Enantioselective Halolactonization Reactions using BINOL-Derived Bifunctional Catalysts: Methodology, Diversification, and Applications

A general protocol is described for inducing enantioselective halolactonizations of unsaturated carboxylic acids using novel bifunctional organic catalysts derived from a chiral binaphthalene scaffold. Bromo- and iodolactonization reactions of diversely substituted, unsaturated carboxylic acids proceed with high degrees of enantioselectivity, regioselectivity, and diastereoselectivity. Notably, these BINOL-derived catalysts are the first to induce the bromo- and iodolactonizations of 5-alkyl-4( Z)-olefinic acids via 5- exo mode cyclizations to give lactones in which new carbon-halogen bonds are created at a stereogenic center with high diastereo- and enantioselectivities. Iodolactonizations of 6-substituted-5( Z)-olefinic acids also occur via 6- exo cyclizations to provide δ-lactones with excellent enantioselectivities. Several notable applications of this halolactonization methodology were developed for desymmetrization, kinetic resolution, and epoxidation of Z-alkenes. The utility of these reactions is demonstrated by their application to a synthesis of precursors of the F-ring subunit of kibdelone C and to the shortest catalytic, enantioselective synthesis of (+)-disparlure reported to date.

High-Content Microfluidic Screening Platform Used To Identify σ2R/Tmem97 Binding Ligands that Reduce Age-Dependent Neurodegeneration in C. elegans SC_APP Model

The nematode Caenorhabditis elegans, with tractable genetics and a well-defined nervous system, provides a unique whole-animal model system to identify novel drug targets and therapies for neurodegenerative diseases. Large-scale drug or target screens in models that recapitulate the subtle age- and cell-specific aspects of neurodegenerative diseases are limited by a technological requirement for high-throughput analysis of neuronal morphology. Recently, we developed a single-copy model of amyloid precursor protein (SC_APP) induced neurodegeneration that exhibits progressive degeneration of select cholinergic neurons. Our previous work with this model suggests that small molecule ligands of the sigma 2 receptor (σ2R), which was recently cloned and identified as transmembrane protein 97 (TMEM97), are neuroprotective. To determine structure-activity relationships for unexplored chemical space in our σ2R/Tmem97 ligand collection, we developed an in vivo high-content screening (HCS) assay to identify potential drug leads. The HCS assay uses our recently developed large-scale microfluidic immobilization chip and automated imaging platform. We discovered norbenzomorphans that reduced neurodegeneration in our C. elegans model, including two compounds that demonstrated significant neuroprotective activity at multiple doses. These findings provide further evidence that σ2R/Tmem97-binding norbenzomorphans may represent a new drug class for treating neurodegenerative diseases.

Synthetic Analogues of the Snail Toxin 6-Bromo-2-mercaptotryptamine Dimer (BrMT) Reveal That Lipid Bilayer Perturbation Does Not Underlie Its Modulation of Voltage-Gated Potassium Channels

Drugs do not act solely by canonical ligand-receptor binding interactions. Amphiphilic drugs partition into membranes, thereby perturbing bulk lipid bilayer properties and possibly altering the function of membrane proteins. Distinguishing membrane perturbation from more direct protein-ligand interactions is an ongoing challenge in chemical biology. Herein, we present one strategy for doing so, using dimeric 6-bromo-2-mercaptotryptamine (BrMT) and synthetic analogues. BrMT is a chemically unstable marine snail toxin that has unique effects on voltage-gated K⁺ channel proteins, making it an attractive medicinal chemistry lead. BrMT is amphiphilic and perturbs lipid bilayers, raising the question of whether its action against K⁺ channels is merely a manifestation of membrane perturbation. To determine whether medicinal chemistry approaches to improve BrMT might be viable, we synthesized BrMT and 11 analogues and determined their activities in parallel assays measuring K⁺ channel activity and lipid bilayer properties. Structure-activity relationships were determined for modulation of the Kv1.4 channel, bilayer partitioning, and bilayer perturbation. Neither membrane partitioning nor bilayer perturbation correlates with K⁺ channel modulation. We conclude that BrMT's membrane interactions are not critical for its inhibition of Kv1.4 activation. Further, we found that alkyl or ether linkages can replace the chemically labile disulfide bond in the BrMT pharmacophore, and we identified additional regions of the scaffold that are amenable to chemical modification. Our work demonstrates a strategy for determining if drugs act by specific interactions or bilayer-dependent mechanisms, and chemically stable modulators of Kv1 channels are reported.

Synthesis of (+)-Disparlure via Enantioselective Iodolactonization

The BINOL-amidine organic catalyst 1 was previously shown to promote highly efficient enantioselective halolactonization reactions of olefinic acids. As part of these studies, it was discovered that the enantioenriched iodolactones could be easily converted into enantioenriched cis-1,2-disubstituted epoxides. This halolactonization-epoxidation sequence was applied to the synthesis of (+)-disparlure, which resulted in the shortest catalytic enantioselective synthesis to date, requiring only five steps and proceeding in 33% yield.

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Sigma 2 Receptor/Tmem97 Agonists Produce Long Lasting Antineuropathic Pain Effects in Mice

Neuropathic pain is an important medical problem with few effective treatments. The sigma 1 receptor (σ1R) is known to be a potential target for neuropathic pain therapeutics, and antagonists for this receptor are effective in preclinical models and are currently in phase II clinical trials. Conversely, relatively little is known about σ2R, which has recently been identified as transmembrane protein 97 (Tmem97). We generated a series of σ1R and σ2R/Tmem97 agonists and antagonists and tested them for efficacy in the mouse spared nerve injury (SNI) model. In agreement with previous reports, we find that σ1R ligands given intrathecally (IT) produce relief of SNI-induced mechanical hypersensitivity. We also find that the putative σ2R/Tmem97 agonists DKR-1005, DKR-1051, and UKH-1114 (Ki ∼ 46 nM) lead to relief of SNI-induced mechanical hypersensitivity, peaking at 48 h after dosing when given IT. This effect is blocked by the putative σ2R/Tmem97 antagonist SAS-0132. Systemic administration of UKH-1114 (10 mg/kg) relieves SNI-induced mechanical hypersensitivity for 48 h with a peak magnitude of effect equivalent to 100 mg/kg gabapentin and without producing any motor impairment. Finally, we find that the TMEM97 gene is expressed in mouse and human dorsal root ganglion (DRG) including populations of neurons that are involved in pain; however, the gene is also likely expressed in non-neuronal cells that may contribute to the observed behavioral effects. Our results show robust antineuropathic pain effects of σ1R and σ2R/Tmem97 ligands, demonstrate that σ2R/Tmem97 is a novel neuropathic pain target, and identify UKH-1114 as a lead molecule for further development.

Small molecule modulator of sigma 2 receptor is neuroprotective and reduces cognitive deficits and neuroinflammation in experimental models of Alzheimer's disease

Accumulating evidence suggests that modulating the sigma 2 receptor (Sig2R) can provide beneficial effects for neurodegenerative diseases. Herein, we report the identification of a novel class of Sig2R ligands and their cellular and in vivo activity in experimental models of Alzheimer's disease (AD). We report that SAS-0132 and DKR-1051, selective ligands of Sig2R, modulate intracellular Ca²⁺ levels in human SK-N-SH neuroblastoma cells. The Sig2R ligands SAS-0132 and JVW-1009 are neuroprotective in a C. elegans model of amyloid precursor protein-mediated neurodegeneration. Since this neuroprotective effect is replicated by genetic knockdown and knockout of vem-1, the ortholog of progesterone receptor membrane component-1 (PGRMC1), these results suggest that Sig2R ligands modulate a PGRMC1-related pathway. Last, we demonstrate that SAS-0132 improves cognitive performance both in the Thy-1 hAPP^Lond/Swe+ transgenic mouse model of AD and in healthy wild-type mice. These results demonstrate that Sig2R is a promising therapeutic target for neurocognitive disorders including AD.

Identification of the gene that codes for the σ2 receptor

The σ2 receptor is an enigmatic protein that has attracted significant attention because of its involvement in diseases as diverse as cancer and neurological disorders. Unlike virtually all other receptors of medical interest, it has eluded molecular cloning since its discovery, and the gene that codes for the receptor remains unknown, precluding the use of modern biological methods to study its function. Using a chemical biology approach, we purified the σ2 receptor from tissue, revealing its identity as TMEM97, an endoplasmic reticulum-resident transmembrane protein that regulates the sterol transporter NPC1. We show that TMEM97 possesses the full suite of molecular properties that define the σ2 receptor, and we identify Asp29 and Asp56 as essential for ligand recognition. Cloning the σ2 receptor resolves a longstanding mystery and will enable therapeutic targeting of this potential drug target.

Norbenzomorphan Scaffold: Chemical Tool for Modulating Sigma Receptor-Subtype Selectivity

Some norbenzomorphans exhibit high affinity for sigma 1 and sigma 2 receptors, and varying the position of substituents on the aromatic ring of this scaffold has a significant effect on subtype selectivity. In particular, compounds bearing several different substituents at C7 of the norbenzomorphan ring system exhibit a general preference for the sigma 1 receptor, whereas the corresponding C8-substituted analogues preferentially bind at the sigma 2 receptor. These findings suggest that the norbenzomorphan scaffold may be a unique chemical template that can be easily tuned to prepare small molecules for use as tool compounds to study the specific biological effects arising from preferential binding at either sigma receptor subtype. In the absence of structural characterization data for the sigma 2 receptor, such compounds will be useful toward refining the pharmacophore model of its binding site.

Synthesis of the Pentacyclic Core of Citreamicin η

The citreamicins comprise a novel class of polycyclic xanthone natural products that have not yet yielded to total synthesis. A concise 11-step synthesis of the pentacyclic core of citreamicin η is now reported that features the use of a general approach for the synthesis of 1,4-dioxygenated xanthones. The synthesis also showcases improved techniques for effecting regioselective bromination of certain substituted phenols and coupling of acetylides with hindered ketones.

Norbenzomorphan Framework as a Novel Scaffold for Generating Sigma 2 Receptor/PGRMC1 Subtype-Selective Ligands

A novel structural class with high affinity and subtype selectivity for the sigma 2 receptor has been discovered. Preliminary structure-affinity relationship data are presented showing that 8-substituted 1,3,4,5-tetrahydro-1,5-methanobenzazepine (norbenzomorphan) derivatives elicit modest to high selectivity for the sigma 2 over the sigma 1 receptor subtype. Indeed, piperazine analogue 8-(4-(3-ethoxy-3-oxopropyl)piperazin-1-yl)-1,3,4,5-tetrahydro-1,5-methanobenzazepine-2-carboxylate (SAS-1121) is 574-fold selective for the sigma 2 over the sigma 1 receptor, thereby establishing it as one of the more subtype-selective sigma 2 binding ligands reported to date. Emerging evidence has implicated the sigma 2 receptor in multiple health disorders, so the drug-like characteristics of many of the selective sigma 2 receptor ligands disclosed herein, coupled with their structural similarity to frameworks found in known drugs, suggest that norbenzomorphan analogues may be promising candidates for further development into drug leads.

Diastereoselective addition of monoorganocuprates to a chiral fumarate: reaction development and synthesis of (-)-dihydroprotolichesterinic acid

Recent studies of diastereoselective conjugate additions of monoorganocuprates, Li[RCuI], to chiral γ-alkoxycrotonates and fumarates are disclosed. This methodology was applied to the shortest total synthesis of (-)-dihydroprotolichesterinic acid to date, but several attempts to prepare other succinate-derived natural products, such as pilocarpine and antrodin E, were unsuccessful.

Applications of Ring Closing Metathesis. Total Synthesis of (±)-Pseudotabersonine

A novel approach to the Aspidosperma family of alkaloids was developed and applied to a concise total synthesis of (±)-pseudotabersonine that was accomplished in 11 steps. Key transformations include a stepwise variant of a Mannich-like multicomponent assembly process, a double ring-closing metathesis sequence, and a one-pot deprotection/cyclization reaction.

Approaches to polycyclic 1,4-dioxygenated xanthones. Application to total synthesis of the aglycone of IB-00208

Hexacyclic xanthone natural products such as IB-00208 present a formidable challenge in organic synthesis. A new approach to polycyclic 1,4-dioxygenated xanthones from benzocyclobutenones has been developed and applied to the first total synthesis of the aglycone of IB-00208. The 22-step synthesis features an acetylide stitching process that joins an aryl aldehyde with an angularly fused benzocyclobutenone, which was prepared by a ring-closing metathesis reaction. The resulting acetylenic benzocyclobutenone diol underwent a Moore rearrangement to give an intermediate that was further elaborated to the aglycone of IB-00208 as a mixture of hydroquinone–quinone tautomers.

Evolution of a strategy for preparing bioactive small molecules by sequential multicomponent assembly processes, cyclizations, and diversification

A strategy for generating diverse collections of small molecules has been developed that features a multicomponent assembly process (MCAP) to efficiently construct a variety of intermediates possessing an aryl aminomethyl subunit. These key compounds are then transformed via selective ring-forming reactions into heterocyclic scaffolds, each of which possesses suitable functional handles for further derivatizations and palladium-catalyzed cross coupling reactions. The modular nature of this approach enables the facile construction of libraries of polycyclic compounds bearing a broad range of substituents and substitution patterns for biological evaluation. Screening of several compound libraries thus produced has revealed a large subset of compounds that exhibit a broad spectrum of medicinally-relevant activities.

Enantioselective total syntheses of citrinadins A and B. Stereochemical revision of their assigned structures

The concise, enantioselective total syntheses of (-)-citrinadin A and (+)-citrinadin B in a total of only 20 and 21 steps, respectively, from commercially available starting materials are described. Our strategy, which minimizes refunctionalization and protection/deprotection operations, features the highly diastereoselective, vinylogous Mannich addition of a dienolate to a chiral pyridinium salt to set the first chiral center. The absolute stereochemistry of this key center was then relayed by a sequence of substrate-controlled reactions, including a highly stereoselective epoxidation/ring opening sequence and an oxidative rearrangement of an indole to furnish a spirooxindole to establish the remaining stereocenters in the pentacyclic core of the citrinadins. An early stage intermediate in the synthesis of (-)-citrinadin A was deoxygenated to generate a dehydroxy compound that was elaborated into (+)-citrinadin B by a sequence of reaction identical to those used to prepare (-)-citrinadin A. These concise syntheses of (-)-citrinadin A and (+)-citrinadin B led to a revision of their stereochemical structures.