Recent Advances in Dynamic Kinetic Resolution by Chiral Bifunctional (Thio)urea- and Squaramide-Based Organocatalysts

Catalytic enantioselective intramolecular hydroamination of alkenes using chiral aprotic cyclic urea ligand on manganese (II)

Asymmetric catalysis for enantioselective intramolecular hydroamination of alkenes is a critical method in the construction of enantioenriched nitrogen-containing rings, often prevalent in biologically active compounds and natural products. Herein, we demonstrate a facile enantioselective intramolecular hydroamination of alkenes for the synthesis of chiral pyrrolidine, piperidine, and indoline moieties, using a manganese (II) chiral aprotic cyclic urea catalyst. The cyclic ligand hinders the inversion of the N atom of the urea and effectively discriminate between the enantiomers of substrates. High-resolution mass spectrometry, deuterium labeling experiments, and molecular orbital energy analysis clearly reveal the intermediates and mechanism of the transformation. As a key step, oxygen coordination by chiral aprotic urea presents a robust control over the asymmetric intra-HA reaction through the involvement of a convergent assembly of two vital intermediates (Mn-N and C-Mn-Br), providing access to chiral cyclic amine systems in high yields with excellent enantioselectivity.

Assessing Squarates as Amine-Reactive Probes

Probes that covalently label protein targets facilitate the identification of ligand-binding sites. Lysine residues are prevalent in the proteome, making them attractive substrates for covalent probes. However, identifying electrophiles that undergo amine-specific, regioselective reactions with binding site lysine residues is challenging. Squarates can engage in two sequential conjugate addition-elimination reactions with amines. Nitrogen donation reduces the second reaction rate, making the mono squaramide a mild electrophile. We postulated that this mild electrophilicity would demand a longer residence time near the amine, affording higher selectivity for binding site lysines. Therefore, we compared the kinetics of squarate and monosquaramide amine substitution to alternative amine bioconjugation handles. The data revealed that N-hydroxy succinimidyl esters react 4 orders of magnitude faster, consistent with their labeling promiscuity. Squarate reactivity can be tuned by a substitution pattern. Electron-withdrawing groups on the vinylogous ester or amide increase reaction rates. Dithionosquarates react more rapidly than squarates, while vinylogous thioester analogs, dithiosquarates, react more slowly. We assessed squarate selectively using the UDP-sugar processing enzyme GlfT2 from Mycobacterium tuberculosis, which possesses 21 surface-exposed lysines. The reaction predominately modified one lysine proximal to a binding site to afford covalent inhibition. These findings demonstrate the selectivity of squaric esters and squaramides, which is a critical feature for affinity-based chemoproteomic probes.

Merging kinetic resolution with C-H activation: an efficient approach for enantioselective synthesis

In the last two decades tremendous progress has been made in transition-metal (TM)-catalyzed C-H bond functionalization, paving the way to design complex molecules. Despite significant advances, enantioselective C-H activation is still in the age of infancy. For the enantioselective synthesis, several TM catalyst based approaches are well known, including kinetic resolution (KR) and its advanced versions [dynamic kinetic resolution (DKR) and parallel kinetic resolution (PKR)]. These strategies have recently been successfully applied synergetically with the TM catalyzed C-H activation to achieve enantioselective synthesis in a more economical and sustainable way. This review will summarize the recent advancements made towards merging KR with TM-catalysed C-H activation for enantioselective synthesis.

A novel chiral DMAP–thiourea bifunctional catalyst catalyzed enantioselective Steglich and Black rearrangement reactions

A novel chiral DMAP–thiourea bifunctional catalyst has been prepared and applied in the highly enantioselective Steglich and Black rearrangement reactions.

Stereoconvergent 1,4-Addition Reaction of 5H-Oxazol-4-ones with E,Z Isomeric Mixture of Alkylidene β-Ketoesters Catalyzed by Chiral Guanidines

A 1,4-addition reaction of 5H-oxazol-4-ones to alkylidene β-ketoesters, which was catalyzed by using chiral guanidines via a dynamic kinetic resolution process that involved geometric isomerization of the alkylidene β-ketoesters, was developed. This method allowed using E,Z isomeric mixture of various acyclic alkylidene β-ketoesters to obtain the products stereoselectively. The relation between the geometry and the stereoselectivity of products was investigated, and the derivatization of the products to the corresponding α-acyl-γ-lactone was performed.