COOPERATIVE CATALYSIS IN GLYCOSIDATION REACTIONS WITH O-GLYCOSYL TRICHLOROACETIMIDATES AS GLYCOSYL DONORS

Strong hydrogen bonds between two groups can lead to strong coordination complexes. In the last years, this observation was exclusively and very successfully employed in organocatalysis and, with the help of chiral hydrogen-bonding catalysts, to asymmetric synthesis. However, none of these hydrogen-bonding catalysts, possesses sufficient acidity to activate under standard conditions typical glycosyl donors as, for instance, O-glycosyl trichloroacetimidates. Generally, acids with a relatively low pK_a value (≤ 5) are employed to this aim; commonly, catalytic amounts of TMSOTf or BF₃·OEt₂ are used.¹

Recently, cooperativity between acid catalysts and strongly hydrogen-bonding cocatalysts was observed, that was successfully applied to asymmetric synthesis.² Hence, the study of eventual cooperativity in glycoside bond formation with O-glycosyl trichloroacetimidates as efficient glycosyl donors is of interest. As these donors – different from most other commonly used glycosyl donors – are accessible to activation with catalytic amounts of an acid, they are ideally suited for these studies. Obviously, it is hoped that not only the reaction rate but also the product yield and the anomeric selectivity will be positively influenced by the hydrogen-bond mediated interaction of the cocatalyst with the substrates (glycosyl donor and acceptor) and the catalyst. Our results along these studies will be discussed.