Dithiols disulfides, cyclic

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

Zinc/hydrochloric acid and sodium boron hydride Dithiols from cyclic disulfides s. 56... 

Dithiosucrose, available by adapuition of a known synthetic method (see VoL22, Chapter 11, Ref.l2), has been converted to the cyclic disulfide 47 by atmospheiic oxidation (in the presence of sodium carbonate) and subsequent acetylation. The redox reaction, dithiol disulfide, was studied in order to examine the conformations of disaccharides in solutitm. ... 

From the reactions of sulfur and carbon disulfide with cyclic ketone-derived enamines (570-573) 3H-l,2-dithiole-3-thiones were obtained, whereas the addition of carbon disulfide to other enamines gave a-dithio-pyrones (574), through initial dimerization of the enamine. 

The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxides has been examined by Still and coworkers192-194. These authors also report that rearrangement to cyclic sulfenates, with subsequent reaction by homolysis of the S—O bond, appears to be a particularly favorable process. For example, ultraviolet irradiation of a solution of 8-methylthiochroman-4-one 1-oxide (133) in benzene for 24h afforded a single crystalline product which was assigned the disulfide structure 134 (equation 54). More recently, Kobayashi and Mutai195 have also suggested a sulfoxide-sulfenate rearrangement for the photochemical conversion of 2,5-diphenyl-l,4-dithiin 1-oxide (135) to the 1,3-dithiole derivatives 136 and 137 (equation 55). 

The cycloaddition of alkynes with the tributylphosphine-carbondisulfide adduct 131 results in the in situ formation of the ylides 132 which react with aldehydes to give the novel 2-arylidene or 2-alkylidene-l,3-dithioles 133 (Scheme 36) [132]. Concerning ylides C-substituted by sulfur we can also mention a publication on the behavior of various keto-stabilized ylides towards acyclic and cyclic a s-disulfides allowing the synthesis of substituted thiazoles, thiols, and dithiols [133]. 

Second, the oxidation of a [l,n]-dithiol by Ti(III)-Fl202 at pH 7 produces the cyclic disulfide as shown in Scheme 3.26. The hnal product of the dithiol oxidation forms a rather stable anion-radical. 

Otto, Sanders, and coworkers have utilized disulfide exchange to generate dynamic libraries of diastereomeric receptors [3]. DCLs made from a racenfic dithiol led to numerous cyclic structures including four cyclic tetramers, with the RR,RR,RR,RR diastereomer being the most stable (along with its all-5 enantiomer). Upon addition of N(CH3)J, the meso-diastereomer shown below was amplified 400-fold (Fig. 5.3). The structure of the diastereomer was confirmed by NMR and re-equilibration... 

Also the preparation of methyltriphenylphosphonium dichromate ((Ph3PMe)2Cr207) and the application of this compound as efficient, inexpensive, stable and mild reagent for coupling a variety of aliphatic and aromatic thiols and aliphatic dithiols to their corresponding acyclic and cyclic disulfides has been published <2006JSF441> 1,2-dithiane was obtained by this procedure in 90% yield. 

The well-known instability of the disulfide anion radicals, (R SR2)-, is apparently explained by the antibonding electron population, presumably in the framework of the disulfide bond. In some cases, however, these anion radicals turned out to be more or less stable (Breitzer et al. 2001). Two examples in Schemes 3-22 and 3-23 deserve to be distinguished. Firstly, one-electron reduction of naphthalene-1,8-disulfide using sodium in dimethoxyethane generates the corresponding anion radical, Scheme 3-22. Second, the oxidation of a [l,n]-dithiol by Ti(III)-H202 at pH 7 produces the cyclic disulfide according to Scheme 3-23 ... 

In the synthesis of a series of cyclic aromatic disulfide oligomers using oxidative coupling of dithiols with oxygen catalyzed by copper salts and an amine, an effective, easy, and rapid method for the synthesis of macrocyclic aromatic disulfide oligomer from 4,4/-oxybis(benzenethiol) by cyclodepoly-... 

Sulfur is an interesting atom because of the specific characteristics of the simple substance and compounds containing sulfur. Further, their functions are indispensable for current technologies. Studies on sulfur compounds are valuable not only in chemistry but also in biochemistry as well. Many cyclic monomers containing sulfur such as cyclic disulfides, thiiranes, thietanes, and arene dithiols were reported to form cyclic polymers by the ring-opening polymerization. The natural source of cyclic polymers is believed to be liquid, elemental sulfur [177-179]. 

A review on cyclic disulfides includes coverage of 1,2-dithioles and dithiolanes [94MI97], Reaction of the 1,2-dithiolane (108) with Me2S=CHR results in ring expansion to give (109)... 

Propane-1,3-Dithiol [HS(CH2)3SH]. In acetonitrile the electrochemical oxidation of thiols (i SH) is facilitated via the presence of bases (H20/H0 ). Ease of oxidation is further facilitated with dithiols that can form cyclic disulfides. Figure 11.5 illustrates cyclic voltammograms at a gold electrode for... 

Cyclic disulfides. Reaction of a dithiol (I) with an aqueous solution oflead acetate gives a lead dithiolate (2) in nearly quantitative yield. These react with sulfur in benzene solution at room temperature to give the cyclic disulfide (3) in high yield. 

The oxidative dimerization of a,o -dithiols carried out by a large number of oxidizing reagents is a well-known and widely used method for the synthesis of cyclic disulfides. An alternative one-step conversion of the acyclic bisthio-cyanates 297 using TBAF in dry THF at room temperature to form cyclic disulfides in moderate yields, including a limited number of 1,2-dithiolanes 298 (Scheme 54), has been developed <1999TL6489>. The available mechanistic information indicates that the process, induced by so-far-unreported nucleophilic attack by fluoride, tolerates ester and ketal groups, while silyl ethers are cleaved. 

The chemistry of Iipoic acid is described in the chapter (pp. 1394-5). A thiol ester of exactly the kind described in the problem is indeed involved and it is deacylated, though by CoASH rather than an alcohol, and the dithiol is indeed recycled as the disulfide. The oxidation to the cyclic disulfide is carried out in Nature by FAD. In the chapter we suggest that the oxidation of the dithiol to the disulfide is a separate step from the transesterification. You may well think that the evidence in this problem suggests that the oxidation may take place first. 

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