Efficient access to 3′-O-phosphoramidite derivatives of tRNA related N6-threonylcarbamoyladenosine (t6A) and 2-methylthio-N6-threonylcarbamoyladenosine (ms2t6A)

Abstract

An efficient method of ureido linkage formation during epimerization-free one-pot synthesis of protected hypermodified N⁶-threonylcarbamoyladenosine (t⁶A) and its 2-SMe analog (ms²t⁶A) was developed. The method is based on a Tf₂O-mediated direct conversion of the N-Boc-protecting group of N-Boc-threonine into the isocyanate derivative, followed by reaction with the N⁶ 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 t⁶A-phosphoramidite monomer was obtained in a less laborious manner and in a remarkably better yield of 74%.