5-Endo-Dig Electrophilic Cyclization of α-Alkynyl Carbonyl Compounds: Synthesis of Novel Bicyclic 5-Iodo- and 5-Bromofuranopyrimidine Nucleosides

5-Endo-dig electrophilic cyclization of 5-alkynyl-2'-deoxyuridines with N-iodosuccinimide or N-bromosuccinimide in acetone at room temperature gives 3-(2'-deoxy-beta-D-ribofuranosyl)-5-halo-2,3-dihydrofuro[2,3-d]pyrimidin-2-ones that usually precipitate from the reaction mixture (86-74%).