ABSTRACT
A new approach to the synthesis of 1,2-cis glycosides is the use of C-2 chiral auxiliaries that feature two key elements (Scheme 1.1).5 The rst is a nucleophilic thiophenyl group that can attack the intermediate oxa-carbenium ion (1a) and form an intermediate sulfonium ion (1b).6 The second is a chiral substituent that directs the thiophenyl head group to the equatorial (β) face and ensures that the trans-decalin system is formed as opposed to the cis-decalin. The equilibrium between the oxa-carbenium ion and β-sulfonium ion can be controlled by the selection of protecting groups and constitution pattern of the chiral auxiliary.7 Subsequent addition of a glycosyl acceptor will result in attack from the α-face and formation of the 1,2-cis-linked disaccharide in high yield and with excellent stereoselectivity. The methodology is applicable to the solid-phase synthesis of oligosaccharides8 and holds considerable promise to become a general methodology for the stereoselective synthesis of 1,2-cis glycosides.
