Dithiols, formation

The Tmb thiol protection has also been exploited for the peptide dithiol formation on a resin by using dilute TFA.1641 The use of the mild oxidant CCLt in NMP1 061 for the oxidation of a resin-bound dithiol peptide is an interesting variation to this synthetic strategy.1641... 

In a separate pathway, MTP synthase is regenerated (grouped within a grey box in Fig. 4.12) by transferring sulfur to the small subunit. In Escherichia coli, MoeB catalyses the adenylation of the carboxyl of the C-terminal Gly residue of MoaD, in a reaction very similar to that catalysed by the El enzyme that activates ubiquitin in the ubiquitin/proteasome pathway referred to earlier. The AMP-activated MoaD is then sulfurated by sulfide transfer from an unknown donor. It is assumed that Cu is inserted directly after dithiolate formation. In the last two steps, adenylated MPT is formed from Mg-ATP, and, finally, in the presence of molybdate (MoOa ), bound MPT-AMP is hydrolysed, Cu is released and Mo is transferred to the MPT dithiolate, and the MoCo cofactor is released. 

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... 

Thieno[3,4-d][l,3]dithiol-2-one, 1-nitro-synthesis, 6, 1013 Thieno[3,4-c]furan, 4,6-dichloro-synthesis, 6, 1013 Thieno[3,4-c]furan, tetraphenyl-formation, 4, 1060 synthesis, 4, 1060 UV spectra, 4, 1044 Thieno[3,2-6]furans structure, 4, 1039 synthesis, 6, 1020 Thieno[3,4-c]furans... 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Quite a number of mixed sulfur-nitrogen macrocycles have been prepared, but these have largely been by the methods outlined in Chaps. 4 and 5 for the respective heteroatoms. An alternative method, involves the formation of a Schiff base, followed by reduction to the fully saturated system, if desired. An interesting example of the Schiff base formation is found in the reaction formulated in (6.12). Dialdehyde 14 is added to ethylenediamine in a solution containing ferrous ions. Although fully characterized, the yield for the reaction is not recorded. To avoid confusion with the original literature, we note the claim that the dialdehyde [14] was readily prepared in good yield by reaction of the disodium salt of 3-thiapentane-l, 5-diol . The latter must be the dithiol rather than the diol. 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

Carbon disulfide can act as an electrophilic agent with enamines at room temperature. Therefore, treatment of an enamine with both elemental sulfur and carbon disulfide in a polar solvent can result in the formation of a 3H-l,2-dithiole-3-thione (such as 108) and/or a 2H-l,3-dithiole-2-thione (such as 109) (135,139,140). These products are the result of competing... 

Addition of hydrogen sulfide results in formation of monomeric gem dithiols or trimeric thioketals (511,512). The initially reported thione formation (513-515), analogous to hydration of morpholinocyclohexene... 

There has been recent interest in naphtho-fused dithiepines as chiral acyl anion equivalents, particularly since the starting dithiol 128 can be obtained in enan-tiomerically pure form (89TL2575). This is transformed using standard methods into the dithiepine 129, but showed only moderate diastereoselectivity in its addition to carbonyl compounds. On the other hand, as we have seen previously for other systems, formation of the 2-acyl compound 130 and reduction or addition of a Grignard reagent gave the products 131 with much better stereoselectivity (91JOC4467). 

A good yield of 492 was obtained by reaction of 369 with dithiol 491 (Eq. 47). This reaction is probably the only example of the eight-membered ring formation by denitrocyclization reaction described so far (91ZOR185). 

Dithiols and dienes may react spontaneously to afford dithiols or dienes depending on the monomer dithiol ratio.221 However, the precise mechanism of radical formation is not known. More commonly, pholoinilialion or conventional radical initiators are employed. The initiation process requires formation of a radical to abstract from thiol or add to the diene then propagation can occur according to the steps shown in Scheme 7.17 until termination occurs by radical-radical reaction. Termination is usually written as involving the monomer-derived radicals. The process is remarkably tolerant of oxygen and impurities. The kinetics of the tbiol-ene photopolymerizalion have been studied by Bowman and... 

Platinum blue formation, 2,265 Platinum complexes, S, 351-500 acetylacetone reactions, 2,380 acetylides reactions, 5,402 alcohols, 5,465 alkene-1,2-dithiolates optica] recording systems, 6,126 alkenes, 5,403 bonding, 5,403... 

A range of 4-substituted l,3-dithiole-2-thiones (71) and 2,6-substituted 1,4-dithiafulvalenes (73) were synthesised from 4-substimted 1,2,3-thiadiazoles (72). Reaction of (72) with NaH in a mixture of CS2 and acetonitrile led to the formation of (71), whereas absence of CS2 gave fulvalenes (73). This route was found to be very efficient for the preparation of 4-formyl-1,3-dithiole-2-thione (71 R = CHO), which was previously difficult to prepare, and thus allowed the synthesis of the novel 2,6(7)-bisformyltetraAiafulvalene (74) <96T3171>. 

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]. 

The synthesis of halogenated TTF is most often based on the lithia-tion/electrophilic halogenation of either the TTF core itself [50] or the five-membered l,3-dithiole-2-thione [36,51]. This procedure has been recently expanded to the analogous l,3-diselenole-2-thione [52,53], allowing for the formation of a wide variety of iodo- and bromotetraselenafulvalenes or dithiadiselenafulvalenes shown in Schemes 6 and 7. 

Insertion of a C=0 or C=S group between two thiol or two selenol functions of a 1,2- or 1,3-dithiol or diselenol results in formation of the corresponding five- or six-membered heterocycles. 

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