Carbohydrate-based therapeutics is now established, and the research and development pipeline for these agents is well outlined, all of which promises a robust future for carbohydrate drugs.
To fully realize the potential of carbohydrate-based therapeutics, new chemical methods to discover, characterize, and supply carbohydrates is needed.
3.1.1 Challenges and Opportunities. The one characteristic of carbohydrates that most vividly demonstrates both the opportunities and challenges of carbohydrate chemistry is the potential of this class of biomolecules to form diverse structures.
With oligosaccharides, carbohydrates of low-to-intermediate complexity, the shear number of possible isomers becomes difficult to calculate. For example, considering a relatively small tetrasaccharide and using only nine common monosaccharides found in humans, there is greater than 15 million possible isomers that can be assembled (93).
Traditionally, carbohydrates have been prepared by synthesis or natural product isolation.
3.1.2 Solution Phase. In general, saccha-rides are a challenge to synthesize because monosaccharides have both regiochemistry and stereochemistry in both open and closed ring forms. If monosaccharides are combined, either a- or j3-covalent bonds can link the resulting molecules. And if many monosacchar-ides are covalently bonded, the linkages may be linear or branched, including multiple branch points with a given monosaccharide. Also, oligosaccharide synthesis requires multiple selective protection and deprotection steps.
The inevitable comparison of polysac-chrides to other biopolymer types helps put the challenge into perspective. Nucleic acids are made in a linear manner through chemical and biological synthetic techniques. Likewise, protein sequences are also linear and can be easily determined, produced, and manipulated through a combination of recombinant DNA technology and solid phase synthesis.
Nature has evolved very sophisticated mechanisms for making exact polypeptides and polynucleotides. In contrast, saccharides are made in an apparent random fashion with a diverse set of enzymes competing to produce very diverse products. Although random in appearance, carbohydrates do possess precise structures that are on the surface of complex large molecules and that affect function. These precise carbohydrate structures are targets of isolation and synthesis.
The creation of the glycosidic linkage between carbohydrate monomers is a strategically important step in oligosaccharide synthesis and requires stereocontrol of the newly formed linkage. Glycosides that have a trans relationship of the C-l and C-2 substituents are synthesized by taking advantage of neigh-
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