Carbamates Hofmann rearrangement

Whereas the Hofmann rearrangement of the amidoalcohols 452 with the usual NaOCl did not occur, reaction of the oxygen-bridged 452 (X = 0) with bis(acetoxy)iodobenzene under mild conditions led to a 9 1 mixture of the corresponding cyclic and acyclic carbamate derivatives 197 and 453 (X = 0) (Equation 50). In the similar reaction of the methylene-bridged analog 452 (X = CH2), the product of perhydro-3,l-benzoxazin-2-one type 197 (X = CH2) was obtained exclusively <20030PP429>. 

Methyl Carbamate Formation via Modified Hofmann Rearrangement Reactions Methyl N-(p-Methoxyphenyl)carbamate. 

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. 

A variation of the Hofmann rearrangement treated a p-hydroxy primary amide with PhI(02CCF3)2 in aqueous acetonitrile, giving an isocyanate via -CON-I, which reacts with the hydroxyl group intramolecularly to give a cyclic carbamate. Note that carbamates are converted to isocyanates by heating with Montmorillonite KIO. ... 

The first asymmetric total synthesis of the hasubanan alkaloid (+)-cepharamine was completed by A.G. Schultz et al. In order to construct the c/s-fused A/-methylpyrrolidine ring, the advanced tetracyclic lactone was first converted to the primary carboxamide by treatment with sodium amide in liquid ammonia. Next the Hofmann rearrangement was induced with sodium hypobromite in methanol initially affording the isocyanate, which upon reacting with the free secondary alcohol intramolecularly gave the corresponding cyclic carbamate in excellent yield. 

Keiiior, J. W., Huang, X. Methyi carbamate formation via modified Hofmann rearrangement reactions Methyi N-(p-methoxyphenyi)carbamate. Org. Synth. 2002, 78, 234-238. 

Hofmann rearrangement Primary amides are transformed into methyl carbamates by NBS-DBU in refluxing methanol. 

METHYL CARBAMATE FORMATION VIA MODIFIED HOFMANN REARRANGEMENT REACTIONS METHYL N-(p-METHOXYPHENYL)CARBAMATE (Carbamlc acid, (4-methoxyphenyl)-, methyl ester)... 

METHYL CARBAMATE FORMATION VIA MODIFIED HOFMANN REARRANGEMENT REACTIONS METHYL N-(p-METHOXYPHENYL)CARBAMATE. 

Carbamates. On Hofmann rearrangement of an amide RCONH2 the treatment with KF/AI2O3 in MeOH leads to RNHCOOMe. ... 

Methyl carbamates. A modified Hofmann rearrangement involves heating primary amides with NBS and NaOMe in methanol (8 examples, 85-100%). 

A series of alkyl carbamates of 1-protected indole-3-methylamines 403 have been prepared from the corresponding acetamides 402 in good to excellent yields via (diacetoxyiodo)benzene-promoted Hofmann rearrangement (Scheme 3.161). This procedure has been further extended to the preparation of alkyl carbamates of thiophenylmethylamines and pyrrolylmethylamines [502]. 

A very mild procedure for the Hofmann rearrangement of aromatic and aliphatic carboxamides 409 is based on the use of (tosylimino)phenyl-X. -iodane, PhINTs, as the oxidant (Scheme 3.165) [506]. Owing to the mild reaction conditions, this method is particularly useful for the Hofmann rearrangement of substituted benzamides 409 (R = aryl), which usually afford complex reaction mixtures with other hypervalent iodine oxidants. The mild reaction conditions and high selectivity in the reaction of carboxamides with PhINTs allow the isolation of the initially formed labile isocyanates 410, or their subsequent conversion into stable carbamates 411 by treatment with alcohols. Based on the previously reported mechanistic studies of the Hofmann rearrangement using other hypervalent iodine reagents [489,490,496], it is assumed that the reaction... 

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. 

Scheme 9.103. The Hofmann rearrangement, which results in the loss of carbon dioxide (CO2) and the production of an amine from oxidation of an amide with bromine (Br ) or chlorine (CI2).The oxidation produces the A -halo-compound, which, on treatment with base, undergoes rearrangement to generate the N-alkylisocyanate (cyclohexylisocyanate in this case). Treatment of the Af-alkyhsocyanate with water, produces the transient carbamic acid. The carbamic acid undergoes spontaneous decarboxylation in water to carbon dioxide (CO2) and cyclohexylamine (Chapter 10). Details of the Hofinann rearrangement will be discussed subsequently. But see Hofmann, A. W. Chem. Ber., 1881,7 /, 2725.

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