Ketoses show positive Fehling’s and Tollens’s tests not only by oxidation to α-dicarbonyl compounds, but through a second process: Ketoses isomerize to aldoses in the presence of base. The aldose then undergoes oxidation by the Fehling’s or Tollens’s solution. Using any ketose in Figure 24-2, propose a base-catalyzed mechanistic pathway to the corresponding aldose.

Figure 24-2

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CH,OH H- -ОН CH,OH D-(-)-Erythrulose CH,OH CH,OH H FOH HO H H- H FOH CH-OH CH,OH D-(+)-Ribulose D-(+)-Хylulose CH,OH CH,OH CH;OH CH,OH O: H- O- Но- -H H- -ОН Но- -H H- H- HO- Но- -H- Но- -H- H OH H- -ОН H- -ОН H- OH CH2OH CH,OH CH,OH ČH,OH D-(+)-Psicose D-(-)-Fructose D-(+)-Sorbose D-(-)-Тagatose