6.57 There are many examples of carbocation rearrangements that cannot be classified as a methyl shift or hydride shift.

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6 57 There Are Many Examples Of Carbocation Rearrangements That Cannot Be Classified As A Methyl Shift Or Hydride Shift 1 (240.5 KiB) Viewed 52 times

6.57 There are many examples of carbocation rearrangements that cannot be classified as a methyl shift or hydride shift. For example, con- sider the following rearrangement that was employed in the synthesis of isocomene, a tricyclic compound isolated from the perennial plant Isocoma wrightii. 13 $-$ (a) Identify the carbon atom that is migrating and draw a mechanism for this carbocation rearrangement. (b) This process generates a new chiral center, yet only one configura- tion is observed. That is, the process is said to be diastereoselec- tive, because only one of the two possible diastereomeric carbo- cations is formed. Draw the diastereomeric carbocation that is not formed, and provide a reason for its lack of formation (Hint: You may find it helpful to construct a molecular model.) (c) In this case, a tertiary carbocation is rearranging to give a differ- ent tertiary carbocation. Suggest a driving force for this process (explain why this process is favorable).