Rearrangement of (-)-β-caryophyllene. A product analysis and force field study

Article abstract of DOI:10.1021/ja00141a009

The rearrangement of (-)-β-caryophyllene (1) with sulfuric acid in ether has been reinvestigated. In the early stage, a multitude of products is formed. Of these, fourteen hydrocarbons (3, 4, 6-17) and four alcohols (5, 18-20) were obtained pure. From the very beginning, the tricycloundecenes 15-17 assumulate quickly. This indicates that a previously unknown rearrangement of 1 by protonation of the exocyclic double bond followed by transannular ring closure is a main reaction, and that the conformation βα is involved. Three hydrocarbons (3, 4, 8) and three alcohols (5, 18, 19) proved to be stable. They are the main components in the late stage. A complete rearrangement scheme followed from a force field analysis using MMP2. First, all conformations up to 4 kcal above the global minimum of each product and each carbenium ion necessary for its formation were detected automatically using the search program HUNTER. Second, the conformation favoring a specific transformation was searched for. For transannular cyclizations, the conformation with the shortest distance between the atoms to be bound and, for ring openings and 1,2-shifts, the conformation with the smallest dihedral angle between the empty p-orbital and the bond to be broken were selected. In all cases, the selection criteria proved valid: The stereochemistry of the conformations selected matched the stereochemistry of the products observed. On the basis of the results presented, a fully automated search program for favorable rearrangement paths may be developed.