Chemoselectivity in reactions of an α-diazo-β-diketone with some conjugative double-bond systems

Reactions of 2-diazo-1,3-diphenylpropane-1,3-dione with alpha,beta -unsaturated aldehydes and ketones, and keto-imines, in refluxing anhydrous toluene indicate that benzoyl(phenyl)ketene, which is generated by the thermal Wolff rearrangement of 2-diazo-1,3-diphenylpropane-1,3-dione, shows a pronounced tendency to form chemospecific [2 + 4] Diels-Alder adducts with the carbonyl group in alpha,beta -unsaturated aldehydes and ketones, and the imine group in keto-imines. The reactivity in reactions of the alpha -diazo-beta -diketone with these conjugative double-bond systems is C=N > C=O > C=C. However, benzoyl(phenyl)ketene reacts with alpha,beta -unsaturated imines to produce chemospecific [2 + 2] cycloadducts: beta -lactams.