Ortho-quinone monoketal

The oxidative activation of arenes is a powerful and versatile synthetic tactic that enables dearomatization to give useful synthons. Central to this chemistry are hydroxylated arenes or arenols, the phenolic functions of which can be exploited to facilitate the dearomatizing process by two-electron oxidation. Suitably substituted arenols can hence be converted, with the help of oxygen- or carbon-based nucleophiles, into ortho-quinone monoketals and ortho-quinols. These 6-oxocyclohexa-2,4-dienones are ideally functionalized for the construction of many complex and polyoxygenated natural product architectures. Today, the inherent and multiple reactivity of arenol-derived ortho-quinone monoketals and ortho-quinols species is finding numerous and, in many cases, biomimetic applications in modern organic synthesis. 

I 75 Oxidative Conversion ofArenols into ortho-Quinols and ortho-Quinone Monoketals... 

Here again, the cycloaddition is endo-selective, with only regioisomers 79 being formed, and, when using 2- and/or 3-substituted furans, only the unsubstituted furan double-bond reacts in these inverse electron-demand Diels-Alder processes [134-136]. Indoles, pyrroles, and thiophenes can also be made to react as dienophiles with ortho-quinone monoketals... 

Oxidation of orr/to-alkoxyphenols in alcohol solvents leads to highly reactive ortho-quinone monoketals, which undergo spontaneous dimerization unless appropriately substituted. In one useful example of such a [4 -f 2]-cycloaddition, monoketal (XXXVIII), derived from constant current electrolysis of phenol (XXXVII), dimerized to give the neolignan natural product asatone (XXXIX) in 34% yield [45] ... 

Fig. 14 Isoxazolines synthesis through 1,3-dipolar [3+2] cycloaddition reactions between ortho-quinone monoketals and benzonitrile oxides...

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