Base-Induced Sommelet–Hauser Rearrangement of azetidinic acid-derived ammonium salts

What is it about?

The reaction proceeded in good yield to afford the desired alpha-aryl azetidinic acid esters without any detectable amount of the competitive [1,2] Stevens rearrangement products. The ring-strain of four-membered N-heterocycles unstabilizes an ammonium enolate form and enables the efficient genaration of a carbanionic ylide, which is a desired intermediate for the Sommelet–Hauser rearrangement. This effect would enhance the rate of the Sommelet–Hauser rearrangement. As a further synthetic application, the asymmetric rearrangement involving N-to-C chirality transmission followed by the regio- and stereo-controlled nucleophilic ring-openings of the products were demonstrated. The corresponding acyclic quaternary alpha-aryl amino acid esters were obtained with excellent enantio-purities by these successive stereoselective transmissions.

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Why is it important?

Although the Sommelet–Hauser rearrangement still has structural limitations, our method provides efficient access to various types of -aryl amino acid derivatives. This study will expand the synthetic scope and utility of not only the Sommelet–Hauser rearrangement but also analogous sigmatropic rearrangements.