Cytosine is silylated by the general method of Wittenberg. It is reacted with 1,2,3,4,6-penta-O-acetyl-β-D-glucopyranose in 1,2 dichloroethane followed by tin tetrachloride in 1,2-dichloroethane (or acetonitrile) according to the method of Vorbruggen and Niedballa to provide the 1-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)cytosine 33.1b (Xi=β-cytidin-1-yl). Following methanolysis with sodium methoxide in methanol the 1-(β-D-glucopyranosyl) cytosine is obtained. Following protection of the heterocyclic amine this is converted into the morpholino subunit 32.2b (Xi=β-N4-benzoylcytidin-1-yl) by the general procedure.
