Asymmetric Reactions with Heterodienophiles

In contrast to fhe intensive studies on the carbonyl-DA reaction, fhe catalytic 

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In a more recent study, Koizumi et al. employed terpene derivatives as chiral auxiliaries attached to thioaldehydes [405], but these heterodienophiles gave only moderate diastereoselectivities in reactions with cyclopentadiene. Mazzan-ti et al. have investigated the hetero Diels-Alder reaction of thioketones bearing an asymmetric silicon atom directly attached to the thiocarbonyl moiety which induced diastereoselectivities up to 50% de [406]. 

Since Danishefsky demonstrated that activated dienes, such as siloxydiene (commonly referred to as Danishefsky s diene) react with a wide spectrum of aldehydes to afford 5,6-dihydro-y-pyrones in 1982 [l],the hetero-Diels-Alder reaction has attracted a great deal of attention over the last two decades [2,3]. The use of asymmetric catalysis in these reactions is overwhelmingly associated with heterodienophiles. Especially, the cyclocondensations of activated dienes with aldehydes or their derivatives are of particular importance, providing a multitude of opportunities for the highly efficient regio- and stereoselective construe-... 

A review of the cycloaddition reactions of o-benzoquinones as carbodiene, heterodiene, dienophile, or heterodienophile has been published. In the Diels-Alder reaction of furans with masked o-benzoquinones (145), the furans unexpectedly behaved as dienophiles to yield cycloadducts (146) (Scheme 56). Masked benzoquinones behave as dienes which undergo Diels-Alder reactions with electron-rich dienophiles such as enol ethers and thienol ethers.The asymmetric Diels-Alder reactions of 5-substituted and 5,6-disubstituted (S)-2-(p-tolylsulflnyl)-l,4-benzoquinones with cyclopentadiene and fran -piperylene show complete regio- and jr-facial selectivities. The hetero-Diels-Alder reactions of o-benzoquinones with tetracyclone produce cyclopenta[I ][l,4]benzodioxinone derivatives in high yield. 

Jimenez et al. studied the asymmetric Diels-Alder reactions of 1-aryl-1,2-diaza-l,3-butadienes 114, heterodienes derived from sugars, with diethyl azodicarboxylate (115), a heterodienophile [85]. The reactions were performed without solvent in a focused microwave reactor for periods of a few hours. The reaction is stereoselective... 

A number of examples have been reported of the asymmetric hetero Diels-Alder reaction of heterodienes or heterodienophiles such as imines, nitroso or carbonyl compounds.The chiral auxiliary is commonly attached to the nitrogen atom of the imine or nitroso compound, or a chiral ester or amide substituent may provide the necessary asymmetric induction. As an example, the a-cWoronitroso dienophile 132, bearing a sugar-derived auxiliary, has been found to be effective for the formation of cyclic hydroxylamines with high optical purity (3.95). The initial cycloadduct breaks down readily (see Scheme 3.32), via an iminium ion with subsequent methanolysis to release the chiral auxiliary and the product 133. 

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