Sigmatropic rearrangements of allyl ammonium ylides

Allylic ammonium ylides generated in the presence of a strong base undergoes [2,3]-sigmatropic rearrangement. The following reactions are the illustrative examples [108-115]  

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Chem. Soc. Perkin Trans. 11999, 2439—2447. Blid, J. Somfai, P. Lewis add mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides. Tetrahedron Lett. 2003, 44, 3159-3162. 

Sigmatropic rearrangement of allylic ammonium ylides mediated by the combination of Lewis acid-nonionic phosphazene base was reported by Blid and Somfai [14], The... 

Scheme 7.11 [2,3J-Sigmatropic rearrangement of allylic ammonium ylides...

The [2,3]-Stevens rearrangement is a thermal sigmatropic rearrangement of an ammonium ylide (38) to form unnatural amino acid derivatives 39 (Scheme 12). Traditionally, the ammonium ylides have been formed through alkylation of aminoesters 36 with aUcyl halides 37 to form quaternary salts followed by treatment with base. Although effective, the harsh conditions lead to side products and limited substrate scope. More recently, the coupling of diazoesters 40 and allylic amines 41 in the presence of metals like copper, rhodium, and palladium has been developed for the direct constmction of ammonium ylides 38 via metal carbenoid intermediates. " Although this approach represented an advance over the traditional alkylation chemistry, the use of diazoesters still limits the synthetic utility of these reactions. 

Rautenstrauch V. Sigmatropic reactions in carbanions. II) The rearrangement of allylic ammonium ylides. Helv. Chim. Acta 1972 55(6) 2233-2240. 

Concerted [2,3]-sigmatropic rearrangements of allyl and benzyl ammonium ylides, allyl sulfonium ylides, allyl sulfoxides, selenoxides, amine oxides and anions of allyl ethers have been reported. These are illustrated by die following general examples ... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

A valuable part of the [2,3]-sigmatropic rearrangement of ammonium ylides is the fact that stereochemical information can be transferred. For example, Kaiser and co-workers stereoselectively alkylated the C-6 position of penicillin using the nitrogen ylide 46 derived from lactam 45.28 Quatemization of 45 with allyl bromide followed by ylide generation using sodium hydride effected the [2,3]-rearrangement. This resulted in the exclusive formation of P-lactam 47 in 75% yield. 

The reaction of dichlorocarbene with A, A -diethyl-3-methyl-2-butenamine (100) produced A V-diethyl-4-methyl-2-pentenamide (103) as the major product.51 The formation of this material was attributed to the generation of ammonium ylide 101 followed by a [l,2]-allylic shift to give intermediate 102 which then hydrolyzed during workup to produce amide 103. No product resulting from a [2,3]-sigmatropic rearrangement of ylide 101 was detected in the crude reaction mixture. 

There are a number of [2,31-sigmatropic rearrangements related to the Mislow-Evans rearrangement but differing in the heteroatom moiety. While these are not the main focus of the current chapter, they present unique opportunities for synthetic applications, and several recent examples are oudined in the following. Another closely related process, the [2,3]-Meisenheimer rearrangement of allylic IV-oxides, is discussed separately in Chapter IS on ammonium ylide rearrangements. 

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

Sulfonium and ammonium salts are prepared readily by alkylation of sulfides or amines. For example, allylation of the amine 318 gave the intermediate ammonium salt 319 (3.205). Deprotonation to the ylide with sodium hydride was followed by [2,3]-sigmatropic rearrangement at room temperature to give the peni-cilhn derivative 320. In a synthesis of y-cyclocitral, the sulfonium salt 321 was prepared by alkylation of 1,3-dithiane and was converted to the ylide 322 and hence the rearranged product 323 (3.206). 

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