Additive-controlled asymmetric iodocyclization enables enantioselective access to both α- and β-nucleosides

Deciphering the impact of Zn-incorporation on M-NC (M = Fe, Co, Ni, Cu) type catalysts for enhanced HER and OER performance

The catalytic activity is mainly controlled by the local environment of the active site, where the chemical reaction occurs. Through selective inter-mixing of different elements, it is possible to fine-tune the electronic and geometric properties of the active site with precision leading to significant enhancement of both catalytic activity and selectivity. This research work focuses on modeling efficient catalysts for electrocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) based on bimetallic MNC type materials. Introducing zinc into single-atom catalysts like Fe-N-C, Co-N-C, Ni-N-C, and Cu-N-C allows us to develop dual-atom MNC catalysts. The HER and OER activities of M-Zn-N-C type catalysts show that zinc significantly improves catalytic performance. A comprehensive orbital interaction analysis of Zn-containing and Zn-free MNC catalysts reveals that the incorporation of zinc has a profound impact on the electronic structure of the transition metals at the active site. Specifically, zinc activates the low-lying dx2-y2 and dz2 orbitals of the transition metals, positioning them near the valence band maximum (VBM) enhances their interaction with the pz orbitals of oxygen in adsorbed species, leading to a significant reduction in overpotential values for the oxygen evolution reaction (OER). In the case of the hydrogen evolution reaction (HER), zinc incorporation modifies the interaction between the dz2 orbital of the transition metals and the s-orbital of hydrogen. This modification reduces the in-phase overlap, optimizing the interaction and resulting in a lower reaction barrier. This detailed analysis provides insight into the mechanisms by which zinc incorporation enhances the catalytic activity of MNC catalysts for both OER and HER. Therefore, our findings explain the intrinsic reaction mechanism of MNC catalysts and provide insights into designing dual-atom catalysts for electrochemical applications.

Radical Strategy Towards N-glycosides: Current Advances and Future Prospects

N-glycosides exhibit diverse biological and pharmacological activities, making their efficient synthesis crucial for both biological research and drug development. Traditional acid-promoted N-glycosylation methods, which rely on the formation of oxocarbenium intermediates, often face significant challenges. These methods are water-sensitive and typically require neighboring group participation to achieve high selectivity. Furthermore, they depend on acid activation, rendering them incompatible with alkyl amine. Additionally, low-nucleophilicity amides often need to be converted into their TMS-derivatives to enhance reactivity, limiting the direct use of such substrates. In contrast, radical-based strategies have emerged as a promising alternative, addressing many of these limitations and leading to notable advances in N-glycosylation. This review explores the unique properties of N-glycosides, the inherent challenges of traditional N-glycosylation techniques, and the transformative advantages offered by radical-based approaches. Specifically, it highlights recent advancements in radical-mediated N-glycosylation, including photoredox radical strategies, radical/ionic hybrid approaches, and metallaphotoredox catalysis, accompanied by a detailed discussion of the underlying mechanisms. Finally, the ongoing challenges and potential future directions of N-glycoside synthesis using radical strategies are presented.

Chemoselectivity Switch between Enantioselective [2,3]-Wittig Rearrangement and Conia-Ene-Type Reactions of Propargyloxyoxindoles

Despite the existence of three competing reactions for propargyloxyoxindoles, we report a chemoselectivity switch between enantioselective propargyl [2,3]-Wittig rearrangement and Conia-ene-type reactions, with suppression of the [1,2]-Wittig-type rearrangement. Using C1-symmetric imidazolidine-pyrroloimidazolone pyridine as the ligand and Ni(acac)2 as the Lewis acid, diverse 3-hydroxy 3-substituted oxindoles containing allenyl groups were obtained in up to 98 % yield and 99 % ee via asymmetric propargyl [2,3]-Wittig rearrangement. In the presence of AgOTf-Duanphos, chiral spiro dihydrofuran oxindoles were given in up to 98 % yield and 91 % ee through a Conia-ene-type reaction.

A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Asymmetric Total Synthesis of Anti-HBV Drug Entecavir: Catalytic Strategies for the Stereospecific Construction of Densely Substituted Cyclopentene Cores

We have successfully accomplished a catalytic asymmetric total synthesis of entecavir, a first-line antihepatitis B virus medication. The pivotal aspect of our strategy lies in the utilization of a Pd-catalyzed enyne borylative cyclization reaction, enabling the construction of a highly substituted cyclopentene scaffold with exceptional stereoselectivity. Additionally, we efficiently accessed the crucial 1,3-diol enyne system early in our synthetic route through a diarylprolinol organocatalyzed enantioselective cross-aldol reaction and Re-catalyzed allylic alcohol relocation. By strategically integrating these three catalytic protocols, we established a practical pathway for acquiring valuable densely heteroatom-substituted cyclopentene cores.

Direct Synthesis of α- and β-2'-Deoxynucleosides with Stereodirecting Phosphine Oxide via Remote Participation

2'-Deoxynucleosides and analogues play a vital role in drug development, but their preparation remains a significant challenge. Previous studies have focused on β-2'-deoxynucleosides with the natural β-configuration. In fact, their isomeric α-2'-deoxynucleosides also exhibit diverse bioactivities and even better metabolic stability. Herein, we report that both α- and β-2'-deoxynucleosides can be prepared with high yields and stereoselectivity using a remote directing diphenylphosphinoyl (DPP) group. It is particularly efficient to prepare α-2'-deoxynucleosides with an easily accessible 3,5-di-ODPP donor. Instead of acting as a H-bond acceptor on a 2-(diphenylphosphinoyl)acetyl (DPPA) group in our previous studies for syn-facial O-glycosylation, the phosphine oxide moiety here acts as a remote participating group to enable highly antifacial N-glycosylation. This proposed remote participation mechanism is supported by our first characterization of an important 1,5-briged P-heterobicyclic intermediate via variable-temperature NMR spectroscopy. Interestingly, antiproliferative assays led to a α-2'-deoxynucleoside with IC50 values in the low micromole range against central nervous system tumor cell lines SH-SY5Y and LN229, whereas its β-anomer exhibited no inhibition at 100 μM. Furthermore, the DPP group significantly enhanced the antitumor activities by 10 times.

Boronic Acid-Catalyzed Regio- and Stereoselective N-Glycosylations of Purines and Other Azole Heterocycles: Access to Nucleoside Analogues

In the presence of an arylboronic acid catalyst, azole-type heterocycles, including purines, tetrazoles, triazoles, indazoles, and benzo-fused congeners, undergo regio- and stereoselective N-glycosylations with furanosyl and pyranosyl trichloroacetimidate donors. The protocol, which does not require stoichiometric activators, specialized leaving groups, or drying agents, provides access to nucleoside analogues and enables late-stage N-glycosylation of azole-containing pharmaceutical agents. A mechanism involving simultaneous activation of the glycosyl donor and acceptor by the organoboron catalyst has been proposed, supported by kinetic analysis and computational modeling.