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1.3: LBA Catalysts

  • Page ID
    168769
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    The combination of Lewis acids and chiral Brønsted acids affords LBA catalysts. In this system, the coordination of the Lewis acids to the heteroatom of the chiral Brønsted acid results in increase the acidity of the latter. For examples, the LBA, derived from optically active monoalkylated-1,2-diaryl ethane- 1,2-diol and SnCl4, has been found to be an effective catalyst for the enantioselective protonation of silyl enol ethers and ketene disilyl acetals (Scheme \(\PageIndex{1}\)).

    clipboard_efc48f226dc8c3c466c4fa5b796dbe5eb.png

    Examples:

    clipboard_ebd3c1510392b466e8e975585f3a29bcd.png
    Scheme \(\PageIndex{1}\)

    Based on the related X-ray crystal structure, the following transition states , controlled by a linear O-H— π bonding interaction, are proposed for the stereochemical course of the reactions (Scheme \(\PageIndex{2}\)).

    clipboard_e7252813e6a4bc460272b050d88cbcb50.png
    Scheme \(\PageIndex{2}\)

    The chiral catechol-derived LBA 1 has been employed as an artificial cyclaze for the cyclization of various 2-(polyprenyl)phenol derivatives with good yield and enantioselectivity. For example, a short total synthesis of (-)-chromazonarol can be accomplished with 88% enantioselectivity (Scheme \(\PageIndex{3}\)).

    clipboard_ed818931b9f59c16fe182ece2495a2766.png
    Scheme \(\PageIndex{3}\)

    clipboard_e5e8d5a676c1aa2bd1c188a79cedb93c6.png

    clipboard_efdd5e0fa70e89afdcd74868b27a9212e.png
    Scheme \(\PageIndex{4}\)

    In addition, LBAs have been used as powerful catalysts for allylation reactions. For examples, LBA 2has been used as an effective catalyst for allylation of aldehydes with high diasterofacial selectivity (Scheme \(\PageIndex{4}\)).


    This page titled 1.3: LBA Catalysts is shared under a CC BY-SA license and was authored, remixed, and/or curated by Tharmalingam Punniyamurthy (National Programme on Technology Enhanced Learning (NPTEL) ) .

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