Stereochemistry Wagner-Meerwein rearrangement

Cram, D. Studies in Stereochemistry. I. The Stereospecific Wagner-Meerwein Rearrangement of the Isomers of 3-Phenyl-2-butanol. J. Amer. chem. Soc. 71, 3863 (1949). 

Shono, T., Fujita, K., Kumai, S. Stereochemistry of migrating carbon in Wagner-Meerwein rearrangement. Tetrahedron Lett. 1973, 3123-3126. 

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 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. 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

The predominantly syn stereochemistry of the products arising from the bicyclo[3.2.1] octyl cation IV would results from the large ring strain in II, the chair conformation of which, (but not boat) facilitate the Wagner-Meerwein type rearrangement. 

---