Highly Enantioselective Chlorination of β-Keto Esters and Subsequent S<sub>N</sub>2 Displacement of Tertiary Chlorides: A Flexible Method for the Construction of Quaternary Stereogenic Centers
Highly enantioselective chlorination of beta-oxo esters and subsequent stereospecific substitution of tertiary chlorides are described. Enantioselective chlorination of beta-keto esters and malonates was performed using a chiral Lewis acid catalyst prepared from Cu(OTf)(2) and the newly developed spirooxazoline ligand 2 to yield the desired a-chlorinated products with high enantioselectivity (up to 98% ee). Nucleophilic substitution of the resulting chlorides proceeded smoothly to afford a variety of chiral molecules such as alpha-amino, alpha-allcylthio, and alpha-fluoro esters, without loss of enantiopurity. The results of X-ray crystallographic analysis proved that Walden inversion occurs at the chlorinated tertiary carbon center. These results supported the fact that the substitution proceeds via an S(N)2 mechanism.
Williamson Ether Synthesis with Phenols at a Tertiary Stereogenic Carbon: Formal Enantioselective Phenoxylation of β‐Keto Esters
The enantioselective formation of α‐aryloxy‐β‐keto esters is described for the first time. Lewis acid catalyzed enantioselective chlorination of β‐keto esters and subsequent SN2 reactions with phenols yielded α‐aryloxy‐β‐keto esters with up to 96 % ee. Favorskii rearrangement of α‐chloro‐β‐keto esters was also found to give 1,2‐diesters with slightly reduced enantiopurity.
首次描述了α-芳氧基-β-酮酸酯的对映选择性形成。路易斯酸催化β-酮酯的对映选择性氯化,随后与酚进行S N 2反应,生成ee高达96%的α-芳氧基-β-酮酯 。还发现α-氯-β-酮基酯的Favorskii重排产生1,2-二酯,其对映体纯度略有降低。