Catalytic Asymmetric Synthesis. Группа авторов
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A broad range of benzyl alcohols (both tertiary and secondary alcohols) was kinetically resolved, wherein the amide group reacted as the electrophile (Scheme 2.27).
The oxa‐Pictet‐Spengler reaction was reported by several groups. List employed nitrated confined IDP 10b as the chiral strong Brønsted acid catalyst for the oxa‐Pictet‐Spengler reaction between 2‐arylethanols and aldehydes to furnish 1‐substituted isochromans with 90–99% ee (Scheme 2.28) [77].
Scheme 2.26. Enantioselective synthesis of isoindolinones.
Source: Based on [75].
Scheme 2.27. Kinetic resolution of 2‐amido benzyl alcohols.
Scheme 2.28. Oxa‐Pictet‐Spengler reaction of 2‐arylethanols.
Source: Based on [77].
Subsequently, Scheidt developed a cooperative catalyst system consisting of achiral hydrogen donor 21 and CPA 6b, and achieved an oxa‐Pictet‐Spengler reaction (Scheme 2.29). The reaction proceeded via oxocarbenium ions bearing chiral counteranion intermediate [78].
Seidel developed a dual catalyst system employing both chiral amine HCl salt and a chiral bisthiourea to generate oxocarbenium ion intermediate from aldehyde and realized an oxa‐Pictet‐Spengler reaction between tryptophol and aldehydes (Scheme 2.30) [79].
Seidel was the first to report an oxa‐Pictet‐Spengler reaction with ketals by developing novel chiral carboxylic acid 24, bearing a urea moiety derived from 1,2‐diaminocyclohexane (Scheme 2.31) [80]. They measured the pK a value of the catalysts in CH3CN and found that 24 was one order of magnitude more acidic than CPA 6e: 12.7 for 24 and 13.6 for CPA 6e.
Scheme 2.29. Oxa‐Pictet‐Spengler reaction.
Source: Based on [78].
Scheme 2.30. Oxa‐Pictet‐Spengler reaction between tryptophol and aldehydes.
Source: Based on [79].
Scheme 2.31. Oxa‐Pictet‐Spengler reaction with ketal.
2.4. CYCLOADDITION REACTIONS
2.4.1. Diels‐Alder Reactions
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 (