Recent Advances in the Metal-Catalyzed Ring Expansions of Three- and Four-Membered Rings

New catalytic ring-expansion reactions of strained ring (hetero- and carbocyclic) substrates reported in the last six years (2006–2012) are presented. As evident from the diversity of new approaches, this is a vibrant area of research. Metals ranging from magnesium to gold have been used as catalysts. Some of these reactions allow access to enantioenriched products by employing catalysts decorated with chiral organic motifs (primarily C-2 symmetric).