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چکیده
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Enantioselective allylic oxidation of alkenes using organic peresters and copper chiral complexes has been a subject of great interest during the last decade [1]. This reaction provides an access to chiral allylic alcohols which are key intermediates in synthesis of natural products. [2] Herein we report new several methods for synthesis of chiral bisoxazoline ligands with an axis-unfixed biphenyl backbone in fair to excellent yields. These ligands and their α-hydroxylamide precursors existed as a mixture of two diastereomers in equilibrium in solution. Interestingly, in this study, crystallization-induced asymmetric transportation of these ligands and their â-hydroxylamide precursors was observed and also when these ligands were coordinated with copper (I), only one of diastereomer complexes, (S,aS,S)-complexes, was formed [3]. In addition for first time, these copper complexes have been studied as catalysts for the enantioselective allylic oxidation of olefins. The reaction has been investigated under various conditions, and high enantioselectivity of allylic benzoates, up to 95% ee, 99% yield, were obtained by using 10 mol % complex of (S,S,S) biphenyl bisoxazoline with copper (I) hexaflurophosphate and p-nitro tert-butyl perbenzoate as oxidant [4].
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