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Pyrithione
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Pyrithione
Pyrithione is the common name of an organosulfur compound with molecular formula C
5H
5NOS, chosen as an abbreviation of pyridinethione, and found in the Persian shallot. It exists as a pair of tautomers, the major form being the thione 1-hydroxy-2(1H)-pyridinethione and the minor form being the thiol 2-mercaptopyridine N-oxide; it crystallises in the thione form. It is usually prepared from either 2-bromopyridine, 2-chloropyridine, or 2-chloropyridine N-oxide, and is commercially available as both the neutral compound and its sodium salt. It is used to prepare zinc pyrithione, which is used primarily to treat dandruff and seborrhoeic dermatitis in medicated shampoos, though is also an anti-fouling agent in paints.
The preparation of pyrithione was first reported in 1950 by Shaw and was prepared by reaction of 2-chloropyridine N-oxide with sodium hydrosulfide followed by acidification, or more recently with sodium sulfide. 2-chloropyridine N-oxide itself can be prepared from 2-chloropyridine using peracetic acid. Another approach involves treating the same starting N-oxide with thiourea to afford pyridyl-2-isothiouronium chloride N-oxide which undergoes base hydrolysis to pyrithione. 2-Bromopyridine can be oxidised to its N-oxide using a suitable peracid (as per 2-chloropyridine), both approaches being analogous to that reported in Organic Syntheses for the oxidation of pyridine to its N-oxide. A substitution reaction using either sodium dithionite (Na
2S
2O
4) or sodium sulfide with sodium hydroxide will allow the replacement of the bromo substituent with a thiol functional group.
The alternative strategy is to form the mercaptan before introducing the N-oxide moiety. 2-Mercaptopyridine was originally synthesized in 1931 by heating 2-chloropyridine with calcium hydrosulfide, an approach similar that first used to prepare pyrithione. The analogous thiourea approach via a uronium salt was reported in 1958 and provides a more convenient route to 2-mercaptopyridine. Oxidation to the N-oxide can then be undertaken.
Pyrithione is found as a natural product in the Allium stipitatum plant, an Asian species of onion, also known as the Persian shallot. Its presence was detected using positive ion mass spectrometry using a DART ion source and the disulfide dipyrithione (2,2'-disulfanediylbis(pyridine)-1,1'-dioxide) has been reported from the same species. Dipyrithione can be prepared in a laboratory by oxidation of pyrithione with chlorine in the presence of sodium hydroxide:
Dipyrithione is used as a fungicide and bactericide, and has been reported to possess novel cytotoxic activity by inducing apoptosis. However, as apoptosis only occurs in higher organisms, this mechanism isn't relevant to the antifungal and bacteric idal properties of pyrithione.
Pyrithione exists as a pair of prototropes, a form of tautomerism whereby the rapid interconversion of constitutional isomers involves the shift of a single proton, in this case between the sulfur and oxygen atoms (shown in the infobox above).
Salts of the conjugate base of pyrithione can also be considered to exhibit tautomerism by notionally associating the sodium ion with whichever heteroatom bears the negative charge of the anion (as opposed to the formal charges associated with the N-oxide); however, considering the anion alone, this could also be described as an example of resonance.
Pyrithione is a weak acid with pKa values of −1.95 and +4.6 (thiol proton), but is a markedly stronger acid than either of its parent compounds (pyridine-N-oxide and pyridine-2-thiol), both of which have pKa > 8. It is only slightly soluble in water (2.5 g L−1) but is soluble in many organic solvents (including benzene, chloroform, dichloromethane, dimethylformamide, dimethylsulfoxide, and ethyl acetate) and slight solubility in others (diethyl ether, ethanol, methyl tert-butyl ether, and tetrahydrofuran).
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Pyrithione
Pyrithione is the common name of an organosulfur compound with molecular formula C
5H
5NOS, chosen as an abbreviation of pyridinethione, and found in the Persian shallot. It exists as a pair of tautomers, the major form being the thione 1-hydroxy-2(1H)-pyridinethione and the minor form being the thiol 2-mercaptopyridine N-oxide; it crystallises in the thione form. It is usually prepared from either 2-bromopyridine, 2-chloropyridine, or 2-chloropyridine N-oxide, and is commercially available as both the neutral compound and its sodium salt. It is used to prepare zinc pyrithione, which is used primarily to treat dandruff and seborrhoeic dermatitis in medicated shampoos, though is also an anti-fouling agent in paints.
The preparation of pyrithione was first reported in 1950 by Shaw and was prepared by reaction of 2-chloropyridine N-oxide with sodium hydrosulfide followed by acidification, or more recently with sodium sulfide. 2-chloropyridine N-oxide itself can be prepared from 2-chloropyridine using peracetic acid. Another approach involves treating the same starting N-oxide with thiourea to afford pyridyl-2-isothiouronium chloride N-oxide which undergoes base hydrolysis to pyrithione. 2-Bromopyridine can be oxidised to its N-oxide using a suitable peracid (as per 2-chloropyridine), both approaches being analogous to that reported in Organic Syntheses for the oxidation of pyridine to its N-oxide. A substitution reaction using either sodium dithionite (Na
2S
2O
4) or sodium sulfide with sodium hydroxide will allow the replacement of the bromo substituent with a thiol functional group.
The alternative strategy is to form the mercaptan before introducing the N-oxide moiety. 2-Mercaptopyridine was originally synthesized in 1931 by heating 2-chloropyridine with calcium hydrosulfide, an approach similar that first used to prepare pyrithione. The analogous thiourea approach via a uronium salt was reported in 1958 and provides a more convenient route to 2-mercaptopyridine. Oxidation to the N-oxide can then be undertaken.
Pyrithione is found as a natural product in the Allium stipitatum plant, an Asian species of onion, also known as the Persian shallot. Its presence was detected using positive ion mass spectrometry using a DART ion source and the disulfide dipyrithione (2,2'-disulfanediylbis(pyridine)-1,1'-dioxide) has been reported from the same species. Dipyrithione can be prepared in a laboratory by oxidation of pyrithione with chlorine in the presence of sodium hydroxide:
Dipyrithione is used as a fungicide and bactericide, and has been reported to possess novel cytotoxic activity by inducing apoptosis. However, as apoptosis only occurs in higher organisms, this mechanism isn't relevant to the antifungal and bacteric idal properties of pyrithione.
Pyrithione exists as a pair of prototropes, a form of tautomerism whereby the rapid interconversion of constitutional isomers involves the shift of a single proton, in this case between the sulfur and oxygen atoms (shown in the infobox above).
Salts of the conjugate base of pyrithione can also be considered to exhibit tautomerism by notionally associating the sodium ion with whichever heteroatom bears the negative charge of the anion (as opposed to the formal charges associated with the N-oxide); however, considering the anion alone, this could also be described as an example of resonance.
Pyrithione is a weak acid with pKa values of −1.95 and +4.6 (thiol proton), but is a markedly stronger acid than either of its parent compounds (pyridine-N-oxide and pyridine-2-thiol), both of which have pKa > 8. It is only slightly soluble in water (2.5 g L−1) but is soluble in many organic solvents (including benzene, chloroform, dichloromethane, dimethylformamide, dimethylsulfoxide, and ethyl acetate) and slight solubility in others (diethyl ether, ethanol, methyl tert-butyl ether, and tetrahydrofuran).
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