Abstract
New recyclable monoaza-15-crown ethers have been synthesized starting from (R,R)-(+)- and (S,S)-(−)-hydrobenzoin. These macrocycles proved to be efficient and reusable phase transfer catalysts in a few asymmetric reactions under mild conditions. The asymmetric epoxidation of trans-chalcone took place with up to 81% ee, while using other chalcone derivatives, the products were formed with 68–88% ee. The hydrobenzoin-based lariat ethers were also tested in the cyclopropanation of a few electron deficient olefins using diethyl bromomalonate to afford the product with good enantioselectivities (54–75% ee). The catalysts were recovered by salt formation, followed by extraction, and were reused without the loss of the activity and effect on the enantioselectivity.Graphic The synthesis of hydrobenzoin-based monaza crown ethers and their application as recyclable enantioselective catalysts.
Highlights
Asymmetric phase transfer catalysis has become a topic of great scientific interest in the last 30 years
Six new hydrobenzoin-based monoaza crown ethers have been synthesized, of which (S,S)-4a and (R,R)-4a and (S,S)-4c and (R,R)-4c are pairs of enantiomers, the outcome of the asymmetric reactions may be influenced by choosing the proper enantiomer of the catalyst
A Based on chiral high performance liquid chromatography (HPLC), the absolute configuration was assigned by comparison of the specific rotations with literature data [59]
Summary
Asymmetric phase transfer catalysis has become a topic of great scientific interest in the last 30 years. A binaphthyl-based fluorous phase transfer catalyst has been used
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