Wagner-Meerwein rearrangement, during

Concentrated sulfuric acid in dichloromethane solution appears to provide the optimal conditions to induce Wagner-Meerwein rearrangement during protonation of cycloprop-... 

Dehydration to olefins, which sometimes accompanies the reaction of alcohols with DAST [95, 108], is seldom as extensive as with a-fluoroamines (FAR and 1,1,2,3,3,3 hexafluoropropyldiethylamine) but occurs in a few cases to the exclusion of fluonnation, thus, 9a-fluoro-11-hydroxysteroids give 9a fluoro-A -steroids [127, 128] Dehydration accompanied by Wagner-Meerwein rearrangement occurs during the fluonnation of testosterone [129] Intermolecular dehydration to form ethers in addition to fluorides is observed in the reaction of benzhydryl alcohols [104] (Table 6)... 

The formation of the isocorrolecarbaldehyde 5 may be best explained if one invokes 1,2-diol 4 as an intermediate of the McMurry coupling which would then react in a titanium(IV) chloride induced Wagner-Meerwein rearrangement to yield the isocorrole 5 as a minor product during... 

In spite of the known tendency of norbornene and related systems to undergo rearrangements of the Wagner-Meerwein type during eleetro-philic addition, no such rearrangement was observed when norbornene underwent cycloaddition with the acridizinium or the. A/ -methylenium benzamide cation. As Schmidt correctly pointed out, this lack of rearrangement is an argument for a concerted reaction. Alternatively, if the cycloaddition is nonsynchronous, the time interval between step 1 and step 2 must be very short. 

Also in agreement with a cationic intermediate but not with concerted addition is the fact that Wagner-Meerwein rearrangements sometimes occur during hydration.6... 

Wagner-Meerwein rearrangements comprise reorganization of the carbon skeleton in eliminations that lead to olefins, in additions to multiple bonds, and in nucleophilic substitutions. Their common characteristic is that after or during formation of an electron deficiency on a carbon atom an alkyl or aryl group moves to that site from a neighboring carbon atom. The electron deficiency can occur as follows ... 

Similar reactions called Wagner-Meerwein rearrangements were discovered over one hundred years ago. The mechanistic explanation is credited to Frank Whitmore of Penn State who carried out a systematic study of rearrangements during the 1930s. 

Perhaps the most spectacular of the natural carbocation rearrangements is the concerted sequence of 1,2-methyl and 1,2-hydride Wagner-Meerwein shifts that occurs during the formation oflanosterol from squalene. Lanosterol is then the precursor of the steroid cholesterol in animals. 

During the acid catalyzed esterification of dicyclopentadiene 47, a rapid Wagner-Meerwein type rearrangement occurs simultaneously, according to the reaction scheme 2 (Eq. 15.5.2). The ester compound contains exclusively the dicyclopentenyl component in the exo-form. 

Simpler cases of Wagner-Meerwein processes are also known. For example, alkyl migration during addition of HX to alkenes (Chapter 6, Scheme 6.19) has already been noted. Similarly, to the extent that the same carbocations are generated during nucleophilic substitution reactions, the same processes occur Indeed, almost identical rearrangments will be encountered again in Chapter 8 in the discussion of derivatives of alcohols as they are here with alkyl halides. 

Many of the most interesting rearrangements involving 1,2-shifts were discovered during structural studies of naturally occurring compounds such as terpenes by H. Meerwein and G. Wagner. 

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