Thiols from sulfonic acids

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

General Reaction Chemistry of Sulfonic Acids. Sulfonic acids may be used to produce sulfonic acid esters, which are derived from epoxides, olefins, alkynes, aHenes, and ketenes, as shown in Figure 1 (10). Sulfonic acids may be converted to sulfonamides via reaction with an amine in the presence of phosphoms oxychloride [10025-87-3] POCl (H)- Because sulfonic acids are generally not converted directiy to sulfonamides, the reaction most likely involves a sulfonyl chloride intermediate. Phosphoms pentachlotide [10026-13-8] and phosphoms pentabromide [7789-69-7] can be used to convert sulfonic acids to the corresponding sulfonyl haUdes (12,13). The conversion may also be accompHshed by continuous electrolysis of thiols or disulfides in the presence of aqueous HCl [7647-01-0] (14) or by direct sulfonation with chlorosulfuric acid. Sulfonyl fluorides are typically prepared by direct sulfonation with fluorosulfutic acid [7789-21-17, or by reaction of the sulfonic acid or sulfonate with fluorosulfutic acid. Halogenation of sulfonic acids, which avoids production of a sulfonyl haUde, can be achieved under oxidative halogenation conditions (15). 

Figure 4.8. The sulfur cycle where S° is elemental sulfur, H2S is hydrogen sulfide, S2032" is thiosulfate, SO32" is sulfite, SO/- is sulfate, R-OSO3H represents a sulfate ester, R-SO3H a sulfonic acid, R-S-R a thioether, and R-SH a thiol. (Adapted from Coyne MS. Soil Microbiology An Experimental Approach. Boston Delmar Publishers 1999.)...

An important drug in the present context is the mineralocorticoid receptor antagonist spironolactone (7.74, Fig. 7.12). Among its many metabolic reactions, spironolactone is readily hydrolyzed at the thioester bond (Fig. 7.12, Reaction a) to form deacetyl-spironolactone (7.75, Fig. 7.12), a metabolite found in a variety of tissues [155 -157]. This thiol compound, which is also a potent mineralocorticoid antagonist, promotes the mechanism-based inactivation of hepatic, adrenal, and testicular cytochrome P450 isozymes. There is now good evidence to indicate that this behavior is the result of microsomal 5-oxidation (see Chapt. 7 in [7]). When spironolactone was incubated with liver microsomes from rats pretreated with dexamethasone (an inducer of CYP3A), the sulfinic and sulfonic acid derivatives were characterized [158]. Perhaps the importance of the 5-deacetylation of spironolactone... 

The alkyl sulfonic acid is made by oxidizing the thiol, which in turn comes from the halide. 

A very convenient method has been devised [40] for the conversion of thiols to ethyldithio derivatives as a routine procedure for protection of thiol-substituted organic acids. It uses a DMAP-catalysed exchange of the ethylthio group between a thiol and ethyl 2-pyridyl disulfide, prepared from the commercially available 2,2 -dithiobispyridine. Filtration through a macroporous sulfonic acidic resin and evaporation of the solvent yielded the disulfide directly. 

Sulfur compounds I, II and IV, methyl sulfonic acid, ethylsulfonic acid and benzenesulfonic acid could have been derived from a number of precursors in the parent coal samples. Firstly, mercaptans and thiols will form sulfonic acids when oxidized with peroxides. However, the presence of methylmercaptan, ethylmercaptan and benzenethiol in an exhaustively extracted coal sample is highly unlikely and we believe that the sulfonic acids did not arise from these compounds. In addition, there is a possibility that these sulfonic acids may have come from pendant or terminal thioether groups. 

Sulfur compound VIII, a carboxytrimethylbenzene sulfonic acid, could have come from an aryl disulfide, a thiol or could be derived from the further oxidation of compound IX, a carboxytrimethyldibenzothiophene-1,1-dioxide. This latter possibility is indicated by the lower concentration of compound IX relative to compound VIII in the oxidation products of coals containing mineral matter. Once again the catalytic effect of the mineral component of coal is indicated. 

Cysteic acid is obtained in nearly quantitative yield from cysteine with aqueous hydrogen peroxide in the presence of iron(II) ions.397 Molybdates and tungstates have also been used as effective catalysts for similar transformations.398 An excellent route for the oxidation of 2-thioethanol to isothionic acid has been developed.399 Heteropolyoxometallates supported on alumina400 can also be used to oxidize a range of organo-sulfur compounds. For example, alkyl monosulfides to sulfoxides and sulfones, and thiols to sulfonic acids are a few possibilities (Figure 3.98). 

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