Thermodynamic control glycoside formation

As we saw in Section 23.14, the preparation of glycosides by acid-catalyzed condensation of a carbohydrate with a simple alcohol—the Fischer glycosidation—is thermodynamically controlled and favors the formation of pyranose over furanose rings. The anomeric effect causes the a stereoisomer to predominate over the p. 

Generally, the products derived from the open-chain form are insignificant in the reaction mixture and stereocontrol between the anomeric forms can be achieved. It was observed that the anomeric oxygen in the equatorial alkoxide 276 has enhanced nucleophilicity compared with the axial [284]. Often referred to as the kinetic anomeric effect, this phenomenon can govern the diastereoselective formations of the equatorial glycosides [579]. The anomeric effect, however, favors the formation of the thermodynamically more stable axial glycosides and chelation control can also play a role in determining anomeric stereoselectivity. 

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