Approaches to Chemical Synthesis of Pectic Oligosaccharides

Many aspects of pectin biosynthesis and the physical properties of pectic polysaccharides can be better understood with the aid of small, well-defined oligosaccharide fragments of these macromolecules. Synthetic chemists have contributed to the study of pectin by preparation of fragments representing all three major types of pectic polysaccharide: homogalacturonan (HG), rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II). Such molecules have been synthesized by sequential coupling of building blocks, the so-called glycosyl donors (GD) and glycosyl acceptors (GA), which aimed at the formation of specific glycosidic linkages as they are present in the target oligosaccharides. Challenges in synthesis of pectic oligosaccharides are associated with often poor stereoselectivity of glycosylation reactions between GA and GD, in particular for the construction of 1, 2-ris-glycosidic linkages, high degrees of branching of oligo-saccharide chains of target molecules and the nature of many monosaccharide components of pectin, which are often acidic and sometimes rare branched-chain sugars. Preparation of carbohydrate building blocks, including de novo syntheses of unusual sugars, protecting group strategies for GA and GD, glycosylation methodologies and general strategies for oligosaccharide assembly are described with the focus on pectin fragments. Synthetic routes to fragments of each type of pectic polysaccharides are discussed in detail in separate sections and structures of all currently known synthetic pectin fragments are summarized. The unsolved problems and future prospects for improved access to synthetic pectin fragments are also discussed.