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%.