This chapter presents several strategies in oligosaccharide synthesis including linear syntheses, convergent syntheses, and “One-pot” syntheses. A linear synthesis is one in which a monosaccharide, by the successive addition of a single monosaccharide, is transformed into the desired oligosaccharide. A protected glycosyl donor (A, X activated) is treated with an acceptor alcohol (B, Y inactive) to generate a disaccharide (AB, Y inactive). After the conversion of Y into X, the process is repeated until the final target oligosaccharide is reached. A reverse approach, which benefits from the fact that the glycosyl donor is more readily acquired and can thus be used in excess, is in common use. A convergent synthesis assembles the oligosaccharide from smaller, preformed components. The convergent approach, often dubbed a block synthesis, has several advantages over its linear counterpart. The smaller components can often be obtained from readily available disaccharides (lactose, cellobiose), an expensive monosaccharide can be introduced late in the synthetic sequence, and generally, there are fewer overall linear steps. A tantalizing goal in oligosaccharide synthesis is the assembly of the molecule in question in “one-pot,” viz., by the stepwise addition of building blocks to the growing chain, with no need for manipulation of protecting groups and anomeric activating groups and with complete stereochemical control.
