Sulfenic acids, reduction

Disulfides can be reduced to two thiols (Fig. 5.14). The best example is the reduction of oxidized glutathione (GSSG) back to the reduced form (GSH) (Fig. 5.14), which is mediated by glutathione reductase. In addition, exchange can occur with other thiols mediated by protein disulfide isomerase. In principle, sulfenic acids can probably also be reduced back to thiols, but because of the reactivity of the sulfenic acid, this is not generally observed. 

The reduction of the disulfide dication (36) to the disulfide can be achieved upon treatment with two equivalents of KI or Bu 4N I . However, treatment with KBr followed by aqueous workup gives acyclic dimers. The reaction mechanism outlined in Scheme 14 shows the formation of sulfenyl bromide (46) which is hydrolyzed into the sulfenic acid (47) which in turn dimerizes into the thiosulfinate intermediate. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Instead, cyclization occurred between a sulfenic acid at azetidinone C4 and an alkyne moiety at nitrogen [206]. The ( )-2-alkylidenepenams 374 were selectively obtained by thermolysis of appropriate 4-rert-butylthioazetidinone precursors, through the intermediacy of transient sulfenic acids 373. The high stereoselectivity of the reaction is suggestive of a pericyclic ring closure. Sulfoxide reduction of 374 was achieved with PBZ3 in DMF. 

Possible mechanisms for reduction of sulfenic acids and selenenic adds by... 

To a lesser extent, the reduction of cystine to cysteine may result from a hydrolysis which is probably not enzymic. This produces a molecule of sulfenic acid and one molecule of cysteine according to ... 

Types of compounds are arranged according to the following system hydrocarbons and basic heterocycles hydroxy compounds and their ethers mercapto compounds, sulfides, disulfides, sulfoxides and sulfones, sulfenic, sulfinic and sulfonic acids and their derivatives amines, hydroxylamines, hydrazines, hydrazo and azo compounds carbonyl compounds and their functional derivatives carboxylic acids and their functional derivatives and organometallics. In each chapter, halogen, nitroso, nitro, diazo and azido compounds follow the parent compounds as their substitution derivatives. More detail is indicated in the table of contents. In polyfunctional derivatives reduction of a particular function is mentioned in the place of the highest functionality. Reduction of acrylic acid, for example, is described in the chapter on acids rather than functionalized ethylene, and reduction of ethyl acetoacetate is discussed in the chapter on esters rather than in the chapter on ketones. 

Lithiated allyl sulfide 256-Li reacted with m-xylylene dibromide 257 to afford the bis-sulfide 258. A double Mislow-Evans rearrangement of 258 and subsequent reductive trapping of the sulfenate ester provided the symmetric trans-diol 255. Monoprotection, Sharpless asymmetric epoxidation, and reductive ring-opening of the epoxide gave 1,3-diol 259 in 97% ee. After several functionalizations, the arene was subjected to Birch reduction to provide 1,4-cyclohexadiene 254. Ozonolysis of the diene, followed by reductive workup and treatment of the resulting 1,3-diketone with acid furnished pyra-... 

Peptide chemistry has made headlines this year with the publication of the first total synthesis of an enzyme. Ribonuclease A, a peptide or rather protein with 124 amino acid residues was prepared by two independent groups and by two quite different procedures, the now well-established Merrifield automated solid-state synthesis and a fragment condensation method with 19 preassembled parts. Regarding the Merrifield method, improved attachment and removal of the polymeric peptide support have been reported. A new peptide synthesis proceeds through sulfen-amides by an oxidation-reduction process . Peptides can also be obtained from N-protected aminothiolic aryl esters and sulfen-amino acids with selective removal of a protective o-nitrophenyl-sulfenyl group... 

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