Phenols diacetoxyiodo benzene

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). 

Hypervalent iodobenzenes have long been known as oxidants and their chemistry is summarized in an early volume of the present series . Some of them are shown in Chart 2. Herein, both (diacetoxyiodo)benzene [PhI(OAc)2] (347) and [di(trifluoroacetoxy)iodo]benzene [PhI(OCOCF3)2] (348) are most frequently and widely used for phenolic oxidation. l-(ferf-Butylperoxy)-l,2-benziodoxol-3(177)-one (349) is also used for phenol oxidation. Many invaluable books and reviews on hypervalent iodobenzene-promoted oxidation of phenols have appeared . 

SCHEME 73. Mechanism of phenol oxidation with (diacetoxyiodo)benzene... 

The preparation of iodonium phenolates 410 was first reported in 1977 via a reaction of phenols 409 with (diacetoxyiodo)benzene followed by treatment with pyridine (Scheme 2.119) [553]. The system of an iodonium phenolate is stabilized by the presence of at least one electron-withdrawing substituent on the aromatic ring. Monosubstituted iodonium phenolates 410 are relatively unstable and easily rearrange to iodo ethers 411 under heating. Such a 1,4 aryl migration is a very common phenomenon for iodonium ylides of types 405-408 according to mechanistic and computational studies it is an intramolecular rearrangement via a concerted mechanism [554,555]. 

Further examples of synthetic application of the para-alkoxylation reaction include the preparation of dimethoxy ketal 231, which is an essential precursor in the enantioselective synthesis of the potent antifungal agent (-)-jesterone [297], the synthesis of various dimethoxy ketals of para- and ort/to-benzoquinones [298] and the methoxylation of various phenolic substrates, such as 232, using (diacetoxyiodo)benzene in methanol (Scheme 3.95) [299-301]. 

A number of S Ar reactions have also been developed in which the electrophile is generated by an oxidative process. For example, She and coworkers recently used a (diacetoxyiodo)benzene-promoted phenol oxidation to generate the cationic species (24) that undergoes cychzation to the natural product— gymnothehgnan N (25, Scheme 1.7) [15]. Other oxidative synthetic methods have been developed for electrophilic halogenation [16], aminations [17], and nitrations [18]. 

When phenols such as 72 are treated with (diacetoxyiodo)benzene in hexafluoro-isopropanol, phenoxenium ions 73 are produced. A subsequent 1,2-shift involving the migration of the more electron-rich substituent produces a dienone 74 (Scheme 22) [20, 21]. Although the allyl group appears to be the best substituent for the 1,2-shift, the reaction can also be performed with an alkyl or aryl group as the migrating substituent. The process also extends to bicyclic phenols 75, yielding dienones of type 76 as reaction products. 

Dearomatization of phenols can also trigger a tandem Prins-pinacoi process. When treated with (diacetoxyiodo)benzene, phenols 92 are converted into spiro-dienones... 

The tp50-rearrangement of arylsilanes can be triggered by dearomatization. When treated with (diacetoxyiodo)benzene, phenols such as 113 are transformed into dienones 116 as shown in Scheme 31 [27], During this process, the phenoxenium ion 114 is produced. This is then trapped by the aryl moiety to produce intermediate 115, and an attack of trifluoro-isopropanol on the silicon atom induces the rearomatization. 

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