Asymmetric Synthesis with Organoselenium Compounds - The Past Twelve Years

The discovery and synthetic applications of novel organoselenium compounds and their reactions proceeded rapidly during the past fifty years and such processes are now carried out routinely in many laboratories. At the same time, the growing demand for new enantioselective processes provided new challenges. The convergence of selenium chemistry and asymmetric synthesis led to key developments in the 1970s, although the majority of early work was based on stoichiometric processes. More recently, greater emphasis has been placed on greener catalytic variations, along with the discovery of novel reactions and a deeper understanding of their mechanisms. The present review covers the literature in this field from 2010 to early 2023 and encompasses asymmetric reactions mediated by chiral selenium-based reagents, auxiliaries, and especially, catalysts. Protocols based on achiral selenium compounds in conjunction with other species of chiral catalysts, as well as reactions that are controlled by chiral substrates, are also included.

Synthesis of the Marine Alkaloid Cylindricine C and Serendipitous Synthesis of Its 2,13-Di-epi Stereoisomer

A new approach to the marine alkaloid cylindricine C afforded its previously unreported (±)-2,13-di-epi stereoisomer as the major product along with a minor amount of the racemic parent alkaloid. Key steps included a stereoselective dianion alkylation of a monoester of 1,2-cyclohexanedicarboxylic acid and an annulation based on the tandem conjugate addition of a primary amine to an acetylenic sulfone, followed by intramolecular acylation of the resulting sulfone-stabilized carbanion. The cis-azadecalin moiety thus formed, comprising the cyclohexane A-ring and enaminone B-ring of the products, was further elaborated by the selenenyl chloride-induced cyclofunctionalization of a pendant butenyl substituent with the enaminone moiety, followed by a seleno-Pummerer reaction. Desulfonylation and enaminone reduction afforded the final products. Molecular modeling and X-ray crystallography provided further insight into these processes.

A Gain-of-function Mutation in the Gating Domain of ITPR1 Impairs Motor Movement and Increases Thermal and Mechanical Sensitivity

Inositol 1,4,5-trisphosphate receptor type 1 (ITPR1) is an intracellular Ca²⁺ release channel important for a number of fundamental cellular functions. Consistent with its critical physiological significance, mutations in ITPR1 are associated with disease. Surprisingly, nearly all the disease-associated ITPR1 mutations characterized to date are loss of function. Despite the paucity of ITPR1 gain-of-function (GOF) mutations, enhanced ITPR1 function as a result of dysregulation by ITPR1 interacting proteins is thought to be associated with ataxia, learning and memory impairments, Alzheimer's disease (AD) progression, and chronic pain. However, direct evidence for the role of ITPR1 GOF in disease is lacking. To determine whether GOF in ITPR1 itself has pathological ramifications, we employed a newly developed mouse model expressing an ITPR1 mutation in the gating domain of the channel, D2594K, that markedly increased the channel's sensitivity to activation by IP₃. Behavioral studies showed that the ITPR1-D2594K^+/- mutant mice displayed motor deficits and reduced muscle strength. However, the ITPR1-D2594K^+/- mutation did not significantly alter hippocampal learning and memory and did not change learning and memory impairments when crossed with the 5xFAD AD model mice. On the other hand, ITPR1-D2594K^+/- mice exhibited increased sensitivity to thermal and mechanical stimulation compared to WT. Interestingly, R-carvedilol treatment attenuated the enhanced thermal and mechanical nociception in ITPR1-D2594K^+/- mice. Thus, the ITPR1-D2594K^+/- mutation in the channel's gating domain has a marked impact on motor movements and pain perception, but little effect on hippocampal learning and memory.

Chemistry Related to the Catalytic Cycle of the Antioxidant Ebselen

The antioxidant drug ebselen has been widely studied in both laboratories and in clinical trials. The catalytic mechanism by which it destroys hydrogen peroxide via reduction with glutathione or other thiols is complex and has been the subject of considerable debate. During reinvestigations of several key steps, we found that the seleninamide that comprises the first oxidation product of ebselen underwent facile reversible methanolysis to an unstable seleninate ester and two dimeric products. In its reaction with benzyl alcohol, the seleninamide produced a benzyl ester that reacted readily by selenoxide elimination, with formation of benzaldehyde. Oxidation of ebselen seleninic acid did not afford a selenonium seleninate salt as previously observed with benzene seleninic acid, but instead generated a mixture of the seleninic and selenonic acids. Thiolysis of ebselen with benzyl thiol was faster than oxidation by ca. an order of magnitude and produced a stable selenenyl sulfide. When glutathione was employed, the product rapidly disproportionated to glutathione disulfide and ebselen diselenide. Oxidation of the S-benzyl selenenyl sulfide, or thiolysis of the seleninamide with benzyl thiol, afforded a transient thiolseleninate that also readily underwent selenoxide elimination. The S-benzyl derivative disproportionated readily when catalyzed by the simultaneous presence of both the thiol and triethylamine. The phenylthio analogue disproportionated when exposed to ambient or UV (360 nm) light by a proposed radical mechanism. These observations provide additional insight into several reactions and intermediates related to ebselen.

Synthesis, Catalytic GPx-like Activity, and SET Reactions of Conformationally Constrained 2,7-Dialkoxy-Substituted Naphthalene-1,8-peri-diselenides

Several 2,7-dialkoxy-substituted naphthalene-1,8-peri-diselenides were prepared and tested for catalytic antioxidant activity in an NMR-based assay employing the reduction of hydrogen peroxide with stoichiometric amounts of benzyl thiol. Acidic conditions enhanced their catalytic activity, whereas basic conditions suppressed it. The highest activity was observed with a 2,7-bis(triethyleneglycol) derivative. These compounds serve as mimetics of the antioxidant selenoenzyme glutathione peroxidase. Studies based on NMR peak-broadening effects and EPR spectroscopy indicated that a thiol-dependent SET reaction occurs under the conditions of the assay, which can be reversed by the addition of triethylamine. In contrast, peak broadening induced by proton-catalyzed electron transfer during the treatment of naphthalene-1,8-peri-diselenides with trifluoroacetic acid can be suppressed by the addition of excess thiol. These observations provide new insights into the redox mechanisms of these processes.

Limiting RyR2 open time prevents Alzheimer's disease-related deficits in the 3xTG-AD mouse model

Increasing evidence suggests that Alzheimer's disease (AD) progression is driven by a vicious cycle of soluble β-amyloid (Aβ)-induced neuronal hyperactivity. Thus, breaking this vicious cycle by suppressing neuronal hyperactivity may represent a logical approach to stopping AD progression. In support of this, we have recently shown that genetically and pharmacologically limiting ryanodine receptor 2 (RyR2) open time prevented neuronal hyperactivity, memory impairment, dendritic spine loss, and neuronal cell death in a rapid, early onset AD mouse model (5xFAD). Here, we assessed the impact of limiting RyR2 open time on AD-related deficits in a relatively late occurring, slow developing AD mouse model (3xTG-AD) that bears more resemblance (compared to 5xFAD) to that of human AD. Using behavioral tests, long-term potentiation recordings, and Golgi and Nissl staining, we found that the RyR2-E4872Q mutation, which markedly shortens the open duration of the RyR2 channel, prevented learning and memory impairment, defective long-term potentiation, dendritic spine loss, and neuronal cell death in the 3xTG-AD mice. Furthermore, pharmacologically shortening the RyR2 open time with R-carvedilol rescued these AD-related deficits in 3xTG mice. Therefore, limiting RyR2 open time may offer a promising, neuronal hyperactivity-targeted anti-AD strategy.

One-Pot Synthesis of Aryl Selenonic Acids and Some Unexpected Byproducts

The synthesis of aryl selenonic acids was achieved from diverse aryl bromides via a one-pot method involving metalation, selenation, and oxidation with hydrogen peroxide followed by ion exchange to afford the pure products in 77-90% yield. An o-hydroxymethyl derivative was found to dehydrate readily, affording the first example of a cyclic selenonic ester, while two minor byproducts were isolated and shown by X-ray crystallography to be mixed salts of aryl selenonic acids with either the corresponding aryl seleninic or selenious acid.

Genetically and pharmacologically limiting RyR2 open time prevents neuronal hyperactivity of hippocampal CA1 neurons in brain slices of 5xFAD mice

Neuronal hyperactivity is an early, common manifestation of Alzheimer's disease (AD), and is believed to drive AD progression. Neuronal hyperactivity in the form of baseline activity (or spontaneous Ca²⁺ transients) has consistently been demonstrated in mouse models of AD using two-photon in vivo Ca²⁺ imaging of cortical or hippocampal neurons in anesthetized animals. Notably, these AD-related spontaneous Ca²⁺ transients were hardly detected in acute hippocampal slices, probably due to neuronal damage during brain slicing. To better preserve neuronal activity, we employed the N-methyl-D-glucamine (NMDG) protective brain slicing protocol. We performed confocal in vitro Ca²⁺ imaging of hippocampal CA1 neurons in optimized hippocampal slices. Consistent with previous in vivo studies, our in vitro studies using optimized brain slices also showed that limiting the open duration of the ryanodine receptor 2 (RyR2) by the RyR2 mutation E4872Q or by the R-carvedilol enantiomer prevented and rescued neuronal hyperactivity of hippocampal CA1 neurons from 5xFAD mice. Thus, genetically and pharmacologically limiting RyR2 open time prevented and rescued AD-related neuronal hyperactivity in vitro in optimized brain slices in the absence of anesthetics' influence. Our data also suggest that the NMDG protective brain slicing preparation offers an alternative means to study neuronal hyperactivity of various cell types in different brain regions, especially in regions that are not readily accessible to two-photon in vivo Ca²⁺ imaging.

Limiting RyR2 Open Time Prevents Alzheimer's Disease-Related Neuronal Hyperactivity and Memory Loss but Not β-Amyloid Accumulation

Neuronal hyperactivity is an early primary dysfunction in Alzheimer’s disease (AD) in humans and animal models, but effective neuronal hyperactivity-directed anti-AD therapeutic agents are lacking. Here we define a previously unknown mode of ryanodine receptor 2 (RyR2) control of neuronal hyperactivity and AD progression. We show that a single RyR2 point mutation, E4872Q, which reduces RyR2 open time, prevents hyperexcitability, hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset AD mouse model (5xFAD). The RyR2-E4872Q mutation upregulates hippocampal CA1-pyramidal cell A-type K⁺ current, a well-known neuronal excitability control that is downregulated in AD. Pharmacologically limiting RyR2 open time with the R-carvedilol enantiomer (but not racemic carvedilol) prevents and rescues neuronal hyperactivity, memory impairment, and neuron loss even in late stages of AD. These AD-related deficits are prevented even with continued β-amyloid accumulation. Thus, limiting RyR2 open time may be a hyperactivity-directed, non-β-amyloid-targeted anti-AD strategy.

Yao et al. show that genetically or pharmacologically limiting the open duration of ryanodine receptor 2 upregulates the A-type potassium current and prevents neuronal hyperexcitability and hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset Alzheimer’s disease mouse model, even with continued accumulation of β-amyloid.

The Unexpected Role of SeVI Species in Epoxidations with Benzeneseleninic Acid and Hydrogen Peroxide

Benzeneperoxyseleninic acid has been proposed as the key intermediate in the widely used epoxidation of alkenes with benzeneseleninic acid and hydrogen peroxide. However, it reacts sluggishly with cyclooctene and instead rapidly decomposes in solution to a mixed selenonium-selenonate salt that was identified by X-ray absorption and 77 Se NMR spectroscopy, as well as by single crystal X-ray diffraction. This process includes a selenoxide elimination of the peroxyseleninic acid with liberation of oxygen and additional redox steps. The salt is relatively stable in the solid state, but generates the corresponding selenonic acid in the presence of hydrogen peroxide. The selenonic acid is inert towards cyclooctene on its own; however, rapid epoxidation occurs when hydrogen peroxide is added. This shows that the selenonic acid must first be activated through further oxidation, presumably to the heretofore unknown benzeneperoxyselenonic acid. The latter is the principal oxidant in this epoxidation.

Synthesis and activity of nucleoside-based antiprotozoan compounds

Parasitic protozoa employ a salvage pathway to synthesize purines and generate essential active nucleotides, whereas mammals are capable of their de novo biosynthesis. This difference provides opportunity for the design of potential new antiprotozoan compounds. A series of 47 adenosine analogues was prepared with modifications at the 2-, 6- and 5'-positions, based on the hypothesis that such compounds would serve as substrates for protozoan nucleoside salvage enzymes, while remaining refractory in mammalian cells. The nucleosides were designed to produce toxic metabolites upon cleavage to the corresponding purine base by the parasite. Three 7-deazaguanosine derivatives were prepared with similar objectives. All of these compounds were tested in vitro against T. brucei (African sleeping sickness), T. cruzi (Chagas' disease), L. donovani (leishmaniasis) and P. falciparum (malaria). In order to determine the therapeutic selectivity indices (SI) of the antiprotozoan nucleosides, their cytotoxicities toward a rat myoblast cell line were also determined. One adenosine derivative proved highly effective against P. falciparum (IC₅₀=110nM and SI=1010, while a modified guanosine displayed potent activities against L. donovani (IC₅₀=60nM, SI=2720) and T. brucei (IC₅₀=130nM, SI=1250), as well as moderate activity against T. cruzi (IC₅₀=3.4µM, SI=48). These results provide proof of concept for the nucleoside-based antiprotozoan strategy, as well as potential lead compounds for further optimization and validation.

Comparison of free-radical inhibiting antioxidant properties of carvedilol and its phenolic metabolites

Carvedilol is a widely prescribed drug for the treatment of heart failure and the prevention of associated ventricular arrhythmias. It has also been reported to function as a biological antioxidant via hydrogen atom transfer from its carbazole N-H moiety to chain-propagating radicals. Metabolites of the drug include phenolic derivatives, such as 3-hydroxy-, 4'-hydroxy- and 5'-hydroxycarvedilol, which are also potential antioxidants. A comparison of the radical-inhibiting activities of the parent drug and the three metabolites was carried out in two separate assays. In the first, hydrogen atom transfer from these four compounds to the stable radical DPPH was measured by the decrease in the UV-visible absorption at 515 nm of the latter. The known radical inhibitors BHT, 4-hydroxycarbazole and α-tocopherol were employed as benchmarks in parallel experiments. In the second assay, inhibition of the photoinduced free-radical 1,2-addition of Se-phenyl p-tolueneselenosulfonate to cyclopropylacetylene, along with competing ring-opening of the cyclopropane ring, was monitored by ¹H NMR spectroscopy in the presence of the carvedilol-based and benchmark antioxidants. In both assays, carvedilol displayed negligible antioxidant activity, while the three metabolites all proved superior radical inhibitors to BHT, with radical-quenching abilities in the order 3-hydroxy- > 5'-hydroxy > 4'-hydroxycarvedilol. Among the metabolites, 3-hydroxycarvedilol displayed even stronger activity in both assays than α-tocopherol, the best of the benchmark antioxidants. These results suggest that the radical-inhibiting antioxidant properties that have been attributed to carvedilol are largely or exclusively due to its metabolites and not to the parent drug itself.

Enhanced Cytosolic Ca2+ Activation Underlies a Common Defect of Central Domain Cardiac Ryanodine Receptor Mutations Linked to Arrhythmias

Recent three-dimensional structural studies reveal that the central domain of ryanodine receptor (RyR) serves as a transducer that converts long-range conformational changes into the gating of the channel pore. Interestingly, the central domain encompasses one of the mutation hotspots (corresponding to amino acid residues 3778-4201) that contains a number of cardiac RyR (RyR2) mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) and atrial fibrillation (AF). However, the functional consequences of these central domain RyR2 mutations are not well understood. To gain insights into the impact of the mutation and the role of the central domain in channel function, we generated and characterized eight disease-associated RyR2 mutations in the central domain. We found that all eight central domain RyR2 mutations enhanced the Ca²⁺-dependent activation of [³H]ryanodine binding, increased cytosolic Ca²⁺-induced fractional Ca²⁺ release, and reduced the activation and termination thresholds for spontaneous Ca²⁺ release in HEK293 cells. We also showed that racemic carvedilol and the non-beta-blocking carvedilol enantiomer, (R)-carvedilol, suppressed spontaneous Ca²⁺ oscillations in HEK293 cells expressing the central domain RyR2 mutations associated with CPVT and AF. These data indicate that the central domain is an important determinant of cytosolic Ca²⁺ activation of RyR2. These results also suggest that altered cytosolic Ca²⁺ activation of RyR2 represents a common defect of RyR2 mutations associated with CPVT and AF, which could potentially be suppressed by carvedilol or (R)-carvedilol.

Carvedilol inhibits cADPR- and IP3-induced Ca2+ release

Spontaneous Ca²⁺ waves, also termed store-overload-induced Ca²⁺ release (SOICR), in cardiac cells can trigger ventricular arrhythmias especially in failing hearts. SOICR occurs when RyRs are activated by an increase in sarcoplasmic reticulum (SR) luminal Ca²⁺. Carvedilol is one of the most effective drugs for preventing arrhythmias in patients with heart failure. Furthermore, carvedilol analogues with minimal β-blocking activity also block SOICR showing that SOICR-inhibiting activity is distinct from that for β-block. We show here that carvedilol is a potent inhibitor of cADPR-induced Ca²⁺ release in sea urchin egg homogenate. In addition, the carvedilol analog VK-II-86 with minimal β-blocking activity also suppresses cADPR-induced Ca²⁺ release. Carvedilol appeared to be a non-competitive antagonist of cADPR and could also suppress Ca²⁺ release by caffeine. These results are consistent with cADPR releasing Ca²⁺ in sea urchin eggs by sensitizing RyRs to Ca²⁺ involving a luminal Ca²⁺ activation mechanism. In addition to action on the RyR, we also observed inhibition of inositol 1,4,5-trisphosphate (IP₃)-induced Ca²⁺ release by carvedilol suggesting a common mechanism between these evolutionarily related and conserved Ca²⁺ release channels.

Novel nucleoside-based antimalarial compounds

The malaria-causing parasite Plasmodium falciparum employs a salvage pathway for the biosynthesis of nucleotides, in contrast to de novo biosynthesis that is utilized by the human host. A series of twenty-two 2-, 6- and 5'-modified adenosine ribonucleosides was synthesized, with the expectation that these compounds would generate toxic metabolites instead of active nucleotides by the pathogen, while remaining inert in host cells. Bioassays with P. falciparum (K1 strain) indicated IC50 values as low as 110nM and a selectivity index with respect to cytotoxicity toward an L6 rat myoblast cell line of >1000 for the most potent analogue.

The role of methoxy substituents in regulating the activity of selenides that serve as spirodioxyselenurane precursors and glutathione peroxidase mimetics

A series of o-(hydroxymethyl)phenyl selenides containing single or multiple methoxy substituents was synthesized, and the rate at which each compound catalyzed the oxidation of benzyl thiol to its disulfide with excess hydrogen peroxide was measured. This assay provided the means for comparing the relative abilities of the selenides to mimic the antioxidant selenoenzyme glutathione peroxidase. The mechanism for catalytic activity involves oxidation of the selenides to their corresponding selenoxides with hydrogen peroxide, cyclization to spirodioxyselenuranes, followed by reduction with two equivalents of thiol to regenerate the original selenide with concomitant disulfide formation. A single p-methoxy group on each aryl moiety afforded the highest catalytic activity, while methoxy groups in the meta position had little effect compared to the unsubstituted selenide, and o-methoxy groups suppressed activity. The installation of multiple methoxy groups on each aryl moiety provided no improvement. These results can be rationalized on the basis of dominating mesomeric and steric effects of the p- and o-substituents, respectively.

Oxidation of Disulfides to Thiolsulfinates with Hydrogen Peroxide and a Cyclic Seleninate Ester Catalyst

Cyclic seleninate esters function as mimetics of the antioxidant selenoenzyme glutathione peroxidase. They catalyze the reduction of harmful peroxides with thiols, which are converted to disulfides in the process. The possibility that the seleninate esters could also catalyze the further oxidation of disulfides to thiolsulfinates and other overoxidation products under these conditions was investigated. This has ramifications in potential medicinal applications of seleninate esters because of the possibility of catalyzing the unwanted oxidation of disulfide-containing spectator peptides and proteins. A variety of aryl and alkyl disulfides underwent facile oxidation with hydrogen peroxide in the presence of catalytic benzo-1,2-oxaselenolane Se-oxide affording the corresponding thiolsulfinates as the principal products. Unsymmetrical disulfides typically afforded mixtures of regioisomers. Lipoic acid and N,N′-dibenzoylcystine dimethyl ester were oxidized readily under similar conditions. Although isolated yields of the product thiolsulfinates were generally modest, these experiments demonstrate that the method nevertheless has preparative value because of its mild conditions. The results also confirm the possibility that cyclic seleninate esters could catalyze the further undesired oxidation of disulfides in vivo.

Stereodivergent synthesis of the LFA-1 antagonist BIRT-377 by porcine liver esterase desymmetrization and Curtius rearrangement

BIRT-377 was synthesized by enzymatic desymmetrization in conjunction with Curtius rearrangement, affording a key α-quaternary amine intermediate in high ee.

Structure driven design of novel human ether-a-go-go-related-gene channel (hERG1) activators

One of the main culprits in modern drug discovery is apparent cardiotoxicity of many lead-candidates via inadvertent pharmacologic blockade of K⁺, Ca²⁺ and Na⁺ currents. Many drugs inadvertently block hERG1 leading to an acquired form of the Long QT syndrome and potentially lethal polymorphic ventricular tachycardia. An emerging strategy is to rely on interventions with a drug that may proactively activate hERG1 channels reducing cardiovascular risks. Small molecules-activators have a great potential for co-therapies where the risk of hERG-related QT prolongation is significant and rehabilitation of the drug is impractical. Although a number of hERG1 activators have been identified in the last decade, their binding sites, functional moieties responsible for channel activation and thus mechanism of action, have yet to be established. Here, we present a proof-of-principle study that combines de-novo drug design, molecular modeling, chemical synthesis with whole cell electrophysiology and Action Potential (AP) recordings in fetal mouse ventricular myocytes to establish basic chemical principles required for efficient activator of hERG1 channel. In order to minimize the likelihood that these molecules would also block the hERG1 channel they were computationally engineered to minimize interactions with known intra-cavitary drug binding sites. The combination of experimental and theoretical studies led to identification of functional elements (functional groups, flexibility) underlying efficiency of hERG1 activators targeting binding pocket located in the S4–S5 linker, as well as identified potential side-effects in this promising line of drugs, which was associated with multi-channel targeting of the developed drugs.

Novel carvedilol analogues that suppress store-overload-induced Ca2+ release

Carvedilol is a uniquely effective drug for the treatment of cardiac arrhythmias in patients with heart failure. This activity is in part because of its ability to inhibit store-overload-induced calcium release (SOICR) through the RyR2 channel. We describe the synthesis, characterization, and bioassay of ca. 100 compounds based on the carvedilol motif to identify features that correlate with and optimize SOICR inhibition. A single-cell bioassay was employed on the basis of the RyR2-R4496C mutant HEK-293 cell line in which calcium release from the endoplasmic reticulum through the defective channel was measured. IC50 values for SOICR inhibition were thus obtained. The compounds investigated contained modifications to the three principal subunits of carvedilol, including the carbazole and catechol moieties, as well as the linker chain containing the β-amino alcohol functionality. The SAR results indicate that significant alterations are tolerated in each of the three subunits.

Carvedilol analog modulates both basal and stimulated sinoatrial node automaticity

The membrane voltage clock and calcium (Ca(2+)) clock jointly regulate sinoatrial node (SAN) automaticity. VK-II-36 is a novel carvedilol analog that suppresses sarcoplasmic reticulum (SR) Ca(2+) release but does not block the β-receptor. The effect of VK-II-36 on SAN function remains unclear. The purpose of this study was to evaluate whether VK-II-36 can influence SAN automaticity by inhibiting the Ca(2+) clock. We simultaneously mapped intracellular Ca(2+) and membrane potential in 24 isolated canine right atriums using previously described criteria of the timing of late diastolic intracellular Ca elevation (LDCAE) relative to the action potential upstroke to detect the Ca(2+) clock. Pharmacological interventions with isoproterenol (ISO), ryanodine, caffeine, and VK-II-36 were performed after baseline recordings. VK-II-36 caused sinus rate downregulation and reduced LDCAE in the pacemaking site under basal conditions (P < 0.01). ISO induced an upward shift of the pacemaking site in SAN and augmented LDCAE in the pacemaking site. ISO also significantly and dose-dependently increased the sinus rate. The treatment of VK-II-36 (30 μmol/l) abolished both the ISO-induced shift of the pacemaking site and augmentation of LDCAE (P < 0.01), and it suppressed the ISO-induced increase in sinus rate (P = 0.02). Our results suggest that the sinus rate may be partly controlled by the Ca(2+) clock via SR Ca(2+) release during β-adrenergic stimulation.

Carvedilol analogue inhibits triggered activities evoked by both early and delayed afterdepolarizations

BACKGROUND

Carvedilol and its analogues suppress delayed afterdepolarizations (DADs) and catecholaminergic polymorphic ventricular tachycardias by direct action on the cardiac ryanodine receptor type 2 (RyR2).

OBJECTIVE

To test a hypothesis that carvedilol analogue may also prevent triggered activities (TAs) through the suppression of early afterdepolarizations (EADs).

METHODS

Intracellular Ca(2+) and membrane voltage were simultaneously recorded by using optical mapping technique in Langendorff-perfused mouse and rabbit hearts to study the effect of carvedilol analogue VK-II-36, which does not have significant beta-blocking effects.

RESULTS

Spontaneous intracellular Ca(2+) elevations (SCaEs) during diastole were induced by rapid ventricular pacing and isoproterenol infusion in intact rabbit ventricles. Systolic and diastolic SCaEs were simultaneously noted in Langendorff-perfused RyR2 R4496(+/-) mouse hearts after creating atrioventricular block. VK-II-36 effectively suppressed SCaEs and eliminated TAs observed in both mouse and rabbit ventricles. We tested the effect of VK-II-36 on EADs by using a rabbit model of acquired long QT syndrome, in which phase 2 and phase 3 EADs were observed in association with systolic SCaEs. VK-II-36 abolished the systolic SCaEs and phase 2 EADs, and greatly decreased the dispersion of repolarization and the amplitude of phase 3 EADs. VK-II-36 completely prevented EAD-mediated TAs in all ventricles studied.

CONCLUSIONS

A carvedilol analogue, VK-II-36, inhibits ventricular tachyarrhythmias in intact mouse and rabbit ventricles by the suppression of SCaEs, independent of beta-blocking activity. The RyR2 may be a potential target for treating focal ventricular arrhythmias triggered by either EADs or DADs.

Carvedilol and its new analogs suppress arrhythmogenic store overload-induced Ca2+ release

Carvedilol is one of the most effective beta blockers for preventing ventricular tachyarrhythmias in heart failure, but the mechanisms underlying its favorable antiarrhythmic benefits remain unclear. Spontaneous Ca(2+) waves, also called store overload-induced Ca(2+) release (SOICR), evoke ventricular tachyarrhythmias in individuals with heart failure. Here we show that carvedilol is the only beta blocker tested that effectively suppresses SOICR by directly reducing the open duration of the cardiac ryanodine receptor (RyR2). This unique anti-SOICR activity of carvedilol, combined with its beta-blocking activity, probably contributes to its favorable antiarrhythmic effect. To enable optimal titration of carvedilol's actions as a beta blocker and as a suppressor of SOICR separately, we developed a new SOICR-inhibiting, minimally beta-blocking carvedilol analog, VK-II-86. VK-II-86 prevented stress-induced ventricular tachyarrhythmias in RyR2-mutant mice and did so more effectively when combined with either of the selective beta blockers metoprolol or bisoprolol. Combining SOICR inhibition with optimal beta blockade has the potential to provide antiarrhythmic therapy that can be tailored to individual patients.

Design and synthesis of some biologically interesting natural and unnatural products based on organosulfur and selenium chemistry

Organosulfur and selenium chemistry has provided fertile ground for the discovery of novel synthetic methodology and for the design of bioactive molecules with potential therapeutic applications. Thus, acetylenic sulfones have been employed in novel strategies for the synthesis of nitrogen heterocycles, including several biologically active alkaloids. The conjugate addition of nitrogen nucleophiles containing ester or chloroalkyl substituents to acetylenic sulfones was followed by base-mediated intramolecular alkylation or acylation to afford variously substituted piperidines, pyrrolizidines, indolizidines, quinolizidines, decahydroquinolines, and 4-quinolones. The products include the dendrobatid alkaloids (–)-pumiliotoxin C, indolizidines (–)-167B, 207A, 209B, and 209D, as well as (–)-(ent)-julifloridine, (–)-lasubine II, myrtine, and two recently discovered alkaloids from the medicinal plant Ruta chalepensis , which had not been previously synthesized. Acetylenic sulfones were also incorporated on solid supports and employed in the types of cyclizations mentioned above, as well as for Diels–Alder reactions and a large variety of 1,3-dipolar cycloadditions. Conjugate additions of tertiary cyclic α-vinyl amines to acetylenic sulfones generated zwitterions that underwent exceptionally facile formal aza-Cope rearrangements to afford ring-expanded macrocyclic amines. An iterative version was developed and used in the synthesis of motuporamine A and B. With respect to organoselenium chemistry, two classes of compounds are described that function as novel mimetics of the selenoenzyme glutathione peroxidase (GPx), which protects cells from oxidative stress caused by the formation of peroxides during aerobic metabolism. They include cyclic seleninates and spirodioxyselenuranes, both of which efficiently catalyze the reduction of peroxides with thiols and are of potential value in the mitigation of oxidative stress. Their aromatic derivatives are generally less effective catalysts, but substituent effects can be used to modulate their activities. The mechanism of their catalytic cycles has been elucidated and Hammett plots indicate that the oxidation of Se(II) to Se(IV) is the rate-determining step for both classes. A methoxy-substituted aromatic spirodioxyselenurane provided the fastest rate for a small-molecule selenium compound that we have observed to date for the reduction of hydrogen peroxide with benzyl thiol.

1,3-Dipolar cycloadditions of acetylenic sulfones in solution and on solid supports

Several representative acetylenic sulfones were immobilized on a polymer support derived from Merrifield resin by means of ester linkers that were used to couple free carboxylic acid groups on the solid support with benzylic hydroxyl functions on the arylsulfonyl moieties of the acetylenes. Several examples of reversed ester linkers, using Merrifield resin directly, were also successfully prepared. The 1,3-dipolar cycloadditions of the solid-supported acetylenic sulfones were investigated with a series of 1,3-dipoles, including benzyl azide, ethyl diazoacetate, diazomethane, as well as representative nitrile oxides, nitrile imines, nitrile ylides, nitrones, azomethine imines, azomethine ylides, munchnones, and sydnones. In general, analogous cycloadditions were also performed with acetylenic sulfones in solution phase for comparison. The cycloadditions typically afforded good to excellent yields of the desired products in both solution and solid phase, although the latter reactions sometimes required more vigorous conditions. Except in the case of benzyl azide and diazo compounds, where mixtures of regioisomers were obtained, the other 1,3-dipoles reacted with high regioselectivity and afforded essentially unique regioisomers. Cleavage of the products from the resin was smoothly effected by alkaline hydrolysis, while several attempts at reductive desulfonylation with sodium amalgam or samarium diiodide-HMPA resulted in N-O or C-O scission, in addition to cleavage from the polymer. The method provides access to a number of important classes of heterocycles, including variously substituted and functionalized triazoles, pyrazoles, 1,2-oxazoles, pyrroles, as well as their dihydro and bicyclic analogues. The success of the cycloadditions on polymer supports paves the way to future investigations of sequential transformations leading to libraries of useful heterocycles.