By lead tetraacetate

The oxidation of tetrahydro-j8-carboline to dihydro-jS-carbolinium salts (341-> 342 342a->343) by -butyl hypochlorite (X = Cl), dichromate, or mercuric acetate (cf. Section III,C, 2,b), followed, by treatment with acid, and to j8-carbolinium salts (341 ->344) by lead tetraacetate [X = Pb(OAc)3] (cf. Section III, E, 2, a, ii) in acid solution... 

Thioamsole, oxidation, by dinitrogen tetroxidc, 46, 80 by hydrogen peroxide, 46, 80 by lead tetraacetate, 46, 80 reaction with sodium metaperiodate to form methvl phenyl sulfoxide, 46,75... 

Unique methods based on new principles have been developed within the past 10 years. Threonine (27,28,249) is oxidized by lead tetraacetate or periodic acid to acetaldehyde, which is determined by photometric analysis of its p-hydroxydiphenyl complex or iodometric titration of its combined bisulfite. Serine is oxidized similarly to formaldehyde, which is determined gravimetrically (207) as its dimedon (5,5-dimethyldihydro-resorcinol) derivative or photometric analysis (31) of the complex formed with Eegriwe s reagent (l,8-dihydroxynaphthalene-3,5-disulfonic acid). It appears that the data obtained for threonine and serine in various proteins by these oxidation procedures are reasonably accurate. [Block and Bolling (26) have given data on the threonine and serine content of various proteins. ]... 

Oxidation of dibenzyl and methyl phenyl sulphides by lead tetraacetate in acetic acid was also reported127. 

However, the cyclic-intermediate mechanism cannot account for all glycol oxidations, since some glycols that cannot form such an ester (e.g., 12) are nevertheless cleaved by lead tetraacetate... 

Deoxy-3-fluoro-D-glucose (see Section 11,2), a weak substrate for yeast hexokinase, is phosphorylated enzymically - to give the 6-phosphate 588, which is transformed into 2-deoxy-2-fluoro-D-arabinose 5-phos-phate (589) by lead tetraacetate oxidation. 

A few rate coefficients for oxidation by lead tetraacetate have been reported. These, together with those for some carbohydrates, are referred to in the section on glycols (p. 349). 

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

Hydroxylation at C-5 or C-l3 has also been successfully achieved by lead tetraacetate oxidation, which was extensively studied in connection with isoquinoline alkaloids by Umezawa s group. (+)-Govanine (96) and (+)-discretine (97) were oxidized with lead tetraacetate in acetic acid to afford 5-acetoxy products 100,101, and 102 via p-quinol acetates (e.g., 99) (Scheme 23)... 

Scheme 40. Synthesis of dihydronitidine (229) and its analog by lead tetraacetate oxidation. Reagents a, Mel b, KOH c, Pb(OAc)4, KOAc d, HCl e, CH2N2.

The 3f/-pyrazole-3,5(4//)-diones (7), prepared by lead tetraacetate,29,30 or tert-butyl hypochlorite31,32 oxidation of malonic acid cyclic hydrazides, are generally less stable than the triazoles 5, and are not isolable. The monocarbonyl derivatives (8-10) have been reported.33-36 The unstable 1,3,4-thiadiazole-2,5-dione (11) has been generated and reacted in situ.31,3 ... 

A 1,4-dioxepin fused derivative 122 of the anticancer agent acronycine was prepared by lead tetraacetate oxidation of c/.s-l,3-dihydroxy-l,2-dihydroacronycine followed by treatment with sodium borohydride <00NPL183>. 

Banerjee and coworkers181-184 have been interested in elucidating the reaction mechanism of the oxidation of mandelic acid and its derivatives by lead tetraacetate [Pb(OAc)4]. 

Fig. 2.—Oxidation of sugar derivatives by lead tetraacetate in dry acetic acid 0.00025 mole substance in 100 cc. ratio of Pb(OAc)4 to substance, 15.2/1 water less than 0.05 %.

Other methods for the generation of cation-radicals should also be mentioned. Mesoionic 5,5 -azinobis(l,3-diphenyltetrazole) and related mesoionic compounds give cation-radicals on oxidation by lead tetraacetate. The reaction proceeds in the presence of sodium tetrafluoroborate. The tetra-fluoroborate cation-radical salt is stable and can be purified by column chromatography on silica gel and stored in air for several months without appreciable decomposition (Araki et al. 1999). The cation-radicals of A,A -dimethyldiazines are produced by usual oxidation of the parent diazine... 

According to Ando et al. (2000), the sonolytic acetoxylation of styrene by lead tetraacetate follows the ion-radical mechanism. Lead tetraacetate was not subject to the sonication influence. The ultrasonic effect facilitates electron transfer from styrene (the nonmetallic donor) to lead tetraacetate. 

Oxidation of tertiary alcohols by lead tetraacetate gives alkyl radicals by /3-scission of the initially formed alkoxy radicals. The reaction has been used to alkylate protonated heteroaromatic bases using 1-methyl-cyclohexanol. (Scheme 4). 

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