Thio group

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

The thio group can be removed by a reagent that binds sulfur strongly, eg, triphenylphosphine [603-35-0]. 

Recognition of the thio group s key role in biochemistry has led to studies of l,4-ben2oquinone with glutathione, a tripeptide 7-Glu-Cys-Gly (GSH). The cross-oxidation of the initial addition product by excess quinone leads, under physiological conditions, to all three isomeric products (46), ie, the 2,3-and 2,6-isomers as well as the 2,5-disubstituted l,4-ben2oquinone shown. 

Disubstitution of 2,4-dichloropyrimidine with thiourea proceeds more readily than with hydrosulfide ion, principally because the former intermediate (229) contains the activating, cationic acylated thio group compared to the highly anionized mercapto group in the latter reaction. 

Thioxo-4-oxopyrimidines 110 treated with 2,4,6-trinitrochlorobenzene (76) are first arylated on the exocyclic thio group and the intermediates are then... 

In contrast, there are two dimeric compounds to consider. One is the oxo,oxo derivative 6, the other a mixed oxo,thio 7 (3). The oxo groups in all of 5-7 occupy terminal positions, as usual, whereas the thio group in 7 is in abridging position. Each rhenium atom in 7 is therefore six-coordinate by virtue of a short rhenium-rhenium distance, 277.7 pm. Ignoring that bond, the local geometry about the one rhenium is a severely-distorted square pyramid, whereas the oxo-rhenium moiety has a geometry best described as a trigonal bipyramid. 

Mechanistic studies [123] have shown that thioglycosides can undergo in situ anomerization in the presence of iodonium ion catalysts. It has been demonstrated that this anomerization proceeds by intermolecular exchange of alkyl thio groups. An increase in the steric bulk of the leaving group resulted in incomplete or no anomerization. It has been proposed that this anomerization process is important for the stereochemical outcome of glycosylations [123]. 

Modifications with Thiol Groups. Mercapto or thio groups often have powerful physiological activity and have been claimed to be useful as radiation protective groups. These groups have been introduced into poly(vinyl alcohol) in a variety of ways. 

Partial fluorination of 4-arylthio-l,3-dioxolan-2-ones occurs preferentially at the carbon atom adjacent to the thio group [67]. However, a remarkable solvent effect is encountered. In the more polar solvent, dimethoxyethane substitution occurs, while in the less polar dichloromethane a larger portion of the desulfurization with cleavage of the phenylthio group takes place. This is attributed to the fact that the intermediate radical cation is more stable in the polar solvent and undergoes deprotonation, while in the less polar solvent, the less stabilized radical cation dissociates into a dioxolane cation and a phenylthio radical. 

The fungitoxic N-perhalogenmethylmercapto moiety was introduced for plant protection in 1950 The. two fungicides captan(l) (N-(trichloromethylthio)-tetrahydrophthalimide and folpet(2) (N-(trichlorome-thylthio)phthalimide) contain a perchlorinated methyl-thio group (1 ). About a decade later substitution of fluorine for one of the chlorines in the perchloro-... 

Thiadiazoles having one or two thio groups in the 2- and/or 5-positions react with metals to form bidentate ligands they are widely used as antioxidants. An interesting reaction of mesoionic (95) with acetylene dicarboxylate is the formation of thiophene (97) via the intermediate (96) (Scheme 15) <84CHEC-I(4)545>. 

Figure 9.14 (a) Complementary strands 19 and 20 carrying trityl-protected thio groups can... 

Figure 9.15 (a) Complementary strands 21 and 22 canying trityl-piotected thio groups... 

Figure 9.14 (a) Complementary strands 19 and 20 carrying trityl-protected thio groups can be cross-linked when subjected to reversible redox conditions, (b) MALDI spectra show that 19 and 20 were sequence specifically cross-linked into 19-20 in both methanol and water. 

Even more facile is the cleavage of thio groups from 1.1-dialkylthio- or diaryl-thio-2.4.6-triphenyl-X -phosphorins 196 (see p. 88). 

A couple of additional efforts warrant comment. The reaction between trifluoromethyliodide and 2,5-dimethoxythiophcnol should have produced 2,5-dimethoxyphenyl trifluoromethyl sulfide, but it didn t produce anything. And one more. What about a bare thio group at the 4-position in this 2C-T-family Maybe... 

RhH2(SiPh2SAr)(PMe3)3 with a Si—S bond length of 222.8 pm. Since the silyl group in 3 is trans to the thio group and the isomerization is intramolecular, it is suggested that cis-trans isomerization occurs prior to transfer (equation ll)19. 

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