Total Synthesis of (-)-Mitrephorone A Enabled by Stereoselective Nitrile Oxide Cycloaddition and Tetrasubstituted Olefin Synthesis

A highly enantioselective and diastereoselective total synthesis of the diterpenoid (−)-mitrephorone A is presented. Key to the synthesis are stereocontrolled 1,4-semihydrogenation of a 1,3-diene to a tetrasubstituted double bond, enzyme-catalyzed malonate desymmetrization, and highly diastereoselective nitrile oxide cycloaddition. The streamlined strategy is a considerable improvement to those reported earlier in terms of diastereo- and enantioselectivity. For the first time, the combination of modern Pd-cross-coupling with Cr-catalyzed reduction allows for rapid access to tetrasubstituted olefins with full stereocontrol.

Development of High-Specificity Fluorescent Probes to Enable Cannabinoid Type 2 Receptor Studies in Living Cells

Pharmacological modulation of cannabinoid type 2 receptor (CB₂R) holds promise for the treatment of numerous conditions, including inflammatory diseases, autoimmune disorders, pain, and cancer. Despite the significance of this receptor, researchers lack reliable tools to address questions concerning the expression and complex mechanism of CB₂R signaling, especially in cell-type and tissue-dependent contexts. Herein, we report for the first time a versatile ligand platform for the modular design of a collection of highly specific CB₂R fluorescent probes, used successfully across applications, species, and cell types. These include flow cytometry of endogenously expressing cells, real-time confocal microscopy of mouse splenocytes and human macrophages, as well as FRET-based kinetic and equilibrium binding assays. High CB₂R specificity was demonstrated by competition experiments in living cells expressing CB₂R at native levels. The probes were effectively applied to FACS analysis of microglial cells derived from a mouse model relevant to Alzheimer's disease.

Electrophilic Nrf2 activators and itaconate inhibit inflammation at low dose and promote IL-1β production and inflammatory apoptosis at high dose

Controlling inflammation is critical for preventing many diseases including cancer, autoimmune disorders and hypersensitivity reactions. NF-E2-related factor 2 (Nrf2) is a key transcription factor that controls the cellular antioxidant and cytoprotective response. Moreover, Nrf2 has been implicated in the regulation of inflammatory processes, although the ultimate mechanism by which this is achieved is unknown. Here, we investigated mechanisms of inflammation and cell death pathways induced by a variety of Nrf2 activators including dimethyl fumarate (DMF) and the endogenous metabolite itaconate. We found that exposure of bone marrow-derived dendritic cells (BMDCs) to low concentrations of a variety of electrophilic Nrf2 activators including itaconate prior to Toll-like receptor (TLR) stimulation inhibits transcription of pro-inflammatory cytokines (such as interleukin [IL]-12 and IL-1β) by activation of Nrf2. By contrast, high doses of these electrophilic compounds after TLR activation promote inflammatory apoptosis and caspase-8-dependent IL-1β processing and release independently of Nrf2. Interestingly, tert-butylhydroquinone (tBHQ), a non-electrophilic Nrf2-activator, failed to induce IL-1β production. These results have important implications for clinical application of electrophilic compounds.

Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected Allenylic Amines Employing Ammonia Surrogates

The first iridium catalyzed, enantioconvergent amination of allenylic carbonates is reported. This process utilizes various commercially available carbamates and sulfonamides to generate allenylic amines including commonly employed protected groups (Boc, Fmoc, Cbz, Ts, Ns) in 62-82 % yield and 87-98 % ee. The products generated through this scalable procedure serve as effective linchpins for the rapid, enantiospecific synthesis of a wide range of complex structures.

Ring-fused cyclobutanes via cycloisomerization of alkylidenecyclopropane acylsilanes

A novel Lewis acid-catalyzed cycloisomerization of alkylidenecyclopropane acylsilanes is disclosed. The readily available starting materials participate in tandem Prins addition/ring expansion/1,2-silyl shift to grant access to bicyclo[4.2.0]octanes and bicyclo[3.2.0]heptanes, which are common motifs in terpenoid natural products. Notably, the transformation relies on the ability of acylsilanes to act sequentially as acceptors and donors on the same carbon atom.

A novel Lewis acid-catalyzed cycloisomerization of alkylidenecyclopropane acylsilanes is disclosed that involves tandem Prins addition/ring expansion/1,2-silyl shift to grant access to bicyclo[4.2.0]octanes and bicyclo[3.2.0]heptanes.

The Serendipitous Discovery of a Rose Odorant

Serendipity has played a role in many groundbreaking scientific discoveries. Key to their identification and exploitation is the ability to recognize the unexpected and invest time trying to understand it. Like any other field of scientific research, total synthesis requires determination and perseverance. When the first-generation route towards a target compound fails, new approaches are developed based on insights gained in the initial studies. Careful analysis of data obtained in a 'failed' approach, e.g. when a reaction did not yield the desired or any expected outcome, can lead to spectacularly improved routes and discoveries that have impact beyond the synthesis of the selected target compound. Serendipity has further led to the identification of intriguing properties that materials or single molecules have, as exemplified by the discovery of electrically conductive polymers. During our total synthesis endeavors towards a complex natural product, we identified a small molecule with interesting olfactory properties, which we decided to investigate further.

Molecular Recognition and Cocrystallization of Methylated and Halogenated Fragments of Danicalipin A by Enantiopure Alleno-Acetylenic Cage Receptors

Enantiopure (P)₄- and (M)₄-configured alleno-acetylenic cage (AAC) receptors offer a highly defined interior for the complexation and structure elucidation of small molecule fragments of the stereochemically complex chlorosulfolipid danicalipin A. Solution (NMR), solid state (X-ray), and theoretical investigations of the formed host-guest complexes provide insight into the conformational preferences of 14 achiral and chiral derivatives of the danicalipin A chlorohydrin core in a confined, mostly hydrophobic environment, extending previously reported studies in polar solvents. The conserved binding mode of the guests permits deciphering the effect of functional group replacements on Gibbs binding energies ΔG. A strong contribution of conformational energies toward the binding affinities is revealed, which explains why the denser packing of larger apolar domains of the guests does not necessarily lead to higher association. Enantioselective binding of chiral guests, with energetic differences ΔΔG_293 K up to 0.7 kcal mol^-1 between diastereoisomeric complexes, is explained by hydrogen- and halogen-bonding, as well as dispersion interactions. Calorimetric studies (ITC) show that the stronger binding of one enantiomer is accompanied by an increased gain in enthalpy ΔH but at the cost of a larger entropic penalty TΔS stemming from tighter binding.

Total Synthesis and Structural Revision of a Harziane Diterpenoid

The first total synthesis of nominal harziane diterpenoid 1 is disclosed, whose spectral characteristics did not match those of the reported natural product. Stereochemical analysis and subsequent synthesis of the epimeric tertiary alcohol led to reassignment of configuration of the natural product as shown for 2. At the heart of the synthesis is an enyne cycloisomerization that sets a key quaternary stereocenter within a cyclobutane with high diastereocontrol. The route features strategies for the synthesis of the highly congested 6-5-7-4 carbon skeleton characteristic of the caged harziane diterpenoids.

Asymmetric Synthesis of the Tricyclooctane Core of Trachylobane Natural Products and Related Terpenoids

Enantioselective synthesis of a highly versatile building block en route to trachylobanes and related terpenoids is presented. The synthesis features diastereoselective Diels-Alder cycloaddition reaction, a cyclopropanation/homoquadricyclane rearrangement cascade, and palladium-catalyzed C-H acetoxylation. The targeted tricyclooctane was obtained in 20% yield over 8 steps and in 95% ee. The ketone and alcohol functional groups serve as handles for further elaboration.

Reversible Spatiotemporal Control of Induced Protein Degradation by Bistable PhotoPROTACs

Off-tissue effects are persistent issues of modern inhibition-based therapies. By merging the strategies of photopharmacology and small-molecule degraders, we introduce a novel concept for persistent spatiotemporal control of induced protein degradation that potentially prevents off-tissue toxicity. Building on the successful principle of bifunctional all-small-molecule Proteolysis Targeting Chimeras (PROTACs), we designed photoswitchable PROTACs (photoPROTACs) by including ortho-F4-azobenzene linkers between both warhead ligands. This highly bistable yet photoswitchable structural component leads to reversible control over the topological distance between both ligands. The azo-cis-isomer is observed to be inactive because the distance defined by the linker is prohibitively short to permit complex formation between the protein binding partners. By contrast, the azo-trans-isomer is active since it can engage both protein partners to form the necessary and productive ternary complex. Importantly, due to the bistable nature of the ortho-F4-azobenzene moiety employed, the photostationary state of the photoPROTAC is persistent, with no need for continuous irradiation. This technique offers reversible on/off switching of protein degradation that is compatible with an intracellular environment and, therefore, could be useful in experimental exploration of biological signaling pathways-such as those crucial for oncogenic signal transduction. Additionally, this strategy may be suitable for therapeutic intervention to address a variety of diseases. By enabling reversible activation and deactivation of protein degradation, photoPROTACs offer advantages over conventional photocaging strategies that irreversibly release active agents.

Iridium-Catalyzed Asymmetric Synthesis of Functionally Rich Molecules Enabled by (Phosphoramidite,Olefin) Ligands

The catalytic, asymmetric synthesis of complex molecules has been a core focus of our research program for some time because developments in the area can have an immediate impact on the identification of novel strategies for the synthesis of value-added molecules. In concert with this central interest, we have emphasized the design of ligand scaffolds as a tactic to discover and develop novel chemistry and overcome well-recognized synthetic challenges. Based on our group's work on chiral pool-derived diolefin ligands, we designed and implemented a class of hybrid (phosphoramidite,olefin) ligands, which combines the properties of both phosphoramidite and olefin motifs to impact, fine-tune, and even override the inherent reactivity of the metal center. Specifically, we have utilized these unique modifying ligands to address several recognized limitations in the field of iridium-catalyzed, asymmetric allylic substitution. The methods we have documented typically employ branched, unprotected allylic alcohols as substrates and obviate the need for rigorous exclusion of air and moisture. Following Takeuchi's seminal report demonstrating the high aptitude of Ir(I)-phosphite catalysts for  branch-selective allylic substitution, concerted efforts from numerous research laboratories have led to a broadening of the synthetic utility of this reaction class. The first section of this Account outlines the process leading to our discovery of an unprecedented (phosphoramidite,olefin) ligand and its validation in the first iridium-catalyzed amination of branched, unprotected allylic alcohols. This section continues with our work involving heteroatom-based nucleophiles within inter- and intramolecular etherification, thioetherification and spiroketalization processes. The second section highlights the use of readily available carbon nucleophiles possessing sp, sp², and sp³ hybridization in a series of enantioselective carbon-carbon bond-forming reactions. We describe how alkylzinc, allylsilane, and several classes of organotrifluoroborate nucleophiles can be coupled enantioselectively to enable construction of several key motifs including 1,5-dienes, 1,4-dienes, and 1,4-enynes. Since the unique electronic and steric properties of this class of ligands renders the (η³-allyl)-Ir(III) intermediate highly electrophilic, even weak nucleophiles such as alkyl olefins can be used. We also show that more nucleophilic alkene motifs such as enamines and in situ generated ketene acetals smoothly participate in substitution reactions with allylic alcohols to yield valuable piperidines and γ,δ-unsaturated esters, respectively. The concept of stereodivergent dual catalysis, which synergistically combines chiral amine catalysis with iridium catalysis to furnish α-allylated aldehydes containing two independently controllable stereocenters is then discussed. This process has enabled the independent, stereoselective synthesis of all four possible product stereoisomers from a single set of starting materials, and was highlighted in the stereodivergent synthesis of Δ⁹-tetrahydrocannabinol. This Account concludes with an overview of our organometallic mechanistic studies regarding relevant intermediates within the catalytic cycle of this class of allylic substitution. These studies have allowed us to better understand the origin of the unique characteristics exhibited by this catalyst in comparison to related systems.

Gold(i)-catalyzed stereoselective cyclization of 1,3-enyne aldehydes by a 1,3-acyloxy migration/Nazarov cyclization/aldol addition cascade

Stereoselective synthesis of bicyclo[3.3.0]octenones from chiral 1,3-enyne aldehydes bearing propargylic acetates is described. The method is based on a Au(i)-catalyzed domino sequence with concomitant transfer of chirality involving 1,3-acyloxy migration followed by Nazarov cyclization and an unprecedented aldol addition. The method furnishes densely functionalized bicyclic structures in high yields, with up to 97% ee and good diastereoselectivity.

Stereochemical Revision, Total Synthesis, and Solution State Conformation of the Complex Chlorosulfolipid Mytilipin B

Chlorosulfolipids constitute a structurally intriguing and synthetically challenging class of marine natural products that are isolated from mussels and freshwater algae. The most complex structure from this family of compounds is currently represented by Mytilipin B, isolated in 2002 from culinary mussel Mytilus galloprovincialis, whose initially proposed structure was shown to be incorrect. In this study, we present the synthesis of four diastereomers which allowed the reassignment of eight stereocenters and the stereochemical revision of Mytilipin B, along with the determination of the dominant solution-state conformation.

Pyridyl Radical Cation for C-H Amination of Arenes

Electron-transfer photocatalysis provides access to the elusive and unprecedented N-pyridyl radical cation from selected N-substituted pyridinium reagents. The resulting C(sp² )-H functionalization of (hetero)arenes furnishes versatile intermediates for the development of valuable aminated aryl scaffolds. Mechanistic studies that include the first spectroscopic evidence of a spin-trapped N-pyridyl radical adduct implicate SET-triggered, pseudo-mesolytic cleavage of the N-X pyridinium reagents mediated by visible light.

Total Synthesis of (-)-Mitrephorone A

The first synthesis of (-)-mitrephorone A is disclosed along with discussion and study of synthetic strategies. The natural product includes a highly congested hexacyclic ent-trachylobane diterpenoid framework featuring a rare, embedded oxetane. The synthetic analysis presented dissects a number of approaches for the synthesis of the central oxetane, including carbonyl-olefin photocycloadditions, Prins-type cyclizations, and oxidative ring closures. In the successful route, three [4 + 2] cycloadditions enable rapid construction of all carbocycles. A novel late-stage oxidative cyclization of a hydroxy diosphenol with Koser's reagent furnishes the pivotal oxetane moiety.

Identification of biologically active δ-lactone eicosanoids as paraoxonase substrates

The mammalian paraoxonases (PONs 1, 2 and 3) are a family of esterases that are highly conserved within and between species. They exhibit antioxidant and anti-inflammatory activities. However, their physiological function(s) and native substrates are uncertain. Previous structure-activity relationship studies demonstrate that PONs have a high specificity for lipophilic lactones, suggesting that such compounds may be representative of native substrates. This report describes the ability of PONs to hydrolyze two bioactive δ-lactones derived from arachidonic acid, 5,6-dihydroxy-eicosatrienoic acid lactone (5,6-DHTL) and cyclo-epoxycyclopentenone (cyclo-EC). Both lactones were very efficiently hydrolyzed by purified PON3. PON1 efficiently hydrolyzed 5,6-DHTL, but with a specific activity about 15-fold lower than PON3. 5,6-DHTL was a poor substrate for PON2. Cyclo-EC was a poor substrate for PON1 and not hydrolyzed by PON2. Studies with the PON inhibitor EDTA and a serine esterase inhibitor indicated that the PONs are the main contributors to hydrolysis of the lactones in human and mouse liver homogenates. Studies with homogenates from PON3 knockout mouse livers indicated that >80% of the 5,6-DHTL and cyclo-EC lactonase activities were attributed to PON3. The findings provide further insight into the structural requirements for PONs substrates and support the hypothesis that PONs, particularly PON1 and PON3, evolved to hydrolyze and regulate a class of lactone lipid mediators derived from polyunsaturated fatty acids.