Oxidation of sulfur groups

Cysteine hydrochloride 567 Tin foil is added to a solution of cystine in hydrochloric acid, the metal at first dissolving without evolution of hydrogep. When gas evolution becomes prominent the mixture is diluted and the tin is precipitated by hydrogen sulfide. The filtrate therefrom is evaporated to dryness. 

Sulfoxides can be reduced to sulfides by triphenylphosphine in boiling carbon tetrachloride, yields usually exceeding 80%.568 

General directions for reduction of sulfoxides to sulfides A mixture of the sulfoxide (0.01 mole) and triphenylphosphine (0.02 mole) in carbon tetrachloride (100 ml) is boiled for 2 h, then evaporated. The sulfide is obtained from the residue either by extraction with light petroleum (b.p. 35-37°) or by recrystallization from ethanol (in which the triphenylphosphine is readily soluble). 

Thus have been obtained, inter alia, dimethyl (82%), di-/ -tolyl (100%), bis-(p-hydroxy-phenyl) (27%), bis-(/Mnethoxyphenyl) (90%), bis-(j -bromophenyl) (94%), and bis-Q -nitro-phenyl) sulfide (83 %). 

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The sulfur-rich oxides S 0 and S 02 belong to the group of so-called lower oxides of sulfur named after the low oxidation state of the sulfur atom(s) compared to the best known oxide SO2 in which the sulfur is in the oxidation state +4. Sulfur monoxide SO is also a member of this class but is not subject of this review. The blue-green material of composition S2O3 described in the older literature has long been shown to be a mixture of salts with the cations S4 and Ss and polysulfate anions rather than a sulfur oxide [1,2]. Reliable reviews on the complex chemistry of the lower sulfur oxides have been published before [1, 3-6]. The present review deals with those sulfur oxides which contain at least one sulfur-sulfur bond and not more than two oxygen atoms. These species are important intermediates in a number of redox reactions of elemental sulfur and other sulfur compounds. 

It is usually believed that NO inhibits enzymes by reacting with heme or nonheme iron or copper or via the S-nitrosilation or oxidation of sulfhydryl groups, although precise mechanisms are not always evident. By the use of ESR spectroscopy, Ichimori et al. [76] has showed that NO reacts with the sulfur atom coordinated to the xanthine oxidase molybdenum center, converting xanthine oxidase into a desulfo-type enzyme. Similarly, Sommer et al. [79] proposed that nitric oxide and superoxide inhibited calcineurin, one of the major serine and threonine phosphatases, by oxidation of metal ions or thiols. 

Oxidation reactions can produce hydroxylation on the aromatic rings, such as in the case of the steroids and thiabendazole, or on the aliphatic carbon chain such as in the case of pentobarbitone. They can also cause epoxidation, which leads to normally unstable intermediates that can be hydrolyzed by epoxide hydrolase to dihydrodiols. In addition, they can produce oxidative dealkylation at the alpha carbons of the alkyl groups attached to a nitrogen, sulfur, or oxygen atom of the drug molecule, as in the case of the macrolide antibiotics and trimethoprim. Moreover, they can induce oxidation of sulfur atoms to the corresponding sulfox-... 

Sulfenylation of pyrrole and thiocyanation of pyrrole or of 1-phenylsulfonylpyrrole also provide means for the electrophihc introduction of sulfur groups, at lower oxidation levels, and in contrast to the sulfonations, at the pyrrole a-position. 

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