Ring expansions of alkynyl cyclopentanols with iodine and Koser's reagent.

Equimolar amounts of iodine, Koser's reagent, and 1-bromoethynylcyclopentanols with none, one or two methyl groups in the α-position lead to ring expansions in acetonitrile at room temperature in yields of 75 to 85%. In the so-formed 2-[bromoiodomethylidene]cyclohexanones, the (Z) isomer was preferred exclusively in the unsubstituted compound. The ZE ratio was 7.7 for the dimethyl compound and 3.3 for the monomethyl case. The corresponding bromine and Koser's reagent with 1-iodoethynylcyclopentanol afforded a ring expanded product with ZE ratio of 12:1.