Bismuth(V)‐Mediated Thioglycoside Activation

Abstract

Chemical glycosylation [1,2] is a crucial step in any oligosaccharide [3] synthesis. [4,5] Among the different classes of commonly used glycosyl donors, [2] thioglycosides [6, 7] offer distinct advantages. Thioglycoside donors are relatively simple to prepare, are stable under various reactions for protectinggroup manipulations, and offer orthogonality in their activation in the presence of other glycosyl donors. [8] As a result, a wide variety of promoters have been developed for activation of these donors in the past 20 years: [2] from heavy metal-cation-based promoters (Hg II sulfate), [9] to the current halonium-based reagents [e.g., N-iodosuccinimide/trifluoromethane sulfonic acid (NIS/TfOH), [10] N-bromosuccinimide (NBS), [11] ICl or IBr/AgOTf, [12] etc.], alkylating reagents [methyl triflate (MeOTf)], [13] and organosulfur-based promoters [e.g., dimethyl(thiomethyl)sulfonium triflate (DMTST), [14] methylsulfenyl triflate (MeSOTf), [15] dimethyl disulfide/triflic anhydride (Me 2 S/Tf 2 O), [16] benzenesulfinylpiperidine/triflic anhydride (BSP/TTBP), [17] N-(phenylthio)-ecaprolactam-Tf 2 O, [18] etc.]. A recent method applies singleelectron transfer using ruthenium or iridium-containing catalysts that are active under visible light [19] to activate thioglycosides. Although these methods have been effective in carrying out a range of glycosylations, most of these still have a limited scope. Generally, these activations need excess amounts of promoters, [2,[9][10][11][12][13][14][15][16][17][18] or require a co-promoter to form the reactive intermediates. Moreover, present methods often require extremely low temperatures (< À20 8C) as a result of generating reactive intermediates. Some of the popular halonium-based promoters are challenging to use in the presence of alkenes, [20] because they tend to give various addition by-products, thereby ultimately resulting in the cleavage of the alkenyl moiety. These issues with solubility, undesired by-products, stability, or reagent handling are particularly problematic in the context of the development of robust automated protocols [21,22] for oligosaccharide synthesis. To circumvent some of these issues with current promoter systems, we herein report a straightforward method for the activation of thiopropylglycosides for coupling to various acceptors in good to excellent yields by utilizing a bismuth(V) compound without additional additives/copromoters.In lieu of the available promoters based on heavy-metal cations, bismuth presents interesting possibilities. Bismuth is a post-transition metal and like its neighboring metals such as mercury and lead, it is considered thiophilic as well as soft Lewis acidic. However, unlike Hg and Pb compounds, bismuth [23] is not only inexpensive, but also nontoxic. Unfortunately, despite its popularity as a treatment for digestive problems, [24] the synthetic utility of bismuth compounds remains relatively unexplored. However, the chemistry of this element [25] has gained considerable interest [26,27] over the past decade. [28] V...

Lewis Superacids: Classifications, Candidates, and Applications

“…[180b] The use of Bi V Lewis acids is less advanced. [186] Bismuth pentafluoride is highly oxidizing and ap owerful fluorination agent. [187] Its Lewis acidity is certainly substantial but expected to be lower than that of SbF 5 .…”

Chemical O‐Glycosylations: An Overview

“…Introducing a novel promoter‐free glycosylation technique, Pohl and co‐workers efficiently used sub‐stoichiometric amounts of triphenyl bismuth ditriflate [Ph 3 Bi(OTf) 2 ] to activate various thiopropyl glycosides . The highly robust nature and solubility of bismuth(V) salts made it a highly advantageous glycosylation catalyst, employing just catalytic amounts of the promoter.…”

Multiparameter High-Throughput and in Situ X-ray Diffraction Study of Six New Bismuth Sulfonatocarboxylates: Discovery, Phase Transformation, and Reaction Trends

“…A survey of bismuth carboxylates, [3][4][5][6][7][8][9][10] sulfonates [11][12][13][14][15][16][17][18][19] and sulfonatocarboxylates 20,21,22 demonstrated that IBUs in the crystal structures range from mononuclear complexes to Bi-O clusters with up to 38 bismuth ions to infinite Bi-O chains and even Bi-O layers. While the Bi-O clusters and dinuclear IBUs are mostly formed through reactions at room temperature or hydrolysis at moderate temperatures, respectively, the Bi-O chains were mostly obtained employing hydrothermal synthesis.…”