Quinone-mono-ketals

Quinone-mono-ketals 46 and 47 are also low reactive dienophiles and are sensitive to Lewis-acid catalysts. The use of high pressure overcomes this limitation [17]. As shown in Equation 5.7, cycloadditions with a variety of substituted 1,3-butadienes 48 occur regioselectively and c This approach provides access to a variety of annulated benzenes and naphthalenes after aromatization of adducts 49. 

The high pressure Diels-Alder reactions of quinone-mono-ketals [80]... 

Constant-current electrolysis (CCE) of para-alkoxyphenols (XXXII) in alcohol solvents, usually methanol, provides ready access to the synthetically useful quinone mono-ketals (XXXIII), as in Eq. (13) [41]. In many cases, the yields are superior to those obtained via chemical oxidation of the phenols with T1(N03)3, DDQ, or FeCl3 [42 and references cited therein]. 

Dihydrophenanthrene synthesis. Evans et al. have reported a new route to dihydrophenanthrene derivatives based on the condensation of p-quinone mono-ketals (1) or monosilyl cyanohydrin derivatives (5, 721-722) with the enolate of methyl 3-(3,4,5-trimethoxyphenyl)propionate (2) to give p-quinol ketals (3). These undergo acid-catalyzed cyclization to dihydrophenanthrene derivatives (4). A typical example is formulated in equation (I). The choice of the Lewis acid catalyst is sometimes critical for the cyclization. In the case of the cyclization shown above, use of SnCL, CF3COOH and BF3 etherate resulted in much lower... 

This reaction has been modified by using quinone mono-ketals to control the regioselectivity. In addition, an aza-Nenitzescu reaction involving the condensation between quinones and hydrazones has been developed for the synthesis of indazole derivatives. ... 

Even though a Birch reduction to give the requisite dihydronaphthalene for the Plieninger indolization is obviously successful, it is not an attractive or general method for providing these dihydronaphthalenes. In 1995, Kerr combined a high-pressure Diels-Alder cycloaddition of quinone mono ketals with 1,3-butadienes to give cycloadducts that readily aromatize to 1,4-dihydronaphthalenes (Scheme 3,... 

Quinone imine ketals have also been recognized to be quite useful for heterocycle synthesis. In a series of quinone mono- and bis-ketal chemistry, Swenton and coworkers carried out anodic oxidation of trifluoroacetamido-substituted p-methoxyphenols . For example, the readily available p-methoxyphenol derivative 88 underwent constant current electrolysis (60 mA) in 2% LiC104 in methanol, followed by hydrolysis with 5% aqueous KOH to afford quinone imine ketal 89 in 82% overall yield, through quinone monoketal 90 (Scheme 17). Furthermore, acid treatment of 89 with TsOH provided 5-methoxyindole (91). 

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