Iridium-promoted deoxyglycoside synthesis: stereoselectivity and mechanistic insight

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.

Cobalt's Dual Role in Promoting C3-Glycosylation of Indoles: Unraveling Mechanistic Insights

In this study, we present a cobalt-catalyzed C3-glycosylation of indoles using unfunctionalized glycals, yielding 3-indolyl-C-deoxyglycosides. These compounds hold promise as sodium-dependent glucose cotransporter 2 (SGLT2) inhibitors for treating type 2 diabetes. Control experiments unveiled that cobalt assumes a dual role, facilitating catalytic C-glycosylation while unexpectedly driving the anomerization of α-anomers through endocyclic cleavage of the C1-O5 bond, resulting in the formation of β-C-deoxyglycosides. Furthermore, density functional theory (DFT) calculations shed light on the reaction mechanism, emphasizing the significant role of the pyridine group of indole in stabilizing transition states and intermediates.

Ferrier/Aza-Wacker/Epoxidation/Glycosylation (FAWEG) Sequence to Access 1,2-Trans 3-Amino-3-deoxyglycosides

3-Amino-3-deoxyglycosides constitute an essential class of nitrogen-containing sugars. Among them, many important 3-amino-3-deoxyglycosides possess a 1,2-trans relationship. In view of their numerous biological applications, the synthesis of 3-amino-3-deoxyglycosyl donors giving rise to a 1,2-trans glycosidic linkage is thus an important challenge. Even though glycals are highly polyvalent donors, the synthesis and reactivity of 3-amino-3-deoxyglycals have been little studied. In this work, we describe a new sequence, involving a Ferrier rearrangement and subsequent aza-Wacker cyclization that allows the rapid synthesis of orthogonally protected 3-amino-3-deoxyglycals. Finally a 3-amino-3-deoxygalactal derivative was submitted for the first time to an epoxidation/glycosylation with high yield and great diastereoselectivity, highlighting FAWEG (Ferrier/Aza-Wacker/Epoxidation/Glycosylation) as a new approach to access 1,2-trans 3-amino-3-deoxyglycosides.

Development of α-Selective Glycosylation with l-Oleandral and Its Application to the Total Synthesis of Oleandrin

This letter describes the development of an α-selective glycosylation using l-oleandrose, a 2-deoxysugar that is frequently found in natural products, and its application to the total synthesis of the natural cardiotonic steroids oleandrin and beaumontoside. To improve the reaction diastereoselectivity and to minimize side-product formation, an extensive evaluation and optimization of the conditions leading to α-selective glycosylation of digitoxigenin with l-oleandrose-based donors was conducted. These studies led to the exploration of 8 different phosphine·acid complexes or salts and yielded HBr·PPh₃ as the optimal catalyst, which provided in the cleanest α-glycosylation and produced protected beaumontoside in 67% yield. Subsequent application of these conditions to synthetic oleandrigenin afforded the desired α-product in 69% isolated yield─enabling the completion of the first synthesis of oleandrin in 17 steps (1.2% yield) from testosterone.

Stereoselective Synthesis of 2-Deoxy Glycosides via Iron Catalysis

An Fe-catalyzed 2-deoxy glycosylation method was developed from 3,4-O-carbonate glycals directly at room temperature. This novel approach enabled facile access to alkyl and aryl 2-deoxy glycosides in high yields with exclusive α-stereoselectivity, tolerating various alcohols, phenols, and glycals. The synthetic utility and advantage of this strategy have been demonstrated by the modification of six natural products and the construction of a tetrasaccharide.

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.

o-Cyanobenzoate: A Recyclable and Reusable Stereo-directing Group for β-O-Glycosylation via Pd(0)-catalyzed Ferrier Rearrangement

Inner sphere Tsuji-Trost reaction has found recent application for β-selective Ferrier rearrangement of glycal substrates with alcohol nucleophiles. Herein, we report an efficient and stereoselective synthesis of 2,3-dideoxy-β-O-glycosides from C3-(o-cyanobenzoate) ester protected glycal donors via Ferrier rearrangement under Pd(0)-catalyzed Tsuji-Trost conditions. The synthesized donors indeed reacted with a variety of acceptors to afford the corresponding glycosides in good yields and excellent β-stereoselectivity. The stereochemical outcome of the reactions has been found to be independent of the nature of protecting groups or conformational flexibility of the glycal donors. Furthermore, regeneration of ortho-cyanobenzoic acid post rearrangement makes it a recyclable and reusable stereodirecting group. A preliminary mechanistic study demonstrates the importance of cyano-group for the observed rearrangement and stereoselectivity. Incorporation of the directing group on the benzoate ester has altered the reactivity of the ester group as a leaving group for Tsuji-Trost as well as Ferrier Rearrangement pathway.

Rhenium(V)-catalyzed synthesis of 1,1'-2-deoxy thioglycosides

As stable glycomimetics, thioglycosides are important tools for the investigation of biological processes and discovery of new drugs. In this note, we report a ReOCl₃(SMe₂)(OPPh₃) catalyzed coupling reaction between β-glycosyl thiols (1-thio sugars) and glycals for the preparation of 1,1'-α,β-2-deoxy thioglycosides, which are glycomimetics of natural trehalose and 2-deoxy glycosides. Furthermore, an S-linked trisaccharide was successfully obtained by successive employment of the Re(V) catalyzed thioglycosylation protocol.