Wagner—Meerwein-type carbocation rearrangements

The formation of sesquiterpenes by a carbocation mechanism means that there is considerable scope for rearrangements of the Wagner-Meerwein type. So far, only occasional hydride migrations have been invoked in rationalizing the examples considered. Obviously, fundamental skeletal rearrangements will broaden the range of natural sesquiterpenes even further. That such processes do occur has been proven beyond doubt by appropriate labelling experiments, and... 

The rearrangement of radicals does not occur so often as the rearrangement of carbocations because the difference in stabilities between radicals is smaller than the difference in stabilities between the corresponding carbocations. However, a ring contraction through a Wagner-Meerwein-type rearrangement of a radical was reported in studies related... 

The electrophilic additions of formic and acetic acids to l,5-dimethyl-l,5-cyclooctadiene yielded mainly - 5yu-8-substituted-l,5-dimethylbicyclo[3.2.1]octanes (equation 10) via parallel jr-cyclization and subsequent Wagner-Meerwein (W-M) type rearrangement. Cross TT-cyclization leading to bicyclo[3.3.0]octane derivatives, which were the major adducts in other electrophilic additions to unsubstituted l,5-cyclooctadiene2 22 comprised only a minor route. This different behavior has been interpreted (equation 11) in terms of a significantly larger stability of the tertiary carbocation II than that of the secondary ion in, both ions being the two potential intermediates derived from I by a parallel and a cross jr-cyclization, respectively. 

---