Esters tetraacetate

This third bile acid containing the hydroxyl group at C-23 was isolated from seal bile and snake bile (136,140). It is the only identified naturally occurring tetrahydroxycholanic acid. The acid had previously been isolated by Hammarsten (137,138) who named it a-phocaecholic acid. The free acid has not been prepared in a form suitable for elemental analysis however appropriate analysis may be made of the methyl ester tetraacetate (140). 

Treatment of (89) with lead tetraacetate generates the unstable open-ring aldehyde (90) which is quickly converted to a dimethylacetal (91). Following basic hydrolysis of the methyl ester and acetates, the acetal is cleaved with aqueous acid to produce TxB2. A number of other approaches, including one starting from the Corey aldehyde, have been described (58). 

Rosenheim reaction CHCI3 + lead tetraacetate in green fluorescence not given by esters of provitamin D can... 

The mechanisms of oxidation using bismuthate, periodate or lead tetraacetate, while still not completely understood, are probably similar, involving some type of cyclic ester formation as the first step. ... 

Since 1,5-enediones are usually obtained via pyrylium salts, syntheses of the type found in Section B, 2, a have a rather theroetical interest, save for a few special syntheses. There exist several direct syntheses of l,5-enediones, e.g., from j8-chlorovinyl ketones and j8-diketones or j8-keto esters special pathways to 1,5-enediones have also been described, namely, oxidation with lead tetraacetate or with periodic acid of cyclopentene-l,2-diols. ... 

A mixture of 3-hydroxy-4-phenylfurazan and 1,2,4-oxadiazole 243 was prepared from a-phenyl-a-hydroximino hydroxamic acid by acylation and subsequent treatment with 15% aqueous NaOH (Scheme 164) (25G201). The reaction of tetraacetate 244 with sodium acetate hydrate in glacial acetic acid at 70°C gives 3,4-dihydroxyfurazan (9%) (92URP1752734). a-Hydroximino ester 245 reacts with hydroxylamine to form furazan 246 in 25% yield (Scheme 164) (79JHC689). 

The methyl ester of 2-phenyloxazole-4-carboxylate gives the 5-methyl-derivative when methylated with lead tetraacetate. ... 

Cycloalkoxy radical intermediates are readily generated from a parent alcohol by various methods (e.g., nitrite ester photolysis, hypohalite thermolysis, lead tetraacetate oxidation) (83MI1). Once formed, reactive cycloalkoxy radicals undergo /3-scission to produce a carbonyl compound and a new carbon-centered radical. 

Hydrogenation of the a-unsaturated ester 32 was accompanied by extensive hydrogenolysis and NMR spectroscopy revealed that only 45% of a 3-deoxyhexose tetraacetate was present in the products and only one... 

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

Dehydrogenation of the hydrazide derivative 33 with mercuric oxide in the presence of ethylene diamine tetraacetic acid (EDTA) gave 34 and 35 (77AP588). The latter (35) was prepared from a reaction of ester 36 with the appropriate lactam 37 (Scheme 11). 

Asymmetric aziridination of a,P-unsaturated esters by use of N-nitrenes was studied in great detail by Atkinson and co-workers [34, 35]. Here, lead tetraacetate-mediated oxidative addition of N-aminoquinazolone 30 (Scheme 3.10) to a-methy-lene-y-butyrolactone 32 was reported to proceed with complete asymmetric indue-... 

Lead tetraacetate has been used to convert phenols, with a hydrazone group in the ortho position, to carboxylic esters, for example. 

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... 

The mechanism with lead tetraacetate is generally accepted to be of the free-radical type. First, there is an interchange of ester groups ... 

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. 

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