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

Abstract

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 Z E 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 Z E ratio of 12:1.