Rhamnosylation: diastereoselectivity of conformationally armed donors.

The α/β-selectivity of super-armed rhamnosyl donors have been investigated in glycosylation reactions. The solvent was found to have a minor influence, whereas temperature was crucial for the diastereoselectivity. At very low temperature, a modest β-selectivity could be obtained, and increasing temperature gave excellent α-selectivity. The donors were highly reactive, and activation was observed at temperatures as low as -107 °C. Different promoter systems and leaving groups were investigated, and only activation with a heterogeneous catalyst increased the amount of the β-anomer significantly. By introducing an electron-withdrawing nonparticipating group, benzyl sulfonyl, on 2-O, an increase in β-product was observed.