Synthesis and enzymatic incorporation of a novel, bicyclic pyrimidine nucleoside: a thymidine mimic

Nucleophilic ring-opening and rearrangement reaction of a furanopyrimidine nucleoside with anhydrous hydrazine provided a novel, 6,6-bicyclic pyrimidopyridazin-7-one nucleoside (dH, 4), whose structure was confirmed by X-ray crystallography. This novel nucleoside was converted to its 5′-triphosphate (dHTP) for studies with DNA polymerases and incorporated into a template by using standard phosphoramidite chemistry. In the template, dH directed the incorporation of dATP and to a lesser extent dGTP into the transcript and dHTP was efficiently incorporated at the 3′-end of a primer opposite dA using both exonuclease free Klenow fragment (KF exo-) and Taq DNA polymerases and extended with natural dNTPs.