JoyaPhos: An Atropisomeric Teraryl Monophosphine Ligand

Diaryl-Pyrano-Chromenes Atropisomers: Stereodynamics and Conformational Studies

The dynamic scenario of di-aryls-pyrano-chromenes was investigated using DFT calculations. The symmetry of the chromene scaffold and the presence of two ortho-substituted aryls substituents can generate two syn/anti diastereoisomers and conformational enantiomers with different rotational barriers. The relative conformations and configurations were derived using NOESY-1D experiments. Depending on the energies related to the conformational exchange, the experimental energy barriers were determined through Dynamic NMR, Dynamic HPLC or kinetic studies. The atropisomeric pairs were resolved in the latter scenario, and their absolute configuration was assigned using the ECD/TD-DFT method.

The Asymmetric Buchwald-Hartwig Amination Reaction

Over the past few decades, the Buchwald-Hartwig reaction has emerged as a powerful tool for forging C-N bonds, and has been vital to the pharmaceuticals, materials, and catalysis fields. However, asymmetric Buchwald-Hartwig amination reactions for constructing centered chirality, planar chirality, and axial chirality remain in their infancy owing to limited substrate scope and laggard ligand design. The recent surge in interest in the synthesis of C-N/N-N atropisomers, has witnessed a renaissance in asymmetric Buchwald-Hartwig amination chemistry as the first practical protocol for the preparation of C-N atropisomers. This review highlights reported asymmetric Buchwald-Hartwig amination protocols and provides a brief overview of their chemical practicality.

Enantioselective Synthesis of Axially Chiral Biaryls by Diels-Alder/Retro-Diels-Alder Reaction of 2-Pyrones with Alkynes

The enantioselective synthesis of axially chiral biaryls by a copper-catalyzed Diels-Alder/retro-Diels-Alder reaction of 2-pyrones with alkynes is reported herein. Using electron-deficient 2-pyrones and electron-rich 1-naphthyl acetylenes as the reaction partners, a broad range of axially chiral biaryl esters are obtained in excellent yields (up to 97% yield) and enantioselectivities (up to >99% ee). DFT calculations reveal the reaction mechanism and provide insights into the origins of the stereoselectivities. The practicality and robustness of this reaction are showcased by gram-scale synthesis. The synthetic utilizations are demonstrated by the amenable transformations of the products.

Catalyst Control over Twofold and Higher-Order Stereogenicity by Atroposelective Arene Formation

Contradictory to the first intuitive impression that forging putatively flat aromatic rings evades stereoisomerism, a striking variety of atropisomeric compounds are conceivable by the formation of arenes, offering captivating avenues for catalyst-controlled stereoselective strategies. Since the assembled atropisomeric products that contain one or several rotationally restricted single bonds are characterized by especially well defined molecular architectures, they are distinctly suitable for numerous pertinent applications. In view of the fascinating arene-forming aldol condensation pathways taking place in polyketide biosynthesis (cyclases/aromatases (CYC/ARO)), the versatile small-molecule-catalyzed aldol reaction appeared as an exceptionally appealing synthetic means to prepare various unexplored atropisomeric compounds in our efforts presented herein. In our initial studies, the use of secondary amine organocatalysts provided excellent selectivities in stereoselective arene-forming aldol condensations for a broad range of atropisomeric products, such as biaryls and rotationally restricted aromatic amides. In further analogy to polyketide biosynthesis, it was also conceivable that several aromatic rings are formed in catalytic cascade reactions. The use of small-molecule catalysts thereby enabled us to transfer this concept to the conversion of unnatural and noncanonical polyketide substrates, thus giving access to atropisomers with particular value for synthetic applications. The versatility of the stereoselective aldol reactions with numerous catalytic activation modes further provided a strategy to individually control several stereogenic axes, similar to the various methodologies developed for controlling stereocenter configurations. By the use of iterative building block additions combined with catalyst-controlled aldol reactions to form the aromatic rings, stereodivergent pathways for catalyst-substrate-matched and -mismatched products were obtained. Besides secondary amines, cinchona-alkaloid-based quaternary ammonium salts also proved to be highly efficient in overcoming severe substrate bias. The obtained atropisomeric multiaxis systems, with all of the biaryl bonds suitably restricted in rotation even at high temperatures, are spatially distinctly defined. The helical secondary structure is therefore excellently suited for several captivating applications.While previous catalyst-controlled stereoselective methods distinguish two stereoisomers for each stereogenic unit, catalyst control beyond the realms of this dualistic stereoisomerism remained unexplored. By the selective preparation of O̅ki atropisomers characterized by their sixfold stereogenicity in Rh-catalyzed [2 + 2 + 2] cyclotrimerizations, one out of the six possible stereoisomers resulting from the restricted rotation of a single bond was shown to be catalytically addressable. Catalyst control over higher-order stereogenicity therefore further interconnects conformational analysis and stereoselective catalysis and offers captivating avenues to explore uncharted stereochemical space for creating a broad range of unprecedented molecular motifs.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.