LongLarf

Abstract

A new approach for the oxidation of 5‐methyl cytidine modification in nucleic acids to 5‐formyl cytidine is reported. This method displays excellent chemo‐/bio‐ selectivity, from which the methyl group in cytidine nucleosides could be selectively oxidized to biologically important formyl functionality without interfering with other cellular biomolecules.

Abstract

We report herein a catalyst‐free method to perform decarboxylative conjugated addition and hydrodecarboxylation of aliphatic N‐(acyloxy)phthalimides (redox active esters, RAEs) through photoactivation of electron‐donor‐acceptor (EDA) complex with Hantzsch ester (HE) in N,N‐dimethylacetamide (DMA) solution. The reactions present a green method to decarboxylatively construct carbon‐carbon bond and to perform hydrodecarboxylation with broad substrate scope and functional group tolerance under mild blue light irradiation condition without recourse of popularly used photoredox catalysts.

Asymmetric C−As bond formation: Key catalytic steps were examined for the PCP Ni^II pincer complex‐catalyzed asymmetric hydroarsination reaction. This was investigated with catalytic screening, DFT calculations, conductivity measurements and NMR spectroscopic studies. A unique Ni–Cl–As interaction was identified in the key catalytic intermediate which was not previously reported in other closely related hydrofunctionalization reactions (see scheme).

Abstract

Synthetic challenges have significantly slowed the development of the catalytic asymmetric hydroarsination reaction despite it being a highly attractive C−As bond formation methodology. In addition, there is a poor understanding of the main reaction steps in such reactions which limit further development in the field. Herein, key intermediates of the hydroarsination reaction catalyzed by a PCP Ni^II‐Cl pincer complex are presented upon investigating the reaction with DFT calculations, conductivity measurements, NMR spectroscopy, and catalytic screening. The novel Ni–Cl–As interaction proposed was then contrasted against known Ni^II‐catalyzed hydrophosphination reactions to highlight dissimilarities between them even though P and As share a close group relationship. Lastly, the asymmetric hydroarsination of nitroolefins was further developed to furnish a library of chiral organoarsines in up to 99 % yield and 80 % ee under mild conditions (−20 °C to RT) between 5 to 210 mins.

The direct fluorination of C−H bonds has emerged as a powerful method for accessing functional molecules such as pharmaceuticals or PET radiotracers. This Review provides an overview of the state of play of this field with an emphasis on the advantages and limitations of the main activation modes exploited to date. The discussion brings to light the importance of the fluorination reagent, and the challenges associated with nucleophilic C−H fluorination methods.

Abstract

This Review summarizes advances in fluorination by C(sp²)−H and C(sp³)−H activation. Transition‐metal‐catalyzed approaches championed by palladium have allowed the installation of a fluorine substituent at C(sp²) and C(sp³) sites, exploiting the reactivity of high‐oxidation‐state transition‐metal fluoride complexes combined with the use of directing groups (some transient) to control site and stereoselectivity. The large majority of known methods employ electrophilic fluorination reagents, but methods combining a nucleophilic fluoride source with an oxidant have appeared. External ligands have proven to be effective for C(sp³)−H fluorination directed by weakly coordinating auxiliaries, thereby enabling control over reactivity. Methods relying on the formation of radical intermediates are complementary to transition‐metal‐catalyzed processes as they allow for undirected C(sp³)−H fluorination. To date, radical C−H fluorinations mainly employ electrophilic N−F fluorination reagents but a unique Mn^III‐catalyzed oxidative C−H fluorination using fluoride has been developed. Overall, the field of late‐stage nucleophilic C−H fluorination has progressed much more slowly, a state of play explaining why C−H ¹⁸F‐fluorination is still in its infancy.

Synlett
DOI: 10.1055/s-0039-1690131

The sex-aggregation pheromone sobralene produced by the sandfly Lutzomyia longipalpis is isomerised to verticillenes in the presence of mild acid, thereby providing credence to the proposal that sobralene is a likely shunt metabolite of the taxadiene synthase cascade.
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© Georg Thieme Verlag Stuttgart · New York

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Abstract

The isomerization of epoxides to aldehydes using the readily available Crabtree's reagent is described. The aldehydes were transformed into synthetically useful amines by a one‐pot reductive amination using pyrrolidine as imine‐formation catalyst. The reactions worked with low catalyst loadings in very mild conditions. The procedure is operationally simple and tolerates a wide range of functional groups. A DFT study of its mechanism is presented showing that the isomerization takes place via an iridium hydride mechanism with a low energy barrier, in agreement with the mild reaction conditions.

Abstract

Described is a tetrabutylammonium fluoride‐mediated hydroxyalkylation reaction of phenols with cyclic carbonates. This operationally simple method enables the synthesis of a variety of aryl β‐hydroxyethyl ethers in good to excellent yields with a very small amount of catalyst loading (0.1–1 mol%). Of particular note is the efficient conversion of aromatic diols and phloroglucinol to the corresponding bis‐ and tris‐hydroxyethylated products. To further showcase the versatility of this protocol, guaifenesin was prepared with a single step by the condensation of guaiacol and glycerol carbonate. We also developed a flow ethoxylation process permitting the continuous synthesis of multiflorol.

Synlett
DOI: 10.1055/s-0037-1611911

Nitrogen-containing compounds are an essential motif in all disciplines of chemistry and the efficient synthesis of these frameworks is a highly sought-after goal. Presented here is a summary of recent efforts conducted by our group to develop radical-mediated amination strategies for the formal synthesis of primary amines from alkenes with exclusive anti-Markovnikov regioselectivity. We have found that N-hydroxyphthalimide is an effective reagent capable of supplying both the N and H atoms for alkene hydroamination in a group transfer radical addition-type mechanism. Furthermore, allyl-oxyphthalimide derivatives are similarly capable of radical group transfers and allow for the aminoallylation of an external alkene.
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© Georg Thieme Verlag Stuttgart · New York

Article in Thieme eJournals:
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Pincers et Al: The intrinsic strong σ‐donation and Lewis acidity of a Lewis‐base‐free X‐type PAlP‐pincer Ir complex has been experimentally and theoretically elucidated (X‐ray diffraction study, NMR, IR, and XANES analysis). The thermally stable pincer‐Ir complex with a tetrahydrido‐Ir^V structure showed moderate activity in the catalytic transfer dehydrogenation of cyclooctane.

Abstract

A pincer‐iridium complex bearing a Lewis‐base‐free X‐type alumanyl ligand has been synthesized. X‐ray diffraction, NMR and IR spectroscopy, as well as XANES analysis confirmed its tetrahydrido‐Ir^V structure and Lewis acidity at the Al center as supported by DFT calculations. The resulting complex was applied as a catalyst for the transfer dehydrogenation of cyclooctane.

Chiral sulfones moving forward: The first general, Pd‐catalyzed asymmetric synthesis of α,α‐disubstituted allylic sulfones has been realized. This process is based on a Tsuji–Trost allylic alkylation using both alkyl‐ and arylsulfinate based nucleophiles and a new phosphoramidite ligand and was successfully applied to the formal synthesis of the sesquiterpene (−)‐Agelasidine A.

Abstract

We describe here the first general asymmetric synthesis of sterically encumbered α,α‐disubstituted allylic sulfones via Pd‐catalyzed allylic substitution. The design and application of a new and highly efficient phosphoramidite ligand (L10) proved to be crucial, and a wide variety of challenging allylic sulfones featuring quaternary stereocenters could be obtained in good yields and with good to excellent levels of regio‐ and enantioselectivities under attractive process conditions. The developed methodology employs easily accessible chemical feedstock including racemic allylic precursors and sodium sulfinates. The utility of the method is further demonstrated by the synthesis of the sesquiterpene (−)‐Agelasidine A.