Synthetic potential of ring expansions of 5-membered carbo- & heterocycles: A review

Ring expansion reactions provide efficient synthetic approaches for the synthesis of a wide spectrum of biologically active compounds. This review covers the major synthetic routes attempted during the last two decades for forming large size carbocycles as well as heterocycles including lactams, boracycles, benzocycloheptenones, azaborinine and azepine derivatives via 5-membered ring expansion reactions. A number of rearrangement and cycloaddition reactions such as photochemical Fries rearrangement, Beckmann rearrangement, oxy-Cope rearrangement, Pauson-Khand reaction, Beckwith-Dowd ring expansion, metal-free decarboxylative ring expansion, hydropalladation etc. have been discussed.