Sterically Strained Brønsted Pair Catalysis by Bulky Pyridinium Salts: Direct Stereoselective Synthesis of 2-Deoxy and 2,6-Dideoxy-β-thioglycosides from Glycals

Why is thiol unexpectedly less reactive but more selective than alcohol in phenanthroline-catalyzed 1,2-cis O- and S-furanosylations?

The lack of catalytic stereoselective approaches for producing 1,2-cis S-furanosides emphasizes the critical need for further research in this area. Herein, we present a stereoselective S-furanosylation method, utilizing a 4,7-dipiperidine-substituted phenanthroline catalyst. This developed protocol fills a gap in the field, enabling the coupling of cysteine residues and thiols with furanosyl bromide electrophiles. The process allows for stereoselective access to 1,2-cis S-furanosides. Through computational and experimental investigations, thiol is found to be less reactive than alcohol but exhibits greater stereoselectivity. The 1,2-cis stereoselectivity of O-products depends on the nature of the electrophile, while S-products are obtained with excellent 1,2-cis stereoselectivity, irrespective of the furanose structure. The displaced bromide ion from the glycosyl electrophile influences the reaction's reactivity and stereoselectivity. Alcohol-OH forms a stronger hydrogen bond with bromide ion than thiol-SH, contributing to the difference in their reactivity. The energy difference between forming S-furanoside and O-furanoside transition states is 3.7 kcal mol-1, supporting the increased reactivity of alcohol over thiol. The difference in transition state energies between the major and minor S-product is greater than that for the major and minor O-product. This is consistent with experimental data showing how thiol is more stereoselective than alcohol. The catalyst and reaction conditions utilized for the generation of 1,2-cis O-furanosides in our prior studies are found to be unsuitable for the synthesis of 1,2-cis S-furanosides. In the present study, a highly reactive phenanthroline catalyst and specific reaction conditions have been developed to achieve stereoselective S-linked product formation.

Coupling of unactivated alkyl electrophiles using frustrated ion pairs

Cross-electrophile coupling reactions have evolved into a major strategy for rapidly assembling important organic molecules1. Two readily accessible electrophiles are coupled to form new C-C bonds, providing a key advantage over traditional cross-coupling strategies that require the preformation of reactive organometallic species. Yet, the formation of C(sp3)-C(sp3) bonds that form the core of nearly all organic compounds remains highly challenging with current approaches, calling for the design of innovative new strategies. Here we report a distinct, transition-metal-free platform to form such bonds without the need for activating or stabilizing groups on the coupling partners. The reaction is enabled by an unusual single-electron transfer in a frustrated ion pair, and it can couple fragments containing functional groups that would be challenging in related transition-metal-catalysed processes. Moreover, we could further leverage this new mechanistic manifold in the design of other reactions, showing the broad potential of this type of reactivity. We anticipate that our results will provide a framework for further exploration of this reactivity pattern to tackle challenging problems in organic synthesis.

Anion-Bridged Dual Hydrogen Bond Enabled Concerted Addition of Phenol to Glycal

A noncovalent organocatalytic concerted addition of phenol to glycal is developed for the stereoselective and regioselective construction of biologically important phenolic 2-deoxyglycosides, featuring wide substrate tolerance. The method relies on an anion-bridged dual hydrogen bond interaction which is experimentally proved by Nuclear Magnetic Resonance (NMR), Ultraviolet and visible (UV-vis), and fluorescence analysis. Experimental evidence including kinetic analysis, Kinetic Isotope Effect (KIE) studies, linear free energy relationship, Hammett plot, and density functional theory (DFT) calculations is provided for a concerted mechanism where a high-energy oxocarbenium ion is not formed. In addition, the potential utility of this method is further demonstrated by the synthesis of biologically active glycosylated flavones. The benchmarking studies demonstrate significant advances in this newly developed method compared to previous approaches.

Reagent-controlled chemo/stereoselective glycosylation of ʟ-fucal to access rare deoxysugars

2,6-Dideoxy sugars constitute an important class of anticancer antibiotics natural products and serve as essential medicinal tools for carbohydrate-based drug discovery and vaccine development. In particular, 2-deoxy ʟ-fucose or ʟ-oliose is a rare sugar and vital structural motif of several potent antifungal and immunosuppressive bioactive molecules. Herein, we devised a reagent-controlled stereo and chemoselective activation of ʟ-fucal, enabling the distinctive glycosylation pathways to access the rare ʟ-oliose and 2,3-unsaturated ʟ-fucoside. The milder oxo-philic Bi(OTf)3 catalyst induced the direct 1,2-addition predominantly, whereas B(C6F5)3 promoted the allylic Ferrier-rearrangement of the enol-ether moiety in ʟ-fucal glycal donor, distinguishing the competitive mechanisms. The reagent-tunable modular approach is highly advantageous, employing greener catalysts and atom-economical transformations, expensive ligand/additive-free, and probed for a diverse range of substrates comprising monosaccharides, amino-acids, bioactive natural products, and drug scaffolds embedded with susceptible or labile functionalities.

Origins of Selectivity in Glycosylation Reactions with Saccharosamine Donors

A combination of DFT calculations and experiments is used to describe how the selection of a promoter can control the stereochemical outcome of glycosylation reactions with the deoxy sugar saccharosamine. Depending on the promoter, either α- or β-linked reactive intermediates are formed. These studies show that differential modes of activation lead to the formation of distinct intermediates that undergo highly selective reactions through an SN2-like mechanism.

Stereoselective Synthesis of 2-Deoxythiosugars from Glycals

2-deoxythiosugars are more stable than 2-deoxysugars occurring broadly in bioactive natural products and pharmaceutical agents. An effective and direct methodology to stereoselectively synthesize α-2-deoxythioglycosides catalyzed by AgOTf has been developed. Various alkyl thiols and thiophenols were explored and the desired products were formed in good yields with excellent α-selectivity. This method was further applied to the syntheses of S-linked disaccharides and late-stage 2-deoxyglycosylation of estrogen, L-menthol, and zingerone thiols successfully.