Aqueous Asymmetric Mukaiyama Aldol Reaction Catalyzed by Chiral Gallium Lewis Acid with Trost‐Type Semi‐Crown Ligands |
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Authors: | Hui‐Jing Li Hong‐Yu Tian Yan‐Chao Wu Yong‐Jun Chen Li Liu Dong Wang Chao‐Jun Li |
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Abstract: | The combination of Ga(OTf)3 with chiral semi‐crown ligands ( 1a – e ) generates highly effective chiral gallium Lewis acid catalysts for aqueous asymmetric aldol reactions of aromatic silyl enol ethers with aldehydes. A ligand‐acceleration effect was observed. Water is essential for obtaining high diastereoselectivity and enantioselectivity. The p‐phenyl substituent in aromatic silyl enol ether ( 2 h ) plays an important role and increases the enantioselectivity up to 95% ee. Although aliphatic silyl enol ethers provided low enantioselectivities and silylketene acetal is easily hydrolyzed in aqueous alcohol, the aldol reactions of silylketene thioacetal ( 12 ) with aldehydes in the presence of gallium‐Lewis acid catalysts give the β‐hydroxy thioester with reasonable yields and high diastereo‐ (up to 99 : 1) and enantioselectivities (up to 96% ee). |
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Keywords: | aqueous asymmetric C?C bond‐forming reaction chiral semi‐crown ligand gallium‐Lewis acid catalyst Mukaiyama aldol reaction |
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