Allene epoxidation: synthesis of functionalized lactones by the DMDO oxidation of allenic acids

A series of allenic carboxylic acids has been converted to functionalized lactones by oxidation–cyclization promoted by dimethyldioxirane. These transformations are rationalized by the involvement of unisolated allene oxide and spirodioxide intermediates. The structures of the starting allenic acids and the reaction conditions determine which of these two intermediate species serves as the source of the isolated products. The use of prepared solutions of the oxidant generally proceed via spirodioxides; whereas in situ reactions normally give products derived from the allene oxides. When products are formed directly from allene oxides, keto lactones are formed. The corresponding spirodioxide intermediates give lactones with appropriately situated α-hydroxyketone moieties. An understanding of the regiochemistry of the epoxidations and the subsequent cyclizations of the reactive intermediates is developed, as are methods for the manipulation of the experimental conditions to favor the desired products.