Asymmetric Diels-Alder crotonaldehyde

MacMillan s imidazolidinone salts, used successfully in the organocatalysed Diels-Alder reaction (see Section 8.1) also function as effective catalysts in the asymmetric nitrone cycloaddition with simple monodentate dipolarophiles. Thus acrolein (8.63) and crotonaldehyde (8.99) both react with acyclic C-aryl, N-benzyl nitrones and C-aryl N-alkyl nitrones such as (8.198) with high ees ranging from 90 to 99% in the presence of the perchlorate salt of imidazolidinone (8.91). 

Asymmetric inverse-electron-demand Diels-Alder reactions controlled by the dienophile HOMO and the diene LUMO have seldom been reported. Through a HOMO-activation strategy by the formation of dienamine intermediate H between crotonaldehyde (2a) and chiral amine 36 to raise the dienophile HOMO energy, Chen and coworkers successfully developed the first organocatalytic aU-carbon-based inverse-electron-demand Diels-Alder reaction to produce multifunctional cycloadducts 35 with up to 99% ee (Scheme 38.11) [19]. 

Although the diversity of asymmetric inverse-electron-demand hetero-Diels-Alder reactions has been weU established, examples of aU-carbon-based catalytic asymmetric versions have rarely been reported, and aU faU into the LUMO-lowering strategy. Based on previous applications of dienamine catalysis in asymmetric inverse-electron-demand hetero-Diels-Alder reactions, Li et al. extended this strategy to all-carbon-based asymmetric inverse-electron-demand Diels-Alder reactions (Scheme 1.28) [36]. The products of cyclohexene derivatives with substantial substitution diversity of electron-deficient dienes and crotonaldehyde were obtained with high diastereo- and enantioselectivities (up to 99% ee, dr up to 95 5). 

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