The mechanisms of both ring-expansion reactions were investigated extensively, employing deuterium labelled methylenecyclopropyl amide substrates.The magnesium iodide-mediated ring expansions of secondary methylenecyclopropyl amides have been examined in detail. The scope of the previously reported rearrangement reaction has been successfully extended, permitting the synthesis of a library of heteroaryl-substituted 4-methyl-1,5-dihydro-pyrrol-2-ones. Development of a novel, complimentary synthetic procedure enabled access to the isomeric 4-methylene-pyrrolidin-2-one products.