Nucleic acid related compounds.: 141.: Addition of difluorocarbene to 3′,4′-unsaturated nucleosides:: Synthesis of 2′-deoxy analogues with a 2-oxabicyclo[3.1.0]hexane framework

[GRAPHIC] Treatment of protected 2'-deoxy-3',4'-unsaturated nucleosides derived from adenosine and uridine with difluorocarbene [generated from bis(trifluoromethyl)mercury and sodium iodide] gave fused-ring 2,2-difluorocyclopropane compounds. Stereoselective alpha-face addition to the dihydrofuran ring resulted from hindrance by the protected beta-anomeric nucleobases. A protected uracil compound was converted smoothly into the cytosine derivative via a 4-(1,2,4-triazol-1-yl) intermediate. Removal of the protecting groups gave new difluorocyclopropane-fused nucleoside analogues. The solid-state conformation of the nearly planar furanosyl ring in the uracil compound had a shallow E-2 pucker, and a more pronounced E-1 conformation was present in the furanosyl ring of the cytosine derivative.