Glycosylation with trichloroacetimidates in ionic liquids: a "green" approach to the synthesis of glycosides

Ionic liquids (ILs) have recently emerged as a new class of solvents for a wide range of organic reactions. These low melting salts are often referred to as \u201cgreen solvents\u201d, as they are not volatile and can be, in principle, recovered and recycled. The unique characteristics of these reaction media prompted us to investigate their use as solvents in the glycosylation reaction. In fact, the highly polar microenvironment of ILs could affect the course of the reaction through the stabilization of its charged intermediates. We focussed on the trichloroacetimidate as a suitable activating group due to its wide applicability and effectiveness. Differently protected donors were tested, ranging from disarmed to armed sugars and belonging both to the D- and to the L-series, in order to enlarge the scope of the methodology as widely as possible. The coupling reactions proceeded under mild conditions and at room temperature and afforded the different glycosydes in fair to excellent yields; in some cases the use of the Lewis acid catalyst was not necessary. A systematic study describing the behaviour of \u3b1 and \u3b2 donors in both coordinating and non-coordinating ILs, strongly supported the hypothesis of the coordination of the oxonium ion by the triflate anion of [emim][OTf], addressing the stereochemical outcome of the glycosylation reaction to the \u3b2 product. On the other hand, when the reaction was performed in ILs bearing a non-coordinating anion, such as [emim][BF4] and [bmim][PF6], the prevailing mechanism leaded to the inversion of the anomeric configuration. The effectiveness of iterating the same reaction in a recycled solvent was also verified

Glycosylation with trichloroacetimidates in ionic liquids: influence of the reaction medium on the stereochemical outcome

The glycosylation with trichloroacetimidates derived from different glycopyranoses bearing a nonparticipating group at C-2 was explored in different ionic liquids as solvents. The stereoselectivity of the reaction was significantly affected by the reaction media and by the anomeric configuration of the donor

Trichloroacetimidates as glycosyl donors in recyclable ionic liquids

The trichloroacetimidates of different glycopyranoses were glycosylated in ionic liquids with different acceptors, affording the corresponding glycopyranosides in fair to excellent yields and allowing to recycle the solvent without additional Lewis acid

Lipase-catalysed regioselective acylations in combination with regioselective glycosylations as a strategy for the synthesis of oligosaccharides: Synthesis of a series of fucosyllactose building blocks

A family of five regioisomenc protected fucosyllactoses has been synthesised in only 14 overall steps from the easily available benzyllactoside. The adopted strategy combines enzymatic regioselective protection, introduction of ortho-gonal protecting groups and regioselective glycosylations on partially unprotected lactosidic acceptors

Efficient Synthesis of Unsymmetrical Ureido-Linked Disaccharides

Five nonsymmetrical urea-linked disaccharides, in which two glycopyranoside units are bound at the 1 2, 1 4, and 1-6 positions, were efficiently synthesized. A mild and safe procedure, in which glycosyl isocyanates were coupled with a glycosylamine, was employed

NMR evidence for the participation of triflated ionic liquids in glycosylation reaction mechanisms

A systematic low-temperature NMR study of a glycosylation reaction was performed in the presence of different ionic liquids and acidic catalysts. The influence of the triflate anion derived from [emim][OTf] on the stereochemistry of the glycosylation products was evaluated

Human milk oligosaccharides: an enzymatic protection step simplifies the synthesis of 3 '- and 6 '-O-sialyllactose and their analogues

We describe a chemo-enzymatic synthesis of 3'- and 6'-O-sialyllactose, two trisaccharides occurring in the 'acidic fraction' of the human milk oligosaccharides and endowed with potential antiadhesive activity. The key step is the highly regioselective 6'-O-acylation of benzyllactoside, which gave access to suitably protected lactose building blocks to be used as acceptors in the sialylation reaction. Moreover, the synthesis of the carboxymethyl and sulfo analogues of the title compounds is reported. (C) 2002 Elsevier Science Ltd. All rights reserved