[2+4] CYCLOADDITION REACTIONS OF NEAT DIPIVALOYLKETENE

Various [2 + 4] hetero-Diels-Alder cycloaddition reactions of dipivalo ylketene (2) with polarized multiple bond systems are described. Aldeh ydes and ketones react with 2 to furnish 1,3-dioxin-4-ones 7, imines a nd heterocumulenes (X = C=N) form 1,3-oxazin-4-ones 9 and 11, respecti vely. Analogous reactions of 2 with ethyl vinyl ether, ethoxyacetylene , diethylcyanamide, and adamantanethione leading to [2 + 4] adducts 12 ,14,15 and 16 are also reported. All these cycloadditions proceed in h igh yield at very mild reaction conditions.