Convergent Synthesis Approach for Stereospecific Preparation of Fluorinated Carbohydrates in Exploration of Cell Surface Receptor-Ligand Interactions.
Abstract: Metastasis, where cancer cells lethally spread throughout the body
is facilitated by specific carbohydrate-protein interactions. Cancer cells that detach from
the primary tumor and penetrate blood vessels can survive and move along the blood stream
by adhering to vascular surfaces. This cellular adhesion is mediated by a tetrasaccharide
epitope sialyl Lewis X, that is overexpressed o... read moren most cancer cells and adhesion proteins
(selectins) present on the surface of the activated endothelium. The contribution of the
adhesion molecules to metastasis suggests a potential therapy based on disruption of cancer
endothelial cell-cell interactions. We are interested in targeting selectin mediated cancer
cell adhesion by structural modification of the ligand, sialyl Lewis X using metabolic
approaches. Previous work in our laboratory has shown that replacement of sialic acids on
cell surfaces with fluorinated analogues significantly reduces cellular adhesion to
selectin coated surfaces. The detailed mechanism of the altered adhesion is not known,
although we envisage that fluorinated sialic acid generates a sialyl Lewis X analogue on
cell surfaces that weakly binds to the selectins. In order to investigate the molecular
determinants of decreased adhesion through biophysical characterization of fluorinated
sialyl Lewis X-selectin interactions, access to pure and structurally defined sialyl Lewis
X analogues is essential. The central objective of this thesis is to develop a convergent
and stereocontrolled synthesis of an N-fluoroacyl sialyl Lewis X analogue. Complementary to
the synthetic efforts, the first synthesis of a trifluorobutyryl modified sialyl Lewis X
tetrasaccharide is established. Two independent convergent strategies toward fluorinated
sialyl Lewis X analogues are described. The first approach was based on a [2+2] strategy
for generating a common tetrasaccharide intermediate that can be fluoroacylated after
removal of the temporary protecting group on the sialic acid amine. However, it was not
possible to optimize the glycosylation reaction to obtain this tetrasaccharide intermediate
in high yield. Thus, an alternative approach to the fluorinated sialyl Lewis X was
developed. This method relied on a [2+2] strategy using a novel fluorinated sialic acid
α(2-3) galactose building block and afforded the final fluorinated sialyl Lewis X
construct in improved yield. Biophysical studies can now be carried out to investigate the
protein-ligand interaction in solution.
Thesis (Ph.D.)--Tufts University, 2012.
Submitted to the Dept. of Chemistry.
Advisor: Krishna Kumar.
Committee: Krishna Kumar, Marc D'Alarcao, Clay Bennett, Elena Rybak-Akimova, and David R. Walt.
Keyword: Chemistry.read less