The Claisen rearrangement is a chemical reaction that involves the rearrangement of allyl aryl ethers to form ortho-allyl phenols. This reaction typically occurs when the ether is heated in the presence of a base, leading to the migration of the allyl group to the ortho position of the aromatic ring. This rearrangement is significant in organic chemistry because it allows for the synthesis of complex molecules from simpler precursors. It is widely used in the development of pharmaceuticals and natural products, showcasing the versatility of organic synthesis techniques.