Hydroxy-thiols, from carbonyls

Aggarwal et al.108 reported excellent results with the catalytic asymmetric epoxidation of aldehydes. As shown in Scheme 4-52, a series of thioacetals 137 was prepared from hydroxy thiol 136 and the corresponding carbonyl compound. Among them, compound 138, derived from 136 and acetaldehyde, proved to be the best catalyst for asymmetric epoxidation of aldehydes. 

The reductive carbonylation of propargyl alcohols in the presence of thiols which results in P,y-unsaturated thioesters is in contrast to the formation of 5-hydroxy-2,3-alkadienoic esters from ethynyloxiranes. 

The mechanism of amide- and ester-hydrolyzing enzymes is very similar to that observed in the chemical hydrolysis by a base. A nucleophilic group from the active site of the enzyme attacks the carbonyl group of the substrate ester or amide. This nucleophilic chemical operator can be either the hydroxy group of a serine (e.g., pig fiver esterase, subtifisin, and the majority of microbial lipases), a carboxyl group of an aspartic acid (e.g., pepsin) [3], or the thiol functionality of cysteine (e.g., papain) [4-6]. 

Catalysts 1 and 4 are reported to give the best results for the conjugate addition of thiophenol to 2-phenylacrylates (Scheme 6.2) [17]. The products were obtained with opposite enantioselectivity in the reaction with 1 and 4, respectively, as the catalyst. Based upon a computational analysis, it was proposed that the transition state for the reaction involves hydrogen bonding between the hydroxy group of the catalyst and the carbonyl group of the ester and asymmetric proton transfer from the thiol to enolate anion (Scheme 6.2). 

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