Abstract
A new asymmetric synthesis of both erythro and threo aldols (β-siloxy aldehydes) has been developed based on the respective stereocontrolled rearrangement of optically active threo and erythro epoxy silyl ethers with stoichiometric use of exceptionally bulky, oxygenophilic methylaluminum bis(4-bromo-2,6-di- tert-butylphenoxide) (MABR) under mild conditions. The observed stereoselectivity varies with the electronic effect of silyl substituents rather than their steric effect, and the more electron-withdrawing triphenylsilyl group exhibits better selectivity than trialkylsilyl group. Since enantiomeric erythro and threo epoxy silyl ethers are readily accessible by Sharpless asymmetric epoxidation, this method allows the practical asymmetric synthesis of four possible aldol isomers with high selectivity.
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