CONTROLLING BENZYLIC AND ANOMERIC FUNCTIONALITY AND STEREOCHEMISTRY -METHODOLOGY AND SYNTHESES UTILIZING INTRAMOLECULAR IONIC HYDROGENATION

Intramolecular hydride transfer from a hydroxyl-tethered SiH unit to a stabilised carbonium ion is shown to be a useful method for stereoche mical control in benzylic and carbohydrate systems, as exemplified by racemic and nonracemic syntheses of key intermediates for the pseudopt erosin/secopseudopterosin family, and by the synthesis of 2-deoxy-C-gl ycosides. These syntheses illustrate and compare two alternative metho ds of carbonium ion generation: olefin protonation, and Lewis acid-pro moted sulfone ionisation. Simple model systems are used to show that r ing size restrictions to hydride transfer in a 5-membered ring may be overcome by the appropriate choice of reagent, permitting the reaction to be used for stereochemical control from a vicinal, hydroxyl-bearin g carbon.