Wagner-Meerwein rearrangements derivatives

It was mentioned above that even alkanes undergo Wagner-Meerwein rearrangements if treated with Lewis acids and a small amount of initiator. An interesting application of this reaction is the conversion of tricyclic molecules to adamantane and its derivatives. It has been found that all tricyclic alkanes containing 10 carbons are converted to adamantane by treatment with a Lewis acid such as AICI3. If the substrate contains more than 10 carbons, alkyl-substituted adamantanes are produced. The lUPAC name for these reactions is Schleyer adamantization. Two examples are... 

The C-8-substituted derivatives of camphor are formed by a route from 3,3-dibromocamphor (33) outlined in Scheme 1This involves a series of Wagner-Meerwein rearrangements. 

These variously substituted camphor derivatives have in turn been used in a number of syntheses of natural products, some of which also involve Wagner-Meerwein rearrangements in their later stages. Examples drawn from the syntheses of (+)-a-santalene (34),(-)-3-santalene (35), (-)-copacamphene (36) and (-)-sativene (37) are given in Schemes 12-14. 

The low-temperature stability of bicyclo[2.2.1]heptane derivatives is well documented by the huge literature on liquid-phase norbornyl carbonium chemistry under such conditions, no ring expansion is observed, but instead, multiple Wagner-Meerwein rearrangements take place, preserving the bicyclo[2.2.1]heptane structure in the course of nucleophilic substitution or elimination reactions. This remarkable behavior is at the origin of the nonclassic carbonium ion concept (28). 

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. 

The case history of the Wagner-Meerwein rearrangement may serve as a guideline to an extensive discussion of mechanism and stereochemistry of cationic 1,2 shifts. First principles may be derived from the behavior of free carbocations as they occur in the gas phase (Section 2) and in solvents of extremely low nucleophilicity ( superacids ) (Section 3). While such conditions provide valuable information on energetics and some insight into mechanism, they virtually exclude stereochemical studies. 

S Cascade reactions implementing the Ritter reaction Recent examples of the Ritter reaction have combined this useful synthetic tool with other reactions and rearrangements to produce complex reaction sequences. For example, Bishop has reported the combination of the Ritter reaction with the Wagner-Meerwein rearrangement, which can result in complex and sometimes unpredictable reaction sequences.17 The use of the Ritter reaction to trap cationic intermediates in the CAN-mediated dimerization of alkoxystyrenes has recently allowed the synthesis of a-aminotetralin derivatives by Nair.18... 

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