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13: Carboxylic Acid Esters of N-Hydroxypyridine-2-thione

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    24077
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    The discussion in Chapter 12 focused on reactions of O-thiocarbonyl compounds prepared by derivatization of hydroxyl groups in partially pro­tected carbohydrates. The current chapter is concerned with reactions of another type of thiocarbonyl compound, one prepared by carboxyl group esterification. The compounds of interest are esters of N‑hy­dro­xy­pyri­dine-2-thione, some­times referred to as O-acyl-N-hydroxy-2-thiopyridones or “Bar­ton esters”.

    Barton esters generate carbon-centered radicals in photo­chem­ically initiated reactions in which the esters themselves produce the chain-carrying radicals needed for the chain reaction to continue (Scheme 1).1–4 Radical formation from these esters is followed by loss of carbon dioxide and, in the absence of additional reactants (e.g., a compound with an electron-deficient double bond), formation of a product with a sulfur atom bonded to the carbon atom in the carbohydrate frame­work where the radical was centered (eq 1).5 The carbon–sulfur bond in the product from Barton ester reaction provides greater flexibility in further synthetic transformation than does the carbon–hydrogen bond that forms when tin or silicon hydrides are present. The synthetic potential of this type of reaction is greater than that indi­cated in eq 1 because the intermediate, carbon- centered radical also can add to a mul­tiple bond (eq 2),6,7 abstract a hydrogen atom from a suitable donor (eq 3),8 or undergo other radical reac­tions. In effect, through formation and reaction of N-hydroxy­pyri­dine-2-thione esters it is possible to replace a carboxyl group in a molecule with a carbon-atom chain or with one of a variety of functional groups.

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    Since relatively few carbohydrates contain the carboxyl group needed for radical formation by Barton ester photolysis, the requirement that a carbo­hydrate first be converted into a carboxylic acid places a barrier to the gen­eral usefulness of this procedure. In spite of this limitation a number of addition and group replacement reac­tions exist that are based on radical formation from N‑hydroxypyridine-2-thione esters of carbohydrates that have been modified to contain a carboxyl group.


    This page titled 13: Carboxylic Acid Esters of N-Hydroxypyridine-2-thione is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.

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