a desired isomer is selectively prepared by phosphorolyzing and isomerizing an anomer mixture of a 1 - phosphorylated saccharide derivative while crystallizing one of the isomers to displace the equilibrium . furthermore , using the action of a nucleoside phosphorylase , a nucleoside is prepared from the 1 - phosphorylated saccharide derivative obtained and a base with improved stereoselectivity and a higher yield . this process is an anomer - selective process for preparing a 1 - phosphorylated saccharide derivative and a nucleoside .