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%).