Dipivaloylketene and Its Dimers. versus Cycloaddition Reactions of α-Oxo Ketenes

Article abstract of DOI:10.1021/jo00052a018

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 α-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, respective ly.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 α-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 α-oxo ketenes are discussed.