Oxodiene Diels-Alder reaction

He M, Uc GJ, Bode JW (2006a) Chiral A-heterocyclic carbene catalyzed, enantioselective oxodiene Diels-Alder reactions with low catalyst loadings. J Am Chem Soc 128 15088-15089... 

In 2006, the Bode group documented NHC-catalyzed highly enantiose-lective 1-oxodiene Diels-Alder reactions of a broad range of enones with racemic a-chloroaldehydes as the dienophile precursors. This process affords a diverse set of highly enantioenriched 3,4,6-trisubstituted dihydro-pyran-2-ones from readily available starting materials under mild conditions (room temperature, 1.5 equiv. of NEtj, 6 h). The use of readily accessible racemic a-chloroaldehydes as enolate precursors greatly expands the scope of enantioselective NHC-catalyzed Diels-Alder reactions. It also makes possible, for the first time, asymmetric annulations with exceptional enantiose-lectivity under reliable eonditions with less than 1 mol% of a chiral NHC catalyst (Scheme 7.77). 

In 2011, You and co-workers developed a series of novel bicyclic triazolium salts from readily available (li ,2i )-DPEN. The NHC catalyst derived from the chiral triazolium salt and EtjN is also found to be efficient for enanti-oselective oxodiene Diels-Alder reactions, affording 3,4-dihydropyridinones and their derivatives in good yields, diastereo- and enantioselectivity (up to 97% ee) (Scheme 7.79). 

Schone 3.30 NHC-catalyzed biphasic Diels-Alder reactions of chloroaldehyde bisulfite salts with oxodienes... 

Bode demonstrated the effectiveness of A -heterocyclic carbenes as catalysts for inverse electron demand hetero Diels-Alder reactions involving both azadienes and oxodienes. In the oxodiene case, reaction of enones with a-chloroaldehydes affords substituted dihydropyranones. In one example, addition of catalyst 60 to chloroaldehyde 58 followed by elimination of HCl yields the electron-rich dienophile that readily combines with oxodiene 59 to selectively fiimish cycloadduct 61. ... 

In the absence of a suitable nucleophile, the homoenolate-derived acyl azolium intermediate can undergo deprotonation to generate a nucleophilic azolium enolate. Bode and coworkers showed such enolates to be competent dienophiles in hetero-Diels-Alder reactions with N-sulfonyl azadiene partners, providing dihy-dropyridinone products in very high diastereo- and enantioselectivity [106]. This work was quickly followed up by an analogous hetero-Diels-Alder reaction with oxodienes 130 (Scheme 18.25) [107]. In this case, the enolate intermediates (132)... 

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