Silver(I) tetrafluoroborate as a potent promoter for chemical glycosylation

Abstract

We have identified silver tetrafluoroborate (AgBF 4 ) as an excellent promoter for the activation of various glycosyl donors including glycosyl halides, trichloroacetimidates, thioimidates, etc. Easy handling and no requirement for azeotropic dehydration prior to application makes AgBF 4 especially beneficial in comparison to the commonly used AgOTf. Selective activation of glycosyl halides or thioimidates over thioglycosides or n-pentenyl glycosides, including simple sequential one-pot syntheses, has been also demonstrated. Versatility of glycosyl thioimidates was further explored by converting these intermediates into a variety of other classes of glycosyl donors.With increasing demand for the synthesis of biologically important and therapeutically active oligosaccharides and glycoconjugates, efforts to expand the arsenal of glycosylation methods and techniques have emerged. In spite of significant recent advances in the areas of stereoselective glycosylation 1,2 and expeditious oligosaccharide synthesis, 3,4 the construction of complex oligosaccharides with high efficiency and complete stereoselectivity remains a difficult task.As a part of a program to develop new methods and strategies for glycochemistry, we became interested in glycosyl thioimidates, a class of glycosyl donors with the generic leaving group SCR 1 =NR 2 . 5,6 We have already reported the synthesis of S-benzoxazolyl (SBox) 7,8 and Sthiazolinyl (STaz) 9,10 glycosides and evaluated their properties in stereoselective glycosylations and expeditious oligosaccharide syntheses. Metal salt-based promoters were shown to provide efficient activation of the thioimidoyl moiety for glycosylation. 8,11 Amongst a variety of metal salts investigated, arguably, silver trifluoromethanesulfonate (AgOTf) was one of the best choices for the activation of the SBox and STaz moieties for a variety of synthetic applications.Beyond the scope of our own research program, AgOTf has been commonly employed as an activator for many other classes of glycosyl donors including glycosyl bromides, 12-14 chlorides, 15 trichloroacetimidates, 16 and seleno glycosides. 17 In spite of wide applicability and high versatility of AgOTf, there are some significant drawbacks that limit the usage of this promoter in synthesis. In a majority of applications, AgOTf requires fresh activation by repetitive co-evaporation with toluene followed by extended drying under vacuum directly prior to use. In addition, we noticed that the quality of commercial reagent may significantly vary depending on the supplier and the batch. It should be noted that the use of AgOTf as an *Corresponding

Glycosyl thioimidates as versatile building blocks for organic synthesis

“…While the syntheses of hexoses were accomplished from acetobromosugar and the corresponding sodium thiolates in MeCN, the synthesis of thioribofuranoside derivative was accomplished from the corresponding tetraacetate and trimethylsilylated thiols in the presence of TMSOTf [54]. The synthesis of a range of differently protected SBaz derivatives from the corresponding glycosyl bromides and KSBaz in acetone has also been reported [55]. …”

“…As an example, AgOTf-promoted glycosylation of diacetone galactose acceptor (6-OH) with glycosyl donors of the L-arabino, D-ribo, and D-xylo series in toluene afforded the corresponding disaccharides in 75% (α/β = 4/3), 98% (α/β = 7/2), and 70% (α/β = 3/10) yield, respectively. It was also reported that the SBaz leaving group can be very rapidly activated for glycosylation in the presence of AgBF 4 and is applicable to selective activation over other types of leaving groups involving S -alkyl and O -pentenyl glycosides [55]. It was also observed that the SBaz leaving group can be activated in the presence of MeOTf, but not in the presence of weaker alkylating reagents, which created a basis for selective activation of STaz donors with benzyl bromide or methyl iodide in the presence of glycosyl acceptors equipped with SBaz anomeric group [35].…”

“…Moreover, in comparative hydrolytic stability studies, STaz glycosides exhibited even more stability than their 1- S -ethyl and 1- S -phenyl counterparts in the presence of NBS or NIS/TfOH [27, 75]. Studies were conducted to determine whether thiazolinyl 2,3,4,6-tetra- O -benzyl-1-thio-β-D-glucopyranoside 46 could serve as a common precursor toward the synthesis of an array of glycosyl donors [55]. It was found that the STaz glycoside 46 can be directly converted into the following glycosyl donors: acetate 47 , chloride 1 , bromide 48 , fluoride 49 , and hemiacetal 50 in excellent yields (Scheme 12).…”

“…Many different leaving groups and methods fit into this general concept [61], and thioimidate-based one-pot glycosylation procedures have also emerged [55, 78]. For instance, SBox glycoside 74 was activated over SEt acceptor 21 with AgOTf (Scheme 16) [78].…”

An Easy and Efficient Synthesis of Bisindolylmethanes and Tetraindolylmethane Tröger's Base Catatlyzed by AgBF4

“…Because of the wide range of biological interest in these compounds, this study pursues a mild, efficient and expedient protocol for synthesizing bisindolylmethanes. Silver tetrafluoroborate is a fluorodesulfurization reagent; 57 is used in the synthesis of 1,6-diketones from siloxycyclopropanes; 58 is a potent promoter of chemical glycosylation; 59 causes the silver ion-induced rearrangement of N-chloramines; 60 causes the silver-induced allylation of b-bromo ethers with allylsilanes, 61 and has other im-portant uses. Biffis et al reported the AgBF 4 -NHC carbene complexes induced the reaction between indoles and ethyl phenylpropiolate to produce diaddition product.…”