Novel Syn Intramolecular Pathway in Base-Catalyzed 1,2-Elimination Reactions of -Acetoxy Esters

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• 作者：Jerry R. Mohrig ; Hans K. Carlson ; Jane M. Coughlin ; Gretchen E. Hofmeister ; Lea A. McMartin ; Elizabeth G. Rowley ; Elizabeth E. Trimmer ; Andrew J. Wild ; Steve C. Schultz
• 刊名：Journal of Organic Chemistry
• 出版年：2007
• 出版时间：February 2, 2007
• 年：2007
• 卷：72
• 期：3
• 页码：793 - 798
• 全文大小：105K
• 年卷期：v.72,no.3(February 2, 2007)
• ISSN：1520-6904

文摘

As part of a comprehensive investigation of electronic effects on the stereochemistry of base-catalyzed1,2-elimination reactions, we observed a new syn intramolecular pathway in the elimination of aceticacid from -acetoxy esters and thioesters. ¹H and ²H NMR investigation of reactions using stereospecificallylabeled tert-butyl (2R*,3R*)-3-acetoxy-2,3-²H₂-butanoate (1) and its (2R*,3S*) diastereomer (2) showsthat 23 ± 2% syn elimination occurs. The elimination reactions were catalyzed with KOH or (CH₃)₄NOH in ethanol/water under rigorously non-ion-pairing conditions. By contrast, the more sterically hindered-trimethylacetoxy ester produces only 6 ± 1% syn elimination. These data strongly support anintramolecular (E_i) syn path for elimination of acetic acid, most likely through the oxyanion produced bynucleophilic attack at the carbonyl carbon of the -acetoxy group. The analogous thioesters, S-tert-butyl(2R*,3R*)-3-acetoxy-2,3-²H₂-butanethioate (3) and its (2R*,3S*) diastereomer (4), showed 18 ± 2% synelimination, whereas the -trimethylacetoxy substrate gave 5 ± 1% syn elimination. The more acidicthioester substrates do not produce an increased amount of syn stereoselectivity even though theirelimination reactions are at the E1cb interface.