定义Definition
前手性是指非手性分子（或其中某个中心、某个面）经过一步改变（取代一个基团或在平面一侧加成）即可变成手性的性质。sp³ 碳上两个相同基团为对映异位（pro-R/pro-S），三角平面的两个面为 Re 面与 Si 面。
Prochirality is the property of an achiral molecule (or centre, or face) that becomes chiral in a single step — replacing one group or adding to one face. Two identical groups on an sp³ carbon are enantiotopic (pro-R/pro-S); the two faces of a trigonal centre are Re and Si.
Re 与 Si 面Re and Si faces
从平面一侧观察三角形碳的三个取代基，按 CIP 优先顺序顺时针为 Re 面，逆时针为 Si 面。例如亲核试剂从乙醛的 Re 面或 Si 面加成得到不同对映体的醇。
Viewing a trigonal carbon’s three substituents from one side, clockwise CIP order is the Re face and anticlockwise the Si face; nucleophilic addition to acetaldehyde’s Re or Si face gives opposite enantiomers of the alcohol.
生物化学中的意义Significance in biochemistry
酶能区分对映异位基团：乙醇脱氢酶只从乙醇 CH₂ 中移除 pro-R 氢；柠檬酸是对称分子，但顺乌头酸酶只作用于其中一个 –CH₂COOH 臂（奥格斯顿，1948），解释了同位素标记实验的“异常”。
Enzymes distinguish enantiotopic groups: alcohol dehydrogenase removes only the pro-R hydrogen of ethanol; citrate is symmetric, yet aconitase acts on one specific –CH₂COOH arm (Ogston, 1948), resolving puzzling labelling results.
不对称合成Asymmetric synthesis
大多数不对称合成本质上就是让试剂区分前手性底物的两个面或两个基团。
Most asymmetric syntheses amount to making a reagent discriminate between the two faces or groups of a prochiral substrate.
年表Timeline
- 1948
奥格斯顿提出三点结合解释柠檬酸的不对称代谢
Ogston’s three-point attachment explains asymmetric citrate metabolism
- 1966
汉森提出 pro-R/pro-S 和 Re/Si 术语
Hanson introduces pro-R/pro-S and Re/Si terms