Synthesis and DNA incorporation of an ethynyl-bridged cytosine C-nucleoside as guanosine surrogate.

As a guanosine mimic that lacks the preference for syn or anti conformation a cytosine C-nucleoside was synthesized connecting the nucleobase at the anomeric center by an ethynyl linker. The key step was a Sonogashira cross coupling of 5-iodocytosine with 1'-ethynyl-2'-deoxyribose. The new C-nucleoside incorporated into G/C-alternating oligonucleotides emerged as guanosine substitute, however, with reduced duplex stability. B-Form DNA was strongly stabilized by the new surrogate even in typically Z-DNA forming sequences and in Z-form inducing environment.