Synthesis of chiral γ-lactones via a RuPHOX-Ru catalyzed asymmetric hydrogenation of aroylacrylic acids

Configurationally labile α-bromoacid derivatives for asymmetric preparation of heterocycles

α-Bromoacid derivatives are configurationally labile under various conditions, and the dynamic resolution of them has been recognized as an effective strategy in asymmetric synthesis. This article is a concise review of our efforts on the heteroannulation of α-bromoacid derivatives through nucleophilic substitution and subsequent ring formation with diverse carbon, nitrogen, oxygen, and sulfur nucleophiles. Chiral auxiliary (Xc)-bound α-bromoacid derivatives serve as versatile chiral building blocks for the direct incorporation of a two-carbon unit in cyclization reactions. Eight readily available chiral auxiliaries are investigated and identified to be suitable for the dynamic resolution of α-bromoacid derivatives. We have presented selected results about three distinct dynamic resolutions such as dynamic kinetic resolution (DKR), dynamic thermodynamic resolution (DTR) and crystallization induced dynamic resolution (CIDR) that have been successfully employed in the asymmetric synthesis of fourteen different scaffolds of six- or five-membered heterocycles.

Simultaneously Enhanced Catalytic Activity and Thermostability of a Baeyer-Villiger Monooxygenase from Oceanicola granulosus by Reshaping the Binding Pocket

Enzymatic oxygenation of various cyclic ketones into lactones via Baeyer-Villiger monooxygenases (BVMOs) could provide a promising route for synthesizing fragrances and pharmaceutical ingredients. However, unsatisfactory catalytic activity and thermostability restricted their applications in the pharmaceutical and food industries. In this study, we successfully improved the catalytic activity and thermostability of a Baeyer-Villiger monooxygenase (OgBVMO) from Oceanicola granulosus by reshaping the binding pocket. As a result, mutant OgBVMO-Re displayed a 1.0- to 6.4-fold increase in the activity toward branched cyclic ketones tested, accompanied by a 3 °C higher melting point, and a 2-fold longer half-life time (t1/2 (45 °C)). Molecular dynamics simulations revealed that reshaping the binding pocket achieved strengthened motion correlation between amino acid residues, appropriate size of the substrate-binding pocket, beneficial surface characteristics, lower energy barriers, and shorter nucleophilic distance. This study well demonstrated the trade-off between the enzyme activity and thermostability by reshaping the substrate-binding pocket, paving the way for further engineering other enzymes.

RuPHOX-Ru Catalyzed Asymmetric Cascade Hydrogenation of 3-Substituted Chromones for the Synthesis of Corresponding Chiral Chromanols

An efficient RuPHOX-Ru catalyzed asymmetric cascade hydrogenation of 3-substituted chromones has been achieved under mild reaction conditions, affording the corresponding chiral 3-substituted chromanols in high yields with excellent enantio- and diastereoselectivities (up to 99 % yield, >99 % ee and >20 : 1 dr). Control reactions and deuterium labelling experiments revealed that a dynamic kinetic resolution process occurs during the subsequent hydrogenation of the C=O double bond, which is responsible for the high performance of the asymmetric cascade hydrogenation. The resulting products allow for several transformations and it was shown that the protocol provides a practical and alternative strategy for the synthesis of chiral 3-substituted chromanols and their derivatives.

Bimetallic Ru/Ru-Catalyzed Asymmetric One-Pot Sequential Hydrogenations for the Stereodivergent Synthesis of Chiral Lactones

Asymmetric sequential hydrogenations of α-methylene γ- or δ-keto carboxylic acids are established in one-pot using a bimetallic Ru/Ru catalyst system, achieving the stereodivergent synthesis of all four stereoisomers of both chiral γ- and δ-lactones with two non-vicinal carbon stereocenters in high yields (up to 99%) and with excellent stereoselectivities (up to >99% ee and >20:1 dr). The compatibility of the two chiral Ru catalyst systems is investigated in detail, and it is found that the basicity of the reaction system plays a key role in the sequential hydrogenation processes. The protocol can be performed on a gram-scale with a low catalyst loading (up to 11000 S/C) and the resulting products allow for many transformations, particularly for the synthesis of several key intermediates useful for the preparation of chiral drugs and natural products.

Copper-Catalyzed Radical Enantioselective Carbo-Esterification of Styrenes Enabled by a Perfluoroalkylated-PyBox Ligand

Chiral lactones are found in many natural products. The reaction of simple alkenes with iodoacetic acid is a powerful method to build lactones, but the enantioselective version of this reaction has not been implemented to date. Herein, we report the efficient catalytic radical enantioselective carbo-esterification of styrenes enabled by a newly developed Cu^I -perfluoroalkylated PyBox system. Simple styrenes have been converted to useful chiral lactones, whose synthetic applications are showcased. Mechanistic studies reveal that this reaction is a rare example of an efficient ligand-decelerated system, in which the ligand decelerates the reaction, but the reaction is still efficient with reduced amounts of ligand. This uncommon catalytic system may inspire further consideration of the effect of ligands in asymmetric catalysis.

Nickel-Catalyzed Asymmetric Hydrogenation of γ-Keto Acids, Esters, and Amides to Chiral γ-Lactones and γ-Hydroxy Acid Derivatives

A highly efficient asymmetric hydrogenation of a series of γ-keto acid derivatives, including γ-keto acids, esters, and amides, using a Ni-(R,R)-QuinoxP* complex as the catalyst has been developed to afford chiral γ-hydroxy acid derivatives with excellent enantioselectivities, up to 99.9% ee. This method provides not only an economical one-pot approach for the synthesis of chiral γ-lactones but also access to (S)-norfluoxetine, an inhibitor of neural serotonin reuptake and an essential intermediate for pharmaceutical synthesis.

Kinetic resolution of azaflavanones via a RuPHOX-Ru catalyzed asymmetric hydrogenation

The kinetic resolution of azaflavanones has been established via RuPHOX-Ru catalyzed asymmetric hydrogenation, providing chiral azaflavanones and azaflavanols in high yields with up to >20 : 1 dr and 99.7% ee.