Peracids, chiral, asymmetric oxidation

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

In contrast to asymmetric oxidation of unsymmetrical sulfides with chiral peracids, microbial oxidation usually gives much better results. Thus, optically active phenyl benzyl sulfoxide was prepared by oxidation of the parent sulfide via fermentation with Aspergillus niger, NRRL 337, with 18% optical purity (42). Similarly, asymmetric... 

In 1967, Henbest et al. reported the first asymmetric epoxidation with an optically active peracid as the chiral oxidant [3], Since then, many optically active peracids have been used for this purpose but enantioselectivity remains low (<20% ee). This is probably because the substrate and the asymmetric center in the peracid are distant from each other in the transition state of the epoxidation as shown in Figure 6B.1 [4],... 

Often, the diastereoselectivity may be attributed to the presence of one or more discrete functional groups, as in the epoxidation of chiral (E)-crotylsilanes which represents a key step for the asymmetric synthesis of substituted tetrahydrofurans (i. e., 35 37). Both catalyzed and uncatalyzed peracid oxidation conditions result in high anti selectivity, a phenomenon which is associated with the phenyldimethylsilyl and free hydroxyl groups. Epoxidation of the 0-protected species gives a 1 1 mixture of syn and anti isomers [94TL6453]. 

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