The Preparation of Optically Active β-Keto Sulfoxides by Means of an Enantiomer-differentiating Reaction of α-Lithio Aryl Methyl Sulfoxides with Chiral Carboxylates

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Abstract
The reaction of α-lithio aryl methyl sulfoxides with a limited amount of chiral carboxylates (R2–CO–O–R*) was found to be an enantiomer-differentiating reaction, affording the corresponding optically active β-keto sulfoxides (3), together with optically active aryl methyl sulfoxides which have the opposite configuration. The optical purity and the predominant configuration of 3 obtained were assigned by means of a combination of polarimetric analysis and NMR. The degree of enantioselectivity of this reaction was affected by the nature of the ester moiety R2, indicating an increase in optical yields, from 1.3%, where R2 was ethyl (3b), to 70.3%, where R2 was t-butyl (3f). The stereochemical course of the reaction has been discussed by considering a six-membered cyclic transition state. Furthermore, the repeated recrystallizations of several β-keto sulfoxides thus obtained, such as (−)-3e, (−)-3f, (−)-3g, and (−)-3p, were found to give the corresponding highly optically pure β-keto sulfoxides.