Disulfides using sodium borohydride

Williams and Rastetter also accomplished an elegant synthesis of ( )-hyalodendrin (83) in 1980 [39]. Beginning with the sarcosine anhydride-derived enolic aldehyde 78, silyl protection of the enal enabled alkylation of the glycine center with benzyl bromide and thiolation using LDA and monoclinic sulfur a la Schmidt. After protection of the thiol with methylsulfenyl chloride and deprotection of the silyl ether, the enol was sulfenylated with triphenylmethyl chlorodisulfide to afford bis(disulfide) 82 as a 2 1 mixture of diastereomers favoring the anti isomer. Reduction of the disulfides with sodium borohydride and oxidation with KI3 in pyridine afforded ( )-hyalodendrin (83) in 29 % yield (Scheme 9.4). 

The X-ray crystal structure of 6-chloro-2,3-dihydro-7-methyl-5-methyIene-2//, 3H, 5H- l,4-dithiepin-l,l,4,4-tetraoxide has been published and a short intermolecular contact across an inversion centre noted <00AX(Qel09>. An experimentally direct and efficient approach to 1,3-dithiepins has been reported using 1 qi-alkyldihalides and carbon disulfide and sodium borohydride, to generate the sulfide nucleophile . 

Cysteine is readily converted to the corresponding disulfide, cystine, even under mild oxidative conditions, such as treatment with I2 or potassium hexacyanoferrate (III). Reduction of cystine to cysteine is possible using sodium borohydride or thiol reagents (mercaptoethanol, dithiothreitol) ... 

Disulfides can be either reduced to two thiols or desulfurized. The former reaction was achieved in high yields using lithium aluminium hydride [680, 681], lithium triethylborohydride [100] and sodium borohydride [682]. 

The interest in papain increased enormously after the publication by Kimmel and Smith D ] of a modification of the purification procedure of Balls and Lineweaver [5j. This modification permitted the isolation of pure papain from papaya dried latex and was used for many years as the standard method for the production of papain. The crystalline papain of Kimmel and Smith consists of three components active papain, reversibly oxidized papain, and irreversibly oxidized papain. Reversibly oxidized papain can be converted into active papain by reduction of the active-site thiol by low molecular weight thiols [10], sodium borohydride [11], or CN [12]. In active papain, the Cys-25, which is essential far catalytic activity, is present in a reduced form, while in reversibly oxidized papain the Cvs-25 forms a mixed disulfide with cysteine. Drenth et al. reported that in irreversibly oxidized papain the Cys-25 has been oxidized to the sulfuric acid... 

Gallenbeck et al. (25) has summarized the work on reductive alkylation using formaldehyde and sodium borohydride to yield dimethylated proteins. Oxidation and reduction reactions involving thiol and disulfide groups have been discussed by Ryan (13) and Feeney (11). 

Sodium borohydride reduces disulfides to thiols, which can then be used to reduce nitro groups. Based on the redox properties of 1,2-dithiolane, lipoamide (17) was used for the selective reduction of mono-substituted nitrobenzenes to the corresponding anilines. Lipoamide (17) can also be immobilized on hydrophilic polymers such as polyvinylamine, polyethyleneimine and chitosan. These polymeric reducing catalysts can be recycled and are easy to separate from the reaction mixture. The system has been used to reduce nitroarenes to anilines. ... 

Radioactive [35S]-phosphorothioates have been used for in vivo evaluation of chronic or cumulative toxicity of oligonucleotides. To achieve site-specific 35S-labeling in phosphoramidite chemistry, either a mixture of [35S]/carbon disulfide/pyridine/triethylamine [329] or 35S-la-beled Beaucage reagent [330] have been used in the sulfurization step. 3H- and 14C-labeled compounds have been tools in investigations of the metabolic fate of the nucleobases of an oligomer [331]. 3H may be inserted at the 5 -end by formation of the 5 -carboxyaldehyde by Mofatt-Pfitzner reagent, followed by reduction with [3H]-sodium borohydride in 2-propanol... 

Disulfides can be reduced chemically, electrochemically or enzymatically. Chemical reduction is often used prior to derivatisation of thiols to fluorophores, but it can also be used with HPLC-ED. Common reducing agents are mercapto-ethanol, DTT, dithioerythritol, cyanide and sodium borohydride, but their efficiency varies towards different disulfides. Cyanide or borohydride rapidly reduce cystine at room temperature, but require over 24 h to reduce cysteine-D-penicillamine disulfide completely, whilst the sterically hindered D-penicillamine disulfide is not reduced at all. ... 

Another improvement implemented over the past years deals with sample preparation. In plasma and blood, only 5-15% of the total homocysteine is free. The rest is bound to disulfide bridges, either to other thiols or to various proteins. A reduction step is thus necessary for a meaningful monitoring of the homocysteine concentration. By switching from sodium borohydride to tris(2-carboxyethyljphosphine (TCEP), the disulfide reducing agent most widely used today [595], sample preparation was simplified. 

More careful oxidation of thiols, by the use of iodine, results in the formation of disulfides, the sulfur analogs of peroxides (Section 9-8). Disulfides are readily reduced back to thiols by mild reducing agents, such as aqueous sodium borohydride. 

Numerous reducing agents have been applied for the cleavage of disulfides in peptides. Reduction with sodium in liquid ammonia, with cysteine, 2-mer-captoethanol, 2-mercaptoacetic acid, 2-mercaptoethylamine (cysteamine), borohydrides and trialkylphosphines were used both for preparative and for... 

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