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In comparison with metal‐based Lewis acid catalysts, chiral Brønsted acid catalysts had limited application to reactive substrates because of the moderate acidity of phosphoric acid. In order to overcome this drawback, Yamamoto introduced an NHTf moiety in place of the OH moiety of CPA 6 to generate a stronger Brønsted acid in 2006. As shown in Section 2.2.1 (Figure 2.2), acidity of phosphoramide 14 is seven orders of magnitude stronger than CPA 6. CPA 6 did not promote the Diels‐Alder reaction between α,β‐unsaturated ketone and siloxydiene. N‐triflyl chiral phosphoramide 14c catalyzed the Diels‐Alder reaction smoothly to furnish the cycloadducts in 43–>99% yields with 82–92% ee (Scheme 2.32) [21, 81].

Scheme 2.32. Diels‐Alder reaction.

Source: [21, 81].

List reported a highly enantioselective Diels‐Alder reaction between cyclopentadiene and 9‐anthracenylmethyl cinnamates [82]. C‐H acid 25 was employed in combination with ketene silyl acetal as the silylating agent (Scheme 2.33). Silylium ion was identified as the chiral Lewis acid catalyst. The reaction was proposed to proceed by way of the silylium binaphthyl‐allyl‐tetrasulfonate (BALT) anion intermediate (Figure 2.8). Although use of bulky 9‐anthracenylmethyl ester was required, List later reported that a simple α,β‐unsaturated ester could be employed in the Diels‐Alder reaction with cyclopentadienes using IDPi catalysts 11 (Scheme 2.34) [83].

Scheme 2.33. Diels‐Alder reaction by C–H acid.

Figure 2.8. Silylium binaphthyl‐allyl‐tetrasulfonate anion intermediate.

List reported an enantioselective Diels‐Alder reaction between cyclopentadiene and enals, which is based on a multisubstrate screening approach [84]. Whereas α‐substituted enals gave exo adducts selectively, β‐substituted enals gave endo adducts preferentially catalyzed by 11j (Scheme 2.35a). List subsequently reported a Diels‐Alder reaction between cross‐conjugated cyclohexadienones and cyclopentadiene using confined chiral monophosphate 11k. Up to five stereocenters were constructed in 66–98% yields with high diastereoselectivity and with 84–95% ee (Scheme 2.35b) [85].

Scheme 2.34. Diels‐Alder reaction between simple ester and cyclopentadiene catalyzed by IDPi.

Source: Based on [83].

Scheme 2.35. Diels–Alder reaction between cyclopentadien and enals (a), and cyclopentadienone (b) using IDPi (

Source: Based on [85]).

Numbers of enantioselective Diels‐Alder reactions have been reported, but reactions with unbiased benzoquinones have remained a formidable challenge. Masson developed the Diels‐Alder reaction between quinone and diene carbamate to afford dihydronaphthalene‐1,4‐diols catalyzed by SPINOL‐derived CPA 13c (Scheme 2.36a) [86]. Simply changing the amount of quinone resulted in the selective formation of the redox isomers with high stereoselectivity (Scheme 2.36b).

Scheme 2.36. Redox‐divergent Diels‐Alde reaction leading to dihydronaphthalene‐1,4‐diols (a) (

Source: Based on [86]

), and redox isomers (b).

In order to generate a stronger chiral Brønsted acid, Ishihara employed a BBr₃‐CPA 6m complex that is expected to function as Lewis acid‐assisted Brønsted acid (LBA) [50], to promote the Diels‐Alder reaction between α‐substituted acroleins and 1,2‐dihydropyridines, which gave cycloadducts with 92–98% ee (Scheme 2.37, Figure 2.9) [87].

Scheme 2.37. Diels‐Alder reaction promoted by Lewis acid‐assisted Brønsted acid.

Source: Based on [87].

Figure 2.9. Lewis acid‐assisted chiral Brønsted acid.

Source: Based on [87].