Synthesis of Acetylene Linked Double-Nucleobase Nucleos(t)ide Building Blocks and Polymerase Construction of DNA Containing Cytosines in the Major Groove

(Cytosin-5-yl)ethynyl derivatives of pyrimidine and 7-deazaadenine 2-deoxyribonucleosides and nucleoside triphosphates (dNTPs) were prepared in one step by the aqueous Sonogashira coupling of unprotected halogenated nucleos(t)ides with 5-ethynylcytosine. The modified dNTPs were good substrates for DNA polymerases suitable for primer extension or PCR construction of DNA bearing acetylene-linked cytosine(s) in the major groove mimicking the flipped-out nucleotide.