8: Carboxylic Acids & Esters
- Page ID
- 24041
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.