Acid-catalyzed interconversion

Considerations of entropies of activation played a major role in a recent discussion of the acid-catalyzed interconversion of 1,2-cyclo-hexanedione and its enol (Bakule and Long, to be published). The reaction is more complex than the usual keto-enol interconversion because the ketone form, but not the enol, is found to be hydrated. 

Draw a stepwise mechanism for the acid-catalyzed interconversion of two glucose anomers by mutarotation. 

Markey SP, Dudding T, Wang TC (2000) Base- and acid-catalyzed interconversions of O-acyl- and N-acyl-... 

Markey, S. P, Budding, T., and Wang, T. C. L. (2000). Base- and acid-catalyzed interconversions of O-acyl- and N-acylethanolamines a cautionary note for lipid analyses. J. Lipid Res. 41, 657-662. 

FIGURE 16.3 Kinetics of acid-catalyzed interconversion of PTXl (a), PTX6 (b), and PTX2 (c). Peaks of pectenotoxins were obtained by selected ion monitoring (SIM) LC-MS. 

SCHEME 2 Proposed acid-catalyzed interconversion of spiroketal regioisomers. 

Show each step in the mechanism of the acid-catalyzed interconversion of / )- and (S)-2-methyl-cyclohexanone. 

The preferential formation of the more substituted alkene is the common pattern in such reactions, but this product distribution could be the result of an acid-catalyzed interconversion of the products. That is, there may be an equilibrium-controlled and not a rate-controlled product distribution in acid-catalyzed reactions. 

McMurry, J. E. The Total Synthesis of Copacamphene and its Acid-catalyzed Interconversion with Sativene. J. Org. Chem. (USA) 36, 2826 (1971). 

Step 1 Keto-enol tautomerism. The acid catalyzes interconversion of the... 

Similarly to many other great discoveries, the molybdic acid catalyzed interconversion of epimeric aldoses is the result of serendipity. Thus, when D-talose, obtained from another, lesser-known reaction also discovered by V. Bflik, namely the stereoselective, molybdate-catalyzed hydroxylation of glycals [3], was repeatedly recrystalHzed (in the presence of a catalyst that had not been completely removed), it was not possible to obtain it in a pure condition, as the aldose slowly epimerized to D-galactose [4]. A simple verification of the explanation of this originally imdesirable interconversion, the treatment of D-galac-... 

Since the first observation of the molybdic acid catalyzed interconversion of epi-meric aldoses it has been clear that the mechanism of the reaction is different to the enediol pathway of the Lobry de Bruyn-Alberda van Ekenstein (LdB-AvE) isomerization reaction of aldoses that provides 2-ketoses as primary major products [31]. Moreover, the interconverting aldoses do not incorporate deuterium if the molybdic acid catalyzed reaction is performed in deuterium oxide [32]. Furthermore, using specifically the tritium-labeled aldoses that were available in the 1970s,it has been proved that during the interconversion D-[l- H]glucose transforms to D-[2- H]mannose and vice versa. The results suggested that the process is intramolecular and involves a simultaneous transfer of the hydrogen atoms hnked to the carbon atoms C-1 and C-2. 

Notice that the steps are reversed in the base- and acid-catalyzed interconversions. In the base-catalyzed reaction, the base removes a proton from an a-carbon in the first step and the oxygen is protonated in the second step. In the acid-catalyzed reaction, the oxygen is protonated in the first step and the proton is removed from the a-carbon in the second step. Notice also that the catalyst is regenerated in both the acid- and base-catalyzed reactions. 

In Summary a, -Unsaturated aldehydes and ketones are more stable than their noncon-jugated counterparts. Either base or acid catalyzes interconversion of the isomeric systems. Reactions typical of alkenes and carbonyl compounds are also characteristic of a,)8-unsaturated aldehydes and ketones. 

---