Hydroxylation with lead tetraacetate

Rao prepared 2a-methyl-5a-cholestan-2i -ol (5) by reaction of methyl-magnesium iodide with 5a-cholestan-2-one (4). The 2i -configuration of the hydroxyl group was established by converting (5) to the 2a-methyl-2j5,19-epoxide (6) with lead tetraacetate and iodine in boiling benzene. 

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)... 

Oxidation of one molar proportion with sodium pieriodate produces two equivalents of formic acid, in accordance with the existence of hydroxyl groups attached to four contiguous carbon atoms. This oxidation (and also that carried out with lead tetraacetate) gives an aldehyde, whose semicar-bazone has an analysis corresponding to that of the semicarbazone of an ethyl formyl-methyl-furoate (XII). By oxidation of aldehyde XII with silver oxide in alkaline solution, 2-methyl-3,4-furandicarboxylic acid (XIV) was obtained this was identical with the compound described by Alder and Rickert.20 The identity was confirmed by preparation of the respective dianilides. The acid XIV has also been prepared by the reaction between the sodium salt of ethyl acetoacetate and ethyl bromopyruvate.9... 

Neither has oxidation, with lead tetraacetate, of the sirup obtained by dehydration of the D-galactose condensate VIII so far resulted in isolation of the expected dialdehyde. On the other hand, when the anhydride from the D-glucose condensate (XXXIV) was oxidized with lead tetraacetate, an appreciable amount of dialdehyde (XXXVI) was isolated. This discrepancy in behavior is probably attributable to the trans position of the hydroxyl groups of the anhydride derived from D-galactose as compared with the cis configuration for the anhydride from D-glucose. 

As regards the l,l-bis(acylamido)-l-deoxyalditols, the first structural study was that of Fletcher and coworkers,69 who conducted oxidations with lead tetraacetate. These authors postulated an empirical relationship between the position of the oxidation curve in a graph and the number of hydroxyl groups present. They compared the curves afforded by a series of l,l-bis(acetamido)-l-deoxyalditols with those of alditols having the same number of free hydroxyl groups, and observed a close correspondence. At present, periodate oxidation is widely applied to these mono- or di-saccharide derivatives. Under controlled conditions, the methylation technique can also be applied.42... 

A novel synthesis of the sesquiterpene ( )-cinnamodial (148) utilizes the furan ring as a latent 1,4-dialdehyde synthon (81JA3226). The triol (141) was thus oxidized to the ketofuran (142). Oxidation of the furan moiety with lead tetraacetate afforded a 90% yield of epimeric diacetates (143) which when exposed briefly to DBU gave dienone (144). Epoxidation of (144) and exposure of the epoxide (145) to p-toluenesulfonic acid gave the bis-acetal (146). Reduction of this intermediate to a diol and hydrolysis of the bis-acetal furnished dialdehyde (147). Acetylation of the secondary hydroxyl group completed the synthesis of cinnamodial (Scheme 32). 

Thallinm(ni) °, particnlarly as its triflnoroacetate salt , has been successfully used for the synthesis of phenols. This method can be carried out in a single step and is subject to isomer orientation control" . The aromatic compound to be hydroxylated is first thallated with thallium trifluoroacetate (TTFA)" and, by treatment with lead tetraacetate followed by triphenylphosphine and then dilute NaOH, it is converted to the corresponding phenol (equation 57). Table 1 shows some examples of these transformations . ... 

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