Anthrone, Michael reactions

In place of active methylene compounds having a nitrile group, malonates, 13-ketoesters, 1,3-diketones, 1,1-disulfones, nitro compounds, Mel drum acid, and anthrone can also be used as the Michael donors for these ruthenium-catalyzed aldol and Michael reactions. The reaction proceeds well in acetonitrile under mild and neutral conditions (Eq. 9.59) [83]. 

It may be possible that the anthrone undergoes the Michael Reaction with acrolein, and the cyclized product is then oxidized by anthraquinone and the latter is reduced to anthrone, as shown by the partial illustration of the mechanism. This route is consistent with the relatively low yield of 50-60% ... 

Scheme 3.13 Michael reaction of cyclopentadiene or anthrone with a,P-unsaturated aldehydes.

Recently, Tan reported related bicyclic guanidine 20 as a chiral Br0nsted base to promote the highly enantioselective Diels-Alder reaction of various anthrones and maleimides (Scheme 5.41) [75]. Interestingly, use of dithranol led to the exclusive formation of the enantio-enriched Michael adducts. 

Chiral bicyclic guanidine (221) has been identified as an excellent catalyst for reactions between anthrones (219) and various dienophiles, such as (220). The catalyst can tolerate a range of substituents and substitution patterns, making several anthrone derivatives suitable for this reaction. Both Diels-Alder and Michael adducts were obtained in excellent yields, high regioselectivities, and high enantioselectivities (<99% ee). This is the first case of a highly enantioselective base-catalysed anthrone Diels-Alder reaction.263... 

The cycloaddition is ascribable to the oxyanion of hydrogen-bonded enolate (ArO —HNEt3 ) rather than to the hydrogen-bonded enol (ArOH—NEtj). An enantioselective version of the reaction was achieved by using a homochiral amine [27]. Similarly the reactions with less reactive dienophiles such as dimethyl fumarate, fumaronitrile, maleonitrile and methyl acrylate give the Diels-Alder adducts quantitatively when the cycloadditions are carried out in THF or CHCI3 in the presence of EtjN, while in MeOH Michael adducts were isolated. Experimental evidence supports the hypothesis that the base-catalyzed cycloadditions of anthrone with dienophiles are concerted Diels-Alder processes [25b]. 

Anthrones [204] and 3-substituted oxindoles [205] possess activated methylenes which have been able to react under asymmetric iminium catalysis with a,p-unsaturated aldehydes. The reaction with 3-substituted oxindoles is especially attractive, since chiral quaternary stereocenters are generated. For this purpose, chiral primary amine thiourea catalyst 132 has been demonstrated as a very efficient promoter for the addition of 3-alkyl substituted oxindoles to P-aryl substituted enals in the presence of benzoic acid as cocatalyst in toluene at rt to afford the corresponding Michael adducts in good diastereoselectivities (dr up to >19/1) and good enantioselectivities (73-93% ee) (Scheme 2.75) [205a], P-Alkyl substituted enals are not suitable partners for the reaction affording very low diastereo- and enanti-... 

Kim and coworkers have developed a binaphthyl-modified chiral squaramide (20) bearing both central and axial chiralities [89]. The Michael addition of anthrone to nitroalkenes catalyzed by squaramide 20 afforded the products in high yield and good enantioselectivity (Scheme 10.17). Notably, the reaction at lower temperature (0 °C) resulted in a reduced enantioselectivily. This transformation has also been shown to be catalyzed by cinchona alkaloid and thiourea derivatives [90-92]. 

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