Ketones a-hydroxylation

TABLE 4-21. Asymmetric Oxidation of Prochiral Ketone Enolates to a-Hydroxyl Ketones Using 147... 

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... 

Trost et al. [11] reported another impressive example of bimetallic catalysts in which a Zn-Zn homobimetallic complex (17, Scheme 7) serves as an effective catalyst for direct aldol reactions [11-13]. The proposed structure of the catalyst was verified by mass spectrometry and the best ratio of Et2Zn and the ligand. The chemical yield was moderate in the reaction of methyl ketones (1) (Scheme 7, top) [11,12], but a highly atom-economic system was achieved when a-hydroxylated ketones (10) were used as a substrate (Scheme 7, bottom) [13]. Excellent diastereo- and enantioselectivity were obtained under mild conditions. In contrast to the case of Shibasaki s heteropolymetallic catalyst, syn-1,2-diols (syn-11) were obtained as the major diastereomers. 

Enantioselective a-hydroxylation of carbonyl compounds,2 Useful enantiose-lectivity (60-95% ee) obtains in the oxidation of enolates of a number of carbonyl compounds (ketones, esters, amides) with the simplest member of this series, ( + )-or (— )-l. This reagent, however, is not useful for enantioselective oxidations resulting in tertiary a-hydroxyl ketones. For this purpose, the 8,8-dichloro derivative (2) of ( + )-l is markedly superior, as shown in equation (I). This derivative can also be... 

The species R2CO2 formed in (f) could be a dioxirane or, alternatively, a hydroxylated ketone according to (i). Further experimental work is necessary to gain more insight. 

Benzoin condensation/ sometimes known as the benzoin reaction/ is the condensation between aromatic aldehydes to form a-hydroxyl ketones (i.e., benzoins) in the presence of a catalyst. It should be pointed out that benzoin itself is not produced from benzaldehyde by acid or base catalysis or under thermal or free-radical conditions in fact, benzoin is generated from benzaldehye in the presence of a catalyst. The first benzoin condensation catalyzed by cyanide ion was reported by Stange in 1824, and has been extensively stud-... 

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