Effective anomerisation of 2 '-deoxyadenosine derivatives during disaccharide nucleoside synthesis

The formation of a disaccharide nucleoside (11) by O3'-glycosylation of 5'-O-protected 2'-deoxyadenosine or its N-6-benzoylated derivative has been observed to be accompanied by anomerisation to the corresponding alpha-anomeric product (12). The latter reaction can be explained by instability of the N-glycosidic bond of purine 2'-deoxynucleosides in the presence of Lewis acids. An independent study on the anomerisation of partly blocked 2'-deoxyadenosine has been carried out. Additionally, transglycosylation has been utilized in the synthesis of 3'-O-beta-D-ribofuranosyl-2'deoxyadenosines and its alpha-anomer.