Thiols characteristic chemical properties

A characteristic of organic sulfur compounds, especially volatile (low molecular mass) thiols, is their disagreeable odors. For example, 3-methyl-1-butanethiol and 2-butene-1-thiol are ingredients of a skunk s perfume, and methanethiol or ethanethiol is usually added, in small amounts, to natural gas, which is odorless by itself, so that leaks can be readily detected. The chemical properties of thiols and sulfides differ from those of alcohols and ethers in that thiols are somewhat stronger acids than alcohols and the sulfur atoms of these compounds are considerably more nucleophilic than the oxygen of their analogs. They are excellent nucleophiles in substitution reactions. 

Treatment of pristinamycin IIa with meta-chloroperbenzoic acid afforded a compound to which the structure (79) was initially assigned, resulting from epoxidation of the more substituted double bond (12,13-C). This material did not display chemical properties characteristic of an epoxide as the assumed epoxide moiety remaining after treatment with nucleophilic reagents. Michael-type addition products on the dehydroproline ring were observed after treatment with thiols or amines (see Sect. 5.4.5). 2D-NMR analysis of the product from reaction of pristinamycin IIa with mCPBA showed that a transannular oxidative cyclization had taken place leading to formation of (80). The reaction can be considered to involve initial epoxidation of the 12,13-double bond followed by an intramolecular nucleophilic attack by the 37-hydroxy of the enol ether (Scheme 19). A similar transannular oxidative cyclization reaction has been reported for the reaction of l,5-dimethylcyclooct-4-en-l-ol with meta-chloroperbenzoic acid [125]. 

Acyl residues are usually activated by transfer to coenzyme A (2). In coenzyme A (see p. 12), pantetheine is linked to 3 -phos-pho-ADP by a phosphoric acid anhydride bond. Pantetheine consists of three components connected by amide bonds—pantoic acid, alanine, and cysteamine. The latter two components are biogenic amines formed by the decarboxylation of aspartate and cysteine, respectively. The compound formed from pantoic acid and p-alanine (pantothenic acid) has vitamin-like characteristics for humans (see p. 368). Reactions between the thiol group of the cysteamine residue and carboxylic acids give rise to thioesters, such as acetyl CoA. This reaction is strongly endergonic, and it is therefore coupled to exergonic processes. Thioesters represent the activated form of carboxylic adds, because acyl residues of this type have a high chemical potential and are easily transferred to other molecules. This property is often exploited in metabolism. 

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