Site-Switchable Alkylation of Carbohydrates Controlled by Inexpensive Metal Ion Catalysts

In this study, the challenge on site-switchable alkylation of carbohydrates with benzyl group or conventional alkyl groups was partially solved by utilizing SnCl2 catalysis and CuCl2 catalysis, respectively. For alkylation of glycoside substrates containing an axial–OH at the 2-position and an equatorial–OH at the 3-position, products with equatorial-O selectivity were obtained by utilizing SnCl2 catalysis, and products with axial-O selectivity were obtained by utilizing CuCl2 catalysis. By mechanism studies, we suggested that the selectivity led by both SnCl2 and CuCl2 catalysis is controlled by steric and stereoelectronic effects. The essential reason for the switchable selectivity may lie in the presence or absence of electrons in the outermost S orbitals of a metal ion. In addition, SnCl2-catalyzed alkylation was investigated more extensively in this study, showing broad substrate scope, good selectivity, high reaction efficiency, and high catalytic activity.