Synthesis of vinyl boronates from aldehydes by a practical boron-Wittig reaction

From Acyclic Intramolecular-[4 + 2]- to Transannular Bis-[4 + 2]-Cycloaddition of the Macrodiolide for the Stereoselective Synthesis of the Octahydronaphthalene Core of Polyenic Macrolactam Sagamilactam

The strategy for the synthesis of the octahydronaphthalene core of natural macrolide sagamilactam has unintentionally evolved from the acyclic intramolecular (IMDA) to the transannular (TADA) Diels-Alder reaction. Lewis acid-promoted IMDA of a protected 2Z,8E,10E-4,6,12-trihydroxy-2,8,10-decatrienal model with a diol of 4,6-anti relative configuration, as proposed by DP4+-based computational studies, afforded the cis-octahydronaphthalene diastereomer through the Re-endo approach. The 26-membered macrodiolide generated, under thermal reaction conditions, the trans-octahydronaphthalene by a double TADA reaction along the desired Si-exo orientation.

Ru(0)-catalysed synthesis of borylated polyene building blocks by cross-dimerisation toward cross-coupling

Conjugated and non-conjugated polyenes are important substructures and are often found in biologically active compounds and natural products. Their preparation often needs multiple steps or iterative reactions and as a result, they have poor step economies. In this feature article, we show a new methodology to prepare these substructures by combinations of cross-dimerisation giving borylated polyenes and subsequent cross-coupling reactions. This divergent reaction strategy allows for the opportunity to access many bioactive compounds and natural products as well as some electronic materials.

Photogeneration of α-Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C-I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C-I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C-B and C-Si bond activation.

Latest Developments of the Julia-Kocienski Olefination Reaction: Mechanistic Considerations

Since its discovery, the Julia-Kocienski olefination reaction has over past 30 years become one of the key C-C connective methods that is used in late-stage natural product synthesis. The reaction proceeds under mild reaction conditions, with a wide substrate scope and functional group tolerance range and with high (E) selectivity. In this focused review, we discuss the reaction from a mechanistic point of view and disclose key features that play an important role in reaction selectivity. Finally, the mechanistic aspects of the newly developed modification of the Julia-Kocienski reaction, which allows the formation of both (E) and (Z) olefins from the same reaction partners, are discussed.

Oxidative hydrolysis of aliphatic bromoalkenes: scope study and reactivity insights

We have developed an operationally simple method for the synthesis of dialkyl α-bromoketones from bromoalkenes by utilizing a hypervalent iodine-catalyzed oxidative hydrolysis reaction. This catalytic process provides both symmetrical and unsymmetrical dialkyl bromoketones with moderate yields across a broad range of bromoalkene substrates. Our studies also reveal the formation of Ritter-type side products by an alternative reaction pathway.

Advances Toward Amphidinolides C, F and U: Isolations, Synthetic Studies and Total Syntheses

Amphidinolides C, F, and U, including C2-C4 analogs, are highly cytotoxic marine macrolides, mainly isolated from dinoflagellates of the genus Amphidinium. All these polyketides share a 75 % or more similar structure, highlighted by a macrolactone ring, at least one trans-2,5-substituted-THF motif and a characteristic polyenic side chain. From their isolation and absolute configurational assignment, the total synthesis of these marine macrolides represented an intense challenge to the organic synthesis community over the last 15 years, with around 14 research groups engaged in this inspiring task. In the first part of this review, we present the different approaches to the isolation and characterization of these natural products, including the most recent analogs, which may cast doubt on the biogenetic origin of these compounds. The various synthetic approaches to the total synthesis of C, F, and U amphidinolides are presented in a second part, focusing on key reactions and/or innovative strategies. The review concludes in a third section summarizing the successful approaches leading to the total synthesis of one of the members of this amphidinolide subfamily.

Palladium-Catalyzed Cascade Heck Coupling and Allylboration of Iododiboron Compounds via Diboryl Radicals

Geminal bis(boronates) are versatile synthetic building blocks in organic chemistry. The fact that they predominantly serve as nucleophiles in the previous reports, however, has restrained their synthetic potential. Herein we disclose the ambiphilic reactivity of α-halogenated geminal bis(boronates), of which the first catalytic utilization was accomplished by merging a formal Heck cross-coupling with a highly diastereoselective allylboration of aldehydes or imines, providing a new avenue for rapid assembly of polyfunctionalized boron-containing compounds. We demonstrated that this cascade reaction is highly efficient and compatible with various functional groups, and a wide range of heterocycles. In contrast to a classical Pd(0/II) scenario, mechanistic experiments and DFT calculations have provided strong evidence for a catalytic cycle involving Pd(I)/diboryl carbon radical intermediates.

α-Boryl Carbanions: The Influence of Geminal Heteroatoms in C-C Bond Formation

The wide applications of alpha-boryl carbanions in selective coupling with organohalides, imines/carbonyls and conjugated unsaturated substrates has become an interesting tool for organic synthesis. Strategically, the inclusion of heteroatoms, such as Si, S, N, F, Cl, Br and I in the alpha position opens a new venue towards multifunctionalities in molecular design. Here, a conceptual and practical view on powerful carbanions, containing α-silicoboron, α-thioboron, α-haloboron and α-aminoboron is given, as well as a prespective on their efficient application for selective electrophilic trapping.

A General Method to Access Sterically Encumbered Geminal Bis(boronates) via Formal Umpolung Transformation of Terminal Diboron Compounds

General methods for the preparation of geminal bis(boronates) are of great interest due to their widespread applications in organic synthesis. While the terminal gem-diboron compounds are readily accessible, the construction of the sterically encumbered, internal analogues has remained a prominent challenge. Herein, we report a formal umpolung strategy to access these valuable building blocks. The readily available 1,1-diborylalkanes were first converted into the corresponding α-halogenated derivatives, which then serve as electrophilic components, undergoing a formal substitution with a diverse array of nucleophiles to form a series of C-C, C-O, C-S, and C-N bonds. This protocol features good tolerance to steric hindrance and a wide variety of functional groups and heterocycles. Notably, this strategy can also be extended to the synthesis of diaryl and terminal gem-diboron compounds, therefore providing a general approach to various types of geminal bis(boronates).

Stereoselective Synthesis of Allylic Alcohols via Substrate Control on Asymmetric Lithiation

Allylic alcohols are a privileged motif in natural product synthesis and new methods that access them in a stereoselective fashion are highly sought after. Toward this goal, we found that chiral acetonide-protected polyketide fragments performing the Hoppe-Matteson-Aggarwal rearrangement in the absence of sparteine with high yields and diastereoselectivities rendering this protocol a highly valuable alternative to the Nozaki-Hiyama-Takai-Kishi reaction. Various stereodyads and -triads were investigated to determine their substrate induction. The mostly strong inherent stereoinduction was attributed to a combination of steric and electronic effects.

Carbonyl group directed synthesis of 3-boryl-3-substituted alkenyl oxindoles and tetrasubstituted β-borylenones

Transition metal-free carbonyl directed boron-Wittig reaction of α-bis(boryl)carbanions with the corresponding isatins or with the α-keto esters/amides was achieved to access alkenyl oxindoles in good yield and high stereoselectivity.

Stereoselective Synthesis of Silylated Vinylboronates by a Boron-Wittig Reaction and Their Application to Tetrasubstituted Olefins

The highly stereoselective synthesis of a series of tetrasubstituted mono- as well as disilylated vinylboronates is reported by using the boron-Wittig approach. The condensation between acylsilanes and gem-diborylalkanes gave the desired tetrasubstituted olefins in good to excellent yield and high stereoselectivity. Also, a series of trisubstituted silylated vinyl MIDA-boronates was synthesized by using the boron-Wittig reaction followed by a transesterification reaction. This methodology allows direct incorporation of B(pin) and TMS groups in the anti-position of the olefin in a highly stereoselective manner. Further, sequential Suzuki coupling reaction with the silylated vinyl boronic esters generated all-carbon tetrasubstituted alkenes, which have been applied in the total synthesis of the anticancer drug Tamoxifen and aggregation-induced luminogen agent TPE-TF17.

Multicomponent Reaction: Pd/Cu-Catalyzed Borocarbonylation of Aryldiazonium Salts with Aliphatic Terminal Alkynes to gem-Bis(boryl) Ketones

We report the development of Pd/Cu-catalyzed selective 2,1-borocarbonylation reactions of aliphatic terminal alkynes with aryldiazonium salts and B2 Pin2 to prepare gem-bis(boryl) ketones in one-pot. A series of corresponding products are obtained with good to excellent yields under a carbon monoxide atmosphere (10 bar). In addition, wide functional-group tolerance can be observed. Preliminary mechanistic studies reveal that ethyl acetate serves as a proton source in the reaction.

A unified strategy for the synthesis of aldohexoses by boronate assisted assembly of CH2X2 derived C1-building blocks

A synthetic strategy for all aldohexoses with individually addressable protecting groups from dihalomethane C1-units is reported. The underlying synthesis of C6-sugar alcohols relies on three consecutive Matteson sequences, vinylation and bishydroxylation. Erythro and threo isomers have been realized for every glycol motif by strategic variation of the sequence.

Photocatalyzed Borylcyclopropanation of Alkenes with a (Diborylmethyl)iodide Reagent

Cyclopropane skeletons play a prominent role in the development of organic synthesis and pharmaceutical chemistry. Herein, we report the design and synthesis of a stable, multifunctional (diborylmethyl)iodide reagent (CHI(Bpin)2 ) for the photoinduced cyclopropanation of alkenes, providing an array of 1,2-substituted cyclopropylboronates in good yields. This α-haloboronic ester can be readily synthesized on a multigram scale from commercially available starting materials. Furthermore, the protocol displays high chemo- and diastereoselectivity, excellent functional-group tolerance, and allows for late-stage borylcyclopropanation of complex molecules. Mechanistic studies reveal that the borylcyclopropanation proceeds through a radical addition/polar cyclization pathway mediated by the photocatalyst fac-Ir(ppy)3 and visible light.

A Modular Approach for the Synthesis of Natural and Artificial Terpenoids

Many terpenoids with isoprene unit(s) demonstrating critical biological activities have been isolated and characterized. In this study, we have developed a robust chem-stamp strategy for the construction of the key isoprene unit, which consists of two steps: one-carbon extension of aldehydes to the alkenyl boronates by the boron-Wittig reaction and the rhodium-catalyzed reaction of alkenyl boronates with 2,3-allenols to yield enals. This chem-stamp could readily be applied repeatedly and separately, enabling the modular concise synthesis of many natural and pharmaceutically active terpenoids, including retinal, β-carotene, vitamin A, tretinoin, fenretinide, acitretin, ALRT1550, nigerapyrone C, peretinoin, and lycopene. Owing to the diversified availability of the starting materials, aldehydes and 2,3-allenols, creation of new non-natural terpenoids has been realized from four dimensions: the number of isoprene units, the side chain, and the two terminal groups.

Understanding the Mechanism and Selectivity of 1,1-Diborylalkanes from Alkenes Catalyzed by a Zirconium Complex

The synthesis of 1,1-diborylalkanes from readily available alkenes is an appealing method. The density functional theory (DFT) method was employed to investigate the reaction mechanism of 1,1-diborylalkanes, which was synthesized from alkenes and a borane, and the reaction was catalyzed by a zirconium complex Cp₂ZrCl₂. The entire reaction is divided into two cycles: dehydrogenative boration to form vinyl boronate esters (VBEs) and hydroboration of VBEs. This article focuses on the hydroboration cycle and elaborates on the role of the reducing reagents in the equilibrium of self-contradictory reactivity (dehydrogenative boration and hydroboration). The H₂ and HBpin pathways were investigated as the reducing reagents in the hydroboration process. The calculated results showed that it is more advantageous to use H₂ as a reducing agent (path A). Furthermore, the σ-bond metathesis is the rate-determining step (RDS) with an energetic span of 21.4 kcal/mol. This is consistent with the self-contradictory reactivity balance proposed in the experiment. The reaction modes of the hydroboration process were also discussed. These analyses revealed the origin of selectivity in this boration reaction, in which the σ-bond metathesis of HBpin needs to overcome the strong interaction between HBpin and the Zr metal. Meanwhile, the origin of the selectivity of different positions of H₂ is the interaction between the σ(H¹-H²) → σ*(Zr¹-C¹) overlap and these findings have implications for catalyst design and application.

Unusually cyclized triterpenoids: occurrence, biosynthesis and chemical synthesis

Covering: 2009 to 2021Biosynthetically, most of the syntheses of triterpenes follow the cascade cyclization and rearrangement of the acyclic precursors viz., squalene (S) and 2,3-oxidosqualene (OS), which lead to the very well known tetra- and pentacyclic triterpene skeletons. Aside from these, numerous other triterpenoid molecules are also reported from various natural sources and their structures are derived from "S" and "OS" via some unusual cyclization operations which are different from the usual tetra- and pentacyclic frameworks. Numerous compelling advances have been made and reported in the identification of these unusual cyclized mono-, di-, tri- and tetracyclic triterpenes between 2009 and 2021. Besides a dramatic increase in the newly isolated uncommon cyclized triterpenoids, substantial progress in the (bio)-synthesis of these triterpenes has been published along with significant progress in their biological effects. In this review, 180 new unusual cyclized triterpenoids together with their demonstrated biogenetic pathways, syntheses and biological effects will be categorized and discussed.

Synthesis of gem-Diboromethyl-Substituted Bicyclo[1.1.1]pentanes and Their Application in Palladium-Catalyzed Cross Couplings

We describe the first general transition-metal-free synthesis of gem-diboromethyl-substituted bicyclo[1.1.1]pentane (BCP) and other related C(sp³)-rich carbocyclic benzene bioisosteres from their corresponding p-tosylhydrazones. These novel functionalized benzene bioisosteres demonstrated unique reactivities toward palladium-catalyzed C(sp²)-C(sp³) cross couplings. The overall transformation can be applied to relatively complex substrates with potential utility in drug discovery.

Ruthenium-Catalyzed Butadiene-Mediated Crotylation and Oxazaborolidine-Catalyzed Vinylogous Mukaiyama Aldol Reaction for The Synthesis of C1-C19 and C23-C35 of Neaumycin B

Neaumycin B is a femtomolar inhibitor of U87 human glioblastoma. Using a newly developed anti-diastereoselective ruthenium-catalyzed butadiene-mediated crotylation of primary alcohol proelectrophiles via hydrogen auto-transfer, as well as a novel variant of the catalytic asymmetric vinylogous Mukaiyama aldol (VMA) reaction applicable to linear aliphatic aldehydes and terminally methylated dienyl ketene acetals, preparation of the key C1-C19 and C23-C35 substructures of neaumycin B is achieved in 12 and 7 steps (LLS), respectively.

Boron-Promoted Deprotonative Conjugate Addition: Geminal Diborons as Soft Pronucleophiles and Acyl Anion Equivalents

Conjugate addition of α-boron-stabilized carbanions is an underexplored reaction modality. Existing methods require deborylation of geminal di-/triboryl alkanes and/or the presence of additional activating groups. We report the 1,4-addition of α,α-diboryl carbanions generated via deprotonation of the corresponding geminal diborons. The methodology provided a general route to highly substituted and synthetically useful γ,γ-diboryl ketones. The development of geminal diborons as soft pronucleophiles also enabled their use as acyl anion equivalents via a one-pot tandem conjugate addition-oxidation sequence.

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β‐position can undergo stereospecific syn‐ or anti‐elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β‐functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron‐stabilized anion (d¹) to an aldehyde or ketone (a¹) or diastereoselective 1,3‐rearrangement of suitable boron‐ate‐complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2‐metalate rearrangement of boron‐ate‐complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d¹*) to chiral boronates (a¹*) with a leaving group in α‐position.

Defluorinative C-C Bond-Forming Reaction of Trifluoromethyl Alkenes with gem-(Diborylalkyl)lithiums

We report the transition-metal-free defluorinative C-C bond-forming reaction of trifluoromethyl alkenes with gem-(diborylalkyl)lithiums. This synthetic strategy provides access to a variety of 4,4-difluoro homoallylic diboronate esters, which serve as versatile intermediates in the efficient preparation of valuable gem-difluoroalkene derivatives. Further synthetic modifications are conducted to demonstrate the synthetic utility of the obtained 4,4-difluoro homoallylic diboronate esters.

Assembling the carbon skeleton of A-74528

The 2′-phosphodiesterase inhibitor A-74528, which combines an intriguing biosynthesis with unusual biological activity, is one of the most complex type II polyketides. As a synthetic target, it represents a significant challenge due to its size but also due to a unique carbon skeleton that features a hexacarbocyclic core with an appended pyrone. Here we report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton and the correct installation of all but one stereocenter. Our strategy employs a molybdenum-catalyzed branched allylation to establish the central quaternary carbon and relies on establishing the remaining stereocenters in a substrate-controlled manner. Carbocycles were established using a spiro epoxide annulation, a 1,3-dipolar cycloaddition, followed by an aldol condensation, and a gold-catalyzed hydroarylation. The pyrone was appended to an aldehyde branching off the quaternary stereocenter by a one-carbon homologation and Mukaiyama aldol addition.

We report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton.

Photocatalyzed E→Z Contra-thermodynamic Isomerization of Vinyl Boronates with Binaphthol

The photocatalytic contra-thermodynamic E→Z isomerization of vinyl boronates by using a binaphthol catalyst is disclosed. The reaction, based on the transient formation of a suitable chromophore with a BINOL derivative as the catalyst, allowed geometrical isomerization in good-to-excellent Z/E ratio and excellent-to-quantitative yields. The mechanism of this E→Z contra-thermodynamic isomerization was studied, and the formation of a transient chromophore species is suggested.

The Phospha-Bora-Wittig Reaction

We report the phospha-bora-Wittig reaction for the direct preparation of phosphaalkenes from aldehydes, ketones, esters, or amides. The transient phosphaborene Mes*P═B-NR₂ reacts with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogues of oxaphosphetane intermediates in the classical Wittig reaction. 1,2,3-Phosphaboraoxetanes undergo thermal or Lewis acid-promoted cycloreversion, yielding phosphaalkenes. Experimental and density functional theory studies reveal far-reaching similarities between classical and phospha-bora-Wittig reactions.

Advances in the E → Z Isomerization of Alkenes Using Small Molecule Photocatalysts

Geometrical E → Z alkene isomerization is intimately entwined in the historical fabric of organic photochemistry and is enjoying a renaissance (Roth et al. Angew. Chem., Int. Ed. Engl. 1989 28, 1193-1207). This is a consequence of the fundamental stereochemical importance of Z-alkenes, juxtaposed with frustrations in thermal reactivity that are rooted in microscopic reversibility. Accessing excited state reactivity paradigms allow this latter obstacle to be circumnavigated by exploiting subtle differences in the photophysical behavior of the substrate and product chromophores: this provides a molecular basis for directionality. While direct irradiation is operationally simple, photosensitization via selective energy transfer enables augmentation of the alkene repertoire to include substrates that are not directly excited by photons. Through sustained innovation, an impressive portfolio of tailored small molecule catalysts with a range of triplet energies are now widely available to facilitate contra-thermodynamic and thermo-neutral isomerization reactions to generate Z-alkene fragments. This review is intended to serve as a practical guide covering the geometric isomerization of alkenes enabled by energy transfer catalysis from 2000 to 2020, and as a logical sequel to the excellent treatment by Dugave and Demange (Chem. Rev. 2003 103, 2475-2532). The mechanistic foundations underpinning isomerization selectivity are discussed together with induction models and rationales to explain the counterintuitive directionality of these processes in which very small energy differences distinguish substrate from product. Implications for subsequent stereospecific transformations, application in total synthesis, regioselective polyene isomerization, and spatiotemporal control of pre-existing alkene configuration in a broader sense are discussed.

α-Boryl Organometallic Reagents in Catalytic Asymmetric Synthesis

Recent years have witnessed an increase in the popularity of α-boryl organometallic reagents as versatile nucleophiles in asymmetric synthesis. These compounds have been adopted in chemo- and stereoselective coupling reactions with a number of different electrophiles. The resulting enantioenriched boronic esters can be applied in stereospecific carbon-carbon or carbon-heteroatom bond construction reactions, enabling a two-step strategy for the construction of complex structures with high efficiency and functional group compatibility. Due to these reasons, tremendous effort has been devoted to the preparation of enantiomerically enriched α-boryl organometallic reagents and to the development of stereoselective reactions of related racemic or prochiral materials. In this review, we describe the enantio- or diastereoselective reactions that involve α-boryl organometallic reagents as starting materials or products and we showcase their synthetic utility.

Synthesis of a Biomimetic Tetracyclic Precursor of Aspochalasins and Formal Synthesis of Trichoderone A

Aspochalasins are leucine-derived cytochalasins. Their complexity is associated with a high degree of biosynthetic oxidation, herein inspiring a two-phase strategy in total synthesis. We thus describe the synthesis of a putative biomimetic tetracyclic intermediate. The constructive steps are an intramolecular Diels-Alder reaction to install the isoindolone core of cytochalasins, whose branched precursor was obtained from a stereoselective Ireland-Claisen rearrangement performed from a highly unsaturated substrate. This also constitutes a formal synthesis of trichoderone A.

A modular olefination reaction between aldehydes and diborylsilylmethide lithium salts

We describe the preparation of densely functionalised 1,1-silylborylated trisubstituted alkenes, via a boron-Wittig reaction, between LiC(Bpin)2(SiMe3) and aliphatic or aromatic aldehydes. The condensation of diborylsilylmethide lithium salts with α,β-unsaturated aldehydes provides a direct pathway to synthesize 1,1-silylborylated conjugated dienes and diynes.

Total Synthesis of Stelletins through an Unconventional Annulation Strategy

Marine ecosystems present the largest source of biodiversity on the planet and an immense reservoir of novel chemical entities. Sessile marine organisms such as sponges produce a wide range of complex secondary metabolites, many of these with potent biological activity engineered for chemical defense. That such compounds exert dynamic effects outside of their native context is perhaps not surprising, and the realm of marine natural products has attracted considerable attention as a largely untapped repository of potential candidates for drug development. Only a handful of the more than 15 000 marine natural products that have been isolated to date have advanced to the clinic, and more are to be expected. The rich chemical information encoded in the intricate three-dimensional structures of many marine natural products facilitates highly discriminating interactions with cell signaling pathways, and especially within cancer cells such nuanced effects offer an exciting opportunity for the development of targeted therapies that lack the side effects and general toxicity of conventional chemotherapeutics. The isomalabaricanes are a rare class of marine triterpenoids that have been hailed as promising cytotoxic lead compounds for the treatment of cancer, and they have attracted a flurry of excitement from researchers because of their potent cytotoxicity in certain human cancer cell lines along with a range of other antineoplastic effects. Most notably, their inhibitory activity is highly cell-selective, characterized by large deviations from their mean GI₅₀ concentrations across 3 orders of magnitude in the NCI-60 Human Tumor Cell Lines screen, suggesting mechanistic specificity rather than general and unbridled toxicity. Despite these auspicious preliminary reports, the isomalabaricane scaffold remains largely unexplored as a potential anticancer lead because of lack of material. This Account describes our recent efforts to develop a general, modular synthesis of the isomalabaricanes, as exemplified by the successful total syntheses of rhabdastrellic acid A, stelletin E, and stelletin A. The unorthodox trans-syn-trans configuration of their perhydrobenz[e]indene core severely circumscribes the synthetic methods available for its construction and required several generations of strategy to assemble. Ultimately, a series of unconventional transformations were identified that were capable of building this highly strained motif, and the syntheses of rhabdastrellic acid A and stelletin E were completed in racemic fashion. Subsequently, a second-generation approach to these natural products was developed, rendering the synthesis enantioselective as well as providing access to stelletin A. These synthetic efforts were greatly assisted by computational techniques such as ¹³C NMR prediction, which enabled structural assignments of hydrocarbon diastereomers, as well as relaxed surface scan conformational analysis, which informed a campaign for directed hydrogenation of an alkene. High-throughput experimentation methods were brought to bear during optimization of a late-stage Suzuki coupling on stelletin A. Finally, preliminary structure-activity relationship studies in glioblastoma and nonsmall cell lung cancer cell lines were conducted on stelletin A, revealing that the singular trans-syn-trans perhydrobenz[e]indene core is essential for the cytotoxic activity of the isomalabaricane triterpenoids.

Stereo- and Enantioselective Synthesis of Propionate-Derived Trisubstituted Alkene Motifs

We report a new method for constructing propionate-derived trisubstituted alkene motifs in a stereoselective manner. 1-Substituted 1,1-di(pinacolatoboryl)-(E)-alk-2-enes are generated in situ from 1-substituted 1,1-di(pinacolatoboryl)alk-3-enes through ruthenium(II)-catalyzed double-bond transposition. These species undergo a chiral phosphoric acid catalyzed allylation reaction of aldehydes to produce the E isomers of anti-homoallylic alcohols. On the other hand, the corresponding Z isomers of anti-homoallylic alcohols are obtained when a dimeric palladium(I) complex is employed as the catalyst for this double-bond transposition. Thus, both E and Z isomers can be synthesized from the same starting materials. A B-C(sp² ) bond remaining with the allylation product undergoes the Suzuki-Miyaura cross-coupling reaction to furnish a propionate-derived trisubstituted alkene motif in a stereo-defined form. The present method to construct the motifs with (E)- and (Z)-alkenes are successfully applied to the syntheses of (+)-isotrichostatic acid, (-)-isotrichostatin RK, and (+)-trichostatic acid, respectively.

Progress in the total synthesis of inthomycins

The inthomycin family of antibiotics, isolated from Streptomyces strains, are interesting molecules for synthesis due to their characteristic common oxazole polyene chiral allylic β-hydroxycarbonyl fragments and significant biological activities. The full structural motif of the inthomycins is found in several more complex natural products including the oxazolomycins, 16-methyloxazolomycin, curromycins A and B, and KSM-2690. This review summarises the application of various efforts towards the synthesis of inthomycins and their analogues systematically.

Boron-Wittig olefination with gem-bis(boryl)alkanes

Boron-Wittig reaction as the condensation of lithium α-bis(boryl)carbanions with carbonyl derivatives on route to substituted borylalkenes.