Regioselective modification of unprotected glycosides.

Selective modification of unprotected carbohydrates is difficult due to the similar reactivity of the hydroxyl groups. In carbohydrate synthesis, therefore, even straightforward transformations often require multiple synthetic steps. The development of selective methods for carbohydrate modification is consequently highly desired. This review describes the methods for the regio- and chemoselective carbohydrate modification, with a focus on novel approaches that mainly apply transition metal catalysis and organocatalysis, and discusses the challenges and opportunities in this field.