The important role of oligosaccharides in biological processes1 has been recognized for a long time. Consequently, synthetic oligosaccharides have become indispensable probes for the life sciences. 2 Methods for the chemical synthesis of oligosaccharides are based on a two-step process: The first comprises activation of the anomeric center to generate a glycosyl donor, and the second is its transfer to a glycosyl acceptor. The stereochemical outcome of the reaction depends on complex stereoelectronic effects as well as the presence or absence of groups at O-2 in the glycosyl donor capable of neighboring group participation. 3 Except for rare cases, when the coupling of a glycosyl donor and a glycosyl acceptor occurs as an almost entirely SN2 process, 4 the reaction of the glycosyl donor involves the formation of the oxocarbenium ion (1). Thus, the nonstereospecificity of glycosylation is virtually inherent in the method.

Syntheses of 1, 2-trans-linked oligosaccharides are relatively easy, but not the highly stereoselective syntheses of their 1, 2-cis-linked counterparts. Most difficult are syntheses of β-mannosides and β-rhamnosides. 5 Both the anomeric effect, which favors the formation of the R-mannopyranosyl linkage, and the formation of 1 during the transition state of the reaction are largely responsible for this unfavorable situation. Efforts aimed at overcoming these difficulties continue, and some new approaches have recently been introduced. 6 Nevertheless, there is still a need for more efficient methods of glycosylation not involving the formation of the oxocarbenium ion. We have now discovered that 1, 2-O-cis-stannylene acetals of sugars are powerful nucleophiles capable of displacing, Via the SN2 process, good leaving groups in carbohydrates. This