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.

Oxidative Cyclization of β-Aminoacrylamides Mediated by PhIO: Chemoselective Synthesis of Isoxazoles and 2H-Azirines

“…The reaction proceeded smoothly as indicated by TLC results and furnished two products, which were characterized as isomeric isoxazoles 3s and 5s on the basis of their spectral and analytical data (Scheme ). Obviously, 3s and 5s were formed through chemoselective rearrangement of 2 H -azirine 4s , , and the selectivity might be attributed to the different reactivity of O -atom in a ketone or an amide group . This finding provided an alternative protocol for the synthesis of fully substituted isoxazoles of type 3 and 5 .…”

“…21 Obviously, 3s and 5s were formed through chemoselective rearrangement of 2H-azirine 4s, 25,26 and the selectivity might be attributed to the different reactivity of O-atom in a ketone or an amide group. 27 This finding provided an alternative protocol for the synthesis of fully substituted isoxazoles of type 3 and 5.…”

“…The transformation is initiated from the nucleophilic addition of acetic acid to idosobenzene to form a tricoordinated iodine species, i.e., PIDA . Acrylamide 1 reacts with the in situ generated PIDA to give an intermediate A-1 , which can also be represented by its resonance structures, carbanion A-2 and enolate ion A-3 . , For both electronic and steric effects, carbanion A-2 is a favorable form when R 3 is alkyl or aryl group, wherein carbanion is particularly stabilized by the adjacent electron-withdrawing carbonyl groups and iminium moiety, and undergoes a cyclization to afford intermediate B along with the release of phenyl iodide and acetate anion . The deprotonation of B by acetate anion produces the title 2 H -azirine 4 .…”

Synthesis of various functionalized 2H‐azirines: An updated library

“…The intramolecular cyclization process of 2‐methyl‐1‐phenyl‐3‐(sulfonyloxyimino)butan‐1‐ones under alkaline and acidic conditions were established, and the mechanical characteristics were studied by Ohwada et al They found that the reaction of the 2‐alkyl‐1‐phenyl‐3‐(sulfonyloxyimino)butan‐1‐one system under alkaline conditions (DABCO) resulted in the formation of neutral enols, which resulted 2 H ‐azirines (Scheme 17) [33].…”

Attached Nitrogen Test by 13C–14N Solid-State NMR Spectroscopy for the Structure Determination of Heterocyclic Isomers

“…Alarmingly, however, such an approach is more complicated when one considers that both the oxazole and isoxazole can be prepared from the same starting ketoxime (9, Scheme 1). 29 The 14 N-filtered 13 C NMR spectra of oxazole (10) and isoxazole (11) enable the straightforward differentiation of the heterocyclic isomers (Figure 3E,F). 10 displays two 13 C NMR signals with C−N bonds, while 11 only exhibits one 13 C NMR signal with a C−N bond.…”