Controlling Benzylic and Anomeric Functionality and Stereochemistry: Methodology and Syntheses Utilising 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 stereochemical control in benzylic and carbohydrate systems, as exemplified by racemic and nonracemic syntheses of key intermediates for the pseudopterosin/secopseudopterosin family, and by the synthesis of 2-deoxy-C-glycosides. These syntheses illustrate and compare two alternative methods of carbonium ion generation: olefin protonation, and Lewis acid-promoted sulfone ionisation. Simple model systems are used to show that ring 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-bearing carbon. 1. Introduction 2. Racemic studies 3. Nonracemic synthesis of a key intermediate for the pseudopterosins 4. Other aspects of the intramolecular ionic hydrogenation 4.1. Synthesis of 2-deoxy-C-glycosides 4.2. Ring size effects in the IIH reaction 5. Conclusions