Oxidation by lead tetraacetate

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. ]... 

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. 

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... 

Diacetamides are oxidized by lead tetraacetate, and Hockett and coworkers have studied the behavior of a number of these compounds. 

Quinoline 1-oxide undergoes nucleophilic attack by ozone to yield a hydroxamic acid (128), and 40% of the starting iV-oxide is recovered (Scheme 74). When an excess of ozone is employed the aldehydes (129) and (130) are obtained. Formation of these products has been attributed to electrophilic attack by ozone rather than further oxidation of (128), because in a separate experiment (128) yielded carbostyril on treatment with ozone. Isoquinoline 2-oxide yields 2-hydroxyisoquinolin-l-one, and acridine 10-oxide gives 10-hydroxyacridone and acridone in a similar manner to the above. Likewise, phenanthridine 5-oxide affords mainly 5-hydroxyphenanthridone. Quinoline 1-oxide undergoes oxidation by lead tetraacetate as shown (Scheme 75). 

The unsubstituted 2-benzopyrylium cation 1 was first synthesized by Blount and Robinson (33JCS555) starting with indene. Its oxidation by lead tetraacetate affords homophthalic dialdehyde, which is cyclized to 2-benzopyrylium tetrachloroferrate on treatment with hydrochloric acid and ferric chloride. 

II. Mechanism of Glycol-cleavage Oxidation by Lead Tetraacetate. 11... 

Although carbinols were found not to be oxidized by lead tetraacetate in acetic acid, they were attacked in less polar solvents, and glycol-cleavage was also more rapid in these solvents23 for example, the rate of oxidation of trans-l, 2-cyclohexanediol in tetrachloroethane was 5,000 times that in acetic acid. The kinetics was no longer true second-order in the media of low polarity, but approached this state as the concentration of acetic acid... 

The relationship between rates of oxidation by lead tetraacetate and the stereochemistry of [Pg.23]

In 1942, Hann, Haskins and Hudson48 reported that the di-(o-nitro-benzylidene)-dulcitol described by Tanasescu and Macovski was not oxidized by lead tetraacetate and therefore could not possess the structure (1,2 5,6) arbitrarily assigned to it by the earlier workers. In view of the fact that the diacetal gave the known l,3,4,6-tetraacetyl-2,5-diben-zoyl-dulcitol when it was benzoylated in pyridine and then acetylated under acidic conditions, they regarded it as l,3 4,6-di-(o-nitrobenzyli-dene)-dulcitol, but of course this conclusion is not unequivocal. 

In 1946, Wiggins121 observed that Irvine and Paterson s129 claim that their diisopropylidene-D-mannitol (m. p. 37-39°) was the 1,2 3,4-derivative rested on no firm basis and he conducted a series of experiments in order to prove its constitution more rigidly. He showed that the compound could be oxidized by lead tetraacetate and that the main product was a diisopropylidene-oWeAj/do-D-arabinose, which he converted into crystalline D-arabinose, leaving no doubt that the diisopropylidene-mannitol had a terminal a-glycol group. This conclusion found support... 

Some interesting cycloadditions to 3-pyridyne (355) are illustrated in Scheme 102. IV-Phenylsydnone (356) gives (357), which spontaneously loses carbon dioxide to give the pyrazolopyridine (358). Benzaldehyde phenylhydrazone is oxidized by lead tetraacetate to the nitrilimine dipole (359), which also reacts with 3-pyridyne to give a pyrazolopyridine (360) (7160858). 

Much simpler and more direct is a recently developed route via thallation. An aryithallium compound is oxidized by lead tetraacetate (in the presence of triphenylphosphine, PhjP) to the phenolic ester of trifluoroacetic acid, which on hydrolysis yields the phenol. The entire sequence, including thallation, can be carried out without isolation of intermediates. Although the full scope of the method has not yet been reported, it has two advantages over the diazonium route (a) the speed and high yield made possible by the fewer steps and (b) orientation control in the thallation step. (Review Secs. 11.7 and 11.13.)... 

A -Cholestene-3-one (1) has a highly reactive methylene group and is oxidized by lead tetraacetate in benzene-acetic acid to give the 4a-acetoxy derivative (2)... 

A half-ester of malonic acid such as 111, prepared in 70% yield by partial saponification of the di-ester, is oxidized by lead tetraacetate in benzene at 50° to the a-acetoxy derivative V, which loses carbon dioxide at 200° to give the a-acetoxy ester VI. Yields are in the range 35-80%. That the half-esters are oxidized much... 

Aromatic compounds with a C-H group at the benzylic position are readily oxidized by lead tetraacetate to the corresponding benzyl acetates. Benzylic acetoxyla-tion is preferably performed in acetic acid (Scheme 13.35) [58]. 

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