Liebeskind-Srogl Coupling Research Articles

Correlation of protein binding pocket properties with hits’ chemistries used in generation of ultra-large virtual libraries

Although the size of virtual libraries of synthesizable compounds is growing rapidly, we are still enumerating only tiny fractions of the drug-like chemical universe. Our capability to mine these newly generated libraries also lags their growth. That is why fragment-based approaches that utilize on-demand virtual combinatorial libraries are gaining popularity in drug discovery. These à la carte libraries utilize synthetic blocks found to be effective binders in parts of target protein pockets and a variety of reliable chemistries to connect them. There is, however, no data on the potential impact of the chemistries used for making on-demand libraries on the hit rates during virtual screening. There are also no rules to guide in the selection of these synthetic methods for production of custom libraries. We have used the SAVI (Synthetically Accessible Virtual Inventory) library, constructed using 53 reliable reaction types (transforms), to evaluate the impact of these chemistries on docking hit rates for 40 well-characterized protein pockets. The data shows that the virtual hit rates differ significantly for different chemistries with cross coupling reactions such as Sonogashira, Suzuki–Miyaura, Hiyama and Liebeskind–Srogl coupling producing the highest hit rates. Virtual hit rates appear to depend not only on the property of the formed chemical bond but also on the diversity of available building blocks and the scope of the reaction. The data identifies reactions that deserve wider use through increasing the number of corresponding building blocks and suggests the reactions that are more effective for pockets with certain physical and hydrogen bond-forming properties.

A Platform for the Liebeskind-Srogl Coupling of Heteroaromatic Thioethers for Medicinal-Chemistry-Relevant Transformations.

General and robust conditions for the Liebeskind-Srogl coupling were developed and used in functionalization of medicinal-chemistry-relevant heterocyclic substrates. Applicability in HTE and library synthesis, combined with its orthogonality to other cross-coupling reactions, make it highly attractive for discovery chemistry workflows. Additionally, the results suggest that the nature of the Cu(I)-carboxylate plays a more prominent role in the reaction performance than the nature of Pd-catalysts, which is rather uncommon for Pd-catalysis and can be used in further optimization of Liebeskind-Srogl coupling.

Total Syntheses of Scabrolide A and Yonarolide.

The concise total syntheses of oxidized norcembranoid terpenoids (-)-scabrolide A and (-)-yonarolide have been accomplished in 10 and 11 steps, respectively. The carbocyclic skeleton was efficiently constructed from two chiral-pool-derived fragments, including a [5,5]-bicyclic lactone accessed through a powerful Ni-catalyzed pentannulation of functionalized cyclopentenone with methylenecyclopropane and subsequent fragmentation. Additional features included a Liebeskind-Srogl coupling, induction of a cyclization/elimination cascade by a zinc-amido base, and installation of a sensitive enedione motif by late-stage γ-oxidation.

Synthesis of functionalized flavones from 3-halo-2-(methylthio)-4H-chromen-4-ones

A simple and efficient method for the synthesis of flavones was achieved by selective sulfide Liebeskind-Srogl cross-coupling reaction of 2-(methylthio)-4H-chromen-4-ones and 3-halo-2-(methylthio)-4H-chromen-4-ones with arylboronic acids. This method is the first example of employing Liebeskind-Srogl coupling for the synthesis of flavones. Various aryl groups can be directly installed to the chromone ring. The structure of all the synthesized compounds were assigned by spectroscopic analysis.

Synthesis of (non-classical) C-acyl-glycosides via Liebeskind–Srogl coupling: Scope, limitation, improved synthesis and antioxidant activity of scleropentaside A

We demonstrated that Liebeskind–Srogl coupling is well-suited for the synthesis of (nonclassical)- C -acyl glycosides. The reaction occurs at ambient temperature and is base-free, stereo-retentive and compatible with a broad range of (hetero)aryl boronic acids and carbohydrate-derived thioesters. The synthesis of challenging acylated (nonclassical) C -acyl glycosides is achieved, avoiding functionality compatibility problem and the elimination side reaction. We have improved the synthesis of scleropentaside A and obtained enough quantity of this natural product for the bio-activity study (DPPH assay), which shows that scleropentaside A exhibited moderate antioxidant activity, whereas its dehydrated analog showed no activity at all.

Catalytic asymmetric total syntheses of (R)-bgugaine and (R)-irnidine

An enantioselective total synthesis of ( R )-bgugaine and the first enantioselective total synthesis of ( R )-irnidine are reported. The key steps are the asymmetric ‘clip-cycle’ formation of the pyrrolidine ring in 94:6 e.r., which is common to both natural products, followed by Liebeskind–Srogl coupling and Wolf-Kishner reduction. The route yields ( R )-bgugaine and ( R )-irnidine in 6 steps and in overall yields of 33% and 18% respectively.

Installation of Minimal Tetrazines through Silver-Mediated Liebeskind-Srogl Coupling with Arylboronic Acids.

Described is a general method for the installation of a minimal 6-methyltetrazin-3-yl group via the first example of a Ag-mediated Liebeskind-Srogl cross-coupling. The attachment of bioorthogonal tetrazines on complex molecules typically relies on linkers that can negatively impact the physiochemical properties of conjugates. Cross-coupling with arylboronic acids and a new reagent, 3-((p-biphenyl-4-ylmethyl)thio)-6-methyltetrazine (b-Tz), proceeds under mild, PdCl2(dppf)-catalyzed conditions to introduce minimal, linker-free tetrazine functionality. Safety considerations guided our design of b-Tz which can be prepared on decagram scale without handling hydrazine and without forming volatile, high-nitrogen tetrazine byproducts. Replacing conventional Cu(I) salts used in Liebeskind-Srogl cross-coupling with a Ag2O mediator resulted in higher yields across a broad library of aryl and heteroaryl boronic acids and provides improved access to a fluorogenic tetrazine-BODIPY conjugate. A covalent probe for MAGL incorporating 6-methyltetrazinyl functionality was synthesized in high yield and labeled endogenous MAGL in live cells. This new Ag-mediated cross-coupling method using b-Tz is anticipated to find additional applications for directly introducing the tetrazine subunit to complex substrates.

Ni-Catalyzed Reductive Liebeskind-Srogl Alkylation of Heterocycles.

Herein we present a Ni-catalyzed alkylation of C–SMe with alkyl bromides for the decoration of heterocyclic frameworks. The protocol, reminiscent to the Liebeskind–Srogl coupling, makes use of simple C(sp2)–SMe to be engaged in a reductive coupling. The reaction is suitable for a preponderance of highly valuable heterocyclic motifs. In addition to cyclic bromides, noncyclic alkyl bromides are well accommodated with exquisite levels of retention over isomerization. The protocol is scalable and permits orthogonal couplings in the presence of other functionalization handles.

Biocompatible metal-assisted C–C cross-coupling combined with biocatalytic chiral reductions in a concurrent tandem cascade

In this study, we present a concurrent chemo/biocatalytic one pot reaction cascade by combining a metal (Pd/Cu) assisted Liebeskind-Srogl (L-S) coupling with an enantioselective enzymatic reduction for the production of chiral amines and alcohols. The latter transformation was realized by applying enantiocomplementary alcohol dehydrogenases from Lactobacillus kefir (R-selective) and Rhodococcus ruber (S-selective). Compatibility issues were solved by investigating first the L-S-coupling protocol in water at room temperature. Subsequently, we investigated two different biphasic systems and applied a biomimicking approach to separate enzyme-deactivating components. By using a lipophilic membrane in a smart reactor design, we were able to perform concurrent catalytic cascades with overall concentrations up to 100 mM substrate and to produce 1-phenylethylamine and several chiral alcohols in high yields (up to 81% over 2 steps) and enantiomeric purity ((+) and (-)-enantiomers both with 99% ee).

Chemoselective Synthesis of 4,5‐Diarylpyrrolo[2,3‐d]pyrimidines (6,7‐Diaryl‐7‐deazapurines) by Consecutive Suzuki and Liebeskind–Srogl Cross‐Couplings

AbstractA new chemoselective synthesis of pyrrolo[2,3‐d]pyrimidines (7‐deazapurines) bearing two different aryl groups at positions 4 and 5 was developed based on two orthogonal cross‐couplings. Starting from 7‐benzyl‐protected 4‐(phenylsulfanyl)‐5‐iodopyrrolo[2,3‐d]pyrimidine, the Pd‐catalyzed Suzuki coupling with arylboronic acids proceeded selectively at position 5, followed by the Pd‐catalyzed Cu‐mediated Liebeskind–Srogl coupling at position 4. A small library of 4,5‐diaryl derivatives was prepared.

Synthesis of triterpenoid triazine derivatives from allobetulone and betulonic acid with biological activities

The synthetic transformation and modification of natural products with the aim to improve the biological properties is an area of current interest. The triterpenoids betulin and betulinic acid are very abundant in nature and now are commercially available. In our study, starting from betulin and betulinic acid, we obtained allobetulone and betulonic acid in a few synthetic steps. The ketone function at the A-ring was used as the starting point for the synthesis of a series of 1,2,4-triazine-fused triterpenoids. The alkylation and Liebeskind–Srogl coupling were used for further substitution of 1,2,4-triazines, and the intramolecular hetero Diels–Alder reaction leads to interesting fused thienopyridine derivatives. All new compounds were tested for their cytostatic activities against murine leukemia L1210, human cervix carcinoma HeLa and human lymphoblast CEM tumor cells. The results show that some triterpenoid triazine betulonic acid derivatives have a promising cytostatic activity in vitro and could be used as potential leads for the development of new type of anti-cancer agents. Several compounds were also endowed with anti-HCMV activity in the low micromolar range.

Synthesis of Soai Aldehydes for Asymmetric Autocatalysis by Desulfurative Cross-Coupling

Palladium-catalyzed dehydrosulfurative Liebeskind-Srogl coupling of terminal alkynes with 2-mercapto-1,3-pyrimidine-5-carbaldehyde under base-free conditions provides 2-(alkynyl)-1,3-pyrimidine-5-carbaldehydes, which are substrates for autocatalytic amplification of chirality according to Soai et al. The mercapto aldehyde acceptor is obtained by condensation of Arnold's vinamidinium salt with thiourea.

Copper-catalyzed direct thiolation of xanthines and related heterocycles with disulfides

A novel copper-catalyzed, base-free direct thiolation of xanthines and related heterocycles is described, featuring the use of inexpensive Cu(OAc)2·H2O as the catalyst, O2 as a clean and cheap oxidant, and easy-to-handle disulfides as the thiolation reagents. It works well for both aryl and alkyl disulfides. Moreover, the resultant products can be converted into 8-(hetero)aryl- or alkenyl-substituted xanthines in good yields via the Liebeskind–Srogl coupling reaction.

ChemInform Abstract: Optimized Liebeskind—Srogl Coupling Reaction Between Dihydropyrimidines and Tributyltin Compounds.

AbstractDihydropyrimidothione substrates are successfully coupled with tributyltins to produce potentially biologically active dihydropyrimidines.

Optimized Liebeskind–Srogl coupling reaction between dihydropyrimidines and tributyltin compounds

We developed an optimized Liebeskind–Srogl reaction in order to synthesize potentially biologically active 2-aryl-1,4-dihydropyrimidines. The pallado-catalyzed cross-coupling reaction between dihydropyrimidines and tributyltin derivatives appears particularly efficient (67–95% yields) when using CuBr·Me2S as the copper cofactor.

Synthesis of medium-sized carbocyclic ketones via the intramolecular B-alkyl Liebeskind–Srogl coupling reaction

Synthesis of medium-sized carbocyclic ketones via the intramolecular B-alkyl Liebeskind–Srogl coupling reaction is described. The sequence of hydroboration of ω-alkenyl thiol ester with 9-BBN and the Liebeskind–Srogl reaction results in the formation of medium-sized carbocyclic ketones with good yield.