Catalytic Asymmetric Synthesis of Palmerolide A via Organoboron Methodology

Copper-Catalyzed Umpolung Reactivity of Propargylic Carbonates in the Presence of Diboronates: One Stone Four Birds

Allylation and propargylation are two powerful synthetic strategies for making new substances that have been of significant importance in chemistry, medicine, and material fields. Conventional tactics employ various preformed allylation and propargylation reagents. In this study, a conceptually novel copper-catalyzed and B2pin2-mediated Umpolung reactivity of propargylic carbonates has been achieved for the first time, realizing both allylation and propargylation of aldehydes and ketones without additional reductants. Three types of allylation products and one type of propargylation product are generated efficiently, and all allylation products are formed with syn-configurations predominantly. The choice of ligands plays a vital role in modulating the Umpolung modes. The synthetic applications have been demonstrated in a myriad of further transformations including natural product synthesis, and systematic mechanistic studies have been conducted to reveal detailed insights into the Umpolung processes.

Asymmetric Total Synthesis of (-)-Phaeocaulisin A

The therapeutic properties of Curcuma (ginger and turmeric’s family) have long been known in traditional medicine. However, only recently have guaiane-type sesquiterpenes extracted from Curcuma phaeocaulis been submitted to biological testing, and their enhanced bioactivity was highlighted. Among these compounds, phaeocaulisin A has shown remarkable anti-inflammatory and anticancer activity, which appears to be tied to the unique bridged acetal moiety embedded in its tetracyclic framework. Prompted by the promising biological profile of phaeocaulisin A and by the absence of a synthetic route for its provision, we have implemented the first enantioselective total synthesis of phaeocaulisin A in 17 steps with 2% overall yield. Our route design builds on the identification of an enantioenriched lactone intermediate, tailored with both a ketone moiety and a conjugated alkene system. Taking advantage of the umpolung carbonyl-olefin coupling reactivity enabled by the archetypal single-electron transfer (SET) reductant samarium diiodide (SmI₂), the lactone intermediate was submitted to two sequential SmI₂-mediated cyclizations to stereoselectively construct the polycyclic core of the natural product. Crucially, by exploiting the innate inner-sphere nature of carbonyl reduction using SmI₂, we have used a steric blocking strategy to render sites SET-unreceptive and thus achieve chemoselective reduction in a complex substrate. Our asymmetric route enabled elucidation of the naturally occurring isomer of phaeocaulisin A and provides a synthetic platform to access other guaiane-type sesquiterpenes from C. phaeocaulis—as well as their synthetic derivatives—for medicinal chemistry and drug design.

Bioinformatic and Mechanistic Analysis of the Palmerolide PKS-NRPS Biosynthetic Pathway From the Microbiome of an Antarctic Ascidian

Complex interactions exist between microbiomes and their hosts. Increasingly, defensive metabolites that have been attributed to host biosynthetic capability are now being recognized as products of host-associated microbes. These unique metabolites often have bioactivity targets in human disease and can be purposed as pharmaceuticals. Polyketides are a complex family of natural products that often serve as defensive metabolites for competitive or pro-survival purposes for the producing organism, while demonstrating bioactivity in human diseases as cholesterol lowering agents, anti-infectives, and anti-tumor agents. Marine invertebrates and microbes are a rich source of polyketides. Palmerolide A, a polyketide isolated from the Antarctic ascidian Synoicum adareanum, is a vacuolar-ATPase inhibitor with potent bioactivity against melanoma cell lines. The biosynthetic gene clusters (BGCs) responsible for production of secondary metabolites are encoded in the genomes of the producers as discrete genomic elements. A candidate palmerolide BGC was identified from a S. adareanum microbiome-metagenome based on a high degree of congruence with a chemical structure-based retrobiosynthetic prediction. Protein family homology analysis, conserved domain searches, active site and motif identification were used to identify and propose the function of the ∼75 kbp trans-acyltransferase (AT) polyketide synthase-non-ribosomal synthase (PKS-NRPS) domains responsible for the stepwise synthesis of palmerolide A. Though PKS systems often act in a predictable co-linear sequence, this BGC includes multiple trans-acting enzymatic domains, a non-canonical condensation termination domain, a bacterial luciferase-like monooxygenase (LLM), and is found in multiple copies within the metagenome-assembled genome (MAG). Detailed inspection of the five highly similar pal BGC copies suggests the potential for biosynthesis of other members of the palmerolide chemical family. This is the first delineation of a biosynthetic gene cluster from an Antarctic microbial species, recently proposed as Candidatus Synoicihabitans palmerolidicus. These findings have relevance for fundamental knowledge of PKS combinatorial biosynthesis and could enhance drug development efforts of palmerolide A through heterologous gene expression.

Transition metal catalyzed asymmetric multicomponent reactions of unsaturated compounds using organoboron reagents

Asymmetric multicomponent reactions allow stitching several functional groups in an enantioselective and atom economical manner. The introduction of boron-based reagents as a multicomponent coupling partner has its own merits. In addition to being non-toxic and highly stable, organoboron compounds can be easily converted to other functional groups in a stereoselective manner. In the last decade several transition metal catalyzed asymmetric multicomponent strategies have been evolved using boron based reagents. This review will discuss the merits and scope of multicomponent strategies based on their difference in the reaction mechanism and transition metals involved.

Heck macrocyclization in natural product total synthesis

Covering: 1981-2020 Heck macrocyclization is a logical extension of the award-winning Mizoroki-Heck reaction. Through covalent linking of two otherwise discrete coupling partners, the resultant chimeric substrate is transformed into a large ring with enhanced rigidity and unique functional group disposition. Pioneered in the early 1980s, this methodology has evolved into a competent option for creating diverse macrocycles. Despite its growing influence, hitherto no systematic survey has ever appeared in the literature. The present review delineates the state-of-the-art of Heck macrocyclization in the context of natural product synthesis. Sixteen selected cases, each examined from a different perspective, coalesce into the view that the title reaction is a viable tool for synthesis-enabled macrocycle research.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Methyl-Shifted Farnesyldiphosphate Derivatives Are Substrates for Sesquiterpene Cyclases

New sesquiterpene backbones are accessible after biotransformation of presilphiperfolan-8β-ol synthase (BcBOT2), a fungal sesquiterpene synthase, with non-natural farnesyldiphosphates in which methyl groups are shifted by one position toward the diphosphate terminus. One of the macrocycles formed, a new germacrene A derivative, undergoes a Cope rearrangement to iso-β-elemene. Three of the new terpenoids show olfactoric properties that range from an intense peppery note to a citrus, ozone-like, and fruity scent.

Stereoselective [4+2]-Cycloaddition with Chiral Alkenylboranes

A method for the stereoselective [4+2]-cycloaddition of alkenylboranes and dienes is presented. This transformation was accomplished through the introduction of a new strategy that involves the use of chiral N-protonated alkenyl oxazaborolidines as dieneophiles. The reaction leads to the formation of products that can be readily derivatized to more complex structural motifs through stereospecific transformations of the C-B bond such as oxidation and homologation. Detailed computation evaluation of the reaction has uncovered a surprising role of the counterion on stereoselectivity.

Synthetic studies on palmerolide C: synthesis of an advanced intermediate towards the revised structure of palmerolide C

Herein, we describe the stereoselective synthesis of highly functionalized advanced key intermediates towards the total synthesis of the revised structure of palmerolide C and 10-epi-palmerolide C in a convergent manner.

Design, synthesis, and biological activity of substituted 2-amino-5-oxo-5H-chromeno[2,3-b]pyridine-3-carboxylic acid derivatives as inhibitors of the inflammatory kinases TBK1 and IKKε for the treatment of obesity

The non-canonical IκB kinases TANK-binding kinase 1 (TBK1) and inhibitor of nuclear factor kappa-B kinase ε (IKKε) play a key role in insulin-independent pathways that promote energy storage and block adaptive energy expenditure during obesity. Utilizing docking calculations and the x-ray structure of TBK1 bound to amlexanox, an inhibitor of these kinases with modest potency, a series of analogues was synthesized to develop a structure activity relationship (SAR) around the A- and C-rings of the core scaffold. A strategy was developed wherein R7 and R8 A-ring substituents were incorporated late in the synthetic sequence by utilizing palladium-catalyzed cross-coupling reactions on appropriate bromo precursors. Analogues display IC50 values as low as 210 nM and reveal A-ring substituents that enhance selectivity toward either kinase. In cell assays, selected analogues display enhanced phosphorylation of p38 or TBK1 and elicited IL-6 secretion in 3T3-L1 adipocytes better than amlexanox. An analogue bearing a R7 cyclohexyl modification demonstrated robust IL-6 production in 3T3-L1 cells as well as a phosphorylation marker of efficacy and was tested in obese mice where it promoted serum IL-6 response, weight loss, and insulin sensitizing effects comparable to amlexanox. These studies provide impetus to expand the SAR around the amlexanox core toward uncovering analogues with development potential.

A nano-catalytic approach for C–B bond formation reactions

Nanoparticle-catalysed borylation is one of the most convenient methods for the synthesis of organoboranes to overcome the confines of homogeneous catalysis such as recyclability and heavy metal contamination.

Secondary Metabolites from Polar Organisms

Polar organisms have been found to develop unique defences against the extreme environment environment, leading to the biosynthesis of novel molecules with diverse bioactivities. This review covers the 219 novel natural products described since 2001, from the Arctic and the Antarctic microoganisms, lichen, moss and marine faunas. The structures of the new compounds and details of the source organism, along with any relevant biological activities are presented. Where reported, synthetic and biosynthetic studies on the polar metabolites have also been included.

Acylboranes: synthetic strategies and applications

Acylboranes are an attractive class of compounds, of which the synthesis has very recently been documented as summarised in this review. Access to these compounds provides a path to study their properties and reactivity.

Multicomponent Hetero-[4 + 2] Cycloaddition/Allylboration Reaction: From Natural Product Synthesis to Drug Discovery

Multicomponent reactions (MCR), transformations employing three or more simple substrates in a single and highly atom-economical operation, are very attractive in both natural product synthesis and diversity-oriented synthesis of druglike molecules. Several popular multicomponent reactions were designed by combining two well-established individual reactions that utilize mutually compatible substrates. In this regard, it is not surprising that the merging of two reactions deemed as workhorses of stereoselective synthesis, the Diels-Alder cycloaddition and carbonyl allylboration, would produce a powerful and highly versatile tandem MCR process. The idea of using 1,3-dienylboronates in [4 + 2] cycloadditions as a means to produce cyclic allylic boronates was first reported by Vaultier and Hoffmann in 1987. In their seminal study, a 1-boronodiene was reacted with electron-poor alkenes, and the intermediate cycloadducts were isolated and added to aldehydes in a separate step leading to α-hydroxyalkylated carbocycles via a highly diastereoselective allylboration reaction. The one-pot three-component variant was realized in 1999 by Lallemand and co-workers, and soon after groups led by Hall and Carboni reported heterocyclic variants of the tandem [4 + 2] cycloaddition/allylboration to prepare α-hydroxyalkylated piperidine and pyran containing compounds, respectively. These classes of heterocycles are ubiquitous in Nature and are important components of pharmaceuticals. This Account summarizes the development and evolution of this powerful multicomponent reaction for accessing nonaromatic heterocycles and its many applications in natural products synthesis and drug discovery. The aza[4 + 2]cycloaddition/allylboration MCR was first optimized in our laboratory using 4-boronylhydrazonobutadienes and N-substituted maleimides, and it was exploited in the preparation of combinatorial libraries of polysubstituted imidopiperidines that feature as many as four elements of chemical diversity. Biological screening of these druglike imidopiperidine libraries unveiled promising bioactive agents such as A12B4C3, the first reported inhibitor of the human DNA repair enzyme, polynucleotide kinase-phosphatase (hPNKP). Related applications of this MCR in target-oriented synthesis also led to total syntheses of palustrine alkaloids. The inverse electron-demand oxa[4 + 2] cycloaddition/allyboration variant can take advantage of Jacobsen's chiral Cr(III)salen catalyst, affording a rare example of catalytic enantioselective MCR, one that provides a rapid access to α-hydroxyalkyl dihydropyrans in high enantio- and diastereoselectivity. This process exploits 3-boronoacrolein pinacolate as the heterodiene with ethyl vinyl ether or various 2-substituted enol ethers, along with a wide variety of aldehydes in the allylation stage. This versatile methodology was deployed in total syntheses of thiomarinol antibiotics, goniodiol and its derivatives, and the complex anticancer macrolide palmerolide A. More recent work from our laboratory centered on the regio- and stereoselective Suzuki-Miyaura cross-coupling of the dihydropyranyl boronates, thus providing a glimpse of the potential for new multicomponent variants that merge hetero[4 + 2] cycloadditions of 1-borylated heterodienes with transition metal-catalyzed transformations. This stereoselective MCR strategy holds great promise for provoking continuing applications in complex molecule synthesis and drug discovery, and is likely to inspire new and innovative MCR-based approaches to nonaromatic heterocycles.

Stereoselective and Regiodivergent Allylic Suzuki-Miyaura Cross-Coupling of 2-Ethoxydihydropyranyl Boronates: Synthesis and Confirmation of Absolute Stereochemistry of Diospongin B

Oxygen-containing heterocycles such as pyrans are a common substructure present in a variety of natural products and pharmaceutical drugs. Highly functionalized 4- and 6-aryl/heteroaryl dihydropyran derivatives are assembled by a highly stereoselective, ligand-controlled regiodivergent sp(3)-sp(2) Suzuki-Miyaura cross-coupling of a 2-ethoxy dihydropyranyl boronate derived from a catalytic enantioselective inverse-electron-demand oxa[4 + 2] cycloaddition. The scope and selectivity of this method were assessed along with an application to a concise total synthesis of the diarylheptanoid natural product diospongin B.

Chelation-assisted substrate-controlled asymmetric lithiation-allylboration of chiral carbamate 1,2,4-butanetriol acetonide

The lithiation of 2-(2,2-dimethyl-1,3-dioxolan-4-yl)ethyl diisopropylcarbamate (1) is achieved freely by sec-butyllithium in diethylether with high lk-diastereoselectivity: the bicyclic chelate complexes 3a and 3b are reacted with electrophiles to form optically active precursors 4a and 4b with >95% diastereoselectivity. In addition, tertiary diamines can undergo an external complexation in contest with the internal oxygen ligand, leading to improved stereoselectivities. The further reactions of lithiated carbamates with trans alkenyl-9-BBN derivatives after 1,2 metallate rearrangements, gave the key intermediate α-substituted allylic boranes 7. Subsequent allylboration of aldehydes gave (Z)-anti-homoallylic alcohols 8 in good yield and excellent d.r.

Utilization of Boron Compounds for the Modification of Suberoyl Anilide Hydroxamic Acid as Inhibitor of Histone Deacetylase Class II Homo sapiens

Histone deacetylase (HDAC) has a critical function in regulating gene expression. The inhibition of HDAC has developed as an interesting anticancer research area that targets biological processes such as cell cycle, apoptosis, and cell differentiation. In this study, an HDAC inhibitor that is available commercially, suberoyl anilide hydroxamic acid (SAHA), has been modified to improve its efficacy and reduce the side effects of the compound. Hydrophobic cap and zinc-binding group of these compounds were substituted with boron-based compounds, whereas the linker region was substituted with p-aminobenzoic acid. The molecular docking analysis resulted in 8 ligands with ΔG_binding value more negative than the standards, SAHA and trichostatin A (TSA). That ligands were analyzed based on the nature of QSAR, pharmacological properties, and ADME-Tox. It is conducted to obtain a potent inhibitor of HDAC class II Homo sapiens. The screening process result gave one best ligand, Nova2 (513246-99-6), which was then further studied by molecular dynamics simulations.

Boron-substituted 1,3-dienes and heterodienes as key elements in multicomponent processes

In the last few years, multicomponent reactions involving boron substituted 1,3-dienes have emerged as important tools in synthetic organic chemistry. The most significant recent results and developments obtained in this area are reported in this review.

Palladium-catalyzed synthesis and isolation of functionalized allylboronic acids: selective, direct allylboration of ketones

Textbook revision: allylboronic acids, which are easily prepared from allylic alcohols, react readily and selectively with ketones without Lewis acid catalysts.

Recent developments in asymmetric multicomponent reactions

Multicomponent reactions (MCRs) receive increasing attention because they address both diversity and complexity in organic synthesis. Thus, in principle diverse sets of relatively complex structures can be generated from simple starting materials in a single reaction step. The ever increasing need for optically pure compounds for pharmaceutical and agricultural applications as well as for catalysis promotes the development of asymmetric multicomponent reactions. In recent years, asymmetric multicomponent reactions have been applied to the total synthesis of various enantiopure natural products and commercial drugs, reducing the number of required reaction steps significantly. Although many developments in diastereoselective MCRs have been reported, the field of catalytic enantioselective MCRs has just started to blossom. This critical review describes developments in both diastereoselective and catalytic enantioselective multicomponent reactions since 2004. Significantly broadened scopes, new techniques, more environmentally benign methods and entirely novel MCRs reflect the increasingly inventive paths that synthetic chemist follow in this field. Until recently, enantioselective transition metal-catalyzed MCRs represented the majority of catalytic enantioselective MCRs. However, metal contamination is highly undesirable for drug synthesis. The emergence of organocatalysis greatly influences the quest for new asymmetric MCRs.

Catalytic enantioselective transformations of borylated substrates: Preparation and synthetic applications of chiral alkylboronates

Organoboronic acid derivatives are well-established intermediates for the preparation of alcohols and amines, and in the formation of C–C bonds via different reactions, including homologations, carbonyl allylboration, or transition-metal-catalyzed cross-coupling chemistry. In the past decade, there has been great interest in the development of catalytic enantioselective methods for the preparation of chiral, optically enriched organoboronates as precursors of enantioenriched compounds. While the mainstream strategy remains the late-stage borylation of organic functional groups, our group has focused on an alternate strategy focused on modification of boron-containing substrates. In this way, acyclic and cyclic secondary alkyl- and allyl-boronates were prepared through catalytic enantioselective processes such as [4 + 2] cycloadditions, isomerizations, allylic substitutions, and conjugate additions. The resulting optically enriched boronates have been successfully utilized in the syntheses of complex natural products and drugs. One remaining challenge in the chemistry of secondary alkylboronate derivatives is their cross-coupling, especially with control of stereoselectivity. In this regard, our recent approach featured the conjugate asymmetric borylation of β-boronyl acrylates, providing the first enantioselective preparation of highly optically enriched 1,1-diboronyl derivatives. The chirality of these geminal diboron compounds is conferred through the use of two distinct boronate adducts, which can be coupled chemo- and stereoselectively with a variety of aryl and alkenyl halides under palladium catalysis.