Rearrangements apparent alkyl migration

It is apparent from Table 26 that the rate of rearrangement increases as ortho-alkyl substitution is increased. Table 27 presents these data with statistical and product ratio corrections. The rate coefficients associated with migration to the ortho and para positions are given by k and respectively. It is seen that a single ort/io-alkyl substituent is more effective in increasing the rate of rearrangement to a hydrogen occupied ortho position than is the same substituent in a para position. 

Successive Wagner-Meerwein rearrangements have been utilized in triterpenoid syntheses. A unique and fascinating example of this can be foimd in Corey s synthesis of the triterpenoid oleanene. When 3P-friedelanol (35) is treated with acid, a total of seven 1,2-alkyl and 1,2-hydride migrations occur leading to the formation of 37. The stereospecific shifts are driven by the apparent decrease in steric strain due to the original location of the axial substituents. Intermediate products have been isolated, supporting the assertion that at least some of the steps are not concerted. 

---