Lead tetraacetate with amides

The earliest oxidations were effected with nitrous fumes and later with mercuric oxide and isoamyl nitrite.74 Lead tetraacetate in acetic acid is in many cases the reagent of choice, but the removal of by-products can present some difficulties.75 IV-Haloimides and amides in alcoholic solutions have been reported to yield essentially pure tetrazolium salts76 and have been found specially useful in the preparation of heteroaryl-substituted tetrazolium salt.77,78 The novel formazans 49 have been successfully oxidized to 50 using 7V-chloro succinimide (Eq. II).79 tert-Butyl hypo-... 

In contrast to the reaction of benzamide and other carboxylic acid amides with lead tetraacetate, which has been said to proceed via an acyl nitrene intermediate 46>, methanesulphonamide and 2-biphenylsulphon-amide are completely inert towards this reagent 42>. 

Treatment of amides with bromine in alkaline medium promotes the Hofmann rearrangement, which may or may not involve a free nitrene intermediate." The oxidation of primary amides with lead tetraacetate and the resulting Lossen rearrangement also produces isocyanates, with the possible intervention of an acylni-... 

An extremely interesting and novel method has been described (91TL133). The principle involved is the intramolecular Diels-Alder addition of a 2,4-dienoic acid amide with an azodicarbonyl moiety. /V-Sorbyl-proline (27) was condensed with an acylhydrazine to form (28). Oxidation of this with lead tetraacetate (LTA) in boiling benzene resulted in the piperazinedione (30). This must have come about via (29), which could undergo an intramolecular Diels-Alder reaction. The structure and stereochemistry of (30) were confirmed by X-ray crystallography. The two new chiral centers have the R configuration as shown in (Scheme 9). 

A similar reaction can be effected by the treatment of amides with lead tetraacetate.2 4 In this case the initial isocyanate and the amine formed from it react with the acetic acid liberated from the lead tetraacetate to give, respectively, ureas and amides. If the reaction is carried out in the presence of an alcohol, carbamates are formed (6-8). 

Determination of the structure of an amide of a carbohydrate is not difficult. Examination of acyl derivatives indicates whether a cyclic or an acyclic structure is present. Moreover, the results of oxidations with periodate and lead tetraacetate allow unequivocal determination of these structures alternatively, methylation studies have been used. 

Hofmann rearrangements Primary amides are converted into amines in high yield by reaction with this owdant at room temperature in aqueous acetonitrile (equation I). The reaction is rslated to a recent method using lead tetraacetate (6, 316) however, the intermet iate isocyanate in this case is not trapped as the carbamate, but is hydrolyzed alirectly to the amine. 

The oxidation of primary amides with lead tetraacetate gives isocyanates, when carried out in dimethylformamide carbonates, when carried out in the presence of alcohols and ureas, when carried out in the presence of amines (equation 482) [1182]. 

The characterization of polymer chemical composition is important in numerous practical applications. The polymer identification can be done using various techniques. One of them is the chemical method, which involves reagents that are able to react with the polymer. Oxidation, for example using periodic acid or lead tetraacetate, can be applied to polymers containing 1,2 diol groups, ozonolysis can be applied to polymers containing double bonds, hydrolysis can be applied to esters and amides [4]. 

A mixture of the amide (17) with 3 moles of lead tetraacetate and 4 moles of iodine in chloroform is irradiated at 15° for 5 hrs. 

Oxidative rearrangement. A reaction related to the Hofmann rearrangement of N-hromoamides occurs on oxidation of an aromatic amide with lead tetraacetate lo give the isocyanate or a derived product in moderate to excellent yield. Simple aliphatic amides react rapidly in dimethylformamide containing triethylamine. 

Lead tetraacetate reacts with a primary amide (I) in refluxing benzene or benzene-acetic acid to give an acylamine (5) and a trace of the dialkylurea (6). Spectroscopic... 

Oxidation of hydrocarbons with a tertiary carbon, e.g. adamantane, with lead tetraacetate in trifluoroacetic acid-dichloromethane solution, in the presence of chloride ion, gave high yields of trifluoroacetate functionahzed bridgehead alcohols [57]. Subsequent hydrolysis yielded the free bridgehead alcohols (Scheme 13.34). Another important advantage of this method is the feasible conversion of the intermediate trifluoroacetate into an amide with acetonitrile. 

Primary amides are converted by treatment with lead tetraacetate in acetic acid into arylamines 701 an acyl nitrene is suggested as the intermediate. 

Relatively few reactions of quinoxaline derivatives occur with change of ring size. Isolated examples are noted in the following text. For example, ring contraction to benzimidazole derivatives occurs when 2,3-diphenylquinoxaline (Chapter XV) or 2-halogenoquinoxalines (Chapter X) are treated with potassium amide in liquid ammonia and quinoxalin-2-one is treated with hydrazine (Chapter VX It is also found that oxidation of l-aminoquinoxalin-2-ones with lead tetraacetate give benzo-1,2,4-triazines (Scheme 6) (Chapter V). 

The pyrrole 23 was also oxidized to 26 using nitric acid, chromium(VI) oxide, lead tetraacetate in acetic acid, or phosphorus pentachloride in phosphorus oxychloride followed by hydrolysis in yields of up to 63%. However, hydrogen peroxide in acetic acid gave a mixture of the epoxy ketone 29 and the amide 30 29 was converted to 30 with ammonia, possibly via rearrangement of an intermediate (31)42 Oxidation of the N-methyl derivative of 23 gave pyrrolenineonium salts, which were difficult to purify.42... 

Rearrangement of amides. Primary amides undergo oxidative rearrangement to isocyanates when treated with lead tetraacetate. The reaction is generally carried out in an alcohol (t-butanol generally), in which case the product is isolated as the carbamate. Triethylamine or stannic chloride catalyzes this reaction. This oxidation provides a useful alternative to the classical Hofmann, Schmidt, and Curtius rearrangements. 

An ortho amide 78 of propiolic acid has been prepared it readily condenses with acetylacetone (equation 11). Novel A,A-bis(trimethylsilyl)ynamines 80 are obtained by electrophilic amination of lithium acetylides with the hydroxylamine derivative 79 a,b The action of lead tetraacetate on di(alk-l-ynyl)mercury compounds or alk-1-ynyltrimethylstannanes gives highly unstable alk-l-ynyllead triacetates 81 (R = pj. 

A new method for preparing triphenylarsinimines Ph3As==NX, where X = Ph, CN, Bz, C02Et etc., from triphenylarsine and azides (XN3) under nitrene-producing conditions has been reported. The compounds also result when triphenylarsine reacts with amides in the presence of lead tetraacetate, probably via Ph3As(OAc)2 as an intermediate. 

C-N-0-CHj Oxidation of the parent JV-methoxy amide with lead tetraacetate/ Benzene EPR/ 350 2.0060 N 1.04 3H(CH3) 0.47 79For3... 

An early variation of the classic HR is the use of lead tetraacetate (LTA) to accomplish the rearrangement of primary amides to carbamates via the intermediacy of isocyanates. Discovered independently by Beckwith and Baumgarten, the reaction is proposed to involve nitrene 6 leading to isocyanate 2 (Scheme 1). Thus, in the presence of LTA, cyclohexyl amide (16) is converted to methyl carbamate 17, phthalamide (18) forms dioxoquinazoline 19, and amide ester 20 yields amino acid 22 after hydrolysis of carbamate 21 with retention of configuration. 

Carbamates are also formed by one-stage, two-step processes involving isocyanates as intermediates and precursors. Thus, carbamates are prepared under modified Hofmann rearrangement conditions, using NBS/NaOMe as the reagent [443], by Curtius rearrangement of the acyl azide if the reaction is carried out in alcohol, by the reaction of amides with lead tetraacetate [444], or by the Lossen rearrangement, if the carbamates are not sensitive to the presence of base. 

The alternative route to (+)-swainsonine by Chen and Tsai used a very similar strategy, but instead formed the indolizidine system by making the C-8/C-8a bond (Scheme 87). In this case the amine 647 and commercially available 2,3-isopropyhdene-D-ribono-1,4-lactone (648) were condensed to form an amide, the terminal diol unit of which was cleaved with lead tetraacetate to give the 5-hydroxypyrroHdinone (—)-649 after acid-promoted cyclization. The thionocarbonate derivative (—)-650 was then treated with iodobenzene bis(trifluoroacetate) to deprotect the acylsi-lane. Intermediate (—)-651 then underwent radical-induced cyclization with tributyltin hydride to give the 8-hydroxyindolizidin-3-one 652 in... 

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