Dienophiles, anthrone reactions

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]. 

The use of a boron atom as a covalent template for intramolecularizing the Diels-Alder reaction has been relatively little investigated [20-22]. However Narasaka et al. have reported the use of a boronate template to control regio- and stereoselectivity in an IMDA reaction between anthrone 50 and methyl 4-hydroxybut-2-enoate 3 [20a]. Formation of the boronate tether was achieved by simply heating an equimolar mixture of phenylboronic acid with the diene and dienophile at reflux in pyridine with azeotropic removal of water. After 5 h, a single cycloadduct 51 was obtained in 81% yield. The tether was readily removed oxidatively affording the corresponding diol 52 in excellent yield (Scheme 10-19). 

Simple chiral tertiary amines (90) have been shown to catalyse the Diels-Alder reaction of anthrone (88) and maleimides (89) to yield cycloadducts (91) in excellent yields (up to 96%) and enantioselectivities (up to 95% ee) (Scheme 26). The aminocat-alytic asymmetric ex -Diels-Alder reaction of a-methylene carbonyl dienophiles with 2,4-dienals formed chiral spirocycles in moderate to excellent diastereo- and enantio-selectivity. ... 

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