Project Summery: Reagent Control for Stereoselective Glycosylation Reactions The study of carbohydrates, or glycobiology, holds enormous promise for producing next-generation therapeutics. Progress in this field has been hampered by difficulties in obtaining homogeneous carbohydrates to serve as analytical standards. Since biological samples typically produce carbohydrates as intractable heterogeneous mixtures, synthesis represents the only avenue for the production of pure material. There are several challenges to carbohydrate synthesis, the biggest of which is the lack of general stereoselective glycosylation protocols for assembling carbohydrate backbones. Most approaches to glycosylation are highly substrate specific, and require a considerable amount of expertise to successfully execute. As such, using current technologies, glycosylation reactions need to be approached on a case-by-case basis, and it is not possible to use the same set of coupling partners to selectively synthesize both anomers of a particular glycosidic linkage. The objective of this proposal is to address this issue by developing glycosylation chemistries where selectivity is entirely under control of the promoter. By making the stereochemical outcome of the reaction independent of the donor and acceptor configurations and protecting group patterns this approach will result in a truly general glycosylation protocol. This will be achieved by pursuing the following Specific Aims.
Specific Aim 1 will study an approach to ?-selective glycosylation reactions based on the in situ conversion of hemiacetals into ?-glycosyl sulfonate intermediates. By matching the reactivity of the sulfonate to that of the donor backbone, these intermediates will react with nucleophilic acceptors through SN2-like pathways to afford ?-linked products.
Specific Aim 2 will examine the ability of cyclopropenium cations to promote ?-selective dehydrative glycosylations. Modulating the electronics of the substituents on the cyclopropenium cation backbone will allow this approach to work with a broad range of substrates.
Specific Aim 3 will use NMR spectroscopy to study the mechanisms of the reactions and identify reactive intermediates. Information from these mechanistic studies will be used to both optimize reactions and help create a set of guidelines for selecting what promoter to use with a particular system. Together these studies will result in the creation of a set of promoters that provide absolute control over the selectivity of glycosylation reactions. Since the stereochemical outcome of these reactions will be independent of the configuration of the donor and acceptor, this approach will permit the selective construction of both ?- and ?-linked carbohydrates starting from the same set of coupling partners. Ultimately, the research described in this proposal will lead to an approach to chemical glycosylation that can be routinely used by any synthetic chemist. This will in turn lay the foundations for technologies that permit the creation of oligosaccharide libraries on time frames currently associated with peptide library construction.
Reagent Control for Stereoselective Glycosylation Reactions This research will result in a general approach to selective glycosylation reactions for the construction of biologically important carbohydrates. This proposal is relevant to public health because it will provide a method for the rapid synthesis of carbohydrate libraries to aid in developing next-generation therapeutics. The chemistry developed through this proposal is relevant to the part of the NIH's mission that pertains to basic biomedical research.
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