The present invention relates to large scale preparation of LNA phosphoramidites using a 2-cyanoethyl-N,N,N',N'-tetra-substituted phosphoramidite and a nucleophilic activator, e.g. 2-cyanoethyl-N,N,N',N'-tetraisopropylphosphoramidite and 4,5-dicyanoimidazole. The method is faster and more cost efficient that previously known methods.