A method for preparing a phosphitylated compound comprising the step of: -reacting hydroxyl containing compound with a phosphitylating agent in the presence of an activator having the formula (I) wherein R=alkyl, cycloalkyl, aryl, aralkyl, heteroalkyl, heteroaryl R.sub.1, R.sub.2=either H or form a 5 to 6-membered ring together. X.sub.1, X.sub.2=independently either N or CH Y.dbd.H or Si(R.sub.4).sub.3, with R.sub.4=alkyl, cycloalkyl, aryl, aralkyl, heteroalkyl, heteroaryl B=deprotonated acid. The hydroxyl containing compound is preferably a sugar moiety or a nucleoside or an oligomer derived therefrom. ##STR00001##