Abnormal Claisen Rearrangement

Abstract The first example of abnormal Claisen rearrangement was reported by Lauer and Filbert in 1936. In contrast to the regular Claisen rearrangement (σ migration), the abnormal Claisen rearrangement usually occurs for allyl aromatic ethers. A similar reaction also occurs for the thermal rearrangement of cyclopropyl ketones to homoallylic ketones. The abnormal Claisen rearrangement is believed to proceed via two consecutive processes, i.e., the normal ortho Claisen rearrangement of γ‐alkylallyl aryl ether to an o ‐(α‐alkylallyl) phenol and the isomerization of the resulting phenol. In general, this type of abnormal Claisen rearrangement does not occur smoothly, except when in the presence of Lewis acids FeCl 3 , even though other Lewis acids (e.g., HfCl 4 , GaCl 3 , and ZrCl 4 ) have limited ability to accelerate such reaction. It is reported that the abnormal Claisen rearrangement can be prevented by the application of 1,1,1,3,3,3‐hexamethyldisilazane and N , O bis(trimethylsilyl)acetamide.