Light Fluorous‐Tag‐Assisted Synthesis of Oligosaccharides

Abstract

Oligosaccharides are ubiquitous in biological systems, but largely exist in such microheterogenous mixtures that the chemical synthesis of structurally well-defined carbohydrates is very attractive compared to their isolation from natural sources. The use of solid-phase methods has been highly successful for the automation of peptide and nucleic acid syntheses [1,2]. By contrast, the synthesis of oligosaccharides, including their production on solid phases [3–6] has been complicated by the presence of multiple hydroxyl groups and the need to control the stereochemistry of the glycoside linkage–a process that has not approached the >95% yields as usually required for good peptide and nucleic acid synthetic processes. The use of a soluble light fluorous tag (those that by definition contain fluorine content ≤40%) by molecular weight [7] for the synthesis of such oligosaccharides from smaller carbohydrate building blocks has several unique advantages over more traditional polymeric supports. Automated solid-phase methods suffer from the inherent limitation of biphasic kinetics and therefore require large excesses of building blocks [8–11] at each coupling cycle. Additionally, solid phases are not amenable to convergent synthetic strategies–ones in which two oligosaccharides are built separately and then are linked together–and are more complex to monitor than solution-phase reactions. In solution-phase glycosylation reactions, both coupling partners are in solution; therefore, the need for large excesses of building blocks to improve biphasic kinetics is eliminated. The small tag required, unlike polymeric supports, can also be readily obtained as a homogeneous compound with no risk of retaining reagents between coupling cycles to contaminate subsequent steps. Reaction intermediates can even be readily characterized using standard mass spectrometry, nuclear magnetic resonance (NMR), and chromatographic techniques. At the same time, the strong carbon–fluorine bond renders the requisite fluorocarbon tag inert to a wide variety of reaction conditions to maintain flexibility in the synthetic strategy. However, like a solid phase, the fluorous tag can still be used to simplify the purification of the growing oligosaccharide chain. Heavy fluorous tags (those that contain fluorine content by molecular weight of >60%) can aid small molecule syntheses through purification by liquid–liquid extraction protocols using fluorous solvents; their use in the synthesis of oligosaccharides has been summarized elsewhere [12]. Unlike these heavy tags, the so-called light fluorous tags can be used to mark carbohydrate molecules for isolation from nonfluorous-tagged side products and excess reagents as needed in the various stages of complex oligosaccharide synthesis by a simpler fluorous solid-phase extraction (FSPE) [13]. Given these advantages, much effort has gone into designing a range of protecting groups with minimal fluorous content for use in carbohydrate synthesis. The general use of various fluorous tags and protecting groups in organic synthesis was recently reviewed [14]. In this chapter, we put these fluorous tag design efforts into the context of carbohydrate synthesis and then focus on the construction of oligosaccharides using these tags along with their capacity for applications in automated protocols and in fluorous-based carbohydrate microarrays.