Base-Free Dynamic Kinetic Resolution of Secondary Alcohols with a Ruthenium-Lipase Couple

Bienzymatic Dynamic Kinetic Resolution of Secondary Alcohols by Esterification/Racemization in Water

Dynamic kinetic resolution (DKR) is a key method used to prepare optically pure compounds in 100 % theoretical yield starting from racemic substrates by combining the interconversion of substrate enantiomers with an enantioselective transformation. Various chemoenzymatic DKR approaches have been developed to deracemize secondary alcohols, typically requiring an organic solvent to facilitate enantioselective acylation, primarily catalyzed by lipases, alongside racemization mediated by an achiral, non-enzymatic catalyst. Achieving both steps in an aqueous solution remained elusive. Herein, we report a DKR of racemic sec-alcohols in an aqueous solution requiring only two biocatalysts. The first key to success was to achieve fast racemization in a buffer employing a non-stereoselective variant of an alcohol dehydrogenase (Lk-ADH-Prince) via a hydrogen-borrowing oxidation-reduction sequence. Engineered variants of the acyltransferase from Mycobacterium smegmatis (MsAcT) enabled enantioselective acyl transfer in water. Besides the appropriate choice of the enzymes, identifying a suitable acyl donor was a second key to the success. The DKR was successfully demonstrated using (R)-selective MsAcT variants for a broad range of racemic (hetero)benzylic alcohols with 2,2,2-trifluoroethyl acetate as the acyl donor, yielding (R)-acetates with up to >99 % conv. and high-to-excellent optical purity (83-99.9 % ee). The (S)-acetates were accessible using a stereocomplementary (S)-selective MsAcT variant. Notably, substrate concentrations of up to 400 mM were tolerated in selected cases.

Chemoenzymatic Dynamic Kinetic Resolution Protocol with an Immobilized Oxovanadium as a Racemization Catalyst

An excellent compatible and cost-effective dynamic kinetic resolution (DKR) protocol has been developed by combining a novel immobilized oxovanadium racemization catalyst onto cheap diatomite (V-D) with an immobilized lipase LA resolution catalyst onto a macroporous resin (LA-MR). V-D was prepared via grinding immobilization, which may become a promising alternative for the immobilization of metals, especially precious metals due to its low cost, high efficiency, easy separation, and large reaction interface. The DKR afforded high yield (96.1%), e.e. (98.67%), and Sel (98.28%) under optimal conditions established using response surface methodology as follows: the amount of V-D 10.83 mg, reaction time 51.2 h, and temperature 48.1 °C, respectively, indicating that all the reactions in the DKR were coordinated very well. The DKR protocol was also found to have high stability up to six reuses. V-D exhibited excellent compatibility with LA-MR because the lipase immobilized onto MR did not physically contact with the vanadium species immobilized onto diatomite, thus avoiding inactivation. Considering that lipase, oxovanadium, diatomite, and MR used are relatively inexpensive, and the adsorption or grinding immobilization is simple, the LA-V-MD DKR by coupling LA-MR with V-D is a cost-effective and promising protocol for chiral secondary alcohols.

syn ‐Selective Epoxidation of Chiral Terminal Allylic Alcohols with a Titanium Salalen Catalyst and Hydrogen Peroxide

In the Sharpless asymmetric epoxidation of chiral secondary allylic alcohols, one substrate enantiomer is predominantly converted to the anti‐epoxy alcohol. We herein report the first highly syn‐selective epoxidation of terminal allylic alcohols using a titanium salalen complex as catalyst, at room temperature, and aqueous hydrogen peroxide as oxidant. With enantiopure terminal allylic alcohols as substrates, the epoxy alcohols were obtained with up to 98 % yield and up to >99 : 1 dr (syn). Catalyst loadings as low as 1 mol % can be applied without eroding the syn‐diastereoselectivity. Modification of the allylic alcohol to an ether does not affect the diastereoselectivity either [>99 : 1 dr (syn)]. Inverting the catalyst configuration leads to the anti‐product, albeit at lower dr (ca. 20 : 1). The synthetic potential is demonstrated by a short, gram‐scale preparation of a tetrahydrofuran building block with three stereocenters, involving two titanium salalen catalyzed epoxidation steps.

Progress on the Stereoselective Synthesis of Chiral Molecules Based on Metal-Catalyzed Dynamic Kinetic Resolution of Alcohols with Lipases

Metal/lipase-combo catalyzed dynamic kinetic resolution (DKR) of racemic chiral alcohols is a general and practical process to obtain the corresponding enantiopure esters R with quantitative conversion. The use of known Ru-catalysts as well as newly developed homogeneous and heterogeneous metal catalysts (Fe, V) contributed to make the DKR process more sustainable and to expand the substrate scope of the reaction. In addition to classical substrates, challenging allylic alcohols, tertiary alcohols, C1-and C2-symmetric biaryl diols turned out to be competent substrates. Synthetic utility further emerged from the integration of this methodology into cascade reactions leading to linear/cyclic chiral molecules with high ee through the formation of multiple bonds, in a one-pot procedure.

Acylative Dynamic Kinetic Resolution of Secondary Alcohols: Tandem Catalysis by HyperBTM and Bäckvall's Ruthenium Complex

Non-enzymatic dynamic kinetic resolution (DKR) of secondary alcohols by enantioselective acylation using an isothiourea-derived HyperBTM catalyst and racemization of slowly reacting alcohol by Bäckvall's ruthenium complex is reported. The DKR approach features high enantioselectivities (up to 99:1), employs easy-to-handle crystalline 4-nitrophenyl isobutyrate as the acylating reagent, and proceeds at room temperature and under an ambient atmosphere. The stereoinduction model featuring cation-π system interactions between the acylated HyperBTM catalyst and π electrons of an alcohol aryl subunit has been elaborated by DFT calculations.