Synthesis and Applications of Quinone Monoketals

Simple quinone monoketals, such as dimethyl ketal variants of t5q)e 9/10 (Fig. 7), constitute the second most often used group of simple quinonoids in s5mthesis. They are usually described as monoprotected or masked forms of their quinone counterparts, with which they share some of the basic reactivity features related to the 

Peddinti also reported a simple and efficient one-pot synthesis of benzoxazolic bicyclo[2.2.2]octenones 29-31 by subjecting 2-methoxy-substituted phenolic aldimines of type 26 to a similar treatment with DIB in the presence of the same kind of dienophiles 24, or furans, in MeOH [47], This clever domino reaction starts with a DIB-mediated oxidative cychzation of the phenolic aldimines into the phenolic benzoxazoles 27, which are then converted, with a second equivalent of DIB, into the MOBs 28 that are finally trapped with excess of dienophile to furnish selectively the expected [4+2] cycloadducts 29-31 (Fig. 11) [47]. 

The above examples of intermolecular Diels-Alder reactions should not let us forget that MOBs and other orf/io-quinone monoketals, and their ort/io-quinol 

Z=various subslitaents at position or bdqf substituents at posilion-4 

Another aspect of the versatile reactivity of ortho-qaiaont monoketals was exploited by Chittimalla s and Suzuki s groups [51, 52], who reported examples of [3+2] cycloaddition reactions involving non-dimerizing MOBs of type 10 and benzonitrile oxides 39. The reactions are site- and regioselective, primarily affording isoxazolines 40, as a result of the 1,3-dipolar cycloaddition of the nitrile oxides to the MOBs 2,3-C-C double bond (Fig. 14). 

---