Contrasting C- and O-Atom Reactivities of Neutral Ketone and Enolate Forms of 3-Sulfonyloxyimino-2-methyl-1-phenyl-1-butanones.

Abstract

The mechanisms of intramolecular cyclization of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones (1) under basic (DABCO and t-BuOK) and acidic (AcOH and TFA) conditions were investigated by means of experimental and computational methods. The ketone, enol, and enolate forms of 1 can afford different intramolecular cyclization products (2, 3, 4), depending on the conditions. The results of the reaction of 1 under basic conditions suggest intermediacy of neutral enol (DABCO) and anionic enolate (t-BuOK), while the results under acidic conditions (AcOH and TFA) indicate involvement of neutral ketones, which exhibit reactivities arising from both the oxygen lone-pair electrons (O atom reactivity) and carbon σ-electrons (C atom reactivity). The neutral enol in DABCO afforded 2H-azirine 4. On the other hand, the products (isoxazole 2 and oxazole 3) generated from the ketone form and from the enolate form are the same, but the reaction mechanisms are apparently different. The results demonstrate ambident-like reactivity of neutral ketone in the 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanone system.