Asymmetric Control of the Diastereoselectivity of Glycosylation
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Diastereoselective control of glycosylation still remains a difficult task. Therefore, new glycosylation methods using asymmetric catalysis were developed to control the diastereoselectivity. Two systems were developed and each focused on a separate type of glycosyl donor. In the first system, glycosyl halides were subjected to reaction conditions inspired by Hamilton et al., who effectively had controlled the substitution of a racemic chloroamine by an alcohol. Asymmetric control of glycosylation was achieved through this adapted catalytic system. Both enantiomers of the catalyst ((R) and (S) TRIP) generally displayed b-selectivity with tertiary butyl methyl ether (TBME) as the solvent allowing almost exclusive formation of the β-anomer. However, low and inconsistent yields were obtained.
The second system proposed the use of the same phosphoric acid catalyst (TRIP) to catalyse the glycosylation of glycals. However, this was ineffective as the catalyst was not a strong enough Brønsted acid. These studies then led to the development of two new chiral catalysts which then promoted the glycosylation of glycals, along with the formation of an undesired side product. Attempts were made to reduce the formation of the side product but unfortunately this proved unsuccessful. The diastereoselective outcome displayed between the different catalysts in separate trials was negligible, but the principles developed in this study should lead to the further development of new chiral catalysts for the glycosylation of glycals.