A concise synthesis of eupomatilones 4, 6, and 7 by rhodium-catalyzed enantioselective desymmetrization of cyclic meso anhydrides with organozinc reagents generated in situ

Divergent functionalization of alkenes enabled by photoredox activation of CDFA and α-halo carboxylic acids

Herein we present our studies on the solvent-controlled difunctionalization of alkenes utilizing chlorodifluoroacetic acid (CDFA) and α-halo carboxylic acids for the synthesis of γ-lactones, γ-lactams and α,α-difluoroesters. Mechanistic insights revealed that photocatalytic reductive mesolytic cleavage of the C-X bond delivers elusive α-carboxyl alkyl radicals. In the presence of an olefin molecule, this species acts as a unique bifunctional intermediate allowing for stipulated formation of C-O, C-N and C-H bonds on Giese-type adducts via single electron transfer (SET) or hydrogen atom transfer (HAT) events. These protocols exhibit great efficiency across a broad spectrum of readily available α-halo carboxylic acids and are amenable to scalability in both batch and flow. To demonstrate the versatility of this concept, the synthesis of (±)-boivinianin A, its fluorinated analog and eupomatilone-6 natural products was successfully accomplished.

Cu(II)/SPDO complex-catalyzed asymmetric Baeyer–Villiger oxidation of 2-arylcyclobutanones and its application for the total synthesis of eupomatilones 5 and 6

A novel classical kinetic resolution of 2-aryl-substituted or 2,3-disubstituted cyclobutanones of Baeyer–Villiger oxidation catalyzed by a Cu(ii)/SPDO complex is reported for the first time, producing normal lactones in excellent enantioselectivities (up to 96% ee) and regioselectivities (up to >20/1), along with unreacted ketones in excellent enantioselectivities (up to 99% ee). The current transformation features a wide substrate scope. Moreover, catalytic asymmetric total syntheses of natural eupomatilones 5 and 6 are achieved in nine steps from commercially available 3-methylcyclobutan-1-one.

A novel classical kinetic resolution of Baeyer–Villiger oxidation catalyzed by a Cu(ii)/SPDO complex with excellent enantioselectivity, regioselectivity and wide substrate scope is reported for the first time and explore the synthetic application.

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Synthesis of 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline through nucleophilic addition of organozinc reagents to N,O-acetals

A new approach to access 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline has been developed through nucleophilic addition of organozinc reagents to N,O-acetals. A number of substituted organozinc reagents were amenable for this transformation, and the desired products were obtained with excellent yields. Moreover, Sc(OTf)3 proved to be an effective catalyst for the formation of 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline using such nucleophilic addition.

Synthesis of γ-Lactones via the Kowalski Homologation Reaction: Protecting-Group-Free Divergent Total Syntheses of Eupomatilones-2,5,6, and 3-epi-Eupomatilone-6

A highly efficient synthesis of functionalized chiral γ-butyrolactone scaffolds has been described. The basis of the approach is the Kowalski ester homologation that is modified for our proposed transformation. The newly developed methodology combines a divergent synthetic strategy to permit a straightforward protecting-group-free asymmetric total syntheses of eupomatilones-2,5,6, and 3-epi-eupomatilone-6 in five or six steps from commercial starting materials, making it one of the shortest syntheses reported to date.

Total Synthesis, Stereochemical Assignment, and Divergent Enantioselective Enzymatic Recognition of Larreatricin

A concise and efficient total synthesis of the lignan natural product larreatricin as well as an unambiguous assignment of configuration of its enantiomers are reported, resolving a long-held controversy. Enzyme kinetic studies revealed that different polyphenol oxidases show high and remarkably divergent enantioselective recognition of this secondary metabolite.

Bioinspired total synthesis of (−)-gymnothelignan L

The first asymmetric total synthesis of gymnothelignan L has been accomplished in 14 steps with 11.2% overall yield. The synthetic approach features an anti Evans aldol reaction, a diastereoselective methylation, a Suzuki–Miyaura coupling and a bioinspired desymmetric transannular Friedel–Crafts reaction.

Dual Nickel- and Photoredox-Catalyzed Enantioselective Desymmetrization of Cyclic meso-Anhydrides

The enantioselective desymmetrization of cyclic meso-anhydrides with benzyl trifluoroborates under nickel-photoredox catalysis is described. The reaction tolerates a variety of sterically and electronically different trifluoroborates, as well as structurally unique cyclic anhydrides. The trans isomer of the keto-acid products is also observed at varying levels dependent on the trifluoroborate identity and relative catalyst loading. A mechanism involving decarbonylation and Ni-C bond homolysis of a Ni^II adduct is proposed. This feature allows access to a trans keto-acid as the major product in high enantioselectivity from a cis meso anhydride.

Nickel-Catalyzed Asymmetric Kumada Cross-Coupling of Symmetric Cyclic Sulfates

Nickel-catalyzed enantioselective cross-couplings between symmetric cyclic sulfates and aromatic Grignard reagents are described. These reactions are effective with a broad range of substituted cyclic sulfates and deliver products with asymmetric tertiary carbon centers. Mechanistic experiments point to a stereoinvertive SN2-like oxidative addition of a nickel complex to the electrophilic substrate.

Cobaltate anion couples terminal dienes with trifluoroacetic anhydride: a direct fluoroacylation of 1,3-dienes

Perfluoroketones are useful products and intermediates in medicinal and materials chemistry.

Synthesis of vicinal dimethyl chirons by asymmetric hydrogenation of trisubstituted alkenes

Roche ester derivatives were converted to trisubstituted alkenes with allylic chiral centers. Hydrogenation of these substrates with chiral analogues of Crabtree's catalyst, specifically, an optically active carbene oxazoline derivative, were found to be mostly catalyst controlled. However, the peripheral functionalities and protecting groups had significant effects and could be adjusted to give high stereoselectivities. The upshot of this work is that alpha,omega-functionalized chirons to introduce 1,2-dimethyl functionalities into acyclic chains have been developed.

Enantioselective cross-coupling of anhydrides with organozinc reagents: the controlled formation of carbon-carbon bonds through the nucleophilic interception of metalacycles

The construction of carbon-carbon bonds, particularly with concomitant control of newly formed asymmetric centers, is of paramount importance for the development of synthetic routes to complex organic molecules. While cross-coupling reactions for the generation of sp(2) carbon centers are well established, similar methodology for the formation and control of sp(3)-hydridized carbon stereocenters is extremely limited. We suggest that the nucleophilic interception of metalacycles provides the means to achieve such a transformation, wherein the metal complex serves to activate electrophiles, facilitate nucleophile addition, and ultimately control stereochemistry. One means of accessing these intermediates is through the use of simple meso-carboxylic anhydrides, which upon reaction with transition metals readily generate the desired metalacycles. Interception of the metalacycle with an appropriate carbon-based nucleophile generates an enantioenriched ketoacid, the product of the asymmetric desymmetrization of achiral starting materials. Early successes with achiral nickel catalysts and organozinc reagents provided the foundations for our approach. Alkylation of both succinic and glutaric anhydrides proceeds with a wide range of organozinc nucleophiles, forming 1,4- and 1,5-ketoacids in excellent yields. This reaction manifold has been extensively examined with a detailed kinetic study and mechanistic investigations utilizing mixed zinc reagents and alkene directing groups. This work has highlighted a number of unusual phenomena, including rate-limiting reductive elimination to form an sp(3)-sp(2) carbon-carbon bond. Despite excellent results with the achiral system, to date, all efforts to render the nickel-catalyzed reaction asymmetric have been limited to modest success. Palladium and rhodium complexes, with the use of chiral P-P and P-N ligands, respectively, have been identified as competent catalysts for the enantioselective addition of organozinc reagents to anhydrides. The arylation of a series of succinic anhydrides with Ph2Zn can be achieved in greater than 95% enantioselectivity using a Pd/Josiphos catalyst. Rhodium catalysts have proven amenable for the incorporation of in situ formed organozinc reagents, nucleophiles traditionally troublesome in transition metal catalysis due to the deleterious effects of residual halide ions. Highly functionalized organozinc nucleophiles, including those containing indole and furan, participate in this chemistry to provide the corresponding 1,4- and 1,5-ketoacids in excellent yield with greater than 85% enantioselectivity. This metalacycle interception methodology is currently being expanded to the use of other systems, most notably the asymmetric [2 + 2 + 2] cycloaddition of alkenes, alkynes, and isocyanates. Ongoing studies promise the extension of existing methodology toward the development of modular, fully intermolecular three-component couplings in which both metalacycle formation and nucleophilic interception can be controlled. Ultimately, we envision the use of heterocumulenes in such methodology, providing a route to complex products utilizing CO2 as an inexpensive C1 feedstock.