Phenylglyoxylic acid, oxidation

Mandelic acid undergoes a chiral inversion, possibly by reversible oxidation to phenylglyoxylic acid. When -mandelic acid was administered, some 80% of the dose was recovered as phenylglyoxylic acid in 48 h with 16% as mandelic acid RJS 80 20). However, when racemic mandelic acid was given, 46% was excreted as phenylglyoxylic acid and 47% as i -mandelic acid. 

The oxidation of methyl ketones to a-keto carboxylic acids is rare and is accomplished by treatment with a cold solution of potassium permanganate. However, the reaction is not general acetophenone, p-methyl-acetophenone, and 3,4-dimethylacetophenone are oxidized all the way to the corresponding benzoic acids. On the other hand, 2,4-dimethylaceto-phenone, when shaken with approximately 1% aqueous potassium permanganate at room temperature, gives 6 72% yields of 2,4-dimethyl-phenylglyoxylic acid [555, 559],... 

Hydroxy carboxylic acids and their esters are oxidized to keto acids and their esters, respectively. o-Nitromandelic acid is oxidized to o-nitro-phenylglyoxylic acid with a dilute solution of potassium permanganate at room temperature in 54% yield [886], The oxidation-of ethyl 3-hydroxy-cyclobutanecarboxylate with ruthenium tetroxide and sodium periodate in... 

Tertiary enamines are oxidized to different compounds. 7V-Methyl-pyrrol gives 92% of /V-methylsuccinimide by oxidation with peroxybenzoic acid in chloroform at room temperature [311]. A -Cyclohexenylmorpholine, with singlet oxygen, is oxidized to cyclohexanedione (equation 524) [42], The oxidation of Af-phenylacetylenyl-lV,Af-diethylamine yields the M diethylamide of phenylglyoxylic acid (equation 525) [7S6]. 

By a slight modification of the procedure, crystalline benzoyl-formic acid (phenylglyoxylic acid) can be isolated. The oxidation of 375 g. (2.5 moles) of mandelic acid by cold alkaline potassium permanganate is performed exactly as described in the first paragraph (p. 236). After removal of the manganese dioxide, the combined filtrates are evaporated to 2500-3000 cc., rather than 800-1000 cc. as given in the second paragraph (p. 237). At this point the solution is acidified carefully, so as to precipitate only benzoic acid, by the addition of 100 cc. (190 g.) of concentrated sulfuric acid (sp. gr. 1.84) previously diluted with an equal volume of water and cooled. The benzoic acid (25-30 g.) which... 

The structure assigned to mandelic acid is in accord with its reactions. It is oxidized by an alkaline solution of potassium permanganate in the cold to phenylglyoxylic acid —... 

Although the mechanisms of the oxidations of organic mono- and di-carboxylic acids have been much examined under acidic conditions, the faster reactions in alkaline media have only recently been investigated. In contrast to the reaction in acidic solution where, with mandelic acid as substrate, oxidative decarboxylation occurs to yield benzaldehyde and CO2, at the higher pH the product is phenylglyoxylic acid ... 

The oxidation of ethyl pyruvate and ethyl phenylglyoxylate with per-oxyphthalic acid at 20-22 °C produces, after 48 and 90 h, mixed anhydrides of ethylcarbonic acid and acetic and benzoic acids, respectively [334]. 

While only 1.0 equiv. of PMHS is needed to complete the reduction of some ketones e. g. a,a,a-trifluoroacetophenone (Ic) and methyl phenylglyoxylate (Id), excess PMHS is necessary in most cases. As shown in Table 1, inconplete conversions are mostly observed for the reduction of relatively acidic substrates i. e. y ketoesters If and Ig and /0-ketoamides Ih and li (pKa = 10-13). Therefore, a likely hypothesis is that the [Zn-diamine]/PMHS system is active not only for the reductive reaction of the carbonyl function, but also for the oxidative silylation of any enolisable group. Thus, the enol-silyl ether produced would hydrolyze back in methanol to the free enol, accounting for the consumption of extra equiv. of PMHS. Nevertheless, this hypothesis does not accotmt for the reduction of imine 3a, since no inqirovement in the conversion is noted on doubling the amount of PMHS (Scheme 3, Table 1). Other imines e. g. 3b, are readily reducible with the present Zn-diamine-methanol system. 

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