Samarium diiodide-promoted intramolecular ketone–ester coupling reaction: novel cyclization and ring expansion pathway

When ethyl 2-substituted-1-indanone-2-carboxylates were treated with samarium diiodide (SmI2), ring expansion products such as 3-substituted-1,2-naphthoquinones were isolated. Alcohols were also obtained as the mixture of cis- and trans-isomers of hydroxy and ester substituents. A reaction mechanism involving intramolecular addition of samarium ketyl radicals to ester substituents followed by ring expansion was proposed for the formation of the one-carbon homologated products. Similarly, reaction of ethyl 1-substituted-2-oxo-1-cyclopentanecarboxylates with SmI2 produced 3-substituted-2-hydroxy-2-cyclohexenones along with the corresponding alcohols.