The Structure‐Activity Relationship and Reusability of Chiral Azacrown Ethers Derived from L‐Threitol

Chiral azacrown ethers from threitol were synthesized, and their catalytic activity and reusability were investigated in liquid–liquid and solid–liquid phase-transfer reactions together with some previously synthesized catalysts. Some structure-effect relationships were explored, while the recoverability of the crown ethers was demonstrated. In some cases, the selectivity was preserved through three cycles.

Abstract

In enantioselective reactions, several catalysts can be used, such as chiral crown ethers, which form a special group of them. In addition to the reaction conditions, the structure of the catalyst used also affects the outcome of the reaction. In the case of carbohydrate-based crown ethers, we previously found that the monoaza-15-crown-5 structure is the most effective, and the generated enantiomeric excess is influenced by the substituent of the nitrogen, the protecting groups of the carbohydrate, and the source of chirality. In this study, macrocycles derived from l-threitol were synthesized with a similar structure, and their catalytic activity and reusability were tested and compared in different model reactions along with some previously prepared compounds. Several types of effect-structure relationships were pointed out, while the enantiomeric excess varied widely. We demonstrated the recoverability of the crown ethers by acidic extraction and subsequent liberation of the azacrown moiety. The regeneration process was investigated more thoroughly in scaled-up reactions, where the selectivity of the catalysts preserved for three examined cycles.

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