Alkenes Wagner-Meerwein rearrangement

From 5 the formation of alkene 2 is possible through loss of a proton. However, carbenium ions can easily undergo a Wagner-Meerwein rearrangement, and the corresponding rearrangement products may be thus obtained. In case of the Bamford-Stevens reaction under protic conditions, the yield of non-rearranged olefins may be low, which is why this reaction is applied only if other methods (e.g. dehydration of alcohols under acidic conditions) are not practicable. 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

Wagner-Meerwein rearrangement as part of an alkene isomerization. 

The ready availability of four-membered rings of defined stereochemistry from enone-alkene photocycloadditions has been the origin of several syntheses which employ Wagner-Meerwein rearrangements in subsequent steps. This is exemplified by a neat and very short synthesis of a-caryophyllene alcohol (40) from cyclopentene and 3-methylcyclohexenone which was described some years ago (see Scheme 15). The rearrangement in 40% sulfuric acid proceeded remarkably smoothly. A short synthesis of iso-comene (41 Scheme 16) also Illustrates this strategy... 

The carbonium ion may also be formed from an alkene or alcohol. The carbonium ion formed from any of these starting materials is particularly prone to rearrangement reactions. These are called Wagner-Meerwein rearrangements, and severely limit the synthetic utility of this reaction to form simple alkyl substituted aromatic compounds. The tendency to rearrange may be reduced if the acyl derivative is used instead. This modification is called the Friedel-Crafts acylation reaction, and it has the further advantage that normally only monoacylation occurs, instead of the polyalkylation that happens using the simple Friedel-Crafts reaction. 

Most ir-nucleophiles employed in iminium ion cyclizations have a predetennined postcyclization destiny. For example, aromatic terminators will rearomatize, organosilanes will eliminate silicon through anticipated pathways and acetals and enol ethers will produce carbonyl compounds. However, the cyclizations of simple alkenes have supplied products that are the formal results of eliminations, additions and Wagner-Meerwein rearrangements. Almost exclusively Mannich-type cyclizations of unsaturated amines have been employed to prepare piperidines. 

The acid-catalyzed dehydration of an alcohol to a rearranged alkene is known as a Wagner-Meerwein rearrangement. Propose a mechanism for the following Wagner-Meerwein rearrangement ... 

This concept has been nicely demonstrated in the electrophilic addition of norbomadiene-fused pyrazoles, the product distribution of which was dramatically effected by the electron donating ability of the substituent at N-1. Electron withdrawing substituents inhibited the Wagner-Meerwein rearrangement by destabilizing 47 and therefore resulted in a predominance of alkene addition products 49 and 50. Alternatively, in the absence of an... 

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