Simple thiols

The most striking characteristic of thiols is their appalling odor. Skunk scent, for instance, is caused primarily by the simple thiols 3-methyl-1-butanethiol and 2-butene-l-thiol. Volatile thiols such as ethanethiol are also added to natural gas and liquefied propane to serve as an easily detectable warning in case of leaks. 

Can you think of reasons why substituted dibenzothiophenes are more difficult to desulfurize than thiophene or simple thiols (see Fig. 9.2) Depending on the choice of catalyst, hydrodesulfurization can be accompanied by hydrogenation to various extents. In which of the product streams in the refinery would you choose hydrogenative HDS and in which would you not ... 

Further investigation of the original QM1 also demonstrated that simple thiols could react reversibly although regeneration of QM1 was very slow.25 In contrast, a later model of anthracycline was shown to form stable adducts with a thiol despite the lability of corresponding adducts formed by oxygen and nitrogen nucleophiles.26... 

This is a complex molecule, made up of an adenine nucleotide (ADP-3 -phosphate), pantothenic acid (vitamin B5), and cysteamine (2-mercaptoethylamine), but for mechanism purposes can be thought of as a simple thiol, HSCoA. Pre-eminent amongst the biochemical thioesters is the thioester of acetic acid, acetyl-coenzyme A (acetyl-CoA). This compound plays a key role in the biosynthesis and metabolism of fatty acids (see Sections 15.4 and 15.5), as well as being a building block for the biosynthesis of a wide range of natural products, such as phenols and macrolide antibiotics (see Box 10.4). 

Type I iodothyronine deiodinase is defined as the enzyme which catalyses the (mono)deiodination of the inner or the outer ring of different iodothyronines and which is inhibited by jrM concentrations of PTU [5-8]. In rats and humans, such enzyme activities are present at high levels in liver, kidneys and interestingly also in thyroid, and at low levels in many other tissues [5-8]. The deiodinase is associated with the microsomal fractions of these tissues but is only active in the presence of a cytoplasmic cofactor [5-8], Also, in the absence of cytosol, deiodinase activity is stimulated by simple thiols such as dithiothreitol (DTT) but the physiological cofactor has not yet been identified with certainty. The effects of synthetic and natural thiols will be discussed in Section 2.5. 

Propose syntheses and oxidation products for simple thiols. 

The synthesis of nootkatone occupied many chemists for some years and has given us some excellent examples of fragmentation reactions. However, the synthetic samples of nootkatone failed to deliver the intense grapefruit taste and smell of the material from grapefruits. The reason is simply that nootkatone is not the flavour principle of grapefruit The samples of nootkatone isolated from grapefruit contained minute traces of the true flavour principle—-a simple thiol. Humans can detect 2 x 10-5 p.p.b. (yes, parts per billion) of this compound, so even the tiniest trace is veiy powerful. At least the syntheses allowed chemists to correct an error. 

Oxidation of this simple thiol ester gives a five-membered cyclic disulfide. The reaction is proposed as a model for the behaviour of lipoic acid in living things. Draw a mechanism for the reaction and make the comparison. 

The bond strengths and heats of formation of a variety of simple thiols and thioethers were calculated from appearance potential measurements (excess energy terms being disregarded) and were compared with the corresponding values for simple alcohols and ethers (Harrison et al., 1966). Using deuterium labelling where necessary. 

Dissolved oxygen rapidly oxidizes simple thiols, through a mercaptide ion intermediate, to disulfides, in the presence of traces of certain metallic... 

Amyes TL, Richard JP. Generation and stability of a simple thiol ester enolate in aqueous-solution. J. Am. Chem. Soc. 1992 114 10297-10302. 38. 

It should be noted that the formation of a stable sulfenylthiosulfate is a phenomenon observed with native proteins (Pihl and Lange 1962). When simple thiols such as cysteine are reacted with an excess of tetrathionate, S-sulfocysteine is formed (Inglis and Liu 1970). If thiol is in excess, the sulfenylsulfonate can react with a second molecule of thiol to form a disulfide. 

Acylation of a simple thiol with an alkyl carboxylate is not a very suitable method for preparation of S-alkyl thiocarboxylates. Transesterification is, however, possible if either the thiol or the carboxylic ester is activated. The enhanced reactivity of boron, aluminum and silicon thiolates has been utilized for the synthesis of a large variety of thiocarboxylic S-esters, including hydroxy derivatives (from lactones). a,P-Unsaturated thiol esters, e.g. cinnamoyl or 2-butenoyl derivatives, are also accessible. Michael addition, an undesirable side reaction of thiols, is completely avoided if alkyl trimethylsilyl sulfides ortris(arylthio)boranes are applied. ... 

Although xanthine oxidase does not mediate sulfhydryl oxidations per se, it is activated by the binding of hydrogen sulfide and perhaps other simple thiols (82). Thus, the effects of this enzyme also might be useful in fabricating enzymic flavor-generating systems where it could serve as an assist in limiting free thiols in the medium. 

The authors found that simple thiols also are effective acceptors in this conpling method, and, in the presence of 4 A molecnlar sieves, the glycoside yields were nniformly high. 

Examples of FenSn clusters synthesized in the laboratory from an iron(II) salt reacted with simple thiol ligands and sulfide ion. 

Heavy metal ions such as Hg(II), Pb(II), or Ag(I) hydrolyze thiol esters quite easily with the formation of a metal mercaptide as a reaction product (139, 174, 192). Although in many cases metal chelates can be postulated as intermediates in these hydrolysis reactions, there is some evidence to show that the bond formed by the coordination of the sulfur atom with a heavy metal ion is sufficiently strong to initiate the cleavage of the thiol esters. For example, simple thiol esters having no other donor atoms and therefore incapable of forming metal chelates will readily undergo hydrolysis in the presence of Hg(II) (24). 

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