Enantioselective Rhodium‐Catalyzed Dimerization of ω‐Allenyl Carboxylic Acids: Straightforward Synthesis of C 2 ‐Symmetric Macrodiolides

Asymmetric bifunctionalization of allenes with aryl iodides and amino acids enabled by chiral aldehyde/palladium combined catalysis

Even though catalytic asymmetric bifunctionalization of allenes has been extensively studied, almost all of the reported examples have been achieved in a two-component manner. In this study, we report a highly efficient asymmetric bifunctionalization of allenes with iodohydrocarbons and NH2-unprotected amino acid esters. The adopted chiral aldehyde/palladium combined catalytic system precisely governs the chemoselectivity, regioselectivity, and stereoselectivity of this three-component reaction. A wide range of substituted aryl iodides, allenes and amino acid esters can well participate in this reaction and deliver structurally diverse α,α-disubstituted α-amino acid esters with excellent experimental outcomes. One of the resulting products is utilized for the total synthesis of the molecule (S,R)-VPC01091.

Synthesis of fulgidic acid and the two possible stereoisomers of chaenomic acid D

We synthesized fulgidic acid and the proposed structure for chaenomic acid D. The core part of the two natural products was constructed stereoselectively by the addition of acetic acid to the α,β-unsaturated epoxy alcohol in the presence of a palladium catalyst. Subsequently, the two natural products were synthesized from the intermediate in a few steps. The data for the synthesized fulgidic acid were in good agreement with the reported data. Chaenomic acid was in good agreement with the natural product in the 1H and 13C NMR data, but not in the optical rotation. The 15R-isomer of chaenomic acid was also synthesized, but the 1H and 13C NMR data did not agree with the natural product.

Rhodium-Catalyzed Allylic Addition as an Atom-Efficient Approach in Total Synthesis

ConspectusFinding efficient synthetic methods for the asymmetric synthesis of complex molecules has always been of interest to organic chemists. Creating and controlling the stereochemistry of stereogenic centers bearing branched allylic moieties in organic molecules using a catalytic process is an attractive and successful method for the synthesis of several natural products and medicinally important compounds. Remarkable progress toward their synthesis has been achieved via transition-metal catalysis, especially in the case of allylic substitution and allylic C-H oxidation chemistry. However, for allylic substitution the preinstallation of a leaving group is essential, and for allylic C-H oxidation, stoichiometric amounts of oxidant are required. Besides that, the control of regioselectivity with these methods is often problematic because the linear product can be produced as a major isomer. Our research group has developed a regioselective, enantioselective, and atom economic route toward the more valuable branched product via a Rh-catalyzed coupling of easily accessible alkynes or the double-bond isomeric allenes with pronucleophiles. It was demonstrated that, using this new approach, it is possible to add different pronucleophiles to alkynes or allenes to form branched allylic moieties through C-C and C-heteroatom bond formation. Since new organic reactions offer new opportunities in chemical synthesis and the benchmark for new synthetic methods is their application in target-oriented synthesis, we have demonstrated several successful syntheses of natural products and medicinally relevant targets. For example, in the total syntheses of Quercuslactones, Helicascolides A-C, Epothilone D, Homolargazole, and Thailandepsin B, the Rh-catalyzed hydro-oxycarbonylation of allenes was used as key step via C-O bond formation. Remarkably, the Rh-catalyzed C2-symmetric dimerization strategy was used to synthesize the complex molecules Clavosolide A and Vermiculine, leading to an extreme increase in structural complexity within a single step. For the total syntheses of Centrolobine, Pitavastatin, and Rosuvastatin, C-O bond formation was achieved through the addition of a hydroxy function to the allene moiety. The potential of the addition of nitrogen pronucleophiles to allenes was demonstrated in the total syntheses of Cusparein, Angusterein, Cermicin C, Senepodin G, Homoproline, Pipecolinol, Coniceine, Coniine, Ruxolitinib, Sitagliptin, Abacavir, Glucokinase activators, and Chaetominine. All of these examples testify to the wide applicability of the Rh-catalyzed addition of pronucleophiles to allenes or alkynes in target-oriented synthesis, and in this Account we summarize our contribution.

Catalytic Cyanation of C-N Bonds with CO2/NH3

Cyanation of benzylic C-N bonds is useful in the preparation of important α-aryl nitriles. The first general catalytic cyanation of α-(hetero)aryl amines, analogous to the Sandmeyer reaction of anilines, was developed using reductive cyanation with CO₂/NH₃. A broad array of α-aryl nitriles was obtained in high yields and regioselectivity by C-N cleavage of intermediates as ammonium salts. Good tolerance of functional groups such as ethers, CF₃, F, Cl, esters, indoles, and benzothiophenes was achieved. Using ¹³CO₂, a ¹³C-labeled tryptamine homologue (five steps, 31% yield) and Cysmethynil (six steps, 37% yield) were synthesized. Both electronic and steric effects of ligands influence the reactivity of alkyl nickel species with electrophilic silyl isocyanates and thus determine the reactivity and selectivity of the cyanation reaction. This work contributes to the understanding of the controllable activation of CO₂/NH₃ and provides the promising potential of the amine cyanation reaction in the synthesis of bio-relevant molecules.

Catalytic Asymmetric Hydroalkoxylation of C-C Multiple Bonds

Asymmetric hydroalkoxylation of alkenes constitutes a redox-neutral and 100% atom-economical strategy toward enantioenriched oxygenated building blocks from readily available starting materials. Despite their great potential, catalytic enantioselective additions of alcohols across a C-C multiple bond are particularly underdeveloped, especially compared to other hydrofunctionalization methods such as hydroamination. However, driven by some recent innovations, e.g., asymmetric MHAT methods, asymmetric photocatalytic methods, and the development of extremely strong chiral Brønsted acids, there has been a gratifying surge of reports in this burgeoning field. The goal of this review is to survey the growing landscape of asymmetric hydroalkoxylation by highlighting exciting new advances, deconstructing mechanistic underpinnings, and drawing insight from related asymmetric hydroacyloxylation and hydration. A deep appreciation of the underlying principles informs an understanding of the various selectivity parameters and activation modes in the realm of asymmetric alkene hydrofunctionalization while simultaneously evoking the outstanding challenges to the field moving forward. Overall, we aim to lay a foundation for cross-fertilization among various catalytic fields and spur further innovation in asymmetric hydroalkoxylations of C-C multiple bonds.

Enantioselective Palladium-Catalyzed Hydrophosphinylation of Allenes with Phosphine Oxides: Access to Chiral Allylic Phosphine Oxides

A Pd-catalyzed hydrophosphinylation of alkyl and aryl-oxyallenes with phosphine oxides has been developed for the efficient and rapid construction of a family of chiral allylic phosphine oxides with a diverse range of functional groups. This methodology was further applied in the facile construction of chiral 2H-chromene and later stage functionalization of cholesterol.

Macrolactonization Reactions Driven by a Pentafluorobenzoyl Group*

Macrolactones constitute a privileged class of natural and synthetic products with a broad range of applications in the fine chemicals and pharmaceutical industry. Despite all the progress made towards their synthesis, notably from seco-acids, a macrolactonization promoter system that is effective, selective, flexible, readily available, and, insofar as possible, compatible with manifold functional groups is still lacking. Herein, we describe a strategy that relies on the formation of a mixed anhydride incorporating a pentafluorophenyl group which, due to its high electronic activation enables a convenient access to macrolactones, macrodiolides and esters with a broad versatility. Kinetic studies and DFT computations were performed to rationalize the reactivity of the pentafluorophenyl group in macrolactonization reactions.

Macrodiolide Diversification Reveals Broad Immunosuppressive Activity That Impairs the cGAS-STING Pathway

The development of new immunomodulatory agents can impact various areas of medicine. In particular, compounds with the ability to modulate innate immunological pathways hold significant unexplored potential. Herein, we report a modular synthetic approach to the macrodiolide natural product (-)-vermiculine, an agent previously shown to possess diverse biological effects, including cytotoxic and immunosuppressive activity. The synthesis allows for a high degree of flexibility in modifying the macrocyclic framework, including the formation of all possible stereoisomers. In total, 18 analogues were prepared. Two analogues with minor structural modifications showed clearly enhanced cancer cell line selectivity and reduced toxicity. Moreover, these compounds possessed broad inhibitory activity against innate immunological pathways in human PBMCs, including the DNA-sensing cGAS-STING pathway. Initial mechanistic characterization suggests a surprising impairment of the STING-TBK1 interaction.

Cephalodiones A-D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition

Cephalodiones A-D (1-4), the first example of C₁₉ -norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo[6.4.1.1^2,7 ]tetradeca-3,5,9,11-tetraene-13,14-dione core that was capped in both ends with rigid multicyclic ring systems either C₂ -symmetrically or asymmetrically. Compounds 1-4 were proposed to be biosynthetically produced by the [6+6]-cycloaddition of two identical C₁₉ -norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2-4. Moreover, some compounds showed significant inhibition on Th17 cell differentiation.

Catalytic enantioselective synthesis of macrodiolides and their application in chiral recognition

New types of C₂-symmetric chiral macrodiolides are readily obtained via chiral N,N′-dioxide-scandium(iii) complex-promoted asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles. This protocol provides an array of enantioenriched macrodiolides with 16, 18 or 20-membered rings in moderate to good yields with high diastereoselectivities and excellent enantioselectivities through adjusting the length of the tether at the C3 position of indoles. Density functional theory calculations indicate that the formation of macrocycles is more favorable than that of 9-membered-ring lactones in terms of kinetics and thermodynamics. The potential utility of these intriguing chiral macrodiolide molecules is demonstrated in the enantiomeric recognition of aminols and chemical recognition of metal ions.

An asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles was achieved by using a chiral N,N′-dioxide-scandium(iii) complex.

Rhodium-Catalyzed Cyclization of Terminal and Internal Allenols: An Atom Economic and Highly Stereoselective Access Towards Tetrahydropyrans

A comprehensive study of a diastereoselective Rh-catalyzed cyclization of terminal and internal allenols is reported. The methodology allows the atom economic and highly syn-selective access to synthetically important 2,4-disubstituted and 2,4,6-trisubstituted tetrahydropyrans (THP). Furthermore, its utility and versatility are demonstrated by a great functional-group compatibility and the enantioselective total synthesis of (-)-centrolobine.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

Rhodium-Catalyzed Diastereo- and Enantioselective Tandem Spirocyclization/Reduction of 3-Allenylindoles: Access to Functionalized Vinylic Spiroindolines

A highly selective rhodium-catalyzed tandem spirocyclization/reduction of 3-allenylindoles is reported. By employing a Hantzsch ester as reductant, vinylic spiroindolines are obtained in excellent yields as well as diastereo- and enantioselectivity. In addition, the reaction's synthetic utility is highlighted by broad functional group compatibility and exemplified by a gram scale reaction with subsequent assorted transformations.

Rhodium-catalyzed asymmetric intramolecular hydroarylation of allenes: access to functionalized benzocycles

A rhodium-catalyzed regio- and enantioselective cyclization of tethered allenylbenzenes is reported. Employing a RhI/Josiphos catalytic system a diverse set of 6-membered, α-chiral benzocycles were obtained, scaffolds found in many bioactive compounds. Moreover, a gram scale reaction as well as the application of suitable transformations are demonstrated.

Rhodium-Catalyzed Enantioselective Cyclization of 3-Allenyl-indoles: Access to Functionalized Tetrahydrocarbazoles

A highly selective rhodium-catalyzed cyclization of tethered 3-allenylindoles is reported. In a smooth reaction, 1-vinyltetrahydrocarbazoles are obtained in excellent yields and enantioselectivities. Aside from a great functional group tolerance, this method requires neither the Schlenk technique nor the use of anhydrous solvents. Preliminary mechanistic investigations proved that the reaction proceeds via an intermediary formed spiroindolenine which rapidly undergoes an acid-catalyzed stereospecific migration.

A rhodium catalyzed cycloisomerization and tandem Diels-Alder reaction for facile access to diverse bicyclic and tricyclic heterocycles

A rhodium catalyzed cycloisomerization of 1,6-allenenes successfully prepared six-membered ring exocyclic 1,3-dienes. Further tandem Diels–Alder reaction with various dienophiles provided diverse bicyclic and tricyclic heterocycles. This methodology is characterised by complete atom and step economy, low catalyst loading, and excellent chemo-, regio-, and diastereoselectivity.

A regioselective distal cycloisomerization of 1,6-allenenes was successfully developed to afford six-membered ring exocyclic 1,3-dienes employing a rhodium/diphosphine catalyst system. Deuterium labelling experiments and DFT calculations were performed to provide insights into the reaction mechanism of this unprecedented transformation. In addition, one-pot tandem Diels–Alder reactions with various dienophiles could readily construct diverse bicyclic and tricyclic nitrogen heterocycles, which are ubiquitous core scaffolds for a variety of natural products and bioactives. High efficiency and exclusive chemo and regioselectivities for a broad substrate scope were achieved under mild conditions using a low catalyst loading of 0.5 mol%.