Halocyclization of Unsaturated Guanidines Mediated by Koser’s Reagent and Lithium Halides

The synthesis of halogenated cyclic guanidines through iodine­(III)-mediated umpolung of halide salts is described. Cyclic guanidines of various sizes can be obtained with generally excellent regioselectivities through either a chloro- or a bromocyclization, using Koser’s reagent and the corresponding lithium salt.