Allenyl Ketones as Versatile Michael Acceptors for the Addition of Chelated Enolates

Chelated amino acid ester enolates undergo clean 1,4-addition towards allenyl ketones 9, giving rise to unsaturated (delta)-keto amino acid esters 12 at low temperature. If the reaction is allowed to warm to room temperature, the enolate intermediates A undergo cyclization towards the corresponding (alpha)-pyrones 13.