The invention is also directed to monomeric building blocks of formula ##STR3## wherein R5 and R6 are independently hydrogen or a hydroxy protecting group or R5 and R6 taken together are a 1,2-dihydroxy protecting group, for example a ketal; R3 ' is hydrogen or lower alkyl; R7 is hydrogen or an amine protecting group; R8 is hydrogen or an acid protecting group; and B is a nucleobase or protected n