A General Method to Prepare α,β-Dehydroamino Acid Esters from the Addition of β-Ketoesters to Nitrosobenzenes.

α,β-Dehydroamino acids (ΔAAs) represent important structural motifs in natural products and synthetic peptides, and serve as versatile precursors for unnatural amino acids. This work demonstrates a mild, general, and efficient protocol for ΔAA synthesis via DBU-mediated addition of β-ketoesters to nitrosobenzenes. Notably, nucleophilic 3-benzoyllactones deliver lactone-bearing ΔAAs. Intriguingly, linear 2-arylmethyl 3-benzoyl esters afford (Z)-configured ΔAAs whereas other 2-alkyl 3-benzoyl esters produce (E)-ΔAAs. Mechanistic analysis validates a complex cascade process featuring a [1,3]-H-shift rearrangement.