REARRANGEMENT OF (-)-BETA-CARYOPHYLLENE - A PRODUCT ANALYSIS AND FORCE-FIELD STUDY

The rearrangement of (-)-beta-caryophyllene (1) with sulfuric acid in ether has been reinvestigated. In the early stage, a multitude of prod ucts is formed. Of these, fourteen hydrocarbons (3, 4, 6-17) and four alcohols (5, 18-20) were obtained pure. From the very beginning, the t ricycloundecenes 15-17 accumulate quickly. This indicates that a previ ously unknown rearrangement of 1 by protonation of the exocyclic doubl e bond followed by transannular ring closure is a main reaction, and t hat the conformation beta alpha is involved. Three hydrocarbons (3, 4, 8) and three alcohols (5, 18, 19) proved to be stable. They are the m ain components in the late stage. A complete rearrangement scheme foll owed from a force field analysis using MMP2. First, all conformations up to 4 kcal above the global minimum of each product and each carbeni um ion necessary for its formation were detected automatically using t he search program HUNTER. Second, the conformation favoring a specific transformation was searched for. For transannular cyclizations, the c onformation with the shortest distance between the atoms to be bound a nd, for ring openings and 1,2-shifts, the conformation with the smalle st dihedral angle between the empty p-orbital and the bond to be broke n were selected. In all cases, the selection criteria proved valid: Th e stereochemistry of the conformations selected matched the stereochem istry of the products observed. On the basis of the results presented, a fully automated search prog ram for favorable rearrangement paths m ay be developed.