Sigmatropic rearrangement, Sommelet-Hauser

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

Selective o/t/io-acylation and orrfio-formylation of 2-alkyl pyridines is possible by [2,3] sigmatropic rearrangement (Sommelet-Hauser) of a-pyrrolidinyl-2-alkylpyridine sulfonates (268), prepared by treating the parent base with cyanomethyl benzenesulfonate (76JOC2658). Acid hydrolysis of the rearranged product (269 R = H) yields 3-formyl-2-methylpyridine (270). Methylation of (269 R = H) using NaH-Mel and acid hydrolysis of the reaction mixture gives 3-acetyl-2-methylpyridine (270 R = Me). 

We have mentioned previously as a stable ylide dimethyl sulfonium fluorenide (see Section 2.9). It yielded the first example (1938) of a [2,3] sigmatropic rearrangement of a sulfur ylide [505, 506] (Sommelet-Hauser rearrangement). 

Heterocyclic nitrogen-derived ylids behaviors have been studied.373 For instance, pyridine derivatives lead exclusively to [2,3]-sigmatropic rearrangement (Sommelet-Hauser) products. 

Klunder nicely exploited the Sommelet-Hauser reaction to synthesize dipyridodiazepine derivatives, such as 84, whose structural analogues show potent inhibition of HIV-1 reverse transcriptase.42 The key step here involves a [2,3]-sigmatropic rearrangement across the pyridine ring. 

V. The Cope Rearrangement, the [1.3] Sigmatropic Shift, the Sommelet-Hauser Reaction, and Sulfur-Mediated Ring Expansions... 

Initial work on the Sommelet-Hauser rearrangement was mechanistically inspired. Hauser and Kantor provided a major contribution to the understanding of this transformation. They showed that two ylides were involved in the rearrangement. The more stable ylide (421) is initially generated but equilibrates with the less stabilized one (422). The latter ylide then undergoes a concerted, symmetry-allowed 3,2-sigmatropic shift, giving the triene (423). Aromatization finally affords the orrAo-substituted benzylic amine (424 Scheme 108). 

Sigmatropic rearrangement of the sulfonium ylides from 3-(bromomethyl)thiophene and 2-(bromomethyl)benzo[f)]thiophene has been reported <8980958). Treatment of the bromomethyl compounds with dimethylsulfide gave the sulfonium salts. These were deprotonated (NaOEt or NaH) to generate the ylides. Sommelet-Hauser rearrangement of the ylides gave the products (458) and (459) (Scheme 93). 

The mechanism of the Gassman indole synthesis bears some resemblance to that of the Fischer indole reaction. An Sn2 displacement of the TV-chlorinated aniline with the a-keto sulfide gives rise to a sulfonium ion. After deprotonation, a [2,3]-sigmatropic rearrangement (Sommelet-Hauser rearrangement) takes place where the weak N-S bond is broken and a strong C-C bond is formed. Similar to the Fischer indole reaction, the resulting imine was transformed into indole after re-aromatization and cyclization. 

The Gassman indole synthesis involves a one-pot process in which a hypohalite, a P-carbonyl sulfide derivative, and a base are added sequentially to an anihne or a substituted aniline to provide 3-thioalkoxyindoles. The mechanism of the Gassman indole synthesis involves a [2,3]-sigmatropic rearrangement (Sommelet-Hauser). The sulfur can be easily removed by hydrogenolysis or Raney nickel. 

SCHEME 3.31 Sommelet—Hauser rearrangement involving [2,3] sigmatropic shifts. 

Several variations exist for the classic Sommelet-Hauser rearrangement. The most important concern the use of base-free conditions for the [2,3]-sigmatropic rearrangement based on the fluoride anion induced desilylation of substituted benzyldialkyl-[(trimethylsilyl)methyl] ammonium halides" and the asymmetric versions of Sommelet-Hauser rearrangement that appeared recently in the literature. None of the above has ever been used in a modification of the Gassman indole synthesis. 

Sommelet-Hauser rearrangement portrays a sigmatropic rearrangement using charged species as outlined in Scheme 18.17 [61]. 

Sigmatropic Rearrangements of Benzyl Ammonium Ylides The Sommelet-Hauser Rearrangement... 

Sigmatropic rearrangements of benzyl quaternary ammonium salts are known as the Sommelet-Hauser rearrangements. The treatment of the benzyl quaternary ammonium salt with sodium amide or other alkali metal amide generates benzyl ammonium ylide. The following examples are illustrative [108, 116-119] ... 

The Sommelet-Hauser reaction, a [2, 3] sigmatropic rearrangement of ammonium ylides, is also an attractive method for ring expansion of nitrogen heterocycles. Recently... 

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