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Conjugate addition – enantioselective protonation reactions

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ABSTRACT

The addition of nucleophiles to electron-deficient alkenes represents one of the more general and commonly used strategies for the convergent assembly of more complex structures from simple precursors. In this review the addition of diverse protic and organometallic nucleophiles to electron-deficient alkenes followed by enantioselective protonation is summarized. Reactions are first categorized by the type of electron-deficient alkene and then are further classified according to whether catalysis is achieved with chiral Lewis acids, organocatalysts, or transition metals.

No MeSH data available.


Luo and Cheng’s addition of indoles to vinyl ketones by enamine catalysis.
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C27: Luo and Cheng’s addition of indoles to vinyl ketones by enamine catalysis.

Mentions: Building on their earlier work with α,β-unsaturated aldehydes (vide infra), Luo and Cheng have extensively explored the use of enamine catalysis for conjugate addition–enantioselective protonation of vinyl ketones. Using primary amine catalyst (S,S)-119, the authors were able to catalyze the Friedel–Crafts addition of indoles 117 to vinyl ketones 118 followed by enantioselective protonation (Scheme 27) [52]. During optimization it was found that addition of a weak acid, 2-naphthoic acid, improved both the yield and enantioselectivity of the transformation by facilitating the formation of the iminium ion intermediates. A variety of vinyl ketones 118 were explored for the reaction, and when R4 was benzylic, shorter reaction times could be employed (41–48 h) and higher yields and enantioselectivities were observed (78–86% yield, 93:7 to 97:3 er). Aromatic vinyl ketones were also reactive, but required higher temperatures (40–60 ºC). Various indoles 117 were investigated, and although substitution on both the aromatic ring and nitrogen were accommodated, when R3 was H, lower enantioselectivity was observed (86.5:13.5 to 89:11 er).


Conjugate addition – enantioselective protonation reactions
Luo and Cheng’s addition of indoles to vinyl ketones by enamine catalysis.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979737&req=5

C27: Luo and Cheng’s addition of indoles to vinyl ketones by enamine catalysis.
Mentions: Building on their earlier work with α,β-unsaturated aldehydes (vide infra), Luo and Cheng have extensively explored the use of enamine catalysis for conjugate addition–enantioselective protonation of vinyl ketones. Using primary amine catalyst (S,S)-119, the authors were able to catalyze the Friedel–Crafts addition of indoles 117 to vinyl ketones 118 followed by enantioselective protonation (Scheme 27) [52]. During optimization it was found that addition of a weak acid, 2-naphthoic acid, improved both the yield and enantioselectivity of the transformation by facilitating the formation of the iminium ion intermediates. A variety of vinyl ketones 118 were explored for the reaction, and when R4 was benzylic, shorter reaction times could be employed (41–48 h) and higher yields and enantioselectivities were observed (78–86% yield, 93:7 to 97:3 er). Aromatic vinyl ketones were also reactive, but required higher temperatures (40–60 ºC). Various indoles 117 were investigated, and although substitution on both the aromatic ring and nitrogen were accommodated, when R3 was H, lower enantioselectivity was observed (86.5:13.5 to 89:11 er).

View Article: PubMed Central - HTML - PubMed

ABSTRACT

The addition of nucleophiles to electron-deficient alkenes represents one of the more general and commonly used strategies for the convergent assembly of more complex structures from simple precursors. In this review the addition of diverse protic and organometallic nucleophiles to electron-deficient alkenes followed by enantioselective protonation is summarized. Reactions are first categorized by the type of electron-deficient alkene and then are further classified according to whether catalysis is achieved with chiral Lewis acids, organocatalysts, or transition metals.

No MeSH data available.