Phenols with lead tetraacetate

A typical example of the LCfi " type is the reaction of lead tetraacetate with enol-type substrates such as, among others, phenols and ketones. In these reactions, the acetoxy ligand plays the role of the... 

Lead tetraacetate The phenol, CC, in acetic acid treated with lead tetraacetate to give 6-aeetoxy,6-(3-methylenepent-4-enyl)-cyclohexa-2,4-dienone... 

Takacs [85] has separated the o- and p-benzoquinol acetates and o-quinone diacetate resulting from oxidation with lead tetraacetate of phenols containing an o-isopropyl, sec-butyl or tert.-butyl group. The o-derivatives were separated and purified on silica gel G. A 1 % solution of p-dimethylaminobenzaldehyde in concentrated sulphuric acid was... 

The 1,1-dimetallic compounds, R2C(SnMe3)ZnBr, were oxidized by dry air at —10 to 0°C in the presence of Me3SiCl to give aldehydes or ketones, R2C=0. In a related indirect method, arylthallium bis(trifluoroacetates) (prepared by 12-21) can be converted to phenols by treatment with lead tetraacetate followed by triphenylphosphine, and then dilute NaOH. Diarylthallium trifluoroacetates undergo the same reaction. ... 

Lead tetraacetate has been used to convert phenols, with a hydrazone group in the ortho position, to carboxylic esters, for example. 

It has been reported previously (147), and will be discussed in detail later, that arylthallium ditrifluoroacetates may be converted in good yield to phenols by treatment with lead tetraacetate in trifluoroacetic acid solution followed by addition of 1 mole of triphenylphosphine. The immediate product of the above reaction, an aryl trifluoroacetate, is hydrolyzed... 

Lead tetraacetate oxidation was applied to construct a benzo[c]-phenanthridine skeleton. The Hofmann degradation product 224 derived from the phenolic protoberberine 59a was oxidized with lead tetraacetate to afford the p-quinol acetate 225, which was cyclized to the benzo[c]-... 

A very simple synthesis of coumestrol (228) has been described by Kappe and coworkers (Scheme 46) (74ZN(B)292). It is based upon dehydrogenation of 4-hydroxy-3-phenyl-coumarins to coumestans (720PP233). A number of 2 -hydroxy 3-phenylcoumarins were oxidized with lead tetraacetate to the corresponding coumestans 3-(l-acetoxy-4-methoxy-2-oxo-3,5-cyclohexadienyl)coumarins were obtained as by-products (76BCJ1955). Coumes-tan itself (226) has been obtained by photolysis of the phenol ether (232), which is in turn available from 4-hydroxycoumarin (229) and (diacetoxyiodo)benzene (Scheme 47) (78CB3857) via an iodonium ylide (231). 

The starting material for the present synthesis was Wieland-Miescher ketone (24), which was converted to the known alcohol (25) by the published procedure [10], Tetrahydropyranylation of alcohol (25) followed by hydroboration-oxidation afforded the alcohol (26), which on oxidation produced ketone (27). Reduction of (27) with metal hydride gave the alcohol (28) (56%). This in cyclohexane solution on irradiation with lead tetraacetate and iodine produced the cyclic ether that was oxidized to obtain the keto-ether (29). Subjection of the keto-ether (29) to three sequential reactions (formylation, Michael addition with methyl vinyl ketone and intramolecular aldol condensation) provided tricyclic ether (30) whose NMR spectrum showed it to be a mixture of C-10 epimers. The completion of the synthesis of pisiferic acid (1) did not require the separation of epimers and thus the tricyclic ether (30) was used for the next step. The conversion of (30) to tricyclic phenol (31) was... 

Esters of phenols are obtained by treatment of aromatic hydrocarbons with peroxy acids [292], organic peroxides [1119, 1120], and lead tetraacetate or tetrakis(trifluoroacetate) [435, 449]. Benzene treated with tri-... 

Thallinm(ni) °, particnlarly as its triflnoroacetate salt , has been successfully used for the synthesis of phenols. This method can be carried out in a single step and is subject to isomer orientation control" . The aromatic compound to be hydroxylated is first thallated with thallium trifluoroacetate (TTFA)" and, by treatment with lead tetraacetate followed by triphenylphosphine and then dilute NaOH, it is converted to the corresponding phenol (equation 57). Table 1 shows some examples of these transformations . ... 

Lead tetraacetate, Pb(OAc)4, is weU known to be effective for phenolic oxidation. From the viewpoint of organic synthesis, it is noted that Wessely oxidation of ortfio-substimted phenols with Pb(OAc)4 provides a useful method for the synthesis of cyclohexadienones, as shown in Scheme 170 " . Herein, heterolytic cleavage of the initially formed O—Pb bond followed by nucleophilic attack by RCOOH results in the preferential formation of 2-acyloxycyclohexa-2,4-dienones over 4-acyloxycyclohexa-2,5-dienones. 

SCHEME 170. Phenolic oxidation with lead tetraacetate... 

In acetic acid lead tetraacetate usually oxidizes phenols to the corresponding quinones or their derivatives. The product is highly dependent on the substituents on the aromatic ring. The reaction of dimethoxyphenol with lead tetraacetate furnished the monooxidation product quinone ketal in excellent yield (Scheme 13.37 compare with Scheme 13.32) [55]. 

Compounds presenting the possibility of keto-enol tautomerism react easily with lead tetraacetate to afford products of a-acetoxylation. This reaction is found in the case of ketones and phenols, although it is frequently accompanied by other products derived from alternate mechanistic pathways. 

Oxidation of phenols with lead tetraacetate was extensively studied by the group of Wessely.l " The nature of the products is dependent upon a number of factors, such as the nature and position of the substituents, the nature of the solvent and the ratio lead tetraacetate substrate. In acetic acid. 

It is now generally accepted that the first step involves the formation of aryloxy-lead (IV) triacetates (1). These can decompose homolytically, leading to dimeric products via the aryloxyl radicals (2). They can also decompose heterolytically to cationic aryloxy species, which are then trapped by external nucleophiles. This has been exemplified by the reaction of phenols with lead tetraacetate in the presence of acetic acid or methanol. In the latter case, ortho-mcthoxy derivatives were formed. However, the preferential formation of 6-acetoxy-2,4-dienone derivatives is more likely explained by an intramolecular ligand coupling reaction (3) rather than by occurrence of a cationic... 

Synthesis of phenols and aromatic nitriles.3 In a new phenol synthesis, an aromatic hydrocarbon is first thallated with the reagent in trifluoroacetic acid to give an arylthallium ditrifluoroacetate (which can be isolated) and then oxidized with lead tetraacetate in the presence of 1 equivalent of triphenylphosphine. The resulting aryl trifluoroacetate is then hydrolyzed with dilute base. The triphenyl-... 

Discovery of the dienone-phenol rearrangement of quinol acetates has made possible the synthesis of dihydric phenols that were difficult of access by other routes. The starting materials are obtained from phenols and lead tetraacetate, and with acetic anhydride and sulfuric acid (Thiele acetylation) or with boron trifluoride in ether they give, respectively, di- and mono-acetyl derivatives of resorcinol or hydroquinone.309 When treated with lN-sodium hydroxide, 0-quinol acetates of type (1) undergo nucleophilic addition of an OH" ion, giving resorcinol derivatives (2).310 Occurrence of the reaction is considered... 

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