Simple Wagner-Meerwein Rearrangements

As a first approximation, the camphene part-structure in longifolene may be expected to mimic rearrangements typical of camphene. 

Longifolene on treatment with hydrogen halides undergoes the expected 1,2-shift of the bridge to furnish longibornyl halide (77), the structure of which (X = Cl), as mentioned earlier, was established by X-ray 

Longibornyl bromide (82) on treatment with ethanolic potassium hydroxide at reflux gives a 1 3 mixture of longicyclene (44) and longi-folene (39). The formation of longicyclene reflects an alternative pathway for stabilization open to the ion (80) (and 33) and may be compared with the formation of tricyclene (84) during solvolysis of certain isobomyl esters (83) (5d). 

Under the influence of cupric acetate in refluxing acetic acid, longifolene and longicyclene equilibrate to a 2 1 mixture (3 44), along with formation of some isolongifolene (23) (39). 

On exposure to hot acetic acid containing sulphuric acid, camphene gives an excellent conversion to isobornyl acetate (83, R = Ac). Under similar conditions, longifolene yields isolongifolene (23) as the major ( 60%) product and an approximately 1 3 mixture of acetates of longi- 

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The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

A Potentially Complex Wagner-Meerwein Rearrangement Made Simple... 

Prior to the advent of simple methods for radical deoxygenation, a double Wagner-Meerwein rearrangement (Scheme 23) was used as a method for removing a bridgehead hydroxy group in this series. The success of this sequence indicates the subtle balance of stabilities in this series. 

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. 

Oxidation and Reduction.—Oxidative rearrangement of olefins with thallium(m) nitrate in methanol provides a simple synthesis of aldehydes and ketones in high yields. Cleavage of the intermediate thallium compound proceeds via a transition state with high carbonium ion character, which leads either to carbonyl compounds (354) by Wagner-Meerwein rearrangement or to glycol methyl ethers (355). ... 

Figure 3.11 shows the biosynthesis of (+)-bornyl pyrophosphate (the precursor of (-l-)-borneol (28) and (+)-camphor (29)) and of (+)-sabinene (30) (the precursor of the thujones) from 3R)-20 cyclized in an anti,endo conformation (Wise et al. 1998). Other related products include camphene (31) and 1,8-cineole (32). The chemistry involved in the formation of the final products includes Wagner-Meerwein rearrangements of hydride and (in the case of camphene) a skeletal carbon-carbon bond as well as simple cyclizations. The biosynthesis of (-)-a-and P-pinene (33 and 34) proceeds along similar lines from (3S)-20 and is shown in Figure 3.12. 

Wagner-Meerwein and allylic rearrangements are very common whenever a carbonium ion is formed. For example, in the simple Friedel-Crafts alkylation, the alkyl group always tends to rearrange to give the most stable carbonium ion, which then adds to the aromatic ring. In order to prevent this rearrangement,... 

Apart from simple mechanisms (condensation, hydrogenation, substitutions, alkylations), more complicated reactions can also be performed (Wagner-Meerwein and pinacolone rearrangements, syndieses of heterocycles) [8],... 

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