Communication: Chemoselective Glycosylation Based on Difference in the Reactivities of Ethyl and p-Tolyl Thioglycosides

Abstract

Thioglycosides are very useful glycosyl donors for the synthesis of oligosaccharides.1 Frequent use of these donors is due to their ease of preparation, stability and capability of reaction in the presence of a variety of promoters like iodonium dicollidine perchlorate,2 N-iodosuccinimide–triflic acid (NIS-TfOH),3 dimethyl-(methylthio)sulfonium triflate,4 methyl triflate,5 etc. to form glycosidic bonds. In addition, the armed - disarmed glycosylation strategy6 was also successfully applied and it was claimed that 2-O-substituents play the greater role in activating or deactivating a donor molecule. Thus coupling of an ethyl thioglycoside donor having a 2-O-benzyl substituent with an ethyl thioglycoside acceptor having a 2-O-acyl protecting group, proceeds with high chemoselectivity to give the desired product in good yield.2,3 There are also reports on the concept of active and latent thioglycosyl donors7 caused by activation or deactivation of the anomeric center. The enhanced reactivities of p-acetamidophenyl and ethyl thioglycoside in comparison to p-nitrophenyl thioglycoside are also examples of anomeric activation and deactivation.8 More recently, it was reported9 that the bulky dicyclohexylmethyl thioglycoside donors are much less reactive than the ethyl thioglycosides and this strategy was applied to chemoselective glycosylation utilising donors and acceptors both having either 2-O-benzyl or 2-O-acyl substituents.