Chemical Synthesis of Oligoribonucleotide (ASL of tRNA T. brucei) Containing a Recently Discovered Cyclic Form of 2-Methylthio-N -threonylcarbamoyladenosine (ms ct A).

The synthesis of the protected form of 2-methylthio-N -threonylcarbamoyl adenosine (ms t A) was developed starting from adenosine or guanosine by using the optimized carbamate method and, for the first time, an isocyanate route. The hypermodified nucleoside was subsequently transformed into the protected ms t A-phosphoramidite monomer and used in a large-scale synthesis of the precursor 17nt ms t A-oligonucleotide (the anticodon stem and loop fragment of tRNA from T. brucei). Finally, stereochemically secure ms t A→ms ct A cyclization at the oligonucleotide level efficiently afforded a tRNA fragment bearing the ms ct A unit. The applied post-synthetic approach provides two sequentially homologous ms t A- and ms ct A-oligonucleotides that are suitable for further comparative structure-activity relationship studies.