Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors: remote protecting group effects and temperature dependency.

The stereoselectivity of glycosylation reactions is affected by many factors. Synthesis of 1,2-cis glycosidic linkages (such as α linkages in glucose and galactose like monosaccharides) is challenging due to lack of control of the stereoselectivity. Our systematic study of GalN(3) donors with different combination of protecting groups indicated that acetyl groups at the 3- and 4-positions are particularly important for high α-selectivity. Temperature is also recognized as a major factor in control of stereoselectivity. Mechanisms responsible for these experimental results are discussed and explored using computational methods. A remote participation model of the acetyl groups is proposed to explain the directing effects of the acetyl groups.