as a pronucleophile.
Azlactones 7 are recognized as a useful pronucleophile and widely used in asymmetric synthesis because the hydrolysis of the adducts affords enantio‐enriched α,α‐disubstituted amino acid derivatives (Figure 3.7) [30]. Palomo and co‐workers investigated the enantioselective reactions of the related N‐heterocyclic compounds that were less explored as a pronucleophile, such as thiazolones 8 and imidazolones 9 and 10 [31]. For instance, the group developed the enantioselective addition of thiazolones 8 to nitroalkenes by using chiral bifunctional catalyst 2i having a ureidopeptide moiety as a hydrogen bond donor unit (Scheme 3.10) [31a]. The products of the reaction were readily converted to functionalized tertiary thiols otherwise difficult to synthesize.
Scheme 3.10. Enantioselective addition of thiazolones 8 to nitroalkenes catalyzed by 2i.
Source: Based on [31a].
Protected hydroxy malononitriles 11, known as masked acyl cyanide (MAC) reagents, are among the versatile umpolung synthons [32]. Rawal and co‐workers successfully applied MAC reagents 11 as a pronucleophile in enantioselective addition to enones [33]. The reaction was efficiently catalyzed by amine‐squaramide catalyst 2j (Scheme 3.11). The method was utilized for the total synthesis of (+)‐fornicin C.
Scheme 3.11. Enantioselective addition of MAC reagents 11 to enones catalyzed by 2j. Source: Based on [33].
Takemoto and co‐workers used glyoxylate cyanohydrins 12, which have a similar structure to MAC reagents, as a pronucleophile for the first time in enantioselective reactions [34]. Specifically, the group developed the direct Mannich‐type reaction of 12 mediated by chiral bifunctional catalysts 2k and 2l, both possessing a benzothiadiazine moiety as a strong hydrogen bond donor unit (Scheme 3.12). Interestingly, the tuning of the substituents on the catalysts, namely the choice of 2k and 2l, resulted in the diastereodivergent synthesis. The enantio‐enriched adducts were readily converted into a series of chiral motifs.
Scheme 3.12. Enantioselective addition of glyoxylate cyanohydrins to imines catalyzed by 2k and 2l. Source: Based on [34].
Palomo and co‐workers established a new strategy for enantioselective α‐functionalization of 2‐alkyl azaarenes [35]. The addition of 2‐cyanomethylpyridine N‐oxides 13 to enones proceeded under the influence of amine‐squaramide catalyst 2m to provide the adducts having an all‐carbon quaternary stereogenic center in a highly enantioselective manner (Scheme 3.13). In the reaction system, the N‐oxide moiety played strategic roles as a removable activating group, which enhances the acidity of α‐proton, and a stereodirecting group.
Scheme 3.13. α‐Functionalization of 2‐alkyl azaarene N‐oxides 13. Source: Based on [35].
The same group designed 2‐alkylthio‐4,6‐dioxopyrimidines 14 as barbituric acid equivalents, and developed the enantioselective Michael addition to enones catalyzed by amine‐squaramide 2n (Scheme 3.14) [36]. This is the first example of highly enantioselective synthesis of chiral barbituric acid derivatives with an in‐ring tetrasubstituted stereogenic center.
Scheme 3.14. Enantioselective Michael addition of barbituric acid derivatives 14 to enones catalyzed by 2n.
Source: Based on [36].
3.2.2. Carbon‐Heteroatom Bond Formations
Enantioselective intramolecular addition of a heteroatom pronucleophile to a carbon–carbon double bond is a powerful strategy for the construction of useful heterocyclic frameworks containing a stereogenic center. Asano and Matsubara reported the enantioselective cycloetherification via oxa‐Michael addition mediated by cinchona alkaloid‐thiourea catalyst 2h (Scheme 3.15) [37]. The reaction provided an efficient access to enantio‐enriched 2‐substituted tetrahydrofurans and tetrahydropyrans in a highly enantioselective manner.
Scheme 3.15. Enantioselective cycloetherification via oxa‐Michael addition catalyzed by 2h.
Source: Based on [37].
The same group developed two types of intriguing cascade processes that involve the enantioselective intramolecular oxa‐Michael addition. One is the enantioselective synthesis of spiroketals through the intramolecular hemiacetalization followed by oxa‐Michael addition catalyzed by Takemoto’s catalyst ent‐2a (Scheme 3.16) [38]. The authors suggested that the enantioselectivity of the reaction is largely attributed to the oxa‐Michael addition step while the diastereoselectivity is determined through the kinetic resolution of the chiral hemiacetal intermediates.
Scheme 3.16. Enantioselective synthesis of spiroketals.
Source: Based on [38].
The other process is the enantioselective
