsic substrate reactivity.

An intermolecular amination at the distal methylene carbon has been realized in an appended alkyl chain with electron withdrawing groups. Traceless remote C_sp³–H functionalization has been accomplished using borate esters.     

Oncogenic KRAS G12D mutation promotes dimerization through a second,phosphatidylserine–dependent interface: a model for KRAS oligomerization

KRAS forms transient dimers and higher-order multimers (nanoclusters) on the plasma membrane, which drive MAPK signaling and cell proliferation. KRAS is a frequently mutated oncogene, and while it is well known that the most prevalent mutation, G12D, impairs GTP hydrolysis, thereby increasing KRAS activation, G12D has also been shown to enhance nanoclustering. Elucidating structures of dynamic KRAS assemblies on a membrane has been challenging, thus we have refined our NMR approach that uses nanodiscs to study KRAS associated with membranes. We incorporated paramagnetic relaxation enhancement (PRE) titrations and interface mutagenesis, which revealed that, in addition to the symmetric ‘α–α’ dimerization interface shared with wild-type KRAS, the G12D mutant also self-associates through an asymmetric ‘α–β’ interface. The ‘α–β’ association is dependent on the presence of phosphatidylserine lipids, consistent with previous reports that this lipid promotes KRAS self-assembly on the plasma membrane in cells. Experiments using engineered mutants to spoil each interface, together with PRE probes attached to the membrane or free in solvent, suggest that dimerization through the primary ‘α–α’ interface releases β interfaces from the membrane promoting formation of the secondary ‘α–β’ interaction, potentially initiating nanoclustering. In addition, the small molecule BI-2852 binds at a β–β interface, stabilizing a new dimer configuration that outcompetes native dimerization and blocks the effector-binding site. Our data indicate that KRAS self-association involves a delicately balanced conformational equilibrium between transient states, which is sensitive to disease-associated mutation and small molecule inhibitors. The methods developed here are applicable to biologically important transient interactions involving other membrane-associated proteins.

Studies of membrane-dependent dimerization of KRAS on nanodiscs using paramagnetic NMR titrations and mutagenesis revealed a novel asymmetric ‘α–β’ interface that provides a potential mechanism for the enhanced assembly of KRAS–G12D nanoclusters.     

Exploring benzylic gem-C(sp3)–boron–silicon and boron–tin centers as a synthetic platform

A stepwise build-up of multi-substituted C_sp³ carbon centers is an attractive, conceptually simple, but often synthetically challenging type of disconnection. To this end, this report describes how gem-α,α-dimetalloid-substituted benzylic reagents bearing boron/silicon or boron/tin substituent sets are an excellent stepping stone towards diverse substitution patterns. These gem-dimetalloids were readily accessed, either by known carbenoid insertion into C–B bonds or by the newly developed scalable deprotonation/metallation approach. Highly chemoselective transformations of either the C–Si (or C–Sn) or the C–B bonds in the newly formed gem-C_sp³ centers have been achieved through a set of approaches, with a particular focus on exploiting the synthetically versatile polarity reversal in organometalloids by λ³-aryliodanes. Of particular note is the metal-free arylation of the C–Si (or C–Sn) bonds in such gem-dimetalloids via the iodane-guided C–H coupling approach. DFT calculations show that this transfer of the (α-Bpin)benzyl group proceeds via unusual [5,5]-sigmatropic rearrangement and is driven by the high-energy iodine(iii) center. As a complementary tool, the gem-dimetalloid C–B bond is shown to undergo a potent and chemoselective Suzuki–Miyaura arylation with diverse Ar–Cl, thanks to the development of the reactive gem-α,α-silyl/BF₃K building blocks.

This work explores divergent reactivity of the benzylic gem-boron–silicon and boron–tin double nucleophiles, including the arylation of the C–B bond with Ar–Cl, along with a complementary oxidative λ³-iodane-guided arylation of the C–Si/Sn moiety.     

An effective and versatile strategy for the synthesis of structurally diverse heteroarylsilanes via Ir(iii)-catalyzed C–H silylation

A versatile silylation of heteroaryl C–H bonds is accomplished under the catalysis of a well-defined spirocyclic NHC Ir(iii) complex (SNIr), generating a variety of heteroarylsilanes. A significant advantage of this catalytic system is that multiple types of intermolecular C–H silylation can be achieved using one catalytic system at α, β, γ, or δ positions of heteroatoms with excellent regioselectivities. Mechanistic experiments and DFT calculations indicate that the polycyclic ligand of SNIr can form an isolable cyclometalated intermediate, which leaves a phenyl dentate free and provides a hemi-open space for activating substrates. In general, favorable silylations occur at γ or δ positions of chelating heteroatoms, forming 5- or 6-membered C–Ir–N cyclic intermediates. If such an activation mode is prohibited sterically, silylations would take place at the α or β positions. The mechanistic studies would be helpful for further explaining the reactivity of the SNIr system.

A versatile silylation of heteroaryl C–H bonds is accomplished under the catalysis of a well-defined spirocyclic NHC Ir(iii) complex (SNIr), generating a variety of heteroarylsilanes.     

A (μ-hydrido)diborane(4) anion and its coordination chemistry with coinage metals

A tetra(o-tolyl) (μ-hydrido)diborane(4) anion 1, an analogue of [B₂H₅]⁻ species, was facilely prepared through the reaction of tetra(o-tolyl)diborane(4) with sodium hydride. Unlike common sp²–sp³ diborane species, 1 exhibited a σ-B–B bond nucleophilicity towards NHC-coordinated transition-metal (Cu, Ag, and Au) halides, resulting in the formation of η²-B–B bonded complexes 2 as confirmed by single-crystal X-ray analyses. Compared with 1, the structural data of 2 imply significant elongations of B–B bonds, following the order Au > Cu > Ag. DFT studies show that the diboron ligand interacts with the coinage metal through a three-center-two-electron B–M–B bonding mode. The fact that the B–B bond of the gold complex is much prolonged than the related Cu and Ag compounds might be ascribed to the superior electrophilicity of the gold atom.

A tetra(o-tolyl)(μ-hydrido)diborane(4) anion is facilely prepared via the reaction of tetra(o-tolyl)diborane(4) with NaH. It exhibits a σ-B–B bond nucleophilicity towards NHC-metal halides to give the corresponding η²-B–B bonded metal complexes.     

Organocatalytic asymmetric synthesis of α-amino esters from sulfoxonium ylides

Described here is the first organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides. Without a metal catalyst, this reaction represents an attractive complement to the well-established carbene insertion reactions. As a stable surrogate of diazocarbonyl compounds, sulfoxonium ylides reacted with a range of aryl amines to provide efficient access to α-aryl glycines with excellent enantiocontrol in the presence of a suitable chiral phosphoric acid catalyst. The high stability and weak basicity of sulfoxonium ylides not only enable this protocol to be user-friendly and practically useful, but also preclude catalyst decomposition, which is crucial to the excellent amenability to electron-poor amine nucleophiles. Detailed mechanistic studies indicated that the initial protonation is reversible and the C–N bond formation is rate-determining.

An organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides has been developed to provide efficient access to α-amino esters without involving a metal carbenoid intermediate.     

Organic amine mediated cleavage of Caromatic–Cα bonds in lignin and its platform molecules

The activation and cleavage of C–C bonds remains a critical scientific issue in many organic reactions and is an unmet challenge due to their intrinsic inertness and ubiquity. Meanwhile, it is crucial for the valorization of lignin into high-value chemicals. Here, we proposed a novel strategy to enhance the C_aromatic–C_α bond cleavage by pre-functionalization with amine sources, in which an active amine intermediate is first formed through Markovnikov hydroamination to reduce the dissociation energy of the C_aromatic–C_α bond which is then cleaved to form target chemicals. More importantly, this strategy provides a method to achieve the maximum utilization of the aromatic nucleus and side chains in lignin or its platform molecules. Phenols and N,N-dimethylethylamine compounds with high yields were produced from herbaceous lignin or the p-coumaric acid monomer in the presence of industrially available dimethylamine (DMA).

Pre-functionalization with amine sources mediated the cleavage of C_aromatic–C_α bonds to produce two valuable chemicals with high yields, for the full utilization of the aromatic rings and side-chains in lignin and its platform molecules.     

Antimicrobial α-defensins as multi-target inhibitors against amyloid formation and microbial infection

Amyloid aggregation and microbial infection are considered as pathological risk factors for developing amyloid diseases, including Alzheimer''s disease (AD), type II diabetes (T2D), Parkinson''s disease (PD), and medullary thyroid carcinoma (MTC). Due to the multifactorial nature of amyloid diseases, single-target drugs and treatments have mostly failed to inhibit amyloid aggregation and microbial infection simultaneously, thus leading to marginal benefits for amyloid inhibition and medical treatments. Herein, we proposed and demonstrated a new “anti-amyloid and antimicrobial hypothesis” to discover two host-defense antimicrobial peptides of α-defensins containing β-rich structures (human neutrophil peptide of HNP-1 and rabbit neutrophil peptide of NP-3A), which have demonstrated multi-target, sequence-independent functions to (i) prevent the aggregation and misfolding of different amyloid proteins of amyloid-β (Aβ, associated with AD), human islet amyloid polypeptide (hIAPP, associated with T2D), and human calcitonin (hCT, associated with MTC) at sub-stoichiometric concentrations, (ii) reduce amyloid-induced cell toxicity, and (iii) retain their original antimicrobial activity upon the formation of complexes with amyloid peptides. Further structural analysis showed that the sequence-independent amyloid inhibition function of α-defensins mainly stems from their cross-interactions with amyloid proteins via β-structure interactions. The discovery of antimicrobial peptides containing β-structures to inhibit both microbial infection and amyloid aggregation greatly expands the new therapeutic potential of antimicrobial peptides as multi-target amyloid inhibitors for better understanding pathological causes and treatments of amyloid diseases.

We report a new “anti-amyloid and antimicrobial hypothesis” by discovering host-defense antimicrobial peptides of α-defensins containing β-sheet structures, which possess inhibition functions against amyloid aggregation and microbial infection.     

Inhibition of (dppf)nickel-catalysed Suzuki–Miyaura cross-coupling reactions by α-halo-N-heterocycles

A nickel/dppf catalyst system was found to successfully achieve the Suzuki–Miyaura cross-coupling reactions of 3- and 4-chloropyridine and of 6-chloroquinoline but not of 2-chloropyridine or of other α-halo-N-heterocycles. Further investigations revealed that chloropyridines undergo rapid oxidative addition to [Ni(COD)(dppf)] but that α-halo-N-heterocycles lead to the formation of stable dimeric nickel species that are catalytically inactive in Suzuki–Miyaura cross-coupling reactions. However, the corresponding Kumada–Tamao–Corriu reactions all proceed readily, which is attributed to more rapid transmetalation of Grignard reagents.

Nickel complexes with a dppf ligand can form inactive dinickel(ii) complexes during Suzuki–Miyaura cross-coupling reactions. However, these complexes can react with Grignard reagents in Kumada–Tamao–Corriu cross-coupling reactions.     

Rh(iii)-Catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates

Herein, we report a mild and highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides with allenyl acetates, which has been elusive so far. The reaction proceeds via vinylic C–H activation, regioselective 2,3-migratory insertion, β-oxy elimination followed by nucleophilic cyclization to get direct access to 1,2-dihydroquinoline derivatives. The strategy was also successfully extended to C–H activation of 2-alkenylphenols for constructing chromene derivatives. In the overall [5 + 1] annulation, the allene serves as a one carbon unit. The acetate group on the allene is found to be crucial both for controlling the regio- and chemoselectivity of the reaction and also for facilitating β-oxy elimination. The methodology was scalable and also further extended towards late stage functionalization of various natural products.

A highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates was described for accessing dihyroquinoline and chromene derivatives.     

N–H⋯X interactions stabilize intra-residue C5 hydrogen bonded conformations in heterocyclic α-amino acid derivatives

Nature makes extensive and elaborate use of hydrogen bonding to assemble and stabilize biomolecular structures. The shapes of peptides and proteins rely significantly on N–H⋯O C interactions, which are the linchpins of turns, sheets and helices. The C5 H-bond, in which a single residue provides both donor and acceptor, is generally considered too weak to force the backbone to adopt extended structures. Exploiting the synergy between gas phase (experimental and quantum chemistry) and solution spectroscopies to decipher IR spectroscopic data, this work demonstrates that the extended C5-based conformation in 4-membered ring heterocyclic α-amino acid derivatives is significantly stabilized by the formation of an N–H⋯X H-bond. In this synergic system the strength of the C5 interaction remains constant while the N–H⋯X H-bond strength, and thereby the support provided by it, varies with the heteroatom.

In 4-membered ring heterocyclic α-amino acid derivatives, extended conformations based on intraresidue C5 H-bonds can be stabilized by N–H⋯X H-bonds, making the combined C5–C6γ structures prominent in both gas phase and in weakly polar solutions.