Catalytic Asymmetric Synthesis. Группа авторов
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intermediate, followed by a 6‐endo cyclization to yield tetrahydroquinoline derivatives (Scheme 2.13). An enantioselective C–H activation was proposed.
Radical reactions are an underexplored area in the field of chiral Brønsted acid catalysis. Kim reported a radical addition reaction that used iodoalkane as the radical precursor in the presence of chiral phosphoramide 14c, which furnished addition products in good yields and with moderate to good enantioselectivity (73–84% ee) (Scheme 2.14). (Me3Si)3SiH (TTMSSH) and Et3B were employed as initiators [56].
Scheme 2.10. Friedel‐Crafts alkylation reaction with ketimines and indoles (a) (
Source: Based on [48]
) and furans (b) (
Source: Based on [49]).
Scheme 2.11. Friedel‐Crafts alkylation reaction between N‐H trifluoromethylated ketimines and pyrroles (a) (
Source: [51, 52]
) and 4,7‐dihydroindole (b) (
Source: Based on [53]).
Scheme 2.12. Friedel‐Crafts alkylation reaction between indole and N‐H trifluoromethylated iminoesters.
Source: Based on [54].
Scheme 2.13. Internal redox reaction.
Scheme 2.14. Radical addition to imines.
2.3.2. Reactions with Carbonyl Compounds and Oxonium Salts
List reported a Torgov reaction catalyzed by chiral DSI 9b, which yielded enantioenriched tri‐ and tetracyclic dienes. The Torgov reaction is proposed to proceed by the isomerization of alkene followed by the Prins reaction, a subsequent isomerization, and dehydration (Scheme 2.15). The method was successfully applied to a concise synthesis of (+)‐estrone [57].
List subsequently attempted to perform Prins cyclization, but chiral DSI 9 was not sufficiently acidic enough to promote the Prins cyclization. List developed confined imino‐imidodiphosphates (iIDPs) 19a and 19b as a new class of highly acidic Brønsted acids and reported the Prins cyclization. Both aliphatic and aromatic aldehydes participated in the reaction successfully to furnish tetrahydropyran derivatives with 80–96% ee (Scheme 2.16) [58].
List reported an intramolecular carbonyl‐ene reaction of olefinic aldehyde using confined IDP 10a as chiral Brønsted acid to afford diverse trans‐3,4‐disubstituted five‐membered carbo‐ and heterocycles in 77–96% yields and with trans selectivity (4 : 1 to >20 : 1), and with 84–96% ee (Scheme 2.17) [59].
Rueping reported intermolecular carbonyl‐ene reaction of 1,1‐disubstituted alkene with ethyl trifluoromethylpyruvate using N‐triflyl phosphoramide. But the enophile was limited to trifluoromethylpyruvate [60]. In order to achieve carbonyl‐ene reaction with wider substrate scope, Terada designed stronger Brønsted acid by perfluorination of the BINOL moiety. CPA 20, derived from F10‐BINOL, catalyzed the intermolecular carbonyl‐ene reaction between exocyclic alkene and ethyl glyoxylate to furnish homoallylic alcohols with 62–93% ee (Scheme 2.18) [61].
Scheme 2.15. Torgov reaction.
Scheme 2.16. Prins cyclization.
Source: Based on [58].
Scheme 2.17. Intramolecular carbonyl‐ene reaction.
Source: Based on [59].
In 2010, List independently synthesized chiral phosphoric acids derived from (S)‐SPINOL. A highly enantioselective kinetic resolution of homoaldols was achieved using CPA 13b (STRIP), which is derived from (S)‐SPINOL bearing 2,4,6‐(i‐Pr)3C6H2 groups at 6,6′‐positions, to furnish tetrahydrofuran derivatives in a highly enantioselective manner by transacetalization (Scheme 2.19) [20].
Scheme 2.18. Intermolecular carbonyl‐ene reaction.
Source: Based on [61].
Scheme 2.19. Kinetic resolution of homoaldols.
Source: Based on [20].
The enantioselective allylation of aldehyde is an important reaction for the preparation of homoallylic alcohols. Antilla reported an enantioselective allylation of aromatic and aliphatic aldehydes with allylboronate using CPA 6e to furnish homoallylic alcohols with 73–99% ee (Scheme 2.20a) [62]. Allenyl boronate also participated in the reaction