Benzoquinone monoacetals 1,4-addition

In the case of benzoquinone monoacetals 61, the two substituents at the 4-position are equal, and side-selective addition Re versus Si face) creates a single stereocenter (Scheme 7.17(a)). In the (S, R, R)-18/Cu(OTf)2-catalyzed 1,4-addition, depending on the nature of the R2Zn reagent and the size of the acetal moiety, enantioselectivities ranging from 85-99% were found (Table 7.4). The highest ees are provided by a combination of a small acetal moiety and Me2Zn 99% ee was obtained with 4,4-dimethoxy-5-methyl-2-cyclohexenone, for example. 

The DA reaction of methacrolein with 1,3-dienol derivatives was catalyzed by BINOL derived titanium complexes, giving the endo adduct with high enantiose-lectivity [155]. The reaction was performed in the absence of MS, since MS can act as an achiral catalyst in this reaction and compromise the product enantiomeric excess. Similarly, the DA reaction of juglone with butadienyl acetate catalyzed by BINOL-Ti complex proceeds in only 9% ee in the presence of MS, whereas the reaction proceeds in 76-96% ee in the absence of MS to provide the endo adducts useful for the synthesis of anthracyclines and tetracycline [155,156). The drawback of the above method is the instability of the addition products, owing to their high tendency to aromatize. Corey improved this DA reaction with quinone monoacetal as a supplant of juglone or benzoquinone [157] (Scheme 14.64). 

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