As a part of our work on synthetic studies of chiral monofluoro compounds and molecular design of efficient reagents for optical purity determination, we focused on novel α, α-disubstituted α-fluoroacetic acids (8a-d) (R1=Me, Ph; R2=C≡CPh, Me, Bu). The ethyl esters (13a-d) were prepared by introduction of alkyl groups into the appropriate α-keto acids (9a, b) followed by fluorination of the corresponding hydroxy-esters (12a-d) with diethylaminosulfur trifluoride (DAST). For comparison with Mosher's reagent (2-methoxy-2-trifluoromethylphenylacetic acid, (MTPA)), the ethyl esters (13a-d) were converted into three representative diastereoisomers, (14a-d), (15a-d), and (16a-d), and 19F-nuclear magnetic resonance (19F-NMR) chemical shift differences between pairs of diastereoisomers (Δδ) were obtained. These α, α-disubstituted α-fluoroacetate derivatives have much larger Δδ values than the corresponding derivatives of MTPA, which strongly indicates that the acids (8a-d) can potentially be better reagents for ee determination than MTPA. The influence on the Δδ values of the steric effects which arise upon introduction of the two substituents (R1 and R2) on the fluorine-bearing chiral center is also discussed.
作为我们在手性单
氟化合物的合成研究和高效试剂的分子设计以确定光学纯度工作的一个部分,我们集中研究了新型的α,α-二取代α-
氟乙酸(8a-d)(R1=Me, Ph; R2=C≡CPh, Me, Bu)。通过将烷基引入适当的α-
酮酸(9a, b),然后用
二乙胺硫氟化物(
DAST)对相应的羟基酯(12a-d)进行
氟化,合成了ethyl酯(13a-d)。为了与莫舍尔试剂(2-甲氧基-2-三
氟甲基
苯乙酸,M
TPA)进行比较,将ethy酯(13a-d)转化为三种代表性的二叠体异构体(14a-d)、(15a-d)和(16a-d),并获得了二叠体异构体对之间的
氟-19核磁共振(19F-NMR)
化学位移差(Δδ)。这些α,α-二取代α-
氟乙酸酯衍
生物的Δδ值明显大于相应的M
TPA衍
生物,这强烈表明酸(8a-d)在ee测定中作为试剂的潜力可能优于M
TPA。文中还讨论了在
氟原子手性中心引入两个取代基(R1和R2)后产生的立体效应对Δδ值的影响。