Catalytic Asymmetric Synthesis. Группа авторов
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Название: Catalytic Asymmetric Synthesis
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119736417
isbn:
Scheme 2.5. Pictet‐Spengler reaction between tryptamine and aldehyde (a) (
Source: Based on [40]
), and of indolyl dihydropyridines (b) (
Source: Based on [41]).
The development of novel synthetic reactions and catalyst designs is normally accomplished by empirical optimization. Denmark demonstrated the utility of machine learning for predicting optimized catalyst CPA 6l (TCYP) using the addition reaction between thiol and imino ketones. The predicted enantioselectivity correlated strongly with the experimental values. It is noted that this is the first example of the machine‐learning‐driven catalyst design in the field of asymmetric acid catalysts. The results constitute a potential transformation of empirical selection and optimization of a chiral catalyst by synthetic chemists into a mathematical‐guided process (Scheme 2.6) [42].
Scheme 2.6. Addition of thiol.
Source: Based on [42].
Zhu reported a nucleophilic addition of N‐monosubstituted hydrazones to N‐Boc imines catalyzed by (S)‐6l, which gave β‐amino N,N′‐dialkyldiazenes in excellent yields, and the obtained β‐amino N,N′‐dialkyldiazenes were transformed into vicinal diamines (Scheme 2.7) [43]. N‐Alkyl hydrazones served as the α‐azo carbanion equivalents.
Scheme 2.7. Addition of hydrazone.
Source: Based on [43].
Zhao reported a polyene cyclization reaction using BINOL‐derived chiral phosphoramide (R)‐14b to furnish fused tricyclic compounds (Scheme 2.8). The cyclization reaction was applied to the total synthesis of (–)‐ferruginol [44].
The Friedel‐Crafts alkylation reaction between electron‐rich heterocycles and imine is an important method for the preparation of chiral indole derivatives. You reported a Friedel‐Crafts alkylation reaction between indole and aldimines using BINOL‐derived CPA 6f [45]. Lin synthesized novel CPA from (S)‐1,1′‐spirobiindane‐7,7′‐diol ((S)‐SPINOL), and found that 13c, bearing the 1‐naphthyl moiety at 6,6′‐positions, exerted similar reactivity and enantioselectivity to BINOL‐derived phosphoric acid (Scheme 2.9) [19, 46]. Numerous kinds of Friedel‐Crafts alkylation reactions of indoles with aldimines and other electrophiles have been reported to proceed efficiently using organocatalysts [47].
The construction of a quaternary carbon center is a challenging task. Ishihara developed chiral monopotassium binaphthyl disulfonate 8c as a strong Brønsted acid catalyst and realized Friedel‐Crafts alkylation reaction between indole and ketimine to generate quaternary stereogenic centers (Scheme 2.10a) [48]. Ishihara also developed C1‐symmetric BINOL‐derived bisphosphoric acid 16 and achieved a Friedel‐Crafts alkylation reaction between 2‐methoxyfuran and α‐imino esters (Scheme 2.10b) [49]. The bisphosphoric acid 16 exhibited stronger acidity in comparison with monophosphoric acid due to the intramolecular hydrogen bond network. This is an example of the Brønsted acid‐assisted Brønsted acid catalysis [50].
Scheme 2.8. Polyene cyclization.
Scheme 2.9. Friedel‐Crafts alkylation reaction between indoles and imines.
Source: [19, 46].
In general, N‐aryl ketimines are stable, whereas N‐H imines are labile. In contrast, trifluoromethylated N‐H ketimines are relatively stable. Akiyama reported Friedel‐Crafts alkylation reaction between CF3‐substituted N‐H ketimines and pyrroles using CPA 6e to furnish 2‐pyrrole derivatives with 83–97% ee (Scheme 2.11a) [51, 52]. 4,7‐Dihydroindoles also participated in the Friedel‐Crafts alkylation reaction with CF3‐subsituted ketimine, and subsequent dehydrogenation with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) furnished 2‐substituted indoles with 76–95% ee (Scheme 2.11b) [53].
Ohshima developed C1‐symmetric 3‐monosubstituted BINOL phosphoric acid 17 and achieved a highly enantioselective Friedel‐Crafts alkylation reaction between N‐unprotected α‐iminoesters and indoles (Scheme 2.12) [54]. Interestingly, use of C2‐symmetric 3,3′‐disubstituted BINOL phosphoric acid gave lower enantioselectivity: 93% ee with CPA 17 and 61% ee with CPA 6h.
Akiyama reported an enantioselective synthesis of tetrahydroquinoline derivatives by the internal redox reaction catalyzed by biphenol‐derived CPA 18 [55]. This reaction was proposed to proceed by a [1,5]‐H shift to generate a zwitterionic 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].