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V. Comparing the Reactivity of O-Thiocarbonyl and O-Carbonyl Carbohydrates

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    24075
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    When one considers the success of O-thiocarbonyl compounds as substrates in the Barton- Mc­Combie reaction, a reasonable question to ask is “Why don’t O-carbonyl carbohydrate der­iv­atives (in particular, O-acylated com­pounds) exhibit similar reactivity?” An answer to this ques­tion can be framed in terms of the reactions shown in Scheme 28.223–225 According to this explan­ation, the equilibrium involving addition of Bu3Sn· to a compound with an O-thiocarbonyl group produces a far greater concentration of the adduct radical 77 than the concentration of the radical 76 produced by addi­tion of Bu3Sn· to the corresponding O-carbonyl carbohydrate. The dramat­i­cally greater equili­brium concentration of 77 leads to a corresponding increased rate of carbo­hydrate radical (CARB·) formation.223–225

    II12s28.png

    Detectable reaction of simple esters with Bu3Sn· becomes possible only if the low equi­librium concentration of 76 can be increased in some manner or compensated for by rapid further reaction of this radical (k1 large). Due to these requirements, no reaction takes place under normal Barton-McCombie conditions, but replacement of an acyloxy group with a hydrogen atom does occur when acylated carbo­hy­drates react with (C6H5)3Si· under vigorous conditions [(C6H5)3SiH, 140 oC, 12 h, benzoyl peroxide].226 Also, under quite different conditions (HMPA, H2O, UV light, room temperature) photo­chemical electron transfer leads to reduction of acylated carbo­hy­drates to the corresponding deoxy compounds.227 These and other reactions of esterified carbo­hydrates are discussed in detail in Chapter 8.


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