Structural and mechanistic studies of the base-induced Sommelet-Hauser rearrangement of N-α-branched benzylic azetidine-2-carboxylic acid-derived ammonium salts

The base-induced Sommelet-Hauser rearrangement of N-alpha-branched benzylic azetidine-2-carboxylic acid ester-derived ammonium salts to obtain a-arylazetidine-2-carboxylic acid esters was investigated. The substrates, two diastereomeric salts (1S,2S,1'S)-and (1R,2R,1' S)-2, showed different reactivities. The rearrangement of (1S,2S,1'S)-2a proceeded with a perfect N-to-C chirality transfer to provide (R)-3a in 74% yield with 99% ee. However, the rearrangement of (1R,2R,1'S)-2a under the same conditions afforded (S)-3a in only 15% yield with a lower 66% ee, along with the competitive [1,2] Stevens rearrangement product 4a. Structural and mechanistic studies of this rearrangement were carried out to clarify the exact reason. Our results define the scope and limitations of the Sommelet-Hauser rearrangement and provide unique synthetic access to alpha-aryl amino acid derivatives.