Ammonium zwitterion -rearrangement

Scheme 14.21 Enantio- and diastereoselective synthesis of homoallylic fragments. CHAPTER 15 [2,3]-REARRANGEMENTS OF AMMONIUM ZWITTERIONS Scheme 15.1 Sigmatrnpic 12.31-rearrangements of reactive ammonium zwitterioas. Scheme 15.2 Meisenheimer s original report of an amine N-oxide rearrangement. Scheme 15.3 12.31- and ri.21-Meisenheimer rearrangements of amine jV-oxides.

Today, it is believed that [2,3]-sigmatropic rearrangements of ammonium zwitterions such as 7 proceed through a five-membered pericyclic transition state 8 of a doubly suprafacial migration tScheme IS.31. The transition state is of the Hiickel t e, and since six electrons... 

The [2,3]-rearrangement of amine iV-oxides was utilized in an efficient S5mthesis of 2,6-dimethyl-l,5-heptadien-3-ol acetate 61, a pheromone of the insect Pseudococcus comstocki tScheme lS.12id Dimethylpyridine 55 was converted in two steps into silylated piperidine 56, which was oxidized with m-CPBA to generate cyclic amine A-oxide 57. Sila-Cope elimination furnished O-silylhydroxylamine 58, which was methylated and desilylated in the presence of Mel and CsF to yield acyclic amine A-oxide 59. Heating this ammonium zwitterion facilitated the [2,3]-Meisenheimer rearrangement to O-allylhydroxylamine 60, which was transformed to the desired pheromone 61 in three steps. 

Early exanples of stereoselective [2,3]-Meisenheimer rearrangements were based on diastereoselective reactions of chiral amine JV-oxides such as 68 fScheme 1S.14T Stereochemical information was transferred from the carbon stereocenter in enantioenriched ammonium zwitterion 68 to the single carbon stereocenter in the rearrangement product 69. 

To compare the transfer of chirality in a [2,3]-rearrangement from carbon to carbon versus nitrogen to carbon, Bonin and coworkers devised a clever series of experiments f Scheme 1521). Allylic amines 94 were prepared from N-protected phenylglycinoL Oxidation with m-CPBA yielded stable amine JV-oxides 96, which were isolated in 51-74% de (confirmed by NMR analysis). Ammonium zwitterions 95 proved to be highly stable in dichloromethane... 

Several years later, Liu and coworkers reported another synthesis of cephalotaxine 172 that relied on a distinct [2,3]-Stevens rearrangement fScheme 1 S.4QL ° Proline derivative 173 was transformed into ammonium ylide 174 in the presence of allyl bromide and K2CO3. This zwitterion rearranged to a-allyl aminoester 175. Hydration of the olefin and reduction of the ester furnished diol 176, which was converted to aminoketone 177 via oxidation and aldol condensation. The assembly of this spirocyclic intermediate represented a formal synthesis of cephalotaxine 172. ... 

On the other hand the external asymmetric induction strongly depended on the chiral auxiliary. The careful analysis of the hypothetical zwitterionic intermediates c-a and c-fi indicated the formation of a stereogenic ammonium center. In terms of the well-known 1,3 chirality transfer of 3,3sigmatropic rearrangements, the... 

MacMillan has developed a Lewis acid-catalyzed Bellus-Claisen rearrangement in which a range of allylamines can add to ketenes via Lewis acids activation (253). This activation-addition pathway generates the zwitterionic allyl alkenyl ammonium complex as shovra for 250 which effectively rearranges to amide 251 with excellent syn selectivity. 

MacMillan has developed a Lewis acid-catalyzed addition-rearrangement sequence which facilitates the conjugate addition of allylamine 408 to allenoate ester 409 to generate the zwitterionic intermediate for the Claisen rearrangement. The minimization of steric interactions between the ammonium species and the y-allenyl substituent led... 

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