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The Povarov reaction is an inverse electron‐demand aza‐Diels‐Alder reaction and 2‐azadiene and electron‐rich alkenes. Akiyama reported the Povarov reaction between aldimine bearing a 2‐hydroxyphenyl group on nitrogen and electron‐rich alkene using CPA 6h in 2006 (Scheme 2.38a) [88]. Although presence of the 2‐hydroxyphenyl group on nitrogen was critical for the Povarov reaction, Zhu and Masson reported a three‐component Povarov reaction using enecarbamates as dienophiles catalyzed by 12a (Scheme 2.38b) [89, 90]. Use of the 2‐hydroxyphenyl group was obviated. The mechanism of the Povarov reaction is shown in Figure 2.10. Whereas phosphoryl oxygen formed a hydrogen bond with the 2‐OH moiety in the reaction reported by Akiyama, Zhu and Masson proposed that phosphoryl oxygen would form a hydrogen bond with the N‐H moiety of the enecarbamate.

Scheme 2.38. Povarov reaction with N‐2‐hydroxyphenyl imine (a) (

Source: Based on [88]

) and N‐phenyl imine (b) (

Source: [89, 90]).

Figure 2.10. Transition state model of the aza‐Diels‐Alder reaction.

Masson subsequently reported intramolecular Povarov reactions catalyzed by 6e (Scheme 2.39) [91]. Imines were generated in situ from aldehydes and o‐hydroxyaniline.

Scheme 2.39. Intramolecular Povarov reaction of azadiene.

Source: [91].

Huang succeeded in the construction of tetrahydroquinolines bearing two quaternary stereogenic centers by the reaction between two types of aniline derivatives and pyruvate catalyzed by CPA 12b (Scheme 2.40) [92]. The Mannich reaction and the subsequent Friedel‐Crafts cyclization gave C2‐quaternary carbon centers. “Hybrid” Povarov products were obtained from aniline pairs.

Scheme 2.40. Povarov reaction leading to tetrahydroquinoline with two quaternary stereogenic centers.

Terada reported a hetero‐Diels‐Alder reaction between azopyridine carboxylate and amidodienes by developing a novel chiral carboxylic acid‐monophosphoric acid 26 (Scheme 2.41) [93].

Scheme 2.41. Hetero‐Diels‐Alder reaction between azopyridine carboxylate and amidodienes.

Source: Based on [93].

Schneider reported a hetero‐Diels‐Alder reaction between enamides and in situ generated aza‐o‐quinone‐methide, giving rise to tetrahydroacridines using CPA 6n [94]. Subsequent transformation gave free hexahydroacridines with a total of three new stereogenic centers (Scheme 2.42).

Ishihara developed a chiral magnesium potassium binaphthyl disulfonate cluster as a chiral Brønsted acid catalyst for the cycloaddition reaction between styrene derivatives and aldimines to afford cyclic carbamates with 79–98% ee (Scheme 2.43) [95]. They obtained a 3 : 1 : 4 aqua complex of (R)‐8d/Mg/K as a crystal and determined the structure of the cluster by X‐ray structural analysis and electrospray ionization mass spectrometry (ESI‐MS) analysis (Figure 2.11). Although the cluster itself did not exhibit the catalytic activity, the addition of TfOH restored the catalytic activity through H⁺ exchange. It was found that the strong acidity of the catalyst dissolved MS3Å and took up leached Mg²⁺ and K⁺.

Scheme 2.42. Addition of enamides to ortho‐quinone methide.

Scheme 2.43. Cycloaddition reaction between styrenes and aldimines.

Source: Based on [95].

Figure 2.11. 3 : 1 : 4 Aqua complex of (R)‐8d/Mg/K.

Terada reported an enantioselective synthesis of β‐amino secondary amides by [4+2] cycloaddition reaction between vinyl azides and N‐acyl imines using chiral phosphoramide 14c. The [4+2] cycloaddition was followed by subsequent ring‐opening of iminodiazonium ion intermediate, Schmidt‐type 1,2‐aryl migration, recyclization of the resulting nitrilium ion, and acid hydrolysis (Scheme 2.44) [96].

Scheme 2.44. [4+2] cycloaddition reaction between vinyl azides and N‐acyl imines.

Source: Based on [96].