Synthesis of 5,6-bis(alkyn-1-yl)pyrimidines and related nucleosides

Sonogashira coupling of 5-iodo-1-methyluracil or 5′-O-TBDMS-5-iodo-2′,3′-O-isopropylideneuridine with alkynes gave 5-(alkyn-1-yl) derivatives that underwent 6-lithiation/iodination to give 5-(alkyn-1-yl)-6-iodo-(1-methyluracil or uridine) intermediates (57–80%). Coupling of the 5-(alkyn-1-yl)-6-iodo intermediates gave 5,6-bis(alkyn-1-yl)pyrimidines and protected nucleosides (51–79%). Two of the 5,6-bis(ethynyl)-1,3-dimethyluracil derivatives underwent Bergman cycloaromatization at 130°C with half-lives of 2–8 h.