Thallium in organic synthesis. XLII. Direct oxidation of 4-substituted phenols to 4,4-disubstituted cyclohexa-2,5-dienones using thallium(III) nitrate

Bromide‐Assisted Oxidation of Substituted Phenols with Hydrogen Peroxide to the Corresponding p‐Quinol and p‐Quinol Ethers over WO42−‐Exchanged Layered Double Hydroxides

“…[1,2] Besides electrolytic [3] and photosensitized [4] approaches, procedures for quinol preparation require stoichiometric amounts of oxidants, such as Bi III or Tl III salts, [5,6] oxygen at high pressure, [7] hydrogen peroxide, [8] or particularly electrophilic halogens, [9] such as I III reagents, chlorine, or bromine. In view of the risks associated with handling such reactive species, much attention has recently been devoted to the in situ generation of halonium species from harmless halides by using oxygen or hydrogen peroxide.…”

“…Alkylated phenols undergo the Br À -assisted oxidative methoxylation in moderate to excellent yields, especially when the ortho and para positions are substituted ( Table 2, 1-4; 81-91 %). However, when the ortho or para positions are unsubstituted, stable brominated phenols are the main products (5)(6). This limitation does not hold for the p-alkoxylated phenols (7-10), and high yields of the p-benzoquinone monoketals are obtained, irrespective of the substitution pattern.…”

“…When Tl salts are used as the oxidant, 8 a is not accessible because of Tl III -induced transketalization. [6,13] Reactions with o-alkoxylated phenols, 11, are less selective and give rise to a mixture of products, including the o-quinone monoketal 11 b. The latter is unstable and dimerizes through a Diels-Alder mechanism.…”

Oxidative Conversion of Arenols into ortho ‐Quinols and ortho ‐Quinone Monoketals – A Useful Tactic in Organic Synthesis

“…In particular, McKillop and collaborators extensively studied the use of TNN in MeOH to oxidize 4-and a few 2-substituted phenols; this reaction is referred to herein as the McKillop oxidation ( Figure 14) [78,79]. As expected from Andersson's observations (Section 15.3.1), the 5-methoxylated ortho-quinone dimethyl monoketal 45a is stable, but the aldehyde 42a gave rise to the Diels-Alder dimer 46 under the conditions of the McKillop oxidation.…”

The scope of thallium nitrate oxidative cyclization of chalcones; synthesis and evaluation of isoflavone and aurone analogs for their inhibitory activity against interleukin-5

“…1). 13,16,17 Some findings indicate that the reaction pathway of cyclic oxidation depends on the nature of substituents present at the starting chalcones. They observed that 2 0 -hydroxychalcones substituted with chloro or nitro group at ring A on reaction with TTN gave aurone 3 through the corresponding (a-methoxybenzyl) coumaranones.…”