Chapter 1 introduces that glycals as one type of unsaturated carbohydrates specialized with a double bond between C1 and C2 on the sugar ring. They demonstrated diverse reactivities and broad applications in organic synthesis, owing to their availability, affordable price and chemical structure with defined chiral centers. Glycals could not only be explored as a glycosyl donor in glycosylation which is the core of carbohydrate chemistry, but also be exploited efficiently as natural chiral pools in total syntheses of natural/unnatural products. In chapter 2, we have developed the first successful catalyst-controlled O-glycosylation via palladium catalysis using 3,4-cyclic carbonate glycal as the glycosyl donor. Various 2,3-unsaturated glycosides with C4-OH were synthesized stereoselectively in high yields under mild reaction condition. While hard nucleophiles such as aliphatic alcohols gave β-glycosides, soft nucleophiles such as phenols produced α-glycosides because palladium(II) catalyst coordinated with glycals on the β-face by carbonate group direction. On the other hand, both aliphatic alcohols and phenols generated only β-glycosides directed by hydrogen-bond effect because palladium(0) catalyst coordinated with glycal from α-face due to the steric hindrance. This method serves as a concise approach for constructing glycosides with predictable stereoselectivity and could potentially be applied to the formation of oligosaccharides and natural products.