Anion Relay Chemistry: An Effective Tactic for Diversity Oriented Synthesis

Ru-Catalyzed [3 + 2] Cycloaddition of Nitrile Oxides and Electron-Rich Alkynes with Reversed Regioselectivity

Polarity reversal ("umpolung") of a functional group can override its inherent reactivity and lead to distinct bond-forming modes. Herein we describe a rarely studied cycloaddition between nitrile oxides and electron-rich alkynes with reversed regioselectivity, leading to the useful 4-heterosubstituted isoxazoles. The use of a ruthenium catalyst completely overrides the inherent polarity of nitrile oxides. This reversed regioselectivity was also observed for their reactions with a range of electron-deficient alkynes.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

Enantioselective α-Functionalization of 1,3-Dithianes by Iridium-Catalyzed Allylic Substitution

An iridium-catalyzed asymmetric allylic substitution reaction with 2-alkoxy carbonyl-1,3-dithianes has been achieved with high regio- and enantioselectivities. The transformation provides a new method for the enantioselective α-functionalization of dithianes. The corresponding dithiane-containing products are easily converted into many other derivatives with high yields and enantioselectivities.

Evolution of Anion Relay Chemistry: Construction of Architecturally Complex Natural Products

Multicomponent union tactics in which three or more fragments are rapidly connected are highly prized in the construction of architecturally complex natural products. Anion Relay Chemistry (ARC), a multicomponent union tactic, has just such potential to elaborate structurally diverse scaffolds in a single operation with excellent stereochemical control. Conceptually, the ARC tactic can be divided into two main classes: "Through-Bond," by the relay of negative charge through the bonding system of a molecule; and "Through-Space," by the migration of negative charge across space by a transfer agent. "Through-Space" Anion Relay Chemistry, the focus of this Account, can be further subdivided into two types: Type I ARC, originated from the Tietze-Schaumann-Smith coupling reaction, which for the first time permits controllable Brook rearrangements to construct unsymmetrical adducts, and as such has been successfully employed in the total syntheses of diverse natural products, including the mycoticins, bryostatin 1, spongistatins, rimocidin, indolizidine alkaloids, and enigmazole A; and Type II ARC, central to which is the design of novel bifunctional linchpins that enable rapid assembly of linear and cyclic fragments with diverse architectural features, ranging from polyols, spiroketals, and polyenes to polypropionate scaffolds. Recently, the Type II ARC tactic has been exploited as the key construction tactic in the total syntheses of the spirastrellolides, the cryptocarya acetates, secu'amamine A, mandelalide A, and nahuoic acid C_i (B_ii). This Account will present the evolution of both the Type I and Type II Anion Relay tactics, in conjunction with some prominent applications.

One Amine-3 Tasks: Reductive Coupling of Imines with Olefins in Batch and Flow

Owing to their wide range of biological properties, γ-aminobutyric acid derivatives (GABA) have been extensively studied and found noteworthy industrial applications. However, atom-economical and efficient processes for their production are scarce and would greatly benefit from further investigations. Herein, we demonstrate that an iridium-based photocatalyst promotes the direct reductive cross-coupling of imines with olefins upon irradiation with visible light to give GABA derivatives in good yields and selectivities. We also stress the enabling triple role of tributylamine additive in this process, discuss the advantages of strategies based on proton-coupled electron transfer (PCET) and demonstrate the scale-up of this reaction in continuous flow.

Merging Asymmetric [1,2]-Additions of Lithium Acetylides to Carbonyls with Type II Anion Relay Chemistry

An enantioselective three-component coupling reaction has been developed, enabling the union of a variety of lithium acetylides and electrophiles exploiting an achiral linchpin via an anionic reaction cascade. This Type II Anion Relay Chemistry tactic is initiated via an enantioselective [1,2]-carbonyl addition exploiting BINOL catalysis to access an enantioenriched alkoxide intermediate. Migration of charge across the linchpin via a [1,4]-Brook rearrangement with electrophile capture affords a three-component propargyl ether adduct. Herein, we report the development, scope, and limitations of this reaction sequence.

Unexpected Propargylic Retro-Brook Rearrangements in Alkynes

Retro-Brook rearrangements refer to the intramolecular migration of a silyl group from oxygen to carbon. In this study, we report a novel propargylic retro-Brook rearrangement observed in terminal alkynes bearing a silyl ether moiety. Retro-Brook rearrangements involving [1,2]-, [1,4]-, and [1,5]-migrations are described, affording propargylsilanes in reasonable yield. The reaction mechanism was investigated experimentally by deuterium quenching and rationalized by density functional theory calculations. The terminal alkyne and the subsequent propargyl/allenyl dianion were shown to be crucial for the reaction favoring the retro-Brook rearrangement product over the Brook rearrangement. The second deprotonation at the propargylic position was determined to be the rate-limiting step. In addition, a gas-phase Brook-type rearrangement of the propargylsilanes was observed under GC-MS conditions. This observation was also further confirmed by DFT calculations.

Reactions of 2-Aryl-1,3-Dithianes and [1.1.1]Propellane

Bicyclo[1.1.1]pentanes (BCPs) have sparked the interest of medicinal chemists due to their recent discovery as bioisosteres of aromatic rings. To study the biological activity of this relatively new class of bioisosteres, reliable methods to incorporate BCPs into target molecules are in high demand, as reflected by a flurry of methods for BCP synthesis in recent years. In this work, we disclose a general method for the synthesis of BCP-containing dithianes which, upon deprotection, provide access to BCP analogues of medicinally abundant diarylketones. A broad scope of 2-aryl-1,3-dithianes, including several heterocyclic derivatives, react with [1.1.1]propellane to afford 26 new derivatives in good to excellent yields. Further transformation of the dithiane portion into a variety of functional groups demonstrates the robustness of the products. A computational study indicates that the reaction of 2-aryl-1,3-dithianes and [1.1.1]propellane proceeds via a two-electron pathway.

Dithiane-directed Rh(iii)-catalyzed amidation of unactivated C(sp3)-H bonds

An oxidant-free Rh(iii)-catalyzed direct amidation of alkyl dithianes via C(sp3)-H bond activation utilizing diverse and robust dioxazolone reagents is reported. The reaction hinges on use of a Cp*Rh(iii) complex in combination with an essential amino-carboxylate additive to generate usefully protected 1,3-aminoaldehyde derivatives. The scalability of the reaction was demonstrated as was a series of downstream product functionalizations, including dithiane deprotection, anion alkylation and reductive desulfurization, highlighting the general applicability of this transformation in the synthesis of novel scaffolds and building blocks.

Generation of Dithianyl and Dioxolanyl Radicals Using Photoredox Catalysis: Application in the Total Synthesis of the Danshenspiroketallactones via Radical Relay Chemistry

Visible-light-induced generation of dithianyl and dioxolanyl radicals via selective hydrogen atom transfer (HAT) has been achieved. This radical relay tactic enables remote C(sp³)-H functionalization to permit rapid access to polyol and spiroketal segments, and in turn has been exploited as a key synthetic construct in the total synthesis of the danshenspiroketallactones. The conformational stability of the danshenspiroketallactones has also been defined via experiments and DFT calculations.

Total Synthesis of the Marine Phosphomacrolide, (-)-Enigmazole A, Exploiting Multicomponent Type I Anion Relay Chemistry (ARC) in Conjunction with a Late-Stage Petasis-Ferrier Union/Rearrangement

An effective late-stage large-fragment union/rearrangement exploiting the Petasis-Ferrier protocol, in conjunction with multicomponent Type I Anion Relay Chemistry (ARC) to access advanced intermediates, permits completion of a convergent, stereocontrolled total synthesis of the architecturally complex phosphomacrolide (-)-enigmazole A (1).

Synthetic Access to the Mandelalide Family of Macrolides: Development of an Anion Relay Chemistry Strategy

The mandelalides comprise a family of structurally complex marine macrolides that display significant cytotoxicity against several human cancer cell lines. Presented here is a full account on the development of an Anion Relay Chemistry (ARC) strategy for the total synthesis of (-)-mandelalides A and L, the two most potent members of the mandelalide family. The design and implementation of a three-component type II ARC/cross-coupling protocol and a four-component type I ARC union permits rapid access respectively to the key tetrahydrofuran and tetrahydropyran structural motifs of these natural products. Other highlights of the synthesis include an osmium-catalyzed oxidative cyclization of an allylic 1,3-diol, a mild Yamaguchi esterification to unite the northern and southern hemispheres, and a late-stage Heck macrocyclization. Synthetic mandelalides A and L displayed potent cytotoxicity against human HeLa cervical cancer cells (EC50, 1.3 and 3.1 nM, respectively). This synthetic approach also provides access to several highly potent non-natural mandelalide analogs, including a biotin-tagged mandelalide probe for future biological investigation.

One pot preparation of α-dithioacetal/α-diselenoacetal amides via a dual-C–S/C–Se bond formation and C–C bond cleavage cascade of 3-oxo-butanamides

A new and practical approach to α-dithioacetal/α-diselenoacetal amides has been developed.

Mechanistic investigation on N → Cα → O relay via non-Brook rearrangement: reaction conditions promote synthesis of furo[3,2-c]pyridinones

A comprehensive density functional theory investigation was employed to disclose the effect of reaction conditions on the mechanism and effective anion relay sequence of a NaH promoted non-Brook rearrangement of benzaldehyde and 1-cinnamoylcyclopropanecarboxamides. Two main mechanisms were explored under four different reaction conditions: Na⁺-assisted and nH₂O-Na⁺, 2H₂O-DMSO-Na⁺, and Na⁺-DMSO co-assisted, and the difference relies on the reaction sequence between the concerted ring-opening and recyclization and electrophilic addition. Being different from previous reports, a cooperative participation of water, solvent DMSO and counterion Na⁺ is revealed in the preferential mechanism. The preferred scenario undergoes five major steps: deprotonation, aza-Michael addition, electrophilic addition, NaOH elimination and a concerted ring-opening and recyclization step. The rate-determining step is the concerted ring-opening and recyclization process with an energy barrier of 30.2 kcal mol^-1. We found that the effective anion relay of a non-Brook rearrangement order is N → C^α → O rather than the previously proposed aza-oxy-carbanion. Meanwhile, a mixed type of ARC chemistry through a novel non-Brook rearrangement was disclosed. Moreover, the non-covalent interactions between substrate and reactant extensively affect the anion relay process by hydrogen-bonding (O-HO and C-HO) and electrostatic (Na⁺O) interactions. Thus, our results provide insightful clues to the mechanism of the reaction condition catalyzed non-Brook rearrangement reaction.

New Mandelalides Expand a Macrolide Series of Mitochondrial Inhibitors

Mandelalides A-D (1-4) are macrocyclic polyketides known to have an unusual bioactivity profile influenced by compound glycosylation and growth phase of cultured cells. The isolation and characterization of additional natural congeners, mandelalides E-L (5-12), and the supply of synthetic compounds 1 and 12, as well as seco-mandelalide A methyl ester (13), have now facilitated mechanism of action and structure-activity relationship studies. Glycosylated mandelalides are effective inhibitors of aerobic respiration in living cells. Macrolides 1 and 2 inhibit mitochondrial function similar to oligomycin A and apoptolidin A, selective inhibitors of the mammalian ATP synthase (complex V). 1 inhibits ATP synthase activity from isolated mitochondria and triggers caspase-dependent apoptosis in HeLa cells, which are more sensitive to inhibition by 1 in the presence of the glycolysis inhibitor 2-deoxyglucose. Thus, mandelalide cytotoxicity depends on basal metabolic phenotype; cells with an oxidative phenotype are most likely to be inhibited by the mandelalides.

Palladium-Promoted Neutral 1,4-Brook Rearrangement/Intramolecular Allylic Cyclization Cascade Reaction: A Strategy for the Construction of Vinyl Cyclobutanols

A cascade reaction to build vinyl cyclobutanol rings through activation of vinyl epoxides by palladium, followed by 1,4-Brook rearrangement and intramolecular cyclization with a palladium complex of the resulting carbon anion, is described. Through this cascade reaction, several highly substituted cyclobutanol substrates were achieved in good yields with high stereoselectivities.

Type II Anion Relay Chemistry: Conformational Constraints To Achieve Effective [1,5]-Vinyl Brook Rearrangements

The design, synthesis, and evaluation of bifunctional linchpins, conformationally anchored on six-membered rings to achieve efficient [1,5]-Brook rearrangements involving vinyl silanes have been achieved. The restrained linchpins were subsequently exploited in type II anion relay chemistry (ARC) to permit both alkylations and Pd-mediated cross-coupling reactions (CCR) of sp² stabilized carbanions. DFT calculations were then employed to understand the mechanism and reactivity trends of [1,4]- and [1,5]-vinyl Brook rearrangements to provide insight on the role of the required copper reagent and the substrate geometry.

Metal-Free Formal Oxidative C-C Coupling by In Situ Generation of an Enolonium Species

Much contemporary organic synthesis relies on transformations that are driven by the intrinsic, so-called "natural", polarity of chemical bonds and reactive centers. The design of unconventionally polarized synthons is a highly desirable strategy, as it generally enables unprecedented retrosynthetic disconnections for the synthesis of complex substances. Whereas the umpolung of carbonyl centers is a well-known strategy, polarity reversal at the α-position of a carbonyl group is much rarer. Herein, we report the design of a novel electrophilic enolonium species and its application in efficient and chemoselective, metal-free oxidative C-C coupling.

Stereoselective strategies for the construction of polysubstituted piperidinic compounds and their applications in natural products’ synthesis

An abridged and far-reaching review communication on the construction of the polysubstituted piperidinic core using diverse methodologies for the benefit of organic chemists interested in the total synthesis of biologically active compounds.

Regiodivergent radical oxidative coupling of vinyl ethers with dithiane by copper or iron catalysis

Direct regioselective C–H dithioacetalization of readily available vinyl ethers to yield a variety of substituted dithianes under a copper or iron catalysis has been developed.

Carbon Cationic Relay via Superelectrophiles: Synthesis of Spiro-diazafluorenes

Superelectrophilic-initiated carbon cationic relay reactions of diazafluorenones with phenols were developed to provide strategically novel and atom-economic access to spirodiazafluorenes via tandem Friedel-Crafts reaction, nucleophilic addition, and intramolecular cyclization sequences. A range of spirodiazafluorenes that are difficult to synthesize with traditional protocols has been constructed successfully in middle to high yields using this method.

Total Synthesis of (-)-Mandelalide A Exploiting Anion Relay Chemistry (ARC): Identification of a Type II ARC/CuCN Cross-Coupling Protocol

Anion relay chemistry (ARC), an effective, multicomponent union tactic, was successfully employed for the total synthesis of the highly cytotoxic marine macrolide (-)-mandelalide A (1). The northern hemisphere was constructed via a new type II ARC/CuCN cross-coupling tactic, while the southern hemisphere was secured via a highly efficient four-component type I ARC union. Importantly, the synthesis of 1 showcases ARC as a rapid, scalable coupling strategy for the union of simple readily available building blocks to access diverse complex molecular fragments with excellent stereochemical control.

An Effective Bifunctional Aldehyde Linchpin for Type II Anion Relay Chemistry: Development and Application to the Synthesis of a C16-C29 Fragment of Rhizopodin

The design, synthesis, and validation of a new bifunctional aldehyde linchpin for Type II anion relay chemistry have been achieved. For this linchpin, the initial nucleophilic addition proceeds under Felkin–Anh control to generate the syn-alkoxide, which undergoes a 1,4-Brook rearrangement to relay the negative charge, thus leading to the formation of a dithiane-stabilized carbanion. Subsequent trapping with an electrophile furnishes a tricomponent adduct with an embedded propionate subunit, a ubiquitous structural motif found in polyketides. The utility of this new linchpin is demonstrated with the construction of a potential C16–C29 fragment for the synthesis of rhizopodin, an actin-binding macrolide.

Total Synthesis of (-)-Enigmazole A

A highly convergent, stereocontrolled total synthesis of the architecturally complex marine sponge metabolite (−)-enigmazole A has been achieved. Highlights include an unprecedented late-stage large-fragment Petasis–Ferrier union/rearrangement, a multicomponent Type I Anion Relay Chemistry (ARC) tactic, and a dithiane–epoxide union in conjunction with an oxazole-directed stereoselective reduction.

Type II Anion Relay Chemistry: Exploiting Bifunctional Weinreb Amide Linchpins for the One-Pot Synthesis of Differentiated 1,3-Diketones, Pyrans, and Spiroketals

The design, synthesis, and validation of new highly effective bifunctional linchpins for type II anion relay chemistry (ARC) has been achieved. The mechanistically novel negative-charge migration that comprises the Brook rearrangement is now initiated by a stabilized tetrahedral intermediate, which is generated by nucleophilic addition to a Weinreb amide, rather than by a simple oxyanion that is generated from an epoxide. As a result, the linchpin preserves the carbonyl functionality in the ARC adducts, thus permitting access to functionally complex systems in a single flask without the need for further chemical manipulations. This tactic was validated with the one-pot preparation of monoprotected 1,3-diketones as well as pyran and spiroketal scaffolds, depending on the choice of nucleophile, electrophile, and work-up conditions.

Through-bond/through-space anion relay chemistry exploiting vinylepoxides as bifunctional linchpins

The development of new bifunctional linchpins that permit the union of diverse building blocks is essential for the synthetic utility of anion relay chemistry (ARC). The design, synthesis, and validation of three vinylepoxide linchpins for through-bond/through-space ARC are now reported. For negative charge migration, this class of bifunctional linchpins employs initial through-bond ARC by an SN 2' reaction, followed by through-space ARC exploiting a 1,4-Brook rearrangement. The trans-disubstituted vinylepoxide linchpin yields a mixture of E/Z isomers, whereas the cis-disubstituted and the trans-trisubstituted vinylepoxide linchpins proceed to deliver three-component adducts with excellent E selectivity.

A unified synthetic strategy to the Cryptocarya family of natural products exploiting Anion Relay Chemistry (ARC)

A unified synthetic strategy to the Cryptocarya family of natural products has been achieved employing four-component fragment unions in a "single flask" exploiting Anion Relay Chemistry (ARC). Functionalization of the ARC adducts permits rapid construction of five polyhydroxylated di- and tetrahydropyrone natural products of the Cryptocarya class (1-5), in a total of 7-9 steps from commercially available materials.