Esters of crotonic acid were brominated on a multigramme scale using a free radical procedure. A phase transfer catalysed fluorination transformed these species to the 4-fluorobut-2E-enoates reproducibly and at scale (48–53%, ca. 300 mmol). Asymmetric dihydroxylation reactions were then used to transform the butenoate, ultimately into all four diastereoisomers of a versatile fluorinated C4 building block at high enantiomeric-enrichment. The (DHQ)2AQN and (DHQD)2AQN ligands described by Sharpless were the most effective. The development and optimisation of a new and facile method for the determination of ee is also described; 19F1H} spectra recorded in d-chloroform/diisopropyl tartrate showed distinct baseline separated signals for different enantiomers.
克罗酸酯在多克拉量级上使用自由基程序进行溴化。相转移催化的氟化反应将这些物种可靠地转化为4-氟丁-2-烯酸酯(48-53%,约300 mmol)。然后使用不对称二羟基化反应将烯酸酯转化,最终形成高对映体富集度的多用途氟代C4构建块的四个对映体异构体。Sharpless描述的(DHQ)2AQN和(DHQD)2AQN配体效果最好。还描述了一种新的简便方法的开发和优化,用于确定对映体过量;在二氯甲烷/异丙酯酒石酸酯中记录的19F1H}谱显示不同对映体的基线分离信号。