Stereoselective Glycosylations – Additions to Oxocarbenium Ions

Abstract

Tremendous progress has been made in the construction of oligosaccharides, and many impressive examples of large and complex oligosaccharide total syntheses have appeared over the years. The stability, lifetime, and reactivity of an oxocarbenium ion depend besides the nature of the counterion on the nature and orientation of the functional groups present on the carbohydrate ring. This chapter explores the role of oxocarbenium ions and contact or close ion pairs (CIPs) that features a glycosyl cation, in chemical glycosylation reactions. It deals with the stability, reactivity, and -conformational behavior of glycosyl oxocarbenium ions, and describes their intermediacy in the assembly of (complex) oligosaccharides. The chapter presents sophisticated and detailed DFT computational approaches to study glycosyl oxocarbenium ions. Where many anomeric triflates have been spectroscopically characterized, glycosyl oxocarbenium ions are too reactive to detect by straightforward NMR techniques. NMR spectroscopy has been used to characterize a multitude of covalent reactive intermediates such as anomeric triflates.