Carbon-Ferrier rearrangements in ionic liquids using Yb(OTf)3 as catalyst

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Abstract

The ionic liquids [bmim][BF4] and [bmim][NTf2] are used as efficient recyclable solvents in the ytterbium triflate catalysed carbon-Ferrier rearrangement of triacetyl glucal with allyl silanes, propargyl silane, and silyl enolethers.

The ionic liquids [bmim][BF4] and [bmim][NTf2] are used as efficient recyclable solvents in the ytterbium triflate catalysed carbon-Ferrier rearrangement of triacetyl glucal with allyl silanes, propargyl silane, and silyl enolethers.

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Introduction

Room temperature ionic liquids (RTILs) appear of much current interest as novel reaction media and as one of the most promising alternatives to volatile organic solvents (VOS) [1]. This is mainly due to their unique physical properties which makes them as very attractive solvents for organic synthesis [2], [3], organometallic catalysis [4], [5], as well as for biotransformations [6], [7]. One of their potential advantages is that, in many cases, such solvents could be recycled for performing several times the same reaction on the same substrates or could even be reused for performing similar reactions on different substrates. Several examples of efficient catalysis by lanthanide salts in RTILs have been already described. This includes Friedel Crafts type reactions [8], Diels–Alder catalysis [9], multicomponent condensations [10], thioacetalization [11], or the synthesis of O-glycosides using the Ferrier rearrangement [12]. C-glycosides are very important compounds in bioorganic chemistry [13], [14]. One of the most versatile methods for their preparation is the reaction of sugar derived electrophiles with carbon nucleophiles. The carbon-Ferrier rearrangement generally involves the reaction of glycals under Lewis acid catalysis with nucleophiles such as silyl enolethers [15], allylsilanes [16], [17], and organometallic derivatives [18]. Recently, it has been established that lanthanides salts are also efficient Lewis acid catalysts for this carbon-Ferrier rearrangement [19], [20]. The stereoselectivity of the latter reaction is usually good in favor of the α-anomer, except in cases of conformational control [21]. Finally, it has to be noted that this reaction was one of the key steps of several elegant total synthesis of bioactive natural products [22], [23], [24], [25]. As part of our ongoing studies on the potential uses of RTILs [26], [27], we have studied this carbon-Ferrier rearrangement in ionic liquids, under lanthanide salt catalysis. The purpose of this paper is to demonstrate that the latter reaction is easily performed in 1-butyl-3-methyl imidazolium tetrafluoroborate [bmim][BF4] or 1-butyl-3-methyl imidazolium bis N-trifluoro imidate [bmim][NTf2] and using Yb(OTf)3 at 5 mol% as a catalyst. Commercially available 2,4,6-tri-O-acetyl-d-glucal was selected as a model for the sugar component and using allyl silanes, propargylsilane as well as silyl enolethers as nucleophiles, moderate to good yields of adducts have been obtained with an excellent stereoselectivity. Furthermore, it proved to be possible to recycle and/or reuse the solvent for these reactions.

Section snippets

Experimental

The ionic liquids [bmim][BF4] and [bmim][NTf2] were prepared according to literature procedures [28], [29]. Allyl and propargyl silanes, as well as Yb(OTf)3 are commercially available and were used as received. The silyl enol ethers were prepared according to literature procedures and their purity was checked by 1H NMR before use.

Allylsilanes and propargylsilane

The 2,4,6-tri-O-acetyl-d-glucal was found to react smoothly with allyl trimethylsilane 2a in [bmim][BF4] under Yb(OTf)3 catalysis (5 mol%): after 3 h at room temperature the reaction is completed and the adduct 3a was isolated in 80% yield after extraction with ether followed by a short SiO2 chromatography (Scheme 1, Table 1). High field NMR analysis of the crude reaction mixture indicates that the reaction is highly stereoselective (>95%) in favor of the α-anomer. The spectral data of 3a are in

Conclusion

Yttterbium triflate is an efficient catalyst for the carbon-Ferrier rearrangement in the ionic liquids [bmim][BF4] and [bmim][NTf2]. The reactions of allylsilanes, propargylsilane, or silyl enolethers afford the corresponding C-glycosides in fair to good yields with an excellent stereoselectivity. Furthermore, it is possible to recycle or reuse the ionic liquid used as the solvent of these reactions.

Acknowledgements

One of us (S.A.) thanks to the Ministère des Affaires Etrangères for a fellowship as part of CEFISO/IFCOS. S.C. thanks IFCPAR-CEFIPRA for providing support to visit the CNRS laboratory in Rennes.

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