ent-Verticilide B1 Inhibits Type 2 Ryanodine Receptor Channels and is Antiarrhythmic in Casq2 -/- Mice

Intracellular Ca2+ leak from cardiac ryanodine receptor (RyR2) is an established mechanism of sudden cardiac death (SCD), whereby dysregulated Ca2+ handling causes ventricular arrhythmias. We previously discovered the RyR2-selective inhibitor ent-(+)-verticilide (ent-1), a 24-membered cyclooligomeric depsipeptide that is the enantiomeric form of a natural product (nat-(-)-verticilide). Here, we examined its 18-membered ring-size oligomer (ent-verticilide B1; "ent-B1") in RyR2 single channel and [3H]ryanodine binding assays, and in Casq2 -/- cardiomyocytes and mice, a gene-targeted model of SCD. ent-B1 inhibited RyR2 single channels and RyR2-mediated spontaneous Ca2+ release in Casq2 -/- cardiomyocytes with sub-micromolar potency. ent-B1 was a partial RyR2 inhibitor, with maximal inhibitory efficacy of less than 50%. ent-B1 was stable in plasma, with a peak plasma concentration of 1460 ng/ml at 10 minutes and half-life of 45 minutes after intraperitoneal administration of 3 mg/kg in mice. In vivo, ent-B1 significantly reduced catecholamine-induced ventricular arrhythmias in Casq2 -/- mice in a dose-dependent manner. Hence, we have identified a novel chemical entity - ent-B1 - that preserves the mechanism of action of a hit compound and shows therapeutic efficacy. These findings strengthen RyR2 as an antiarrhythmic drug target and highlight the potential of investigating the mirror-image isomers of natural products to discover new therapeutics. SIGNIFICANCE STATEMENT: The cardiac ryanodine receptor (RyR2) is an untapped target in the stagnant field of antiarrhythmic drug development. We have confirmed RyR2 as an antiarrhythmic target in a mouse model of sudden cardiac death and shown the therapeutic efficacy of a second enantiomeric natural product.

Generality-Driven Catalyst Development: A Universal Catalyst for Enantioselective Nitroalkene Reduction

Cracking the selectivity-generality paradox is among the most pressing challenges in asymmetric catalysis. This obstacle prevents the immediate and successful translation of new methods to diverse small molecules. This is particularly rate-limiting for therapeutic development, where availability and structural diversity are often critical components of successful campaigns. Here we describe the union of generality-driven enantioselective catalysis and the preparation of diverse peptidomimetics. A single new organocatalyst provides high selectivity and substrate generality that is matched only by a combination of metal and organocatalysts. Within organocatalysis, this discovery breaks a 16-year monolithic paradigm, uncovering a powerful new scaffold for enantioselective reduction with behavior that suggests the recognition of a nitroethylene minimal catalaphile.

RyR2 Binding of an Antiarrhythmic Cyclic Depsipeptide Mapped Using Confocal Fluorescence Lifetime Detection of FRET

Hyperactivity of cardiac sarcoplasmic reticulum (SR) ryanodine receptor (RyR2) Ca2+-release channels contributes to heart failure and arrhythmias. Reducing the RyR2 activity, particularly during cardiac relaxation (diastole), is a desirable therapeutic goal. We previously reported that the unnatural enantiomer (ent) of an insect-RyR activator, verticilide, inhibits porcine and mouse RyR2 at diastolic (nanomolar) Ca2+ and has in vivo efficacy against atrial and ventricular arrhythmia. To determine the ent-verticilide structural mode of action on RyR2 and guide its further development via medicinal chemistry structure-activity relationship studies, here, we used fluorescence lifetime (FLT)-measurements of Förster resonance energy transfer (FRET) in HEK293 cells expressing human RyR2. For these studies, we used an RyR-specific FRET molecular-toolkit and computational methods for trilateration (i.e., using distances to locate a point of interest). Multiexponential analysis of FLT-FRET measurements between four donor-labeled FKBP12.6 variants and acceptor-labeled ent-verticilide yielded distance relationships placing the acceptor probe at two candidate loci within the RyR2 cryo-EM map. One locus is within the Ry12 domain (at the corner periphery of the RyR2 tetrameric complex). The other locus is sandwiched at the interface between helical domain 1 and the SPRY3 domain. These findings document RyR2-target engagement by ent-verticilide, reveal new insight into the mechanism of action of this new class of RyR2-targeting drug candidate, and can serve as input in future computational determinations of the ent-verticilide binding site on RyR2 that will inform structure-activity studies for lead optimization.

ent -Verticilide B1 inhibits type 2 ryanodine receptor channels and is antiarrhythmic in Casq2-/- mice

Ca 2+ leak from cardiac ryanodine receptor (RyR2) is an established mechanism of sudden cardiac death (SCD), whereby dysregulated Ca 2+ handling causes ventricular arrhythmias. We previously discovered the RyR2-selective inhibitor ent- (+)-verticilide ( ent -1), a 24-membered cyclooligomeric depsipeptide that is the enantiomeric form of a natural product ( nat -(-)-verticilide). Here, we examined its 18-membered ring-size oligomer ( ent -verticilide B1; " ent -B1") in single RyR2 channel assays, [ 3 H]ryanodine binding assays, and in Casq2 -/- cardiomyocytes and mice, a gene-targeted model of SCD. ent -B1 inhibited RyR2 single-channels and [ 3 H]ryanodine binding with low micromolar potency, and RyR2-mediated spontaneous Ca 2+ release in Casq2-/- cardiomyocytes with sub-micromolar potency. ent -B1 was a partial RyR2 inhibitor, with maximal inhibitory efficacy of less than 50%. ent -B1 was stable in plasma, with a peak plasma concentration of 1460 ng/ml at 10 min and half-life of 45 min after intraperitoneal administration of 3 mg/kg in mice. Both 3 mg/kg and 30 mg/kg ent -B1 significantly reduced catecholamine-induced ventricular arrhythmia in Casq2-/- mice. Hence, we have identified a novel chemical entity - ent -B1 - that preserves the mechanism of action of a hit compound and shows therapeutic efficacy. These findings strengthen RyR2 as an antiarrhythmic drug target and highlight the potential of investigating the mirror-image isomers of natural products to discover new therapeutics.

Significance statement

The cardiac ryanodine receptor (RyR2) is an untapped target in the stagnant field of antiarrhythmic drug development. We have confirmed RyR2 as an antiarrhythmic target in a mouse model of sudden cardiac death and shown the therapeutic efficacy of a second enantiomeric natural product.

The selective RyR2 inhibitor ent-verticilide suppresses atrial fibrillation susceptibility caused by Pitx2 deficiency

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major cause of stroke and morbidity. The strongest genetic risk factors for AF in humans are variants on chromosome 4q25, near the paired-like homeobox transcription factor 2 gene PITX2. Although mice deficient in Pitx2 (Pitx2+/-) have increased AF susceptibility, the mechanism remains controversial. Recent evidence has implicated hyperactivation of the cardiac ryanodine receptor (RyR2) in Pitx2 deficiency, which may be associated with AF susceptibility. We investigated pacing-induced AF susceptibility and spontaneous Ca2+ release events in Pitx2 haploinsufficient (+/-) mice and isolated atrial myocytes to test the hypothesis that hyperactivity of RyR2 increases susceptibility to AF, which can be prevented by a potent and selective RyR2 channel inhibitor, ent-verticilide. Compared with littermate wild-type Pitx2+/+, the frequency of Ca2+ sparks and spontaneous Ca2+ release events increased in permeabilized and intact atrial myocytes from Pitx2+/- mice. Atrial burst pacing consistently increased the incidence and duration of AF in Pitx2+/- mice. The RyR2 inhibitor ent-verticilide significantly reduced the frequency of spontaneous Ca2+ release in intact atrial myocytes and attenuated AF susceptibility with reduced AF incidence and duration. Our data demonstrate that RyR2 hyperactivity enhances SR Ca2+ leak and AF inducibility in Pitx2+/- mice via abnormal Ca2+ handling. Therapeutic targeting of hyperactive RyR2 in AF using ent-verticilide may be a viable mechanism-based approach to treat atrial arrhythmias caused by Pitx2 deficiency.

In Vivo Pharmacokinetic and Pharmacodynamic Properties of the Antiarrhythmic Molecule ent-Verticilide

The unnatural verticilide enantiomer (ent-verticilide) is a selective and potent inhibitor of cardiac ryanodine receptor (RyR2) calcium release channels and exhibits antiarrhythmic activity in a murine model of catecholaminergic polymorphic ventricular tachycardia (CPVT). To determine verticilide's pharmacokinetic and pharmacodynamic properties in vivo, we developed a bioassay to measure nat- and ent-verticilide in murine plasma and correlated plasma concentrations with antiarrhythmic efficacy in a mouse model of CPVT. nat-Verticilide rapidly degraded in plasma in vitro, showing >95% degradation within 5 minutes, whereas ent-verticilide showed <1% degradation over 6 hours. Plasma was collected from mice following intraperitoneal administration of ent-verticilide at two doses (3 mg/kg, 30 mg/kg). Peak C max and area under the plasma-concentration time curve (AUC) scaled proportionally to dose, and the half-life was 6.9 hours for the 3-mg/kg dose and 6.4 hours for the 30-mg/kg dose. Antiarrhythmic efficacy was examined using a catecholamine challenge protocol at time points ranging from 5 to 1440 minutes after intraperitoneal dosing. ent-Verticilide inhibited ventricular arrhythmias as early as 7 minutes after administration in a concentration-dependent manner, with an estimated potency (IC50) of 266 ng/ml (312 nM) and an estimated maximum inhibitory effect of 93.5%. Unlike the US Food and Drug Administration-approved pan-RyR blocker dantrolene, the RyR2-selective blocker ent-verticilide (30 mg/kg) did not reduce skeletal muscle strength in vivo. We conclude that ent-verticilide has favorable pharmacokinetic properties and reduces ventricular arrhythmias with an estimated potency in the nanomolar range, warranting further drug development. SIGNIFICANCE STATEMENT: ent-Verticilide has therapeutic potential to treat cardiac arrhythmias, but little is known about its pharmacological profile in vivo. The primary purpose of this study is to determine the systemic exposure and pharmacokinetics of ent-verticilide in mice and estimate its efficacy and potency in vivo. The current work suggests ent-verticilide has favorable pharmacokinetic properties and reduces ventricular arrhythmias with an estimated potency in the nanomolar range, warranting further drug development.

Enantioselective Synthesis of cis- and trans-Cycloheptyl β-Fluoro Amines by Sequential aza-Henry Addition/Ring-Closing Metathesis

The synthesis of 7-membered carbocyclic β-fluoroamines is accomplished by a combination of the enantioselective aza-Henry reaction of aliphatic N-Boc imines and ring-closing metathesis. Use of reductive denitration gives both diastereomers of the β-fluoro amine carbocycle, each with high enantiomeric excess.

Secondary Orbital Effect Involving Fluorine is Responsible for Substrate-Controlled Diastereodivergence in the Catalyzed syn-aza-Henry Reaction of α-Fluoronitroalkanes

The fluorine atom is a powerful, yet enigmatic influence on chemical reactions. True to form, fluorine was recently discovered to effect diastereodivergence in an enantioselective aza-Henry reaction, resulting in a very rare case of syn-β-amino nitroalkane products. More bewildering was the observation of an apparent hierarchy of substituents within this substrate-controlled behavior: Ph>F>alkyl. These cases have now been examined comprehensively by computational methods, including both non-fluorinated and α-fluoro nitronate additions to aldimines catalyzed by a chiral bis(amidine) [BAM] proton complex. This study revealed the network of non-covalent interactions that dictate anti- (α-aryl) versus syn-selectivity (α-alkyl) using α-fluoronitronate nucleophiles, and an underlying secondary orbital interaction between fluorine and the activated azomethine.

Structure-Activity Relationships for the N-Me- Versus N-H-Amide Modification to Macrocyclic ent-Verticilide Antiarrhythmics

The synthesis of all N-Me and N-H analogues of ent-verticilide is described, enabling a structure-activity relationship study based on cardiac ryanodine receptor (RyR2) calcium ion channel inhibition. The use of permeabilized cardiomyocytes allowed us to correlate the degree of N-methylation with activity without concern for changes in passive membrane permeability that these modifications can cause. A key hypothesis was that the minimal pharmacophore may be repeated in this cyclic oligomeric octadepsipeptide (a 24-membered macrocycle), opening the possibility that target engagement will not necessarily be lost with a single N-Me → N-H modification. The effect in the corresponding 18-membered ring oligomer (ent-verticilide B1) was also investigated. We report here that a high degree of N-methyl amide content is critical for activity in the ent-verticilide series but not entirely so for the ent-verticilide B1 series.

Preparation of N-Aryl Amides by Epimerization-Free Umpolung Amide Synthesis

Amide synthesis is one of the most widely practiced chemical reactions, owing to its use in drug development and peptide synthesis. Despite the importance of these applications, the attendant effort to eliminate waste associated with these protocols has met with limited success, and pernicious α-epimerization is most often minimized but not eliminated when targeting challenging amides (e.g., N-aryl amides). This effort has focused on what is essentially a single paradigm in amide formation wherein an electrophilic acyl donor reacts with a nucleophilic amine. Umpolung amide synthesis (UmAS) emerged from α-halo nitroalkane reactions with amines and has since been developed into a method for the synthesis of enantiopure amides using entirely catalytic, enantioselective synthesis. However, its inability to forge N-aryl amides has been a longstanding problem, one limiting its application more broadly in drug development where α-chiral N-aryl amides are increasingly common. We report here the reaction of α-fluoronitroalkanes and N-aryl hydroxyl amines for the direct synthesis of N-aryl amides using a simple Brønsted base as the promoter. No other activating agents are required, and experiments guided by mechanistic hypotheses outline a mechanism based on the UmAS paradigm and confirm that the N-aryl amide, not the N-aryl hydroxamic acid, is the direct product. Ultimately, select chiral α-amino-N-aryl amides were prepared with complete conservation of enantioenrichment, in contrast to a parallel demonstration of their ability to epimerize using the conventional amide synthesis alternative.

Enantioselective iodolactonization to prepare ε-lactone rings using hypervalent iodine

Despite the rapid growth of enantioselective halolactonization reactions in recent years, most are effective only when forming smaller (6,5,4-membered) rings. Seven-membered ε-lactones, are rarely formed with high selectivity, and never without conformational bias. We describe the first highly enantioselective 7-exo-trig iodolactonizations of conformationally unbiased ε-unsaturated carboxylic acids, effected by an unusual combination of a bifunctional BAM catalyst, I₂, and I(iii) reagent (PhI(OAc)₂:PIDA).

We describe the first highly enantioselective 7-exo-trig iodolactonizations of conformationally unbiased ε-unsaturated carboxylic acids, effected by an unusual combination of a bifunctional BAM catalyst, I₂, and I(iii) reagent (PhI(OAc)₂:PIDA).

Exercise Causes Arrhythmogenic Remodeling of Intracellular Calcium Dynamics in Plakophilin-2-Deficient Hearts

BACKGROUND

Exercise training, and catecholaminergic stimulation, increase the incidence of arrhythmic events in patients affected with arrhythmogenic right ventricular cardiomyopathy correlated with plakophilin-2 (PKP2) mutations. Separate data show that reduced abundance of PKP2 leads to dysregulation of intracellular Ca2+ (Ca2+i) homeostasis. Here, we study the relation between excercise, catecholaminergic stimulation, Ca2+i homeostasis, and arrhythmogenesis in PKP2-deficient murine hearts.

METHODS

Experiments were performed in myocytes from a cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout murine line (PKP2cKO). For training, mice underwent 75 minutes of treadmill running once per day, 5 days each week for 6 weeks. We used multiple approaches including imaging, high-resolution mass spectrometry, electrocardiography, and pharmacological challenges to study the functional properties of cells/hearts in vitro and in vivo.

RESULTS

In myocytes from PKP2cKO animals, training increased sarcoplasmic reticulum Ca2+ load, increased the frequency and amplitude of spontaneous ryanodine receptor (ryanodine receptor 2)-mediated Ca2+ release events (sparks), and changed the time course of sarcomeric shortening. Phosphoproteomics analysis revealed that training led to hyperphosphorylation of phospholamban in residues 16 and 17, suggesting a catecholaminergic component. Isoproterenol-induced increase in Ca2+i transient amplitude showed a differential response to β-adrenergic blockade that depended on the purported ability of the blockers to reach intracellular receptors. Additional experiments showed significant reduction of isoproterenol-induced Ca2+i sparks and ventricular arrhythmias in PKP2cKO hearts exposed to an experimental blocker of ryanodine receptor 2 channels.

CONCLUSIONS

Exercise disproportionately affects Ca2+i homeostasis in PKP2-deficient hearts in a manner facilitated by stimulation of intracellular β-adrenergic receptors and hyperphosphorylation of phospholamban. These cellular changes create a proarrhythmogenic state that can be mitigated by ryanodine receptor 2 blockade. Our data unveil an arrhythmogenic mechanism for exercise-induced or catecholaminergic life-threatening arrhythmias in the setting of PKP2 deficit. We suggest that membrane-permeable β-blockers are potentially more efficient for patients with arrhythmogenic right ventricular cardiomyopathy, highlight the potential for ryanodine receptor 2 channel blockers as treatment for the control of heart rhythm in the population at risk, and propose that PKP2-dependent and phospholamban-dependent arrhythmogenic right ventricular cardiomyopathy-related arrhythmias have a common mechanism.

Resolving Bromonitromethane Sourcing by Synthesis: Preparation at the Decagram Scale

The accessibility of bromonitromethane has declined in recent years, limiting its viability as a reagent for chemical synthesis. The reinvestigation and optimization of a variety of preparations, and the development of safe operating principles, are described. The reproducible protocol described here leverages the effectiveness of hydroxide for nitromethane bromination while respecting its incompatibility with the product it forms. This careful balance was achieved at scales up to 56 g, resulting in a reproducible procedure that provides straightforward, sustainable, and affordable access to this critical reagent.

Fluorine-induced diastereodivergence discovered in an equally rare enantioselective syn-aza-Henry reaction

Attention to the aza-Henry reaction, particularly over the past two decades, has resulted in a wide range of effective catalysts for the enantio- and diastereoselective versions, driven by the versatility of the β-amino nitroalkane products as precursors to secondary amines and vic-diamines. Despite this broad effort, syn-diastereoselective variants are exceedingly rare. We have discovered a subset of α-fluoro nitroalkane additions that are characterized by an unusual crossover in diastereoselection, often delivering the products with high selectivities. We report here a rigorous comparative analysis of non-fluorinated and α-fluoro nitroalkanes in their additions to azomethines. Both homogeneous and heterogeneous catalysis were applied to probe the possibility that this phenomenon might be more widely operative in the enantioselective additions of fluorine-substituted carbon nucleophiles. A complete correlation within four categories is described that uncovered a clear trend, while revealing a dramatic and distinct reversal of diastereoselection that would normally go undetected.

Ring Size as an Independent Variable in Cyclooligomeric Depsipeptide Antiarrhythmic Activity

Hit-to-lead studies employ a variety of strategies to optimize binding to a target of interest. When a structure for the target is available, hypothesis-driven structure-activity relationships (SAR) are a powerful strategy for refining the pharmacophore to achieve robust binding and selectivity characteristics necessary to identify a lead compound. Recrafting the three-dimensional space occupied by a small molecule, optimization of hydrogen bond contacts, and enhancing local attractive interactions are traditional approaches in medicinal chemistry. Ring size, however, is rarely able to be leveraged as an independent variable because most hits lack the symmetry required for such a study. Our discovery that the cyclic oligomeric depsipeptide ent-verticilide inhibits mammalian cardiac ryanodine receptor calcium release channels with submicromolar potency provided an opportunity to explore ring size as a variable, independent of other structural or functional group changes. We report here that ring size can be a critical independent variable, suggesting that modest conformational changes alone can dramatically affect potency.

DFT-Based Stereochemical Rationales for the Bifunctional Brønsted Acid/Base-Catalyzed Diastereodivergent and Enantioselective aza-Henry Reactions of α-Nitro Esters

A pair of chiral bis(amidine) [BAM] proton complexes provide reagent (catalyst)-controlled, highly diastereo- and enantioselective direct aza-Henry reactions leading to α-alkyl-substituted α,β-diamino esters. A C₂-symmetric ligand provides high anti-selectivity, while a nonsymmetric congener exhibits syn-selectivity in this example of diastereodivergent, enantioselective catalysis. A detailed computational analysis is reported for the first time, one that supports distinct models for selectivity resulting from the more hindered binding cavity of the C₁-symmetric ligand. Binding in this congested pocket accommodates four hydrogen bond contacts among ligands and substrates, ultimately favoring a pre-syn arrangement highlighted by pyridinium-azomethine activation and quinolinium-nitronate activation. The complementary transition states reveal a wide range of alternatives. Comparing the C₁- and C₂-symmetric catalysts highlights distinct electrophile binding orientations despite their common hydrogen bond donor-acceptor features. Among the factors driving unusual high syn-diastereoselection are favorable dispersion forces that leverage the anthracenyl substituent of the C₁-symmetric ligand.

The Formation of Impossible Rings in Macrocyclooligomerizations for Cyclodepsipeptide Synthesis: The 18-from-12 Paradox

A reinvestigation into the macrocyclooligomerization (MCO) of a tetradepsipeptide is reported, uncovering a paradox in which the MCO of depsipeptide monomers can produce "impossible" ring sizes: a 12-atom chain produced the expected 24-membered ring, alongside unexpected 18- and 30-membered cyclic oligomeric depsipeptides (CODs). We report an alternative preparation of authentic 18- and 36-membered macrocycles for this case using a stepwise synthesis that provides definitive analytical characterization for each ring size. Our investigation yields a recharacterization and reassignment of two macrocycles originally reported in this MCO series, along with updated yields and isothermal titration calorimetry data after implementation of new critical protocols for purification and subsequent analysis. Initial studies to probe this mechanistic conundrum are described.

RYR2 Channel Inhibition Is the Principal Mechanism of Flecainide Action in CPVT

RATIONALE

The class Ic antiarrhythmic drug flecainide prevents ventricular tachyarrhythmia in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by hyperactive RyR2 (cardiac ryanodine receptor) mediated calcium (Ca) release. Although flecainide inhibits single RyR2 channels in vitro, reports have claimed that RyR2 inhibition by flecainide is not relevant for its mechanism of antiarrhythmic action and concluded that sodium channel block alone is responsible for flecainide's efficacy in CPVT.

OBJECTIVE

To determine whether RyR2 block independently contributes to flecainide's efficacy for suppressing spontaneous sarcoplasmic reticulum Ca release and for preventing ventricular tachycardia in vivo.

METHODS AND RESULTS

We synthesized N-methylated flecainide analogues (QX-flecainide and N-methyl flecainide) and showed that N-methylation reduces flecainide's inhibitory potency on RyR2 channels incorporated into artificial lipid bilayers. N-methylation did not alter flecainide's inhibitory activity on human cardiac sodium channels expressed in HEK293T cells. Antiarrhythmic efficacy was tested utilizing a Casq2 (cardiac calsequestrin) knockout (Casq2-/-) CPVT mouse model. In membrane-permeabilized Casq2-/- cardiomyocytes-lacking intact sarcolemma and devoid of sodium channel contribution-flecainide, but not its analogues, suppressed RyR2-mediated Ca release at clinically relevant concentrations. In voltage-clamped, intact Casq2-/- cardiomyocytes pretreated with tetrodotoxin to inhibit sodium channels and isolate the effect of flecainide on RyR2, flecainide significantly reduced the frequency of spontaneous sarcoplasmic reticulum Ca release, while QX-flecainide and N-methyl flecainide did not. In vivo, flecainide effectively suppressed catecholamine-induced ventricular tachyarrhythmias in Casq2-/- mice, whereas N-methyl flecainide had no significant effect on arrhythmia burden, despite comparable sodium channel block.

CONCLUSIONS

Flecainide remains an effective inhibitor of RyR2-mediated arrhythmogenic Ca release even when cardiac sodium channels are blocked. In mice with CPVT, sodium channel block alone did not prevent ventricular tachycardia. Hence, RyR2 channel inhibition likely constitutes the principal mechanism of antiarrhythmic action of flecainide in CPVT.

Substituted Imidazoline Synthesis: A Diastereo- and Enantioselective aza-Henry Route to a Human Proteasome Modulator

The first enantio- and diastereoselective synthesis of Tepe's human proteasome modulator is described. Routes to this and other highly substituted chiral imidazolines generally produce racemic material. Key to the route disclosed here is a gram-scale anti-selective aza-Henry reaction of an α-alkyl α-nitro ester nucleophile, catalyzed by a Bis(Amidine) [BAM] chiral proton complex, delivering the key intermediate in high yield as a single stereoisomer. The adduct is reduced to the amino ester and converted to an imidazoline.

Direct Observation and Analysis of the Halo-Amino-Nitro Alkane Functional Group

Conventional amide synthesis is a mainstay in discipline-spanning applications, and it is a reaction type that historically developed as a singular paradigm when considering the carbon-nitrogen bond-forming step. Umpolung amide synthesis (UmAS) exploits the unique properties of an α-halo nitroalkane in its reaction with an amine to produce an amide. The "umpolung" moniker reflects its paradigm-breaking C-N bond formation on the basis of evidence that the nucleophilic nitronate carbon and electrophilic nitrogen engage to form a tetrahedral intermediate (TI) that is an unprecedented functional group, a 1,1,1-halo-amino-nitro alkane (HANA). Studies probing HANA transience have failed to capture this (presumably) highly reactive intermediate. We report here the direct observation of a HANA, its conversion thermally to an amide functionality, and quantitative analysis of this process using computational techniques. These findings validate the HANA as a functional group common to UmAS and diverted UmAS, opening the door to its targeted use and creative manipulation.

The inverted ketene synthon: a double umpolung approach to enantioselective β2,3-amino amide synthesis

A stereocontrolled synthesis of β2,3-amino amides is reported. Innovation is encapsulated by the first use of nitroalkenes to achieve double umpolung in enantioselective β-amino amide synthesis. Step economy is also fulfilled by the use of Umpolung Amide Synthesis (UmAS) in the second step, delivering the amide product without intermediacy of a carboxylic acid or activated derivative. Molybdenum oxide-mediated hydride reduction provides the anti-β2,3-amino amide with high selectivity.

Catalytic, Enantioselective Synthesis of Cyclic Carbamates from Dialkyl Amines by CO2-Capture: Discovery, Development, and Mechanism

Cyclic carbamates are a common feature of small-molecule therapeutics, offering a constrained hydrogen bond acceptor that is both polar and sterically small. Methods for their preparation most often focus first on amino alcohol synthesis and then reaction with phosgene or its equivalent. This report describes an enantioselective synthesis of cyclic carbamates in which carbon dioxide engages an unsaturated basic amine, facilitated by a bifunctional organocatalyst designed to stabilize a carbamic acid intermediate while activating it toward subsequent enantioselective carbon-oxygen bond formation. Six-membered cyclic carbamates are prepared in good yield with high levels of enantioselection, as constrained 1,3-amino alcohols featuring a chiral tertiary alcohol carbon. Spectroscopic analysis (NMR, DOSY) of various substrate-reagent combinations provides insight into the dominant species under the reaction conditions. Two peculiar requirements were identified to achieve highest consistency: a "Goldilocks" amount of water and the use of a noncrystalline form of the ligand. These atypical features of the final protocol notwithstanding, a diverse range of products could be prepared. Their functionalizations illustrate the versatility of the carbamates as precursors to enantioenriched small molecules.

Enantioselective Organocatalytic Amine-Isocyanate Capture-Cyclization: Regioselective Alkene Iodoamination for the Synthesis of Chiral Cyclic Ureas

Ureas of chiral diamines are prominent features of therapeutics, chiral auxiliaries, and intermediates in complex molecule synthesis. Although many methods for diamine synthesis are available, metal-free enantioselective alkene functionalizations to make protected 1,2- and 1,3-diamines from simple achiral starting materials are rare, and a single reagent that accesses a cross-section of each congener with high enantiomeric excess is not available. We describe a method to synthesize enantioenriched cyclic 5- and 6-membered ureas from allylic amines and an isocyanate using a C2-symmetric BisAmidine (BAM) catalyst that delivers N-selectivity from an ambident sulfonyl imide intermediate, overcoming electronic and steric deactivation at nitrogen. The geometry of 1,2-disubstituted alkenes is correlated to 5-exo and 6-endo cyclizations without altering alkene face selectivity, which is unexpectedly opposite that observed with O-nucleophiles. Straightforward product manipulations to diamine and imidazolidinone derivatives are underscored by the synthesis of an NK1 antagonist.

Continuous Platform to Generate Nitroalkanes On-Demand (in situ) using Peracetic Acid-Mediated Oxidation in a PFA Pipes-in-Series Reactor

The synthetic utility of the aza-Henry reaction can be diminished on scale by potential hazards associated with the use of peracid to prepare nitroalkane substrates, and the nitroalkanes themselves. In response, a continuous and scalable chemistry platform to prepare aliphatic nitroalkanes on-demand is reported, using the oxidation of oximes with peracetic acid and direct reaction of the nitroalkane intermediate in an aza-Henry reaction. A uniquely designed pipes-in-series plug flow tube reactor addresses a range of process challenges including stability and safe handling of peroxides and nitroalkanes. The subsequent continuous extraction generates a solution of purified nitroalkane which can be directly used in the following enantioselective aza-Henry chemistry to furnish valuable chiral diamine precursors in high selectivity, thus, completely avoiding isolation of potentially unsafe low molecular weight nitroalkane intermediate. A continuous campaign (16 h) established that these conditions were effective in processing 100 g of the oxime and furnishing 1.4 L of nitroalkane solution.

Evidence for Ion-Templation During Macrocyclooligomerization of Depsipeptides

The ion-mediated Mitsunobu macrocyclooligomerization (M-MCO) reaction of hydroxy acid depsipeptides provides small collections of cyclic depsipeptides with good mass recovery. The approach can produce good yields of a single macrocycle or provide rapid access to multiple oligomeric macrocycles in good overall yield. While Lewis acidic alkali metal salts are known to play a role in the outcome of MCO reactions, it is unclear whether their effect is due to an organizational (e.g., templating) mechanism. Isothermal titration calorimetry (ITC) was used to study macrocycle-metal ion binding interactions, and this report correlates these thermodynamic measurements to the (kinetically determined) size distributions of depsipeptides formed during a Mitsunobu-based macrocyclooligomerization (MCO). Key trends have been identified in quantitative metal ion-cyclic depsipeptide binding affinity ( K_a), enthalpy of binding (Δ H), and stoichiometry of complexation across discrete series of macrocycles, and they provide the first analytical platform to rationally select a metal-ion template for a targeted size regime of cyclic oligomeric depsipeptides.

1,3,4-Oxadiazole and Heteroaromatic-Fused 1,2,4-Triazole Synthesis using Diverted Umpolung Amide Synthesis

Umpolung Amide Synthesis (UmAS) has emerged as a superior alternative to conventional amide synthesis methods based on carbonyl electrophiles in a range of situations, particularly when epimerization-prone couplings are prescribed. In an unanticipated development during our most recent studies, it was discovered that diacyl hydrazide products from UmAS were not formed as intermediates when using an acyl hydrazide as the amine acceptor. This resulted in a new preparation of 1,3,4-oxadiazoles from α-bromonitroalkane donors. We hypothesized that a key tetrahedral intermediate in UmAS was diverted toward a more direct pathway to the heterocycle product rather than through formation of the diacyl hydrazide, a typical oxadiazole progenitor. In studies reported here, diversion to 1,2,4-triazole products is described, a behavior hypothesized to also result from an analogous tetrahedral intermediate, but one formed from heteroaromatic hydrazine acceptors.

Diastereo- and enantioselective additions of α-nitro esters to imines for anti-α,β-diamino acid synthesis with α-alkyl-substitution

The discovery that a C2-symmetric bis(AMidine) [BAM] catalyst promotes an anti-selective addition of α-substituted α-nitro esters to imines is described, providing α-substituted α,β-diamino ester products with high diastereo- and enantioselectivity. When compared to the function of a BAM catalyst reported previously, the pair offer a rare example of diastereodivergence using a bifunctional Brønsted acid-base organocatalyst.

Biomimetic Desymmetrization of a Carboxylic Acid

The enantioselective desymmetrization of carboxylic acids by chiral Brønsted base catalysis is reported, leading to bridged bicyclic lactones with up to 94% ee. Crystallographic analysis of a substrate-catalyst complex suggests an origin of stereocontrol, reminiscent of functional Brønsted bases in biological settings, and enabled reaction optimization. The products contain an all-carbon quaternary stereocenter and can be derivatized to functionalized cyclopentanes.

A convergent synthesis of 1,3,4-oxadiazoles from acyl hydrazides under semiaqueous conditions

The 1,3,4-oxadiazole is an aromatic heterocycle valued for its low-lipophilicity in drug development. Substituents at the 2- and/or 5-positions can modulate the heterocycle's electronic and hydrogen bond-accepting capability, while exploiting its use as a carbonyl bioisostere. A new approach to 1,3,4-oxadiazoles is described wherein α-bromo nitroalkanes are coupled to acyl hydrazides to deliver the 2,5-disubstituted oxadiazole directly, avoiding a 1,2-diacyl hydrazide intermediate. Access to new building blocks of oxadiazole-substituted secondary amines is improved by leveraging chiral α-bromo nitroalkane or amino acid hydrazide substrates. The non-dehydrative conditions for oxadiazole synthesis are particularly notable, in contrast to alternatives reliant on highly oxophilic reagents to effect cyclization of unsymmetrical 1,2-diacyl hydrazides. The mild conditions are punctuated by the straightforward removal of co-products by a standard aqueous wash.

On-Demand Complex Peptide Synthesis: An Aspirational (and Elusive?) Goal for Peptide Synthesis

Peptide synthesis is a truly interdisciplinary tool, familiar to a broad group of scientists who do not otherwise overlap scientifically. For this reason, some may perceive even complex peptide synthesis to be a "solved problem", while others might argue that immense opportunity remains untapped or simply inaccessible. At the extreme of complexity, what might a concise assessment of the state-of-the-art in peptide synthesis look like? As one of the most practiced forms of synthetic chemistry by chemists and non-chemists alike, what restrictions remain that constrain access to chemical space? Using popular terminology, what forms of peptide synthesis are appropriately termed "on-demand"? The purpose of this Perspective is to appraise synthetic access to complex peptides, particularly those containing unnatural α-amino amides. Several case studies in complex peptide synthesis are summarized here, each selected to characterize the challenges attendant to unnatural α-amino amide synthesis. As peptidic molecules find increasing value in therapeutic development, especially in clinical applications, their impact will ultimately be determined by efficient preparative methods.

Enantioselective Synthesis of β-Fluoro Amines via β-Amino α-Fluoro Nitroalkanes and a Traceless Activating Group Strategy

Preparation of a range of enantioenriched β-fluoro amines (α,β-disubstituted) is described in which the nitrogen and fluorine atoms are attached to sp3-hybridized carbons. The key finding is a chiral bifunctional Brønsted acid/base catalyst that can deliver β-amino-α-fluoro nitroalkanes with high enantio- and diastereoselection. A denitration step renders the nitro group "traceless" and delivers secondary, tertiary, or vinyl alkyl fluorides embedded within a vicinal fluoro amine functional group. A synthesis of each possible stereoisomer of a β-fluoro lanicemine illustrates the potential ease with which fluorinated small molecules relevant to neuroscience drug development can be prepared in a stereochemically comprehensive manner.

Development of an Intermittent-Flow Enantioselective Aza-Henry Reaction Using an Arylnitromethane and Homogeneous Brønsted Acid-Base Catalyst with Recycle

A stereoselective aza-Henry reaction between an arylnitromethane and Boc-protected aryl aldimine using a homogeneous Brønsted acid-base catalyst was translated from batch format to an automated intermittent-flow process. This work demonstrates the advantages of a novel intermittent-flow setup with product crystallization and slow reagent addition which is not amenable to the standard continuous equipment: plug flow tube reactor (PFR) or continuous stirred tank reactor (CSTR). A significant benefit of this strategy was the integration of an organocatalytic enantioselective reaction with straightforward product separation, including recycle of the catalyst, resulting in increased intensity of the process by maintaining high catalyst concentration in the reactor. A continuous campaign confirmed that these conditions could effectively provide high throughput of material using an automated system while maintaining high selectivity, thereby addressing nitroalkane safety and minimizing catalyst usage.

A one-pot amidation of primary nitroalkanes

A two-step, one-pot oxidative amidation of primary nitroalkanes involving tandem halogenation/umpolung amide synthesis (UmAS) is described.

Characterization of Staphylococcus aureus infections in children with Down syndrome

Staphylococcus aureus infections in the Down syndrome (DS) population have not been well characterized. This study determined clinical and molecular characteristics of S. aureus infections in children with DS followed at Texas Children's Hospital (TCH), from 2001 to 2011. Patients were retrospectively identified from an ongoing S. aureus surveillance study. Medical records were reviewed. Isolates were characterized by antimicrobial susceptibility, pulsed-field gel electrophoresis patterns, and detection of PVL genes (pvl), mupA (high-level mupirocin resistance gene), smr (chlorhexidine resistance conferring gene), and Staphylococcal Chromosomal Cassette mec (SCCmec) type. Twenty-six patients with DS had a total of 34 S. aureus infections (8 recurrent); 61% were MRSA. DS patients represented 16.8 per 10,000 community onset S. aureus infections seen at TCH. Among 26 initial infections 17 were skin and soft tissue (SSTI), 7 were outer or middle ear and 2 were invasive infections. Seventeen patients were hospitalized. Thirteen (65%) of 20 available isolates were USA300, 14 were pvl+, 5 were mupA+, and 8 were smr+. Five of 8 (63%) recurrent infections were ear infections. All 4 recurrent ear isolates available for study were smr+, ciprofloxacin non-susceptible and treated with ciprofloxacin otic drops. S. aureus infections among patients with DS were similar in presentation to other patient groups, except for a greater proportion being associated with ear infections. Seventy percent of ear fluid isolates carried antiseptic and fluoroquinolone resistance genes. A study of a greater number of DS patients is warranted to further explore these findings.

Abstract B12: Synergistic anticancer activity of Aurora A kinase and MDM2 antagonists in melanoma

Senescence-inducing therapies can block proliferation of malignant cells and promote anti-tumor immune activity. However, the risk of tumor relapse remains high due to the long lifespan of senescence cells with potential to escape senescence. Here our preclinical studies demonstrate that combining a senescent-inducing aurora kinase A (AURKA) inhibitor alisertib (MLN8237) with an MDM2 antagonist [(-)-nutlin 3a] effectively induces robust p53 activation in senescent Tp53WT tumors accompanied by: 1) tumor cell proliferation arrest; 2) mitochondrial depolarization and tumor cell apoptosis; and 3) tumor cell clearance via CCL5-, CCL1- and CCL9-mediated recruitment of anti-tumor leukocytes. This combined therapy shows adequate bioavailability and low toxicity to the host in the mouse model. Moreover, the prominent preclinical response of patient-derived melanoma tumors to the co-targeting of MDM2 and AURKA provides rationale for further investigation of alisertib and MDM2 inhibitors.

Citation Format: Anna E. Vilgelm, Jeff S. Pawlikowski, Yan Liu, E. Hawkins Oriana, A. Davis Tyler, Kevin P. Weller, Linda W. Horton, Colt M. McClain, Gregory D. Ayers, David Turner, David C. Essaka, Clinton F. Stewart, Jeffrey A. Sosman, Mark C. Kelley, Jeffrey A. Ecsedy, Jeffrey N. Johnston, Ann Richmond. Synergistic anticancer activity of Aurora A kinase and MDM2 antagonists in melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B12.

Enantioselective small molecule synthesis by carbon dioxide fixation using a dual Brønsted acid/base organocatalyst

Carbon dioxide exhibits many of the qualities of an ideal reagent: it is nontoxic, plentiful, and inexpensive. Unlike other gaseous reagents, however, it has found limited use in enantioselective synthesis. Moreover, unprecedented is a tool that merges one of the simplest biological approaches to catalysis-Brønsted acid/base activation-with this abundant reagent. We describe a metal-free small molecule catalyst that achieves the three component reaction between a homoallylic alcohol, carbon dioxide, and an electrophilic source of iodine. Cyclic carbonates are formed enantioselectively.

Adaptation of a small-molecule hydrogen-bond donor catalyst to an enantioselective hetero-Diels-Alder reaction hypothesized for brevianamide biosynthesis

Chiral diamine-derived hydrogen-bond donors were evaluated for their ability to effect stereocontrol in an intramolecular hetero-Diels-Alder (HDA) reaction hypothesized in the biosynthesis of brevianamides A and B. Collectively, these results provide proof of principle that small-molecule hydrogen-bond catalysis, if even based on a hypothetical biosynthesis construct, holds significant potential within enantioselective natural product synthesis.

Enantioselective Synthesis of D-α-Amino Amides from Aliphatic Aldehydes

Bromonitromethane is used in a phase transfer-catalysed enantioselective aza-Henry reaction, leading to d-amino amide bearing an alkyl chain.

Peptides consisting of d-amino amides are highly represented among both biologically active natural products and non-natural small molecules used in therapeutic development. Chemical synthesis of d-amino amides most often involves approaches based on enzymatic resolution or fractional recrystallization of their diastereomeric amino acid salt precursors, techniques that produce an equal amount of the l-amino acid. Enantioselective synthesis, however, promises selective and general access to a specific α-amino amide, and may enable efficient peptide synthesis regardless of the availability of the corresponding α-amino acid. This report describes the use of a cinchona alkaloid-catalyzed aza-Henry reaction using bromonitromethane, and the integration of its product with umpolung amide synthesis. The result is a straightforward 3-step protocol beginning from aliphatic aldehydes that provides homologated peptides bearing an aliphatic side chain at the resulting d-α-amino amide.

Enantioselective Addition of Bromonitromethane to Aliphatic N-Boc Aldimines Using a Homogeneous Bifunctional Chiral Organocatalyst

This report details the enantioselective synthesis of β-amino-α-bromo nitroalkanes with β-alkyl substituents, using homogeneous catalysis to prepare either antipode. Use of a bifunctional Brønsted base/acid catalyst allows equal access to either enantiomer of the products, enabling the use of Umpolung Amide Synthesis (UmAS) to prepare the corresponding L- or D-α-amino amide bearing alkyl side chains - overall, in only 4 steps from aldehyde. The approach also addresses an underlying incompatibility between bromonitromethane and solid hydroxide bases.

Mdm2 and aurora kinase a inhibitors synergize to block melanoma growth by driving apoptosis and immune clearance of tumor cells

Therapeutics that induce cancer cell senescence can block cell proliferation and promote immune rejection. However, the risk of tumor relapse due to senescence escape may remain high due to the long lifespan of senescent cells that are not cleared. Here, we show how combining a senescence-inducing inhibitor of the mitotic kinase Aurora A (AURKA) with an MDM2 antagonist activates p53 in senescent tumors harboring wild-type 53. In the model studied, this effect is accompanied by proliferation arrest, mitochondrial depolarization, apoptosis, and immune clearance of cancer cells by antitumor leukocytes in a manner reliant upon Ccl5, Ccl1, and Cxcl9. The AURKA/MDM2 combination therapy shows adequate bioavailability and low toxicity to the host. Moreover, the prominent response of patient-derived melanoma tumors to coadministered MDM2 and AURKA inhibitors offers a sound rationale for clinical evaluation. Taken together, our work provides a preclinical proof of concept for a combination treatment that leverages both senescence and immune surveillance to therapeutic ends.

Brønsted acid catalyzed phosphoramidic acid additions to alkenes: diastereo- and enantioselective halogenative cyclizations for the synthesis of C- and P-chiral phosphoramidates

The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

Umpolung amide synthesis using substoichiometric N-iodosuccinimide (NIS) and oxygen as a terminal oxidant

Umpolung Amide Synthesis (UmAS) provides direct access to amides from an α-bromo nitroalkane and an amine. Based on its mechanistic bifurcation after convergent C–N bond formation, depending on the absence or presence of oxygen, UmAS using substoichiometric N-iodosuccinimide (NIS) under aerobic conditions has been developed. In combination with the enantioselective preparation of α-bromo nitroalkane donors, this protocol realizes the goal of enantioselective α-amino amide and peptide synthesis based solely on catalytic methods.

Organocatalytic, diastereo- and enantioselective synthesis of nonsymmetric cis-stilbene diamines: a platform for the preparation of single-enantiomer cis-imidazolines for protein-protein inhibition

The finding by scientists at Hoffmann-La Roche that cis-imidazolines could disrupt the protein–protein interaction between p53 and MDM2, thereby inducing apoptosis in cancer cells, raised considerable interest in this scaffold over the past decade. Initial routes to these small molecules (i.e., Nutlin-3) provided only the racemic form, with enantiomers being enriched by chromatographic separation using high-pressure liquid chromatography (HPLC) and a chiral stationary phase. Reported here is the first application of an enantioselective aza-Henry approach to nonsymmetric cis-stilbene diamines and cis-imidazolines. Two novel mono(amidine) organocatalysts (MAM) were discovered to provide high levels of enantioselection (>95% ee) across a broad range of substrate combinations. Furthermore, the versatility of the aza-Henry strategy for preparing nonsymmetric cis-imidazolines is illustrated by a comparison of the roles of aryl nitromethane and aryl aldimine in the key step, which revealed unique substrate electronic effects providing direction for aza-Henry substrate–catalyst matching. This method was used to prepare highly substituted cis-4,5-diaryl imidazolines that project unique aromatic rings, and these were evaluated for MDM2-p53 inhibition in a fluorescence polarization assay. The diversification of access to cis-stilbene diamine-derived imidazolines provided by this platform should streamline their further development as chemical tools for disrupting protein–protein interactions.