AbstractAcylation on the 5î‐position of 2î‐O‐methoxytetrahydropyranyl ribonucleosidesla(B=U) and2a(B=Aan) with levulinic acid, followed by phosphorylation of the 3î‐hydroxy function with the monofunctional phosphorylating agent 2,2,2‐trichloroethyl 2‐chlorophenyl phosphoro‐chloridate (3), afforded the fully‐protected mononucleotides4aand5a, respectively, in reasonable yields. The latter compounds were key‐intermediates in the synthesis of fully‐protected tetradecaribonucleotide15a. The preparation of tetradecamer15awas achieved by using two specific deblocking procedures, i.e. (a) zinc in pyridine/2,4,6‐triisopropylbenzenesulfonic acid to cleave the 2,2,2‐trichloroethyl group and (b) hydrazine in pyridine/acetic acid for the rapid and selective removal of the levulinyl group. Deblocking of the 2‐chlorophenyl groups from15awas successfully accomplished by the action of fluoride ion. Attention is directed to anomalous reactions occurring with the us