Enantioselective Palladium-Catalyzed Total Synthesis of Vitamin E by Employing a Domino Wacker–Heck Reaction

One-pot chemo- and photo-enzymatic linear cascade processes

The combination of chemo- and photocatalyses with biocatalysis, which couples the flexible reactivity of the photo- and chemocatalysts with the highly selective and environmentally friendly nature of enzymes in one-pot linear cascades, represents a powerful tool in organic synthesis. However, the combination of photo-, chemo- and biocatalysts in one-pot is challenging because the optimal operating conditions of the involved catalyst types may be rather different, and the different stabilities of catalysts and their mutual deactivation are additional problems often encountered in one-pot cascade processes. This review explores a large number of transformations and approaches adopted for combining enzymes and chemo- and photocatalytic processes in a successful way to achieve valuable chemicals and valorisation of biomass. Moreover, the strategies for solving incompatibility issues in chemo-enzymatic reactions are analysed, introducing recent examples of the application of non-conventional solvents, enzyme-metal hybrid catalysts, and spatial compartmentalization strategies to implement chemo-enzymatic cascade processes.

Developments in the stereoselective synthesis of benzopyran, benzopyrone and flavonoid based natural product analogues using C-glycosides as an intrinsic chiral synthon

Stereoselective synthesis is essential for propelling mainstream academia toward a relentless pursuit of novel and cutting-edge strategies for constructing molecules with unparalleled precision. Naturally derived benzopyrans, benzopyrones, and flavonoids are an essentially prominent group of oxa-heterocycles, highly significant targets in medicinal chemistry owing to their extensive abundance in biologically active natural products and pharmaceuticals. The molecular complexity and stereoselectivity induced by heterocycles embedded with C-glycosides have attracted considerable interest and emerged as a fascinating area of research for synthetic organic chemists. This present article emphasizes the existing growths in the strategies involving the diastereoselective synthesis of C-glycosylated benzopyrans, benzopyrones, and flavonoids using naturally acquired glycones as chiral synthons.

Total Synthesis of Ganoderma Meroterpenoids Cochlearol B and Its Congeners Driven by Structural Similarity and Biological Homology

Secondary metabolites that have the same biological origin must share some relationship in their biosynthesis. Exploring this relationship has always been a significant task for synthetic biologists. However, from the perspective of synthetic chemists, it is equally important to propose, prove, or refute potential biosynthetic pathways in order to elucidate and understand the biosynthesis of homologous secondary metabolites. In this study, driven by the high structural similarity between the homologous Ganoderma meroterpenoids cochlearol B and ganocin B, two chemically synthetic strategies were designed and investigated sequentially for the synthesis of cochlearol B from ganocin B. These strategies include intramolecular metal-catalyzed hydrogen atom transfer (MHAT) and intramolecular photochemical [2+2] cycloaddition. The aim was to reveal their potential biosynthetic conversion relationship using chemical synthesis methods. As a result, a highly efficient total synthesis of cochlearol B, cochlearol T, cochlearol F, as well as the formal total synthesis of ganocins A-B, and ganocochlearins C-D, has been achieved. Additionally, a novel synthetic approach for the synthesis of 6,6-disubstituted 6H-dibenzo[b,d]pyran and its analogues has been developed through palladium(II)-catalyzed Wacker-type/cross-coupling cascade reactions.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Palladium-Catalyzed Aminocyclization-Coupling Cascades: Preparation of Dehydrotryptophan Derivatives and Computational Study

Dehydrotryptophan derivatives have been prepared by palladium-catalyzed aminocyclization-Heck-type coupling cascades starting from o-alkynylaniline derivatives and methyl α-aminoacrylate. Aryl, alkyl (primary, secondary, and tertiary), and alkenyl substituents have been introduced at the indole C-2 position. Further variations at the indole benzene ring, as well as the C-2-unsubstituted case, have all been demonstrated. In the case of C-2 aryl substitution, the preparation of the o-alkynylaniline substrate by Sonogashira coupling and the subsequent cyclization–coupling cascade have been performed in a one-pot protocol with a single catalyst. DFT calculations have revealed significant differences in the reaction profiles of these reactions relative to those involving methyl acrylate or methacrylate, and between the reactions of the free anilines and their corresponding carbamates. Those calculations suggest that the nature of the alkene and of the acid HX released in the HX/alkene exchange step that precedes C–C bond formation could be responsible for the experimentally observed differences in reaction efficiencies.

Concise Synthesis of (S)-δ-CEHC, a Metabolite of Vitamin E

Carboxyethyl hydroxychromans (CEHCs) are natural metabolites of vitamin E (VE). Their urinary levels can serve as useful biomarkers for the nutritional assessment of VE. Moreover, specific biological activities of CEHCs deserve further investigation. Here, we report a concise method to build up a chiral 6-hydroxychroman scaffold of VE and CEHCs. The first total synthesis of (S)-δ-CEHC was accomplished in 40% overall yield and 97% ee using a chiral synthon derived from naturally occurring (-)-linalool.

Catalytic allylic functionalization via π-allyl palladium chemistry

This review highlights the palladium-catalyzed allylic C–H functionalizations via π-allyl palladium reported from early 2014 to present date.

Catalytic, Enantioselective, Intramolecular Sulfenofunctionalization of Alkenes with Phenols

The catalytic, enantioselective, cyclization of phenols with electrophilic sulfenophthalimides onto isolated or conjugated alkenes affords 2,3-disubstituted benzopyrans and benzoxepins. The reaction is catalyzed by a BINAM-based phosphoramide Lewis base catalyst which assists in the highly enantioselective formation of a thiiranium ion intermediate. The influence of nucleophile electron density, alkene substitution pattern, tether length and Lewis base functional groups on the rate, enantio- and site-selectivity for the cyclization is investigated. The reaction is not affected by the presence of substituents on the phenol ring. In contrast, substitutions around the alkene strongly affect the reaction outcome. Sequential lengthening of the tether results in decreased reactivity, which necessitated increased temperatures for reaction to occur. Sterically bulky aryl groups on the sulfenyl moiety prevented erosion of enantiomeric composition at these elevated temperatures. Alcohols and carboxylic acids preferentially captured thiiranium ions in competition with phenolic hydroxyl groups. An improved method for the selective C(2) allylation of phenols is also described.

Enantioselective Synthesis of Chiral Oxygen-Containing Heterocycles Using Copper-Catalyzed Aryl C-O Coupling Reactions via Asymmetric Desymmetrization

An enantioselective desymmetric aryl C-O coupling reaction was demonstrated under the catalysis of CuI and a chiral cyclic diamine ligand. A series of chiral oxygen-containing heterocyclic units such as 2,3-dihydrobenzofurans, chromans, and 1,4-benzodioxanes with tertiary or quaternary stereocarbon centers were synthesized with this method. DFT calculations were also carried out for a better understanding of the model for enantiocontrol.

Multicomponent reaction through cooperative trio catalysis incorporating enamine, Brønsted acid and metal Lewis acid catalysis: a concise route to access chromans

Chroman formation is achieved through a tricatalytic system incorporating enamine, metal Lewis acid and Brønsted acid catalysis.

Heterogeneous enantioselective synthesis of chromans via the oxa-Michael–Michael cascade reaction synergically catalyzed by grafted chiral bases and inherent hydroxyls on mesoporous silica surface

A heterogeneous enantioselective oxa-Michael–Michael reaction for the synthesis of chromans has been developed on bifunctional catalysts, resulting in excellent catalytic performance.

A facile access to cis-dihydrofurobenzofuran from 2-(2,5-dihydro-furan-2-yl)-phenol

An access to cis-dihydrofurobenzofuran from the Pd-catalyzed intramolecular oxidation of 2-(2,5-dihydro-furan-2-yl)-phenol with moderate to good yields is reported.

The syn/anti-dichotomy in the palladium-catalyzed addition of nucleophiles to alkenes

In this review the stereochemistry of palladium-catalyzed addition of nucleophiles to alkenes is discussed, and examples of these reactions in organic synthesis are given. Most of the reactions discussed involve oxygen and nitrogen nucleophiles; the Wacker oxidation of ethylene has been reviewed in detail. An anti-hydroxypalladation in the Wacker oxidation has strong support from both experimental and computational studies. From the reviewed material it is clear that anti-addition of oxygen and nitrogen nucleophiles is strongly favored in intermolecular addition to olefin-palladium complexes even if the nucleophile is coordinated to the metal. On the other hand, syn-addition is common in the case of intramolecular oxy- and amidopalladation as a result of the initial coordination of the internal nucleophile to the metal.

Total synthesis of (R,R,R)-α-tocopherol through asymmetric Cu-catalyzed 1,4-addition

By introducing a disposable activating substituent at C-3, the asymmetric 1,4-addition to a notoriously unreactive 2-substituted chromenone was achieved with high levels of (2R)-stereoselectivity in the presence of a chiral Cu(I)-phosphoramidite complex as a catalyst. This paved the way for an efficient and conceptually novel synthesis of (R,R,R)-α-tocopherol from readily available starting materials.

Asymmetric palladium-catalysed intramolecular Wacker-type cyclisations of unsaturated alcohols and amino alcohols

The palladium (II)-catalysed reactions of alkenols and aminoalkenols such as oxycarbonylations or bicyclisations are powerful methods for the construction of oxygen and nitrogen-containing heterocyclic compounds. This review highlights recent progress in the development of the asymmetric palladium(II)-catalysed Wacker-type cyclisations of unsaturated polyols and aminoalcohols. The scope, limitations, and applications of these reactions are presented.

Synthesis of chromans via Pd-catalyzed alkene carboetherification reactions

A new method for the construction of 2-substituted and 2,2-disubstituted chromans via Pd-catalyzed carboetherification reactions between aryl/alkenyl halides and 2-(but-3-en-1-yl)phenols is described. These reactions effect formation of a C-O bond and a C-C bond to afford the chroman products in good yield, and also provide stereoselective access to tricyclic chroman derivatives.