Source: Dissertation Abstracts International, Volume: 78-11(E), Section: B.

Adviser: Alanna Schepartz.

This dissertation describes studies of glycosylation and isomerization reactions of carbohydrates in an aqueous solvent system. Glycosylation reactions attach a carbohydrate donor to a functional group of an acceptor molecule; of particular importance to this work will be reactions where the acceptor molecule is also a carbohydrate. In these cases, where the acceptor is a polyol, high levels of site-selectivity are necessary for efficient reactions. Initial studies were aimed towards utilizing short aptamer peptides that were selected from phage display libraries to bind the Thomsen Friedenrich (T-Antigen) disaccharide and alter the selectivity of a glycosylation reaction. To accomplish this, the reaction must be performed in aqueous solvent system where the peptides can bind their target. Previously reported chemical methods for glycosylation were incompatible with aqueous solvent systems, so we utilized a pair of glycosyltransferases to perform the transformation. When the enzymatic glycosylations were performed in the presence of peptides no alteration in site-selectivity was observed.

A method for chemical glycosylation that was compatible with aqueous solvent systems was subsequently developed. Glycosyl fluorides, which are stable in water and can undergo stereoinvertive nucleophilic substitution with anionic nucleophiles, were chosen as donors. Sucrose was selected as an acceptor because it is a readily available non-reducing sugar that is unlikely to undergo competing isomerization reactions. A mixture of water and calcium hydroxide promoted glycosylation, and a mixture of two regioisomers were obtained. Alternatively a combination of trimethylamine and calcium nitrate also promoted glycosylation to produce a single regioisomer in good yield. These studies culminated in a protecting-group free glycosylation that was site-selective and compatible with aqueous solvent.

We also observed that combinations of trimethylamine and calcium chloride could promote the isomerization of glucose into fructose. Of particular interest was that this reaction proceeded via a discrete calcium-fructose complex which formed under the reaction conditions. The scope of this reaction was probed to show the breadth of substrates that it could tolerate. Additional mechanistic studies were performed to show that an ene-diol intermediate formed under the reaction conditions, consistent with previously observed base promoted isomerization.