The Synthesis of Azasugars Using an I2-mediated Carbamate Annulation
The biological activity of azasugars has largely been attributed to their ability to mimic the oxocarbenium ion-like transition state formed during reactions with carbohydrate-processing enzymes and, for this reason, functional and stereochemical modifications of the azasugar scaffold have led to the development of specific and potent glycosidase inhibitors. Given the potential of azasugars as glycosidase inhibitors, we were interested in developing efficient methodology for their synthesis. This thesis highlights synthetic methodology developed to produce amino-imino-hexitols as azasugar scaffolds. Key in the synthesis of the amino-imino-hexitols was the application of a stereoselective Strecker reaction, without the need for chiral Lewis acids or catalysts, and an extension of an I2-mediated carbamate annulation to cyclise functionalised and protected alkenylamines. Sixteen amino-imino-hexitols were synthesized, including ten previously undisclosed substrates with the D-galacto, D-talo, and L-altro configurations. The novel amino-imino-hexitols were then tested for their ability to act as glycosidase inhibitors and substrates of the D-talo configuration showed promising inhibitory effects. Mechanistic considerations of the I2-mediated carbamate annulation are discussed and although the exact annulation mechanism has yet to be determined, experimental studies have revealed that an aziridine is not an intermediate in the reaction. Factors influencing the diastereoselectivity of the carbamate annulation are also explored. Furthermore, an in depth analysis of the high cis-selectivity of the carbamate annulation is investigated using density functional theory to calculate the transition states of iodocyclisations en route to the formation of carbamates. Taken as a whole, the applicability of the carbamate annulation to a variety of alkenylamines and an understanding of the factors controlling the diastereoselectivity of the reaction should make this methodology a valuable addition to the synthetic chemist’s toolbox.