Rh(I)-Catalyzed Regio- and Enantioselective Ring Opening of Vinyl Cyclopropanes.

We describe a Rh(I) catalyzed asymmetric ring opening of racemic vinyl cyclopropanes using aryl boronic acids as C-nucleophiles. When ferrocene-based chiral bisphosphines are used as ligands, the products are obtained with regioselectivities typically 99:1 r.r. and ee's generally between 88 and 96%. A wide range of aryl boronic acids can be used, and the products can be converted into a variety of targets. Preliminary mechanistic studies indicate that Zn(OTf)2 plays a significant role in the reaction by promoting rhodium-ligand complex formation and accelerating the reaction. We expect this method and these mechanistic insights to be useful in the development of new asymmetric methods.

Rhodium-Catalyzed Asymmetric Arylation of Cyclobutenone Ketals.

Complex cyclobutanes are important motifs in both bioactive molecules and natural products, yet their enantioselective preparation has not been widely explored. In this work, we describe rhodium-catalyzed enantioselective additions of aryl and vinyl boronic acids to cyclobutenone ketals. This transformation involves enantioselective carbometalation to give cyclobutyl-rhodium intermediates, followed by ß-oxygen elimination to afford enantioenriched enol ethers. Overall, this addition serves as a surrogate for Rh-catalyzed 1,4-additions to cyclobutenone.

Iridium-Catalyzed Enantioselective Allylic Substitution with Aqueous Solutions of Nucleophiles.

The iridium-catalyzed asymmetric allylic substitution under biphasic conditions is reported. This approach allows the use of various unstable and/or volatile nucleophiles including hydrazines, methylamine, t-butyl hydroperoxide, N-hydroxylamine, α-chloroacetaldehyde and glutaraldehyde. This transformation provides rapid access to a broad range of products from simple starting materials in good yields and up to >99% ee and 20:1 d.r. Additionally, these products can be elaborated efficiently into a diverse set of cyclic and acyclic compounds, bearing up to four stereocenters.

Enantio- and Diastereoselective Suzuki-Miyaura Coupling with Racemic Bicycles.

Herein, we describe a rhodium-catalyzed enantio- and diastereoselective Suzuki-Miyaura cross-coupling between racemic fused bicyclic allylic chlorides and boronic acids. The highly stereoselective transformation allows for the coupling of aryl, heteroaryl, and alkenyl boronic acids and gives access to functionalized bicyclic cyclopentenes, which can be converted into other five-membered-ring scaffolds with up to five contiguous stereocenters. Preliminary mechanistic studies suggest that these reactions occur with overall retention of the relative stereochemistry and are enantioconvergent for pseudo-symmetric allylic chloride starting materials. In addition, a bicyclic allylic chloride starting material without pseudo-symmetry undergoes a highly enantioselective regiodivergent reaction.

Correction: Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes.

[This corrects the article DOI: 10.1039/D1SC06035J.].

Asymmetric Synthesis of Nortropanes via Rh-Catalyzed Allylic Arylation.

Tropane derivatives are extensively used in medicine, but catalytic asymmetric methods for their synthesis are underexplored. Here, we report Rh-catalyzed asymmetric Suzuki-Miyaura-type cross-coupling reactions between a racemic N-Boc-nortropane-derived allylic chloride and (hetero)aryl boronic esters. The reaction proceeds via an unexpected kinetic resolution, and the resolved enantiopure allyl chloride can undergo highly enantiospecific reactions with N-, O-, and S-containing nucleophiles. The method was applied in a highly stereoselective formal synthesis of YZJ-1139(1), a potential insomnia treatment that recently completed Phase II clinical trials. Our report represents an asymmetric catalytic method for the synthesis of YZJ-1139(1) and related compounds.

Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes.

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known. Herein we report the Rh-catalyzed asymmetric hydrometallation of a range of meso-cyclobutenes with salicylaldehydes. The ortho-phenolic group promotes hydroacylation and can be used as a handle for subsequent transformations. The reaction proceeds via asymmetric hydrometallation of the weakly activated cyclobutene, followed by a C-C bond forming reductive elimination. A prochiral, spirocyclic cyclobutene undergoes a highly regioselective hydroacylation. This report will likely inspire the development of other asymmetric addition reactions to cyclobutenes via hydrometallation pathways.

A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes.

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.

An Asymmetric Suzuki-Miyaura Approach to Prostaglandins: Synthesis of Tafluprost.

We report the catalytic asymmetric synthesis of Tafluprost (1), a prostaglandin analogue. This synthesis demonstrates a new approach to prostaglandins involving symmetrization and desymmetrization of a racemic precursor to control the absolute and relative stereochemistry of the cyclopentyl core. Key steps include a diastereo- and enantioselective Rh-catalyzed Suzuki-Miyaura reaction of a racemic bicyclic allyl chloride and an alkenyl boronic acid and a regio- and diastereoselective Pd-catalyzed Tsuji-Trost reaction with an enolate surrogate.

Fragment Screening Hit Draws Attention to a Novel Transient Pocket Adjacent to the Recognition Site of the tRNA-Modifying Enzyme TGT.

Fragment-based lead discovery was applied to tRNA-guanine transglycosylase, an enzyme modifying post-transcriptionally tRNAs in Shigella, the causative agent of shigellosis. TGT inhibition prevents translation of Shigella's virulence factor VirF, hence reducing pathogenicity. One discovered fragment opens a transient subpocket in the preQ1-recognition site by pushing back an aspartate residue. This step is associated with reorganization of further amino acids structurally transforming a loop adjacent to the recognition site by duplicating the volume of the preQ1-recognition pocket. We synthesized 6-carboxamido-, 6-hydrazido-, and 4-guanidino-benzimidazoles to target the opened pocket, including a dihydro-imidazoquinazoline with a propyn-1-yl exit vector pointing into the transient pocket and displacing a conserved water network. MD simulations and hydration-site analysis suggest water displacement to contribute favorably to ligand binding. A cysteine residue, exclusively present in bacterial TGTs, serves as gatekeeper of the transient subpocket. It becomes accessible upon pocket opening for selective covalent attachment of electrophilic ligands in eubacterial TGTs.

Highly enantioselective rhodium-catalyzed cross-coupling of boronic acids and racemic allyl halides.

Although Csp2-Csp2 Suzuki-Miyaura couplings (SMCs) are widely used in small-molecule synthesis, related methods that allow the incorporation of Csp3-hybridized coupling partners, particularly in an asymmetric manner, are less developed. This protocol describes catalytic asymmetric SMC reactions that provide access to enantiomerically enriched cyclic allylic products. The method couples racemic allyl halide starting materials with sp2-hybridized boronic acid derivatives and is compatible with heterocyclic coupling partners. These reactions are catalyzed by a rhodium-ligand complex and typically display very high levels of enantioselectivity (>95% enantiomeric excess (ee)). In this protocol, we detail a procedure using a dihydropyridine-derived allyl chloride for the synthesis of (-)-(S)-tert-butyl-3-(4-bromophenyl)-3,6-dihydropyridine-1(2H)-carboxylate, an intermediate in the synthesis of the anticancer drug niraparib. This procedure affords 1.17 g (86% yield) of the coupling product with 96% ee. The initial experimental setup of the reaction takes 45-50 min, and the reaction is complete within 4-5 h.