The descriptive passage in this chapter describes the solid-phase synthesis of

Chapter 26, Problem 26.28

(choose chapter or problem)

The descriptive passage in this chapter describes the solid-phase synthesis of oligonucleotides. The solid phase technique has also been applied to the automated synthesis of oligosaccharides in what is termed the glycal assembly method. An unsaturated carbohydrate known as a glycal is attached to the solid polystyrene support. The glycal is then converted to an epoxide by treatment with dimethyldioxirane (DMDO). The epoxide serves as the glycosyl donor and undergoes nucleophilic attack by the hydroxyl group of the glycosyl acceptor. The double bond of the new disaccharide is activated and coupled in the same way to allow extension of the oligosaccharide chain. (a) Show a mechanism for the reaction of the glycosyl donor with the glycosyl acceptor.(Hint: Zn2+ acts as a catalyst.)(b) Explain the regioselectivity of the reaction in (a).