Dynamic kinetic resolution of hemiaminals with axially chiral twisted amides

Synthesis of Tetra-Substituted 3-Hydroxyphthalide Esters by Isothiourea-Catalysed Acylative Dynamic Kinetic Resolution

A general and highly enantioselective method for the preparation of tetra-substituted 3-hydroxyphthalide esters via isothiourea-catalysed acylative dynamic kinetic resolution (DKR) is reported. Using (2S,3R)-HyperBTM (5 mol %) as the catalyst, the scope and limitations of this methodology have been extensively probed, with high enantioselectivity and good to excellent yields observed (>40 examples, up to 99 %, 99 : 1 er). Substitution of the aromatic core within the 3-hydroxyphthalide skeleton, as well as aliphatic and aromatic substitution at C(3), is readily tolerated. A diverse range of anhydrides, including those from bioactive and pharmaceutically relevant acids, can also be used. The high enantioselectivity observed in this DKR process has been probed computationally, with a key substrate heteroatom donor O⋅⋅⋅acyl-isothiouronium interaction identified through DFT analysis as necessary for enantiodiscrimination.

Dynamic Covalent Kinetic Resolution

Implemented with the highly efficient concept of Dynamic Kinetic Resolution (DKR), dynamic covalent chemistry can be a useful strategy for the synthesis of enantioenriched compounds. This gives rise to dynamic covalent kinetic resolution (DCKR), a subset of DKR that over the last decades has emerged as increasingly fruitful, with many applications in asymmetric synthesis and catalysis. All DKR protocols are composed of two important parts: substrate racemization and asymmetric transformation, which can lead to yields of >50% with good enantiomeric excesses (ee) of the products. In DCKR systems, by utilizing reversible covalent reactions as the racemization strategy, the substrate enantiomers can be easily interconverted without the presence of any racemase or transition metal catalyst. Enzymes or other chiral catalysts can then be adopted for the resolution step, leading to products with high enantiopurities. This tutorial review focuses on the development of DCKR systems, based on different reversible reactions, and their applications in asymmetric synthesis.

Radical–radical cross coupling reactions of photo-excited fluorenones

Radical–radical cross coupling reactions of photoexcited 9-fluorenones have been accomplished for the first time, leading to the synthesis of 9-alkyl, pyrollidinyl and spiro-THF derivatives of 9-fluorenones.

Efficiency in nonenzymatic kinetic resolution

The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.