Enzyme- and Ruthenium-Catalyzed Dynamic Kinetic Asymmetric Transformation of 1,5-Diols. Application to the Synthesis of (+)-Solenopsin A

Construction of C-B axial chirality via dynamic kinetic asymmetric cross-coupling mediated by tetracoordinate boron

Catalytic dynamic kinetic asymmetric transformation (DyKAT) provides a powerful tool to access chiral stereoisomers from racemic substrates. Such transformation has been widely employed on the construction of central chirality, however, the application in axial chirality remains underexplored because its equilibrium of substrate enantiomers is limited to five-membered metalacyclic intermediate. Here we report a tetracoordinate boron-directed dynamic kinetic asymmetric cross-coupling of racemic, configurationally stable 3-bromo-2,1-azaborines with boronic acid derivatives. A series of challenging C-B axially chiral compounds were prepared with generally good to excellent enantioselectivities. Moreover, this transformation can also be extended to prepare atropisomers bearing adjacent C-B and C-C diaxes with excellent diastereo- and enantio-control. The key to the success relies on the rational design of a reversible tetracoordinate boron intermediate, which is supported by theoretical calculations that dramatically reduces the rotational barrier of the original C-B axis and achieves the goal of DyKAT.

Chemoenzymatic Dynamic Kinetic Asymmetric Transformations of β-Hydroxyketones

Herein we report on the development and application of chemoenzymatic Dynamic Kinetic Asymmetric Transformation (DYKAT) of α-substituted β-hydroxyketones (β-HKs), using Candida antartica lipase B (CALB) as transesterification catalyst and a ruthenium complex as epimerization catalyst. An operationally simple protocol allows for an efficient preparation of highly enantiomerically enriched α-substituted β-oxoacetates. The products were obtained in yields up to 95 % with good diastereomeric ratios.

Catalytic enantioselective OFF ↔ ON activation processes initiated by hydrogen transfer: concepts and challenges

Hydrogen transfer initiated processes are eco-compatible transformations allowing the reversible OFF ↔ ON activation of otherwise unreactive substrates. The minimization of stoichiometric waste as well as the unique activation modes provided by these transformations make them key players for a greener future for organic synthesis. Long limited to catalytic reactions that form racemic products, considerable progress on the development of strategies for controlling diastereo- and enantioselectivity has been made in the last decade. The aim of this review is to present the different strategies that enable enantioselective transformations of this type and to highlight how they can be used to construct key synthetic building blocks in fewer operations with less waste generation.

Chemoenzymatic dynamic kinetic resolution: a powerful tool for the preparation of enantiomerically pure alcohols and amines

Chemoenzymatic dynamic kinetic resolution (DKR) constitutes a convenient and efficient method to access enantiomerically pure alcohol and amine derivatives. This Perspective highlights the work carried out within this field during the past two decades and pinpoints important avenues for future research. First, the Perspective will summarize the more developed area of alcohol DKR, by delineating the way from the earliest proof-of-concept protocols to the current state-of-the-art systems that allows for the highly efficient and selective preparation of a wide range of enantiomerically pure alcohol derivatives. Thereafter, the Perspective will focus on the more challenging DKR of amines, by presenting the currently available homogeneous and heterogeneous methods and their respective limitations. In these two parts, significant attention will be dedicated to the design of efficient racemization methods as an important means of developing milder DKR protocols. In the final part of the Perspective, a brief overview of the research that has been devoted toward improving enzymes as biocatalysts is presented.

Concise Chemoenzymatic Three Step Total Synthesis of Isosolenopsin Through Medium Engineering

A short and efficient total synthesis of the alkaloid isosolenopsin and its enantiomer has been achieved. The key step was a ω-transaminase-catalysed regioselective monoamination of the diketone pentadecane-2,6-dione, which was obtained in a single step through the application of a Grignard reaction. Initial low conversions in the biotransformation could be overcome by optimisation of the reaction conditions employing suitable cosolvents. In the presence of 20 vol.-% N,N-dimethylformamide (DMF) or n-heptane the best results were obtained by employing two enantiocomplementary ω-transaminases originating from Arthrobacter at 30–40 °C; under these conditions, conversions of more than 99 % and perfect stereocontrol (ee > 99 %) were achieved. Diastereoselective chemical reduction (H₂/Pd/C) of the biocatalytic product gave the target compound. The linear three-step synthesis provided the natural product isosolenopsin in diastereomerically pure form (ee > 99 %, dr = 99:1) with an overall yield of 64 %.

Complementary lipase-mediated desymmetrization processes of 3-aryl-1,5-disubstituted fragments. Enantiopure synthetic valuable carboxylic acid derivatives

Desymmetrizaton enzymatic processes have been extensively studied searching for optimal methods of producing enantioenriched monoacetates from prochiral diols and diesters. AK lipase has been found as an excellent biocatalyst for the desymmetriaztion of a series of previously synthesized 3-arylpentane-1,5-diols derivatives. The access to (S)- or (R)-monoacetates in high optical purity (86-99% ee) has been possible by using acetylation or hydrolysis reactions, respectively, where the reaction parameters have been optimized in terms of source and amount of biocatalyst, temperature, solvent, and reaction time. The synthetic potential of enantiopure monoesters has been demonstrated by using these interesting chiral building blocks for the preparation of novel enantiopure carboxylic acid derivatives.