A green to chestnut colouring appears in the reaction medium and in this way the triprotected starting nucleoside derivative is formed, namely the derivative of formula (I) in which B represents the radical of formula (II), guanine derivative, R1 represents a hydrogen atom, R2 represents the 2-chlorophenoxy radical, R3 and R4 represent the 2-chlorophenoxy acetyl radical and R5 is a hydrogen atom.