Chemical Synthesis of Oligonucleotides by Use of Phenylthio Group

Abstract

This review deals with recent studies of oligonucleotide synthesis using the phenylthio group as the phosphate protecting group. Several fundament! properties of the P-S bond in phosphorothioate compounds are described in detail. The chemical synthesis and properties of nucleoside phosphorothioate derivatives, which can be prepared via silyl phosphite intermediates, are described. Several interesting and unique properties of a new useful phosphorylating reagent, S, S-diphenyl phosphorodithioate (PSS) are described with some examples involving an application to oligodeoxyribonucleotide synthesis using the phenylthio group as the 5'-terminal or internucleotidic phosphate protecting group. The potential utility of pyridinium phosphonate (PPN) has been demonstrated as an efficient catalyst for dephenylthiolation of bis(phenylthio)phosphorylated nucleoside derivatives, which is required for stepwise condensation in these oligonucleotide syntheses via the phosphotriester approach. Further application of this thiophosphotriester method to the solid-phase synthesis of oligodeoxyribonucleotides is described. The background of developments of bifunctional condensing reagents such as MDS and DDS and their application to the liquid phase synthesis of oligonucleotides are also reviewed. Several methods using oximate reagents and (Bu3Sn)20 are exemplified for removal of the phenylthio group from fully protected oligonucleotides. The application of the phenylthio group as an activatable protecting group to the chemical synthesis of 7-methylguanosine (MMG)- and trimethylguanosine (TMG)-capped oligonucleotides. Other applications of this activation strategy to the synthesis of naturally occurring nucleic acids are also described. Finally, a plausible mechanism of the present thiophosphotriester method is discussed.