Enolate anions, reduction

Graphite-potassium Support for reduction of ketones, alkylhalides, alkyl sulfonate esters formation of enolate anions reduction of transition metals Bergbreiter and Killough, 1978 and references therein... 

The ff-oxidation of carbonyl compounds may be performed by addition of molecular oxygen to enolate anions and subsequent reduction of the hydroperoxy group, e.g. with triethyl phosphite (E.J. Bailey, 1962 J.N. Gardner, 1968 A,B). If the initially formed a-hydroperoxide possesses another enolizable a-proton, dehydration to the 1,2-dione occurs spontaneously, and further oxidation to complex product mitctures is usually observed. 

A useful alternate procedure which allows the generation and alkylation of the less stable enolate anion has been reported by Stork.This method takes advantage of the fact that the thermodynamically less stable enolate anion formed in the lithium ammonia reduction of a conjugated enone... 

Under the conditions of the Birch reduction, IV-Boc amides such as 60 can be reductively alkylated in high yields, presumably via a dianion intermediate which is protonated by ammonia at C-5 leaving an enolate anion at C-2 <96JOC7664>. Quenching the reaction with alkyl halides or ammonium chloride then affords the 3-pyrrolines 61. 

The reduction of 2-oxoacids bound to different chiral auxiliaries gave the 2-hydroxyacid derivatives in a 64 to 76% yield and 42 to 86% de depending on solvent, proton donor, supporting electrolyte, temperature, and substituent R in the oxoacid. The results are in accordance with an ECE reduction of the 2-oxoamide to an enolate anion, which subsequently undergoes a face-selective protonation to the hydroxy acid [346, 347]. 

The pathway by which the reactions are considered to occur involves attack of the enolate anion at the 1 0 bond of o-iodosyl benzoic acid followed by reductive elimination of o-iodobenzoic acid upon addition of methoxide to the carbonyl group. Ring opening of the epoxide thus formed yields the hydroxy dimethyl acetal ... 

However, these experiments may not have established a mechanism for natural flavoprotein catalysis because the properties of 5-deazaflavins resemble those of NAD+ more than of flavins.239 Their oxidation-reduction potentials are low, they do not form stable free radicals, and their reduced forms don t react readily with 02. Nevertheless, for an acyl-CoA dehydrogenase the rate of reaction of the deazaflavin is almost as fast as that of natural FAD.238 For these enzymes a hydride ion transfer from the (3 CH (reaction type D of Table 15-1) is made easy by removal of the a-H of the acyl-CoA to form an enolate anion intermediate. 

A third enzyme is required to reduce the epoxide to vitamin K (Eq. 15-57). The biological reductant is uncertain but dithiols such as dithiothreitol serve in the laboratory.518 See also Eq. 18-47. Protonation of an intermediate enolate anion would give 3-hydroxy-2, 3-dihydrovitamin K, an observed side reaction product. 

---