GAGs and their derivatives are widely used in biomedical applications, with heparin and heparan sulfate being the most widely used in clinical settings [9,10]. In particular, heparin's remarkable anticoagulant activity has led to its use as an antithrombotic drug. Other biomedical applications of GAGs, including their potential as antiviral agents, are enormous. For example, a heparin decasaccharide and the polysulfonated heparin analog suramin were both reported to inhibit dengue virus infection of host cells [11]. Other viruses such as HIV also appear to be susceptible to polysulfated, negatively charged carbohydrate oligomers such as curdlan sulfate [12] and kakelokelose [13]. Curdlan sulfate is a sulfated semisynthetic polysaccharide that inhibits HIV-1 infection of human peripheral lymphocytes. Kakelokelose, a related polysulfated ^(1,6)-mannose polymer isolated from a marine source, also displays moderate anti-HIV activity. Although no consensus on the mode of activity currently exists, it is clear that the long, negatively charged chains of the polysac-charide bind to specific domains on the viral surface proteins, thereby neutralizing entry into the host cell. With the development of potential therapies based on the control of protein-GAG interactions for modifying cell-cell interaction, viral infection, and cell growth, the chemical preparation of GAG fragments and their analogs becomes increasingly important.

This chapter surveys the chemical preparation of the glycosaminoglycan oli-gosaccharides of hyaluronan, chondroitin sulfate, dermatan sulfate, keratan sulfate, heparin, and heparan sulfate. Despite the biological importance of these ubiquitous carbohydrate polymers, there are surprisingly few reports of the chemical syntheses of GAG oligosaccharides. The multiple functionality of GAGs provides an excellent scaffold on which structure-activity relationships can be studied, but their syntheses present an unparalleled challenge to the synthetic carbohydrate chemist. The de novo construction of these highly functionalized carbohydrates has proven difficult, and only limited synthetic methodologies, exist for their assembly.

For a long time, the lack of well-defined synthetic targets had discouraged organic chemists from the chemical synthesis of GAG oligosaccharides. Only in recent decades have the structures of these biologically active carbohydrate oligo-saccharides been elucidated. This, together with the development of new carbohydrate synthetic methodology that utilizes newly developed protecting groups as well as glycosylation procedures, has made it possible to synthetically access GAG fragments that allow them to be analoged for new drug development.

A. Synthetic Strategies for GAG Preparation

As a result of their high degree of functionality, GAGs represent challenging synthetic targets. All GAGs have structural similarities that form the focal point of any synthetic effort. The disaccharide repeating units are composed of either ^-d-glu-copyranosiduronic acid or a-l-idopyranosiduronic acid and a hexosamine residue of either ^-d-glucosamine or ^-d-galactosamine that is usually N-acetylated. Additionally, the linear, polymeric chains may be O-sulfated and N-sulfated to varying degrees. The order of the glycosidic bond forming events, the choice of starting amino functionality, and the timing of oxidation state adjustment of C6 all represent important considerations that must be addressed at the onset of any synthetic effort. As with any carbohydrate synthesis, multiple orthogonal protecting groups are required to allow reaction to be isolated to a particular site. The choice of protecting groups is crucial inasmuch as the final deprotection sequence may prove problematic [14].

Formation of the ^(1,4)-glycosidic linkage between C4 of the uronic acid moiety and the hexosamine residue is generally more difficult than formation of the corresponding ^(1,3)-linkage. In addition to the steric deactivation of the C4 hy-droxyl group, its nucleophilicity is lowered by the electron-withdrawing properties of the C6 ester in the uronic acid derivative [15]. Therefore, historically, the ^(1,4)-linkage is formed early on in the synthetic sequence to yield disaccharide fragments, which are then further elaborated (Scheme 1). The uronic acid residue is typically masked as a selectively protected form of d-glucose or l-idose prior to glycosidic bond formation. When glycosylation is complete, the uronic acid moiety is produced by selective C6 deprotection and oxidation.

Newly described and efficient glycosylation methodologies that utilize highly reactive glycosyl donors, such as Schmidt's trichloroacetimidate method [16], Kahne's sulfoxide chemistry [17], and the pentenyl glycosylation technique of Fraser-Reid [18], now offer high-yielding glycosylation strategies to offset the poor nucleophilicity of uronic esters in GAG synthesis.

The synthesis of GAGs requires the installation and protection of a 2-deoxy-2-amino functionality. The hexosamine unit is either V-acetyl-^-d-glucosamine or jV-acetyl-^-d-galactosamine, with the former being more extensively employed, largely because of its ready availability and relatively low cost. Galactosamine, a rare and expensive sugar, is less attractive for use as a starting material. Consequently, several methodologies have been developed for the installation of the 2-amino functionality into d-glucal and d-galactal building blocks, both of which are readily available at modest cost. Regardless of the strategy, the j -protecting group employed must survive several chemical manipulations while maintaining facile conversion to the corresponding acetamide at the conclusion of the synthesis. Presumably, the most convenient j -protecting group for GAG synthesis would be the acetate group, since this would eliminate the need for an j -deprotection and acetylation sequence at the conclusion of the synthesis. However, the V-acetate group generally imports poor solubility to the sugar; therefore, a range of alternative V-protecting groups including phthalamide [19], azide [20], trichloroacetyl [21], trichloroethoxy-carbonyl [22], and benzenesulfonamide [23] are currently employed.

The widely used 2-deoxy-2-azido group has been accessed by the azidonitration [24] of d-glucal or d-galactal. In general, this route is more attractive for the preparation of j -acetyl galactosamine derivatives because it produces predominantly two isomers owing to the strong preference for the C2 azido group to exist in the equatorial orientation (presumably due to the sterically disfavored axial approach). On the other hand, azidonitration of d-glucal gives rise to four isomers: 1:1 mixtures of both the gluco- and mannoazidonitrate derivatives (Scheme 2). In either case, the C1 nitrate can then be transformed into a variety of functional groups such as halides, acetates, or methyl ethers. Additionally, the azide is stable under a wide range of glycosylation conditions and can readily be reduced and acetylated at the conclusion of the synthesis.

The iodosulfonamidation method pioneered by Danishefsky and coworkers offers an alternative method of introducing a 2-deoxy-2-amino moiety into d-glucal or d-galactal scaffolds [25-29]. Briefly, trans-diaxial addition of a glycal is achieved by treatment with an iodonium ion source (yym-collidine iodonium perchlorate) followed by trapping with an arylsulfonamide (Scheme 3). The active glycosylating intermediate formed under basic conditions is believed to be the 1,2-sulfonylaziri-

R1 = C02Me

Scheme 1 A typical retrosynthetic analysis for GAGs

CO y

0 0

Post a comment