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8: Carboxylic Acids & Esters

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    24041

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    Carbohydrates containing typical O-acyl groups are unreactive under the reduction condi­tions (AIBN initiation, Bu3SnH, 80-110 oC) normally used for radical reac­tions. This lack of reac­tivity changes when O‑acyl groups become part of the more complex structures found in α-acyl­oxy ketones, methyl oxalyl esters, and p‑cyano­benzoates. For such com­pounds radical reac­tion with Bu3SnH under normal reaction conditions replaces the acyloxy group with a hydrogen atom.

    There are conditions under which a less complex O‑acyl group (e.g., an O‑acetyl or O-ben­zoyl group) is replaced with a hydrogen atom. One set of conditions includes raising the reaction temperature drama­tically, a change with potentially destructive consequences for the compounds involved. A more attractive approach depends upon photo­chem­ically promoted electron transfer to an esterified carbo­hydrate. Electron transfer (both photochemical and nonphotochemical) per­meates the radical reactions of carboxylic acid esters; that is, many of these reactions either involve (or may involve) electron transfer.

    Another way in which O-acyl groups participate in radical reactions is by group migration. When a radical centered at C-1 in a pyranoid or furanoid ring has an O-acyl group attached to C-2, this group will migrate to C-1 when the conditions are properly selected. Such migration provides an effective method for producing 2-deoxy sugars.

    Although esters of carboxylic acids are rich sources for substrates in radical-forming reac­tions, the acids themselves also can produce radicals. Under the proper conditions carboxylic acids generate carboxyl radicals, intermediates that lose carbon dioxide to form carbon-centered radi­cals. Carboxyl radicals are generated by electrolysis of carboxylate anions and by the reaction of carboxylic acids with hypervalent iodine compounds.


    This page titled 8: Carboxylic Acids & Esters 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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