Esters Lead tetraacetate

HETEROCYCLES Copper phcnylace-tylide. Dichlorobis(benzonitrile)palladium. N-Dichloromethylene-N,N-dimcthylammo-nium chloride. Diiminosuccinonitrile. Dimethyl acetylenedicarboxylate. Dipotassium cyanodithioimidocarbonate. Ethoxy-carbonyl isothiocyanate. Ethyldiisopropyl-amine. Ethylene oxide. Hydrogen fluoride. Isocyanomethane-phosphoric acid diethyl ester. Lead tetraacetate. Lithium aluminium hydride. Methylhydrazine. Phosphoryl chloride. Polyphosphate ester. Polyphosphoric acid. Potassium amide. Potassium hydroxide. Tolythiomethyl isocyanide. Tosylmethyl isocyanide, Trichlo-romethylisocyanide dichloride. Trimethyl-silyldiazomethane. 

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. ... 

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. 

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. 

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. 

The exact course of the periodate reaction has not yet been established. That an intermediate complex, compound, or ion is involved has been determined kinetically.28 269 261 262 283-286 The exact structure of this intermediate is still in doubt. The most universally accepted structure is a cyclic ester intermediate propounded by Criegee,27 285 analogous to his cyclic ester intermediate for another agent oxidizing 1,2-glycols, lead tetraacetate. 

LAURIC ACID, VINYL ESTER, 30, 106 Laurone, 31, 68 Lauroyl chloride, 31,68 Laurylmethylamine, 36, 48 Lead acetate trihydrate, 31, 19 Lead sulfide, 31, 19 Lead tetraacetate, 35, 18 Lithium aluminum hydride, 32, 46 33, 33, 82 36, 49... 

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. 

Dihydrocorynantheine was obtained via similar steps from normal cyanoacetic ester 319 (172). Stereoselective transformation of the alio cyanoacetic ester 315 to the normal stereoisomer 319 was achieved by utilizing a unique epimerization reaction of the corresponding quinolizidine-enamine system (174). Oxidation of alio cyanoacetic ester 315 with lead tetraacetate in acetic acid medium, followed by treatment with base, yielded the cis-disubstituted enamine 317, which slowly isomerized to the trans isomer 318. It has been proved that this reversible eipmerization process occurs at C-15. The ratio of trans/cis enamines (318/317) is about 9 1. The sodium borohydride reduction of 318 furnished the desired cyanoacetic ester derivative 319 with normal stereo arrangement. The details of the C-15 epimerization mechanism are discussed by B rczai-Beke etal. (174). 

For the substitution of the angular methyl groups in steroids five methods are known (a) homolysis of N-chloramines [Loffier-Freytag reaction (only C-18)] (b) oxidation of alcohols with lead tetraacetate (c) photolysis of nitrite esters (d) homolysis of hypochlorites (e) the hypoiodite reaction. ... 

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