Dipivaloylketene and its dimers. [2+4] Versus [2+2] cycloaddition reactions of .alpha.-oxo ketenes

Dipivaloylketene (8) is generated by preparative flash vacuum pyrolysis of furandione 6. While dimerization of 8 in apolar and several polar solvents leads to the previously reported [2 + 4] dimer 7, dimerization in the presence of DMSO, tributylphosphine oxide, or pyridine instead surprisingly occurs across the C=O bond of the ketene function, affording dioxinone 9. This is a novel type of alpha-oxo ketene dimerization. The reversibility of both dimerizations and mechanistic pathways for the formation of 9 are discussed. Cycloaddition reactions of 8 with heterocumulenes 10a,b and 12a-c yield [2 + 4] cycloadducts 11a,b and 13a-c, respectively. Oxazinones 13a-c can add a second molecule of 8 to generate novel spiro heterocycles 14a-c. In contrast, oxo ketene 7 reacts with carbodiimides 12a,b, to furnish [2 + 2] adducts 15a,b. This is the first unequivocal example of a direct [2 + 2] cycloaddition reaction of an alpha-oxo ketene. The unexpected formation of these compounds was confirmed by X-ray crystallography (15a). General aspects of [2 + 2] versus [2 + 4] cycloaddition in alpha-oxo ketenes are discussed.