Carbapyranose 1,2-epoxides as useful glycosyl donor mimics for the synthesis of carbasugars

The present thesis work concerns regioselective studies of the nucleophilic addition reactions to carbapyranose 1,2-epoxides with α-gluco configuration under Lewis acidic conditions. While carbapyranose 1,2-epoxides with β-manno configuration react with good yields and regioselectivity both with O- and N-nucleophiles, ring-opening reactions of 1,2-epoxides with α-gluco configuration do not give good results because the opening process is not regioselective and occurs only with some N-nucleophiles as the azido ion and amines. These results were determined by stereoelectronic reasons that make both the C(1) and C(2) attacks unfavourable . Despite of this, carbapyranose 1,2-epoxides with α-gluco-configuration theoretically turn out to be very useful for the synthesis of pseudoglycosides with pseudo-gluco configuration on the carbapyranose unit. In view of this, in my thesis work I have synthesized carbapyranose 1,2-epoxide 3α, starting from the commercially available tri-O-acetyl-D-glucal 2, and I have studied the regio- and stereochemical behaviour of this key product under different reaction conditions in order to direct the ring-opening process. In my studies I observed that a bulky secondary alcohol, such as diacetone-D-glucose, was unsuccessful in the ring-opening reaction, while a primary alcohol such as diacetone-D-galactose led to an unregioselective mixture of C(1) and C(2) adducts with good yields. More regioselective results were obtained with t-butyl alcohol (C(1)/C(2) ratio 80/20), and with the less reactive p-methoxyphenol a complete regioselectivy was observed with the only formation of the C(1) adduct. Finally, I verified that under strongly coordinating reaction conditions, by the presence of an ion such as Li¬¬+ (from LiClO4), a very interesting regioselective ring-opening process occurs with the exclusive formation of the C(1) regioisomer