A-Hydroxy-2-thiopyridone esters

Radical decarboxylation of A-hydroxy-2-thiopyridone esters in the presence of an olefin results in the formation of a carbon-carbon bond. The monoester 5 is not prone to jS-elim-ination, and the faces of the dioxolane ring are encumbered by the methyl ester, so radical trapping occurs preferentially from the side opposite the methyl ester. This results in overall retention of configuration at the reacting carbon. Irradiation of ester 11 in the presence of methyl acrylate affords the tmns-eAkQm 12 after appropriate isolation. Once the first carboxylic acid has been modified, the second can be reacted similarly, whereby the stereochemistry of the first substitution controls the stereochemistry of the second. In this way the resulting product is obtained with overall double retention [11]. 

One great advantage of the decarboxylative halogenation with O-acyl esters of A-hydroxy-2-thiopyridone is that the reaction does not require any heavy metal such as Ag or Hg, unlike the Hunsdiecker reaction [24, 25]. Moreover, decarboxylative bromination of p-methoxybenzoic acid can be also carried out in good yield, while it does not proceed with the Hunsdiecker reaction instead, electrophilic bromination on the aromatic ring occurs. 

A mixture of O-acyl ester (1 mmol) of A-hydroxy-2-thiopyridone and benzoquinone (5-7 mmol) in degassed dichloromethane (20 ml) was irradiated with a 300 W tungsten lamp for 30 min. under a nitrogen atmosphere at 0 °C. After the reaction, the solvent was removed and the residue was chromatographed on silica gel to give 1,2-disubstituted benzoquinone [63]. 

The photolysis of Barton esters (N-hydroxy-2-thiopyridone esters, 43) proved to be an efficient method for generating carbon-centered radicals that are exploited for the regioselective alkylation of electron-deficient olefins a thiopyridyl unit is likewise incorporated into the end products. In a recent application, Barton esters were found useful in the synthesis of natural and unnatural disubstituted maleimides or maleic anhydrides by way of two consecutive radical addition steps, as described in Scheme 3.27 [72]. 

A covalent bond is generally cleaved to its radical fragments at temperatures higher than 800 °C. Covalent bonds that can be cleaved at <150°C are limited to weak bonds whose dissociation energies are under 30-40 kcal/mol [1]. Azo compounds, peroxides, nitrite esters, ester of A-hydroxy-2-thiopyridone etc. fit into this group... 

Fundamentally, O-esters of N-hydroxy-2-thiopyridone are photo-lyzed in the presence of an excess of white phosphorus in a methylene chloride/carbon disulfide medium. On solvent removal, hydrolysis, and oxidation with hydrogen peroxide, good yields of phosphonic acids (Figure 2.10) bearing the carbon functionality of the parent acid are isolated. 

A solution of sulfur (S8, 74 mg, 0.29 mmol) and adamantanecarboxylate ester of N-hydroxy-2-thiopyridone (290 mg, 1 mmol) in dichloromethane (15 ml) was irradiated with a tungsten lamp for 1 h under a nitrogen atmosphere at 0 °C. After the reaction, a solution of NaBH4 (20 mmol) in methanol (10 ml) was added drop wise over 15 min to the reaction mixture at room temperature, and the mixture was stirred for one hour. After the reaction, 1 M sulfuric acid (60 ml) was added to the mixture, and then the solution was extracted with dichloromethane (4 X 20 ml), and dried over Na2S04. After removal of the solvent, the residue was purified by column chromatography on silica gel (eluent hexane/ethyl acetate = 97/3) to provide adamantanethiol in 88% yield [33]. 

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