Design chiral Bronsted acid catalysts

Ishihara and Yamamoto designed a Bronsted acid assisted chiral Bronsted acid catalyst (40), bearing a bis (trifly l)methyl group (Figure 2.31) [152]. The enantios-elective Mannich-type reaction of ketene silyl acetals with aldimines catalyzed by (40) in the presence of stoichiometric achiral proton source gave (S)-P-amino esters in high yields with good enantiomeric excesses. 

Figure 11.2 Design of chiral Bronsted acid catalyst.

Recently, chiral bis-phosphoric acid 77 bearing a new chiral scaffold with triple axial chirality assisted by intramolecular hydrogen-bonding between two phosphoric acid moieties was designed as a new chiral Bronsted acid catalyst by the Terada group [33], Application of this catalyst in the Diels-Alder reaction between substituted acroleins 66 and amido-dienes 76 produced the corresponding cycloadducts 78 with excellent enantioselectivities (Scheme 38.22). In comparison with the mono-phosphoric acid, bis-phosphoric acid 77 showed obviously higher catalytic activity and selectivity. 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

Arylboron compounds with electron-withdrawing aromatic groups such as triarylborons, diarylborinic acids, and arylboronic acids represent a new class of air-stable and water-tolerant Lewis acid or Bronsted acid catalysts in organic synthesis. In particular, arylboronic acids are showing to be powerful tools in the design of chiral boron catalysts. 

The utility of weak-acidic Bronsted acid catalysts for the Diels-Alder reaction is often limited to reactive substrates. To overcome this drawback, Yamamoto and coworkers designed chiral N-triflyl phosphoramide 74 with higher acidity as catalyst for the enantioselective Diels-Alder reaction of ethyl -vinyl ketone (73) with electron-rich silyloxydiene 72 [32]. Various multifunctional cyclohexenes 75 can be obtained in moderate to excellent yields with high enantioselectivities (Scheme 38.21). 

While carboxylic acids have been recognized as one of the most promising acid catalysts in organic synthesis, they had been scarcely utilized for asymmetric Bronsted acid catalysis. This situation was presumably due to the relatively poor reactivity of the carboxylic acid and the difficulty associated with creating a discrete chiral environment around the acidic functionality. Maruoka addressed this problem by designing chiral l,l -binaphthyl-2,2 -dicarboxylic acid derivatives 27 (Figure 7.5). 

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