摘要：

The addition of C-nucleophiles such as Grignard reagents or enolates to chiral alpha - or beta -alkoxy aldehydes or ketones creates a new center of chirality and is therefore diastereogenic. In order to control stereoselectivity, two strategies have been developed: 1) Use of Lewis-acidic reagents which form intermediate chelates, these being attacked stereoselectively from the less hindered side (chelation control); 2) use of reagents incapable of chelation, stereoselective attack being governed by electronic and/or steric factors (non-chelation control). Generally, the two methods lead to the opposite sense of diastereoselectivity. It is possible to predict the outcome by careful choice of organometallic reagents containing elements such as Li, Mg, B, Si, Sn, Cu, Zn, or Ti.

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