Nucleoside phosphoramidites are derivatives of natural or synthetic
nucleosides. They are used to
synthesize oligonucleotides, relatively short fragments of
nucleic acid and their
analogs. Nucleoside phosphoramidites were first introduced in 1981 by Beaucage and Caruthers. In order to avoid undesired side reactions, reactive hydroxy and exocyclic amino groups present in natural or synthetic nucleosides are appropriately protected. As long as a nucleoside analog contains at least one hydroxy group, the use of the appropriate protecting strategy allows one to convert that to the respective phosphoramidite and to incorporate the latter into synthetic nucleic acids. In order to be incorporated in the middle of an oligonucleotide chain using phosphoramidite strategy, the nucleoside analog have to possess two hydroxy groups or, less often, a hydroxy group and another nucleophilic group (amino or mercapto). Examples include, but are not limited to,
alternative nucleotides,
LNA,
morpholino, nucleosides modified at the 2'-position (OMe, protected NH
2, F), nucleosides containing non-canonical bases (
hypoxanthine and
xanthine contained in natural nucleosides
inosine and
xanthosine, respectively, tricyclic bases such as G-clamp, etc.) or bases derivatized with a fluorescent group or a linker arm.