Enol and keto-tautomerism

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. 

Simultaneous ring-chain and keto-enol tautomeric equilibria were observed (90ZOR2489) in solutions of 5-hydroxy-5-phenylpentane-2,4-dione 29. Increase of the solvent polarity and proton-accepting ability in the series... 

Pron et al.569) looked at polyacetylene treated from the gas phase with H2S04 which leads to HS04 counter-ions. They found that the conductivity drops in air with the appearance of C=O bands in the ir, although the rate of decay is much lower than would be expected for undoped samples. The polymer was more rapidly degraded by exposure to water but could be redoped with further acid treatment. Pron et al.570) have also reported hydrolytic instability in polyacetylene with A1C14 as the counterion. In both cases the proposed mechanism involves addition of OH" to the chain and keto-enol tautomerism to form carbonyl groups. 

In the proteins and nucleic acids the configurational mechanism has to be invoked to explain any hydrogen-bond disorder involving )N-H or NH groups where there is no orientational flexibility. This is shown in the two examples, below (the second describing amino/imino and keto/enol tautomeric states) ... 

Problem 27.5 Acrolein, CH2 CHCHO, is prepared by heating glycerol with sodium hydrogen sulfate, NaHS04. (a) Outline the likely steps in this synthesis, which involves acid-catalyzed dehydration and keto-enol tautomerization. (Hinf Which OH is easier to eliminate, a primary or a secondary ) (b) How could acrolein be converted into acrylic acid ... 

The mushroom tyrosinase-catalyzed oxidative decarboxylation of 3,4-dihydroxyphenyl mandelic acid (111, R = H) and a-(3,4-dihydroxyphenyl) lactic acid (111, R = Me) proceeds via the quinone methide intermediate 112. The coupled dienone-phenol rearrangement and keto-enol tautomerism transforms the quinone methide 112 into 1-acyl-3,4-dihydroxyphenyl compounds 113 (equation 48) . ... 

There is controversy over the kind and position of the carbon-oxygen bonds in GO. Besides hydroxyl groups, either ether linkages or keto groups on puckered layers of carbon hexagons are possible, and keto-enol tautomerism allows the carbon layers to remain planar . Carbon-oxygen IR bands of GO may be assigned to any of these... 

Dissolving monosaccharides in aqueous base causes them to undergo a series of enolizations and keto-enol tautomerizations that lead to isomerizations. For example, if a solution of D-glucose containing calcium hydroxide is allowed to stand for several days, a number of products can be isolated, including D-fructose and D-mannose (Fig. 22.5). This type of reaction is called the Lobry de Bmyn-Alberda van Ekenstein ttansfonnadon after the two Dutch chemists who discovered it in 1895. 

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