Spirodienones part 81. The direct preparation of spirodioxole — and spirobenzoxazole - cyclohexadienones by the oxidation of 4 - aryloxyanilines

Oxidative Dearomatization of Phenols and Anilines via λ3- and λ5-Iodane-Mediated Phenylation and Oxygenation

“…Quinonoid compounds are powerful intermediates for organic synthesis, and those bearing a nitrogen functionality obviously expand their versatility as potential precursors and building blocks for the synthesis of biologically relevant natural and non-natural substances [ 29 ]. Among those, quinonoid imines such as quinone imines, quinone imine ketals and quinol imines occupy a valuable position as potentially useful synthons, but their imine function is unfortunately very sensitive to hydrolysis, unless the imine nitrogen is substituted by an acyl or a sulphonyl group [ 30 , 31 ] Most previous studies on quinonoid imines and imides have focused on para -compounds, and their generation was based on electrochemical or chemical oxidation of aniline and anilide parents [ 30 , 31 ]. Iodane reagents have seldom been used for mediating such transformations [ 32 , 33 , 34 ], and the only examples in the ortho -series [ 35 ] are those recently reported by Nicolaou and his co-workers, who exploited Dess–Martin periodinane (DMP) generated ortho -quinone imides in Diels–Alder reactions [ 29 , 36 , 37 ]; DMP belongs to the ArIL 4 type of the λ 5 -iodane reagents.…”

“…Furthermore, this first reaction with aniline 7a showed us that IBX is capable of selectively delivering an oxygen at the positions adjacent to the amine function, in a manner probably similar to the one that we and others have suggested for analogous IBX- and SIBX-mediated ortho - oxygenations of phenols ( Scheme 7 ) [ 4 , 5 , 12 ]. Having thus successfully tested the feasibility of dearomatizing an aniline into an ortho -quinol imine, the same reaction was performed using the anilide 7b in the hope of improving its yield, for quinonoid imides are known to be stable enough for isolation as opposed to imines that are sensitive to hydrolysis and polymerization [ 29 , 30 , 31 , 37 ]. In addition, the lower nucleophilicity of the anilide nitrogen should preclude nucleophilic trapping of the ortho -quinone imine formed.…”

Cyclohexadienone Ketals and Quinols: Four Building Blocks Potentially Useful for Enantioselective Synthesis

“…Coutts finds that upon oxidation of 402 with MnO 2 , the MPB ketal 403 forms (Figure ). This material undergoes an S N 2‘ displacement with MeMgI to afford the phenol 404 . The success of this reaction, in regard to a competitive 1,2-addition to the carbonyl, is most likely a consequence of the aryl ketal and the stability of the resulting magnesium phenoxide.…”

Oxidation of heterocyclic compounds by manganese dioxide (Review)*

“…The previously discovered effectiveness of MnO 2 in the cyclization of substituted phenols with the formation of heterocyclic spiro compounds [108] was recently [9] confirmed for the case of the analogous transformation of 4-(2-hydroxyphenoxy)anilines 113 into the spiro compounds 115. It is considered that the reaction takes place through a biradical, which cyclizes to the ketal of quinone imine 114, where the NH group is readily hydrolyzed to a ketone.…”

“…The low yields of the products 115 (31-40%) are probably due to polymerization of the intermediate radicals and the formation of azobenzene derivatives from them. According to the same paper [9], in compounds 116 the substituted NH group takes part in cyclization, leading to 50-60% yields of the products 117 containing an oxazolidine ring. …”