Upon condensation of 6-thio-6-deoxy-mannosyl donors 1,2-cis products are obtained with a high degree of stereoselectivity. Subsequent reductive removal of the 6-thio functionality gives 1,2-cis rhamnosides. The 1,2-cis-selectivity can be rationalized with a product forming (3)H(4)-oxocarbenium, which is in equilibrium with a bridged sulfonium intermediate.
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