Efficient access to 3'- O -phosphoramidite derivatives of tRNA related N 6 -threonylcarbamoyladenosine (t 6 A) and 2-methylthio- N 6 -threonylcarbamoyladenosine (ms 2 t 6 A).

An efficient method of ureido linkage formation during epimerization-free one-pot synthesis of protected hypermodified N 6 -threonylcarbamoyladenosine (t 6 A) and its 2-SMe analog (ms 2 t 6 A) was developed. The method is based on a Tf 2 O-mediated direct conversion of the N -Boc-protecting group of N -Boc-threonine into the isocyanate derivative, followed by reaction with the N 6 exo -amine function of the sugar protected nucleoside (yield 86-94%). Starting from 2',3',5'-tri- O -acetyl protected adenosine or 2-methylthioadenosine, the corresponding 3'- O -phosphoramidite monomers were obtained in 48% and 42% overall yield (5 step synthesis). In an analogous synthesis, using the 2'- O -( tert -butyldimethylsilyl)-3',5'- O -(di- tert -butylsilylene) protection system at the adenosine ribose moiety, the t 6 A-phosphoramidite monomer was obtained in a less laborious manner and in a remarkably better yield of 74%.