A protecting group strategy for the synthesis of C-glycosides
C-Glycosides are carbohydrate analogues in which a carbon atom substitutes the glycosidic oxygen. This relatively minor structural change results in significant chemical property changes without affecting the physical properties to a great extent. Consequently, they found applications in many areas of biological and medicinal chemistry, including enzyme inhibition and intercellular recognition events, making them highly valued agents for anti-cancer, anti-tumour and anti-viral drugs. Many of the C-glycoside forming a reactions produce more than one stereoisomer, and usually only one is desired, resulting in the need for separation steps. Routes that produce only one stereoisomer do away with the separation step requirement and consequently can enhance the yield (since fewer steps minimise loss), along with saving time and money. The research aim was to investigate and develop methodology for the totally stereoselective synthesis of C-glycosides i.e., the production of one isomer exclusively, using bulky protecting groups. The research that was carried out showed that a number of factors affected the outcome of C-glycosylation reactions, of which the most significant were the choice of solvent, the choice of sugar and the C-glycosylation method used. However, they only became important factors as a result of the bulk of the protecting groups. The choice of solvent and sugar, for example, only became factors because of the individual sugar’s need to relieve the degree of steric hindrance, as a consequence of the bulk of the protecting groups. The way the C-glycosylation method affected the stereochemistry of the reaction was enhanced by the size of the protecting groups. The work showed that if the steric strain that existed (as a result of the large protecting groups) got too severe, it could be relieved either by ring contraction, migration or a combination of both, but only if certain criteria were met. Ultimately, the research was successful and showed that stereoselective C-glycosylation reactions can be forced to be totally stereoselective, as a direct consequence of increasing the size of the protecting groups. This is a result of the steric strain being too severe in one another and there being no path by which it can be relieved, which consequently precludes the anomer’s synthesis.