Lead acetate Mandelic acid

Mandelic acid-derived chiral (a-substituted) acetate enolate addition to aldehydes leading to chiral j5-hydroxycarboxylic acids illustrates the versatility of the readily available ester 63. The addition of phenylmagnesium bromide to methyl (i )-mandelate (63) gives the (i )-diol 152, which is acetylated to (i )-2-acetoxy-l,l,2-triphenylethanol (153) [(/ )-HYTRA]. Deprotonation with LDA at — 78 °C provides an enolate that is then transmetallated with magnesium bromide and further cooled to —115 °C before reaction with an aldehyde to produce 154 as the major diastereomer with a yield of 84-95%. Heating 154 in aqueous methanol containing potassium hydroxide provides the optically active j5-hydroxyacid 156 (Scheme 36) [41- 4]. 

This work was extended to include the lead tetraacetate oxidation of methyl esters of meta- and para-substituted mandelic acids183,184 shown in equation 121. A kinetic study by Banerjee and collaborators showed the kinetic dependence on the ester concentration changed from second order in 1% (v/v) acetic acid in benzene to first order when the solvent contained more than 10% (v/v) acetic acid. These workers observed a significant decrease in AH (from 82.9 to 53.6 kcalmol-1) and in AS (from —5.84 to —35.6 e.u.) when the solvent composition was changed from 1% acetic acid to greater than or equal to 10% acetic acid in benzene. 

In the case of benzal chloride, the carboxylation in conventional diaphragm systems fails, leading to poor yields in phenylacetic and mandelic add [180], At an Al anode, carboxylation occurs because the self-esterification of the first carboxylate anion onto the second chloride group is hindered by the formation of Al complex salts [181]. Yields of phenylmalonic and chlorophenylic acetic acids up to 30% each have been obtained [178],... 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each halfreaction is an a cleavage which leads to a thiamin- bound enamine (center, Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step b followed by a in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an a4 tetramer and one with an ( P)2 quaternary structure. The isolation of ohydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate52 provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase126 in the biosynthesis of the plant hormone indoIe-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25).1243/127... 

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