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[[Image:Hemithioacetal.png|150px|thumb|Hemithioacetal functional group]]
 
'''Hemithioacetal''' is an organic [[functional group]] with the general formula RCH(OR)SR.<ref name=March>{{March6th}}</ref> They form in a spontaneous reaction between a [[thiol]] and an [[aldehyde]]. Since the formerly carbonyl carbon bears four different substituents, hemiacetals are chiral. Hemithioacetals are usually intermediates in the catalytic reactions and usually arise via acid or base [[catalysis]]. The hemithioacetal features [[vicinal (chemistry)|vicinal]] [[hydroxyl]] and [[thioether]] functionalities. Although they are important intermediates, hemithioacetals are usually not isolated since they exist in equilibrium with the thiol and aldehyde:
 
:RCHO + R’SH  <math>\overrightarrow{\leftarrow}</math>   RCH(OH)(SR’)
 
==Isolable hemithioacetal==
Hemithioacetals ordinarily readily dissociate into thiol and aldehyde. Some hemithioacetals have been isolated. The few isolable hemithioacetals are all cyclic, which disfavors dissociation. One example is 2-hydroxy[[tetrahydrothiophene]].<ref>Cox, J.M.; Owen, L.N., J. Chem. Soc. C, Cyclic hemithioacetals: Analogues of thiosugars with sulphur in the ring, 1967, 1130-1134. {{DOI|10.1039/J39670001130}}</ref> Another isolable hemithioacetal can be prepared by addition of thiol to methyl glyoxalate.<ref>Milton, J; Brand, S; Jones, M.F; Rayner, C.M., Tetrahedron Letters, Enantioselective Enzymatic Synthesis of the Anti-Viral Agent Lamivudine, 1995, volume 36, 6961-6964, {{DOI|10.1016/0040-4039(95)01380-Z}}</ref> The stability of hemithioacetal is enhanced in the presence of acid.<ref>Barnett, R. E.; Jencks, W. P, J. Am. Chem. Soc, Diffusion-controlled and concerted catalysis in the decomposition of hemithioacetals, 1969, volume 91, 6758-6765. {{DOI|10.1021/ja01052a038}}</ref> Another class of isolable hemithioacetals are derived from carbonyl groups that form stable hydrates. For example, thiols react with hexafluoroacetone trihydrate to give hemithioacetals, which can be isolated.<ref>Field, L.; Sweetman, B.J.; Bellas, M., Journal of Medicinal Chemistry, Biologically oriented organic sulfur chemistry. II. Formation of hemimercaptals or hemimercaptoles as a means of latentiating thiols, 1969, 12(4), 624-628.  {{DOI|10.1021/jm00304a014}}</ref>
[[File:C4H7(OH)S.png|thumb|140px|left|2-Hydroxy[[tetrahydrothiophene]] is a rare example of a hemithioacetal that can be isolated.]]
 
==Hemithioacetals in nature==
[[Glyoxalase I]], which is part of the glyoxalase system present in the [[cytosol]], catalyzes the conversion of α-oxoaldehyde (RC(O)CHO) and the thiol [[glutathione]] (abbreviated GSH) to S-2-hydroxyacylglutathione derivatives [RCH(OH)CO-SG]. The catalytic mechanism involves an intermediate hemithioacetal adduct [RCOCH(OH)-SG]. The spontaneous reaction forms [[methylglyoxal]]-glutathione hemithioacetal and human glyoxalse I.<ref>Thornalley, P.J., Biochemical Society Transactions, Glyoxalase I - Structure, function and a critical role in the enzymatic defence against glycation, 2003, 31 (6), 1343-1348. ISSN: 03005127</ref>
 
A hemithioacetal is also invoked in the mechanism of prenylcysteine [[lyase]]. In catalytic mechanism, S-farnesylcysteine is oxidized by a [[flavin]] to a thiocarbenium ion. The thiocarbenium ion hydrolyzes to form the hemithioacetal:
: [(RS)C(R’)(H)]<sup>+</sup> +  H2O  →  (RS)C(R’)(H)OH  +  H<sup>+</sup>
After formation, the hemithioacetal breaks into [[hydrogen peroxide]], farnesal, and [[cysteine]].<ref>Digits, J.A.; Pyun, H.-J.; Coates, R.M.; Casey, P.J. Journal of biological chemistry, Stereospecificity and kinetic mechanism of human prenylcysteine lyase, an unusual thioether oxidase, 2002, volume 277, 41086-41093. {{DOI|10.1074/jbc.M208069200}}</ref>
 
==References==
{{Reflist}}
 
[[Category:Acetals]]
[[Category:Functional groups]]

Revision as of 17:39, 29 October 2013

Hemithioacetal functional group

Hemithioacetal is an organic functional group with the general formula RCH(OR)SR.[1] They form in a spontaneous reaction between a thiol and an aldehyde. Since the formerly carbonyl carbon bears four different substituents, hemiacetals are chiral. Hemithioacetals are usually intermediates in the catalytic reactions and usually arise via acid or base catalysis. The hemithioacetal features vicinal hydroxyl and thioether functionalities. Although they are important intermediates, hemithioacetals are usually not isolated since they exist in equilibrium with the thiol and aldehyde:

RCHO + R’SH RCH(OH)(SR’)

Isolable hemithioacetal

Hemithioacetals ordinarily readily dissociate into thiol and aldehyde. Some hemithioacetals have been isolated. The few isolable hemithioacetals are all cyclic, which disfavors dissociation. One example is 2-hydroxytetrahydrothiophene.[2] Another isolable hemithioacetal can be prepared by addition of thiol to methyl glyoxalate.[3] The stability of hemithioacetal is enhanced in the presence of acid.[4] Another class of isolable hemithioacetals are derived from carbonyl groups that form stable hydrates. For example, thiols react with hexafluoroacetone trihydrate to give hemithioacetals, which can be isolated.[5]

2-Hydroxytetrahydrothiophene is a rare example of a hemithioacetal that can be isolated.

Hemithioacetals in nature

Glyoxalase I, which is part of the glyoxalase system present in the cytosol, catalyzes the conversion of α-oxoaldehyde (RC(O)CHO) and the thiol glutathione (abbreviated GSH) to S-2-hydroxyacylglutathione derivatives [RCH(OH)CO-SG]. The catalytic mechanism involves an intermediate hemithioacetal adduct [RCOCH(OH)-SG]. The spontaneous reaction forms methylglyoxal-glutathione hemithioacetal and human glyoxalse I.[6]

A hemithioacetal is also invoked in the mechanism of prenylcysteine lyase. In catalytic mechanism, S-farnesylcysteine is oxidized by a flavin to a thiocarbenium ion. The thiocarbenium ion hydrolyzes to form the hemithioacetal:

[(RS)C(R’)(H)]+ + H2O → (RS)C(R’)(H)OH + H+

After formation, the hemithioacetal breaks into hydrogen peroxide, farnesal, and cysteine.[7]

References

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  1. ↑ Template:March6th
  2. ↑ Cox, J.M.; Owen, L.N., J. Chem. Soc. C, Cyclic hemithioacetals: Analogues of thiosugars with sulphur in the ring, 1967, 1130-1134. Electronic Instrument Positions Staff (Standard ) Cameron from Clarence Creek, usually spends time with hobbies and interests which include knotting, property developers in singapore apartment For sale and boomerangs. Has enrolled in a world contiki journey. Is extremely thrilled specifically about visiting .
  3. ↑ Milton, J; Brand, S; Jones, M.F; Rayner, C.M., Tetrahedron Letters, Enantioselective Enzymatic Synthesis of the Anti-Viral Agent Lamivudine, 1995, volume 36, 6961-6964, Electronic Instrument Positions Staff (Standard ) Cameron from Clarence Creek, usually spends time with hobbies and interests which include knotting, property developers in singapore apartment For sale and boomerangs. Has enrolled in a world contiki journey. Is extremely thrilled specifically about visiting .
  4. ↑ Barnett, R. E.; Jencks, W. P, J. Am. Chem. Soc, Diffusion-controlled and concerted catalysis in the decomposition of hemithioacetals, 1969, volume 91, 6758-6765. Electronic Instrument Positions Staff (Standard ) Cameron from Clarence Creek, usually spends time with hobbies and interests which include knotting, property developers in singapore apartment For sale and boomerangs. Has enrolled in a world contiki journey. Is extremely thrilled specifically about visiting .
  5. ↑ Field, L.; Sweetman, B.J.; Bellas, M., Journal of Medicinal Chemistry, Biologically oriented organic sulfur chemistry. II. Formation of hemimercaptals or hemimercaptoles as a means of latentiating thiols, 1969, 12(4), 624-628. Electronic Instrument Positions Staff (Standard ) Cameron from Clarence Creek, usually spends time with hobbies and interests which include knotting, property developers in singapore apartment For sale and boomerangs. Has enrolled in a world contiki journey. Is extremely thrilled specifically about visiting .
  6. ↑ Thornalley, P.J., Biochemical Society Transactions, Glyoxalase I - Structure, function and a critical role in the enzymatic defence against glycation, 2003, 31 (6), 1343-1348. ISSN: 03005127
  7. ↑ Digits, J.A.; Pyun, H.-J.; Coates, R.M.; Casey, P.J. Journal of biological chemistry, Stereospecificity and kinetic mechanism of human prenylcysteine lyase, an unusual thioether oxidase, 2002, volume 277, 41086-41093. Electronic Instrument Positions Staff (Standard ) Cameron from Clarence Creek, usually spends time with hobbies and interests which include knotting, property developers in singapore apartment For sale and boomerangs. Has enrolled in a world contiki journey. Is extremely thrilled specifically about visiting .
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