Aqueous Asymmetric Mukaiyama Aldol Reaction Catalyzed by Chiral Gallium Lewis Acid with Trost‐Type Semi‐Crown Ligands
2005; Wiley; Volume: 347; Issue: 9 Linguagem: Inglês
10.1002/adsc.200505089
ISSN1615-4169
AutoresHuijun Li, Hongyu Tian, Yan‐Chao Wu, Yong‐Jun Chen, Li Liu, Dong Wang, Chao‐Jun Li,
Tópico(s)Synthetic Organic Chemistry Methods
ResumoAbstract 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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