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Hemithioacetal is an organic functional group with the general formula RCH(OH)SR. They are also called thiohemiacetal. With four substituents on carbon, hemithioacetals are chiral. A related functional group are dithiohemiacetal, with the formula RCH(SH)SR. Although they can be important intermediates, hemithioacetals are usually not isolated since they exist in equilibrium with the thiol and aldehyde:
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Formation and structure
Hemithioacetal form by the reaction of a thiol and an aldehyde:
RCHO + R’SH ⇌ RCH(OH)(SR’)Hemithioacetals usually arise via acid catalysis. They typically are intermediates in the formation of dithioacetals:
RCH(OH)(SR’) + R’SH ⇌ RCH(SR’)2 + H2OIsolable hemithioacetal
Hemithioacetals ordinarily readily dissociate into thiol and aldehyde, however some have been isolated. In general these isolable hemithioacetals are cyclic, which disfavors dissociation, and can often be further stabilized by the presence of acid. An important class are S-glycosides, such as octylthioglucoside, which are formed by a reaction between thiols and sugars. Other examples include 2-hydroxytetrahydrothiophene and the anti-HIV drug Lamivudine. 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.
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.
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:
After formation, the hemithioacetal breaks into hydrogen peroxide, farnesal, and cysteine.